JP2009104834A - Multilayer porous membrane - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a multilayer porous film which has superior oxidation resistance and high permeability and is particularly preferable as a separator for a nonaqueous electrolytic battery in which safety and high output are required. <P>SOLUTION: The multilayer porous film is a lamination composed of a porous layer made of a thermoplastic resin and a porous layer made of a polyphenylene ether, wherein the porous layer made of a polyphenylene ether has air permeability of less than 60 sec/100 ml. <P>COPYRIGHT: (C)2009,JPO&INPIT

Description

  The present invention relates to a separator and the like used for a nonaqueous electrolyte secondary battery having excellent oxidation resistance and high permeability and having high safety and output characteristics.

The porous membrane is widely used as a material separation, selective permeation and isolation material. Among them, polyolefin microporous membranes are used as separators for various batteries, and polyethylene microporous membranes are particularly preferably used as separators for lithium ion secondary batteries. Polyethylene microporous membrane has excellent electronic insulation, ion permeability when impregnated with electrolyte, excellent resistance to electrolyte and oxidation, moderate strength, etc. This is considered to be the reason why they are suitably used because they have a hole blocking effect that cuts off the current at a temperature of about 120 to 150 ° C. at the time of temperature rise and suppresses excessive temperature rise.
In recent years, lithium-ion secondary batteries have been developed for high capacity and high output such as automotive applications, so the danger at the time of short-circuiting has further increased, and the separator has a higher level of safety. Is required. In addition, due to the higher operating voltage, the deterioration of the separator due to the oxidation reaction in the vicinity of the positive electrode has emerged as a major problem, and a solution has been demanded.

In Patent Document 1, a layered body including a layer having an oxygen index of 26 or more and a layer having a shutdown of 140 ° C. or less and a layer having a thermal deformation temperature of 100 ° C. or more is used for a nonaqueous secondary battery. It is disclosed as a separator. An example of a material having an oxygen index of 26 or more is polyphenylene oxide (another name for polyphenylene ether).
Patent Document 2 discloses a separator made of polyphenylene ether.
However, in any of the prior arts, a problem remains in permeability, and a problem remains because a porous film having both oxidation resistance and high permeability cannot be obtained.
JP 2006-269359 A JP 2000-138048 A

  An object of the present invention is to provide a multilayer porous membrane that is excellent in oxidation resistance and has high permeability, and that is particularly suitable as a separator for a nonaqueous electrolyte battery that requires high safety and high output.

The present inventors are a laminate comprising a porous layer made of a thermoplastic resin and a porous layer made of polyphenylene ether, and a multi-layer porous film having an air permeability of less than 60 seconds / 100 ml of the porous layer made of polyphenylene ether, The present inventors have found that the above problems can be solved and have reached the present invention.
That is, the present invention is as follows.
1. A multilayer porous membrane comprising a porous layer made of a thermoplastic resin and a porous layer made of polyphenylene ether, wherein the air permeability of the porous layer made of polyphenylene ether is less than 60 seconds / 100 ml.
2. 1. The air permeability of a porous layer made of polyphenylene ether is smaller than the air permeability of a porous layer made of a thermoplastic resin. 2. The multilayer porous membrane described in 1.
3. 1. The thermoplastic resin is a porous layer made of a polyolefin resin. Or 2. 2. The multilayer porous membrane described in 1.
4). 1. The maximum pore size of a porous layer made of polyphenylene ether is greater than 0.1 μm and not more than 20 μm. ~ 3. A multilayer porous membrane according to any one of the above.
5). 1. The porous layer made of polyphenylene ether is a porous layer made of an ultrafine fiber fiber aggregate having an average fiber diameter of 0.01 μm or more and 10 μm or less. ~ 3. A multilayer porous membrane according to any one of the above.
6.1. ~ 5. A separator for a non-aqueous electrolyte secondary battery comprising the multilayer porous membrane according to any one of the above.
7.6. A non-aqueous electrolyte secondary battery using the separator described in 1.

  The multilayer porous membrane of the present invention is a multilayer porous membrane having excellent oxidation resistance and excellent permeability. In particular, it is possible to provide a multilayer porous membrane that is particularly suitable as a separator for a non-aqueous electrolyte battery that requires particularly high safety and high output.

Hereinafter, the present invention will be described in detail.
<Structure of porous layer made of polyphenylene ether>
The multilayer porous membrane of the present invention has at least one porous layer made of polyphenylene ether. The porous layer made of polyphenylene ether is preferably disposed on the surface layer of one or both sides of the multilayer film. The polyphenylene ether used in the present invention comprises a repeating unit structure of the following formula,

(R ₁ and R ₄ each independently represents hydrogen, chlorine, bromine, iodine, primary or secondary lower alkyl, phenyl, aminoalkyl, hydrocarbon oxy. R ₂ and R ₃ each independently. Represents hydrogen, primary or secondary lower alkyl, and phenyl.) Reduced viscosity (0.5 g / dl, chloroform solution, measured at 30 ° C.) in the range of 0.15 to 1.0 dl / g. Preferred are certain homopolymers and copolymers. A more preferred reduced viscosity is in the range of 0.20 to 0.70 dl / g, most preferably in the range of 0.40 to 0.60 dl / g. A reduced viscosity of 1.0 dl / g or less is preferable because it is excellent in oxidation resistance. When the viscosity is in this range, the polymer concentration and the solution viscosity can be adjusted to an appropriate range when a porous membrane is produced by the electrospinning method.

  Specific examples of the polyphenylene ether include poly (2,6-dimethyl-1,4-phenylene ether), poly (2-methyl-6-ethyl-1,4-phenylene ether), and poly (2-methyl -6-phenyl-1,4-phenylene ether), poly (2,6-diphenyl-1,4-phenylene ether), poly (2,6-dichloro-1,4-phenylene ether) and the like. From the viewpoint of oxidation resistance, a polyphenylene ether copolymer such as a copolymer of 2,6-dimethylphenol and other phenols (for example, 2,3,6-trimethylphenol or 2-methyl-6-butylphenol) is used. A polymer is also mentioned. Among them, poly (2,6-dimethyl-1,4-phenylene ether), a copolymer of 2,6-dimethylphenol and 2,3,6-trimethylphenol is preferable, and poly (2,6-dimethyl- 1,4-phenylene ether) is preferred.

The polyphenylene ether may be used alone, a polymer compatible with the polyphenylene ether may be mixed, or an incompatible polymer, inorganic particles, glass fiber, etc. may be mixed with the polyphenylene ether. Absent. Examples of the polymer compatible with polyphenylene ether include polystyrene.
The air permeability of the porous layer made of polyphenylene ether is less than 60 seconds / 100 ml. It is preferably 40 seconds / 100 ml or less, and more preferably 10 seconds / 100 ml or less. When the air permeability is less than 60 seconds / 100 ml, good output characteristics can be obtained when used as a battery separator. When such a multilayer porous membrane having a porous layer made of polyphenylene ether having a low air permeability is used as a battery separator, and the porous layer made of polyphenylene ether is disposed on the separator surface in contact with the positive electrode active material layer, A remarkably good result can be obtained also in terms of chemical conversion. The reason is not clear, but think as follows. In the battery, the positive electrode active material layer and the separator surface are in close contact with each other, and the ion migration resistance at the interface is much larger than that in the separator. Since the increase in the ion transfer resistance causes an increase in the internal resistance, the voltage actually applied to the battery is higher than the set value, and the positive electrode is in a higher potential state.

  Therefore, the oxidative degradation of the separator is more likely to proceed on the positive electrode surface. However, when the air permeability of the separator is as low as less than 60 seconds / 100 ml, the increase in ion migration resistance at the interface between the positive electrode active material layer and the separator is small, and the state is as low as the separator bulk. Inferred. Therefore, it is speculated that the increase in the internal resistance is suppressed, the increase in the positive electrode potential can be suppressed, and the oxidation deterioration of the separator can be reduced. Any porous structure may be used as long as the air permeability is less than 60 seconds / 100 ml. The pore structure may be composed of a microporous film having a fine three-dimensional network structure or an aggregate of fibers. The fiber aggregate is a structure formed by subjecting fibers to operations such as weaving, knitting, and lamination, and examples thereof include a nonwoven fabric.

A suitable structure includes an aggregate of fibers. Of these, ultrafine fibers having a small average fiber diameter are preferable, and the average fiber diameter of the ultrafine fibers is preferably in the range of 0.01 μm to 10 μm. An average fiber diameter of 10 μm or less is preferred because the resulting porous layer has excellent uniformity and denseness. Moreover, when it uses as a battery separator, it exists in the tendency which can suppress the increase in the ion migration resistance in an interface, and is preferable. If it is 0.01 micrometer or more, it is excellent in intensity | strength. The average fiber diameter of the fibers is preferably 0.03 μm or more and 1 μm or less, and more preferably 0.03 μm or more and 0.5 μm or less.
The thickness of the porous layer made of polyphenylene ether is preferably 0.1 μm or more and less than 20 μm. 1 μm or more and less than 15 μm is more preferable, and 3 μm or more and less than 10 μm is more preferable.

  In the case of 1 μm or more, for example, when used as a battery separator and the surface in contact with the electrode surface is a porous layer made of polyphenylene ether, the electrode active material is a lower layer of the porous layer made of polyphenylene ether (2 counted from the interface with the electrode). It is preferable because contact with the second and subsequent layers can be suppressed, and deterioration of the separator can be suppressed. If the thickness is less than 20 μm, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the battery capacity. The porosity is preferably 40% or more and 99% or less, and more preferably 60% or more and 95% or less. If it is 40% or more, the increase in resistance at the interface tends to be suppressed, and if it is 99% or less, contact with the lower layer of the porous layer made of polyphenylene ether as the electrode active material (second and subsequent layers counted from the interface with the electrode) is suppressed. This is preferable because it tends to suppress the deterioration of the separator. The maximum pore size of the porous layer made of polyphenylene ether is preferably larger than 0.1 μm and not larger than 20 μm, more preferably not smaller than 0.2 μm and not larger than 10 μm. If it is larger than 0.1 μm, the increase in resistance at the interface tends to be suppressed, and if it is less than 20 μm, the electrode active material is in contact with the lower layer of the porous layer made of polyphenylene ether (the second and subsequent layers counted from the interface with the electrode). This is preferable because it can suppress the deterioration of the separator.

<Structure of porous layer made of thermoplastic resin>
The multilayer porous membrane of the present invention has at least one porous layer made of a thermoplastic resin. Examples of the thermoplastic resin include polyolefin resin, fluororesin, polystyrene, ABS resin, vinyl chloride resin, vinyl acetate resin, acrylic resin, polyamide resin, acetal resin, polycarbonate, etc., particularly for non-aqueous electrolyte batteries. When used as a separator, a polyolefin resin is preferred from the viewpoint of resistance to electrolytic solution.
The polyolefin resin refers to a polyolefin resin used for normal extrusion, injection, inflation, blow molding and the like, such as ethylene, propylene, 1-butene, 4-methyl-1-pentene, 1-hexene, and 1-octene. Homopolymers and copolymers, multistage polymers, and the like can be used. In addition, polyolefins selected from the group of these homopolymers, copolymers, and multistage polymers can be used alone or in combination. Representative examples of the polymer include low density polyethylene, linear low density polyethylene, medium density polyethylene, high density polyethylene, ultrahigh molecular weight polyethylene, isotactic polypropylene, atactic polypropylene, polybutene, and ethylene propylene rubber. . When the porous membrane of the present invention is used as a battery separator, a resin having a high melting point polyethylene as a main component (for example, 10 parts by mass or more) is used because it is a low melting point resin and requires high piercing strength. Is preferred. In the present invention, the above polyolefin resins can be used alone or in combination of two or more.

  The porous layer structure made of thermoplastic resin may be any porous layer as long as it is a porous layer including communication holes. The inclusion of the communication hole means that it has a hole structure communicating from one side to the other side, and can be confirmed by measuring gas permeability from one side to the other side. If the air permeability, which is an indicator of gas permeability, is 10,000 seconds or less, it can be said to be a porous layer. The structure of the porous layer may be a microporous film or a collection of fibers having a three-dimensional network structure with a fine pore structure. The fiber aggregate is a structure formed by subjecting fibers to operations such as weaving, knitting, and lamination, and examples thereof include a nonwoven fabric. When used as a battery separator, a microporous membrane is preferred from the viewpoint of reliability.

  The thickness of the porous layer made of a thermoplastic resin is preferably 1 μm or more and 40 μm or less, more preferably 2 μm or more and 35 μm or less, and further preferably 5 μm or more and 30 μm or less. For example, when it is used as a battery separator, if it is 1 μm or more, it is excellent in handling and operability, and if it is 40 μm or less, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery. The porosity is preferably in the range of 25% to 70%, more preferably 30% to 60%. When the porosity is 25% or more, the permeability is not easily lowered. On the other hand, when the porosity is 70% or less, there is little possibility of self-discharge when used as a battery separator, which is reliable. The air permeability is preferably 10 seconds / 100 ml or more and 1000 seconds / 100 ml or less, more preferably 60 seconds / 100 ml or more and 500 seconds / 100 ml or less, and still more preferably 80 seconds / 100 ml or more and 300 seconds / 100 ml or less.

  When the air permeability is 10 seconds / 100 ml or more, self-discharge is small when used as a battery separator, and when the air permeability is 1000 seconds / 100 ml or less, good charge / discharge characteristics are obtained. The piercing strength is preferably 2N or more, more preferably 3N or more, and further preferably 4N or more. If it is 2N or more, it is possible to suppress film breakage due to the dropped active material or the like during battery winding. In addition, there is little fear of short-circuiting due to the expansion and contraction of the electrodes accompanying charging and discharging. Although there is no restriction | limiting in particular in an upper limit, 10 N or less is preferable from a viewpoint which can reduce heat shrink. The maximum pore diameter is preferably from 0.01 μm to 0.5 μm, more preferably from 0.02 μm to 0.3 μm, and even more preferably from 0.03 μm to less than 0.1 μm. If the maximum pore diameter is 0.01 μm or more, the permeability is good and preferable. If it is 0.5 micrometer or less, when it uses as a battery separator, there is little possibility of a self-discharge and it is preferable from a viewpoint of reliability.

<Structure of multilayer porous membrane>
The multilayer porous membrane of the present invention is a laminate comprising a porous layer made of a thermoplastic resin and a porous layer made of polyphenylene ether. The porous layer made of a thermoplastic resin and the porous layer made of polyphenylene ether are preferably bonded. When the multilayer porous film has three or more layers, it is preferable that a porous layer made of polyphenylene ether forms a surface layer from the viewpoint of suppressing oxidative degradation. The total thickness of the multilayer porous membrane is preferably 2 μm or more and less than 60 μm, more preferably 4 μm or more and less than 50 μm, and even more preferably 8 μm or more and less than 40 μm. For example, when used as a battery separator, if it is 2 μm or more, it is excellent in handling and operability, and if it is less than 60 μm, the occupied volume of the separator is reduced, which tends to be advantageous in terms of increasing the capacity of the battery. The air permeability is preferably 10 seconds / 100 ml or more and 1000 seconds / 100 ml or less, more preferably 60 seconds / 100 ml or more and 500 seconds / 100 ml or less, and still more preferably 80 seconds / 100 ml or more and 300 seconds / 100 ml or less. When the air permeability is 10 seconds / 100 ml or more, self-discharge is small when used as a battery separator, and when the air permeability is 1000 seconds / 100 ml or less, good charge / discharge characteristics are obtained. The piercing strength is preferably 2N or more, more preferably 3N or more, and further preferably 4N or more. If it is 2N or more, it is possible to suppress film breakage due to the dropped active material or the like during battery winding. In addition, there is little fear of short-circuiting due to the expansion and contraction of the electrodes accompanying charging and discharging. Although there is no restriction | limiting in particular in an upper limit, 10 N or less is preferable from a viewpoint which can reduce heat shrink.

As the structure of the multilayer porous membrane, the air permeability of the porous layer made of polyphenylene ether is preferably lower than the air permeability of the thermoplastic resin layer. The maximum pore size of the porous layer made of polyphenylene ether is preferably larger than the maximum pore size of the thermoplastic resin. When used as a battery separator, the structure of the porous layer made of polyphenylene ether and the porous layer made of a thermoplastic resin of the multilayer porous film of the present invention are differentiated, so that excellent oxidation resistance and output characteristics are easily developed.
An example of the manufacturing method of the multilayer porous membrane of this invention is demonstrated.
The multilayer porous membrane of the present invention may be achieved by using an existing method for producing a microporous membrane or a method for producing a fiber assembly. Moreover, after manufacturing each porous layer independently, you may make it a laminated body in a subsequent process, and you may make it porous after making a laminated body, and you may manufacture a porous layer on a porous layer.

Examples of the method for producing a porous layer made of a thermoplastic resin include the following examples.
(1) An extraction method in which a plasticizer that can be easily extracted and removed in a subsequent process is added to a thermoplastic resin, and then molded, and then the plasticizer is removed with a suitable solvent to make it porous (2) A crystalline polymer material After forming, stretching method that selectively stretches structurally weak amorphous parts to form micropores (3) Perform molding by adding filler to thermoplastic resin, and then perform thermoplastic resin by stretching operation Interfacial exfoliation method in which the interface with the filler is exfoliated to form micropores (4) Phase conversion method in which a thermoplastic resin is dissolved in a good solvent, cast into a sheet, and then contacted with a poor solvent to form a porous layer When used as a battery separator, from the viewpoint of a dense and uniform pore structure, high strength, etc., in the extraction method (1), in particular, in the extraction method (1), either before plasticizer extraction or plasticizer extraction, or A production method in which both are combined with stretching is preferred.

Specifically, a production method including the following steps (a) to (d) is exemplified.
(A) A step of melt-kneading a polyolefin resin, a plasticizer, an antioxidant, and the like in a nitrogen atmosphere.
(B) A step of extruding the melt, forming it into a sheet and cooling and solidifying it.
(C) A step of stretching in at least a uniaxial direction.
(D) A step of extracting the plasticizer.
The order and number of these steps are not particularly limited, but (a) step → (b) step → (c) step → (d) step order, (a) step → (b) step → (c) The order of process → (d) process → (c) process or (a) process → (b) process → (d) process → (c) process is preferable, and (a) process → (b) process → ( The order of c) process → (d) process, (a) process → (b) process → (c) process → (d) process → (c) process is more preferable.

  In addition, if necessary, known additives such as metal soaps such as calcium stearate and zinc stearate, ultraviolet absorbers, light stabilizers, antistatic agents, antifogging agents, and coloring pigments also impair film formation. And can be mixed and used within a range not impairing the requirements and effects of the present invention. Furthermore, polymer materials other than polyolefin resins and other organic and inorganic materials can be blended without impairing the film forming property and within the range not impairing the requirements and effects of the present invention.

Examples of the method for producing a porous layer made of polyphenylene ether include the following.
(1) Interfacial exfoliation method in which a filler is added to polyphenylene ether, molding is performed, and then the interface between polyphenylene ether and filler is exfoliated by a stretching operation to form micropores. (2) Polyphenylene ether is dissolved in a good solvent. A phase change method in which a porous layer is formed by contact with a poor solvent after casting into a sheet

As a method for producing a porous layer made of polyphenylene ether, the following examples can be given when producing a non-woven fabric of fiber aggregates.
(1) A dry method in which the card used in the spinning process is used, or fibers dispersed in the air are laminated in a sheet shape, and the fibers are bonded by one or more bonding methods.
(2) Wet method in which fibers are dispersed in a dispersion liquid such as water by paper making, and these are accumulated in a sheet form, and the fibers are bonded by one or more bonding methods. (3) Melting of polyphenylene ether or A spunbond method in which spun continuous fibers are laminated on a moving screen by melting and the fibers are bonded by one or more bonding methods. (4) A polyphenylene ether solution is formed in an electrostatic field formed between the electrodes. Electrospinning method to obtain an aggregate of fibers by discharging using a nozzle, etc., by thinning and solidifying the solution by electrostatic force and volatilization of the solvent and depositing ultrafine fibrous material on the collector When used as a separator, an aggregate of ultrafine fibers is preferable from the viewpoint of obtaining a dense and uniform pore structure, and the electrostatic spinning method (4) is preferable.

As an example of a method for producing a porous layer made of polyphenylene ether, a method for producing by an electrospinning method will be described.
First, the step of preparing a polymer solution in which polyphenylene ether is dissolved in a solvent will be described.
The solvent is not particularly limited as long as it is a substance that has a boiling point of 250 ° C. or less at normal pressure and is a liquid at room temperature and can dissolve polyphenylene ether. For example, chloroform, 1,2-dichloroethane, 1, Halogenated hydrocarbon solvents such as 1,2-trichloroethane and 1,1,2,2-tetrachloroethane are exemplified.

In addition, aromatic hydrocarbon solvents such as benzene, toluene and o-xylene, cyclic ether solvents such as tetrahydrofuran and dioxane, phenol solvents such as o-chlorophenol, and N-methyl-2-pyrrolidone (NMP) are listed. It is done. These solvents may be used alone or as a mixed solvent of two or more kinds.
A mixed solvent containing a halogenated hydrocarbon compound and a low-volatile solvent having a boiling point of 140 ° C. or higher has good solubility of polyphenylene ether, and because of low volatilization speed, needle contamination and needle clogging hardly occur, and stable spinning for a long time. Is preferable because it is possible.
The solvent should be a mixed solvent of an aromatic hydrocarbon solvent and at least one solvent selected from N-methyl-2-pyrrolidone, N, N-dimethylformamide, and N, N-dimethylacetamide. It is preferable because it has low toxicity and good solubility of polyphenylene ether, and since it has a low volatilization rate, needle contamination and needle clogging hardly occur and stable spinning can be performed for a long time.

In order to increase the dielectric constant of the solvent, an amount of poor solvent that does not cause polyphenylene ether to precipitate may be mixed. Examples of the poor solvent include N, N-dimethylformamide (DMF), N, N-dimethylacetamide (DMAc) and the like.
In order to increase the electrical conductivity of the solution, the electrospinning solution preferably contains 0.01 to 5 wt% of an organic acid salt or inorganic acid salt, more preferably in the range of 0.1 to 2 wt%. . Examples of inorganic acid salts include lithium chloride, potassium chloride, sodium chloride, monovalent inorganic salts such as lithium bromide, potassium bromide, potassium fluoride, calcium chloride, magnesium chloride, calcium bromide, calcium fluoride, etc. Examples of the divalent inorganic salt and organic acid salt include ammonium salts such as tetrabutylammonium chloride, tetrabutylammonium bromide, and benzyltriethylammonium chloride.

The polymer concentration of the polyphenylene ether solution is 5 wt% or more and 20 wt% or less when a polymer having a weight average molecular weight of about 5 × 10 ⁴ is used, although it depends on the polymer molecular weight used.
When the polymer concentration is 5 wt% or more, it is preferable because the solution viscosity is easy to form a fiber structure. Moreover, when the polymer concentration is 20 wt% or less, the fiber diameter of the obtained fiber is preferably a solution viscosity that tends to be thin.
In addition, when supplying a polymer solution with a nozzle in an electrospinning method, if the solvent is only a low-boiling solvent component having a boiling point of 100 ° C. or less at normal pressure, nozzle contamination and clogging are likely to occur, which is effective during continuous production. Becomes worse. Therefore, it is preferable to use 20 to 100% by weight of a solvent having a boiling point of 140 ° C. or higher as a solvent because needle clogging and needle contamination do not occur and it becomes possible to produce ultrafine fibers stably and stably. A more desirable ratio is 50 to 100% by weight. Examples of the high boiling point solvent having a boiling point of 140 ° C. or higher at normal pressure include N, N-dimethylacetamide, N, N-dimethylformamide, dimethyl sulfoxide, 1,1,2,2-tetrachloroethane, o-chlorophenol, N- Examples include methyl-2-pyrrolidone and o-xylene.

Next, the step of spinning the polymer solution by an electrostatic spinning method will be described. Any convenient method can be used to introduce the solution into the electrostatic field. For example, as shown in FIG. 1, the solution is put into a solution holding tank 1, and the solution is extruded at an arbitrary flow rate through a metal nozzle 3 by a metering pump 2, and at the same time, the high voltage generator 4 causes the nozzle 3 to By applying a voltage, an electrostatic field is formed between the nozzle 3 and the grounded collector 5. The polymer solution extruded in the electrostatic field is stretched by repulsion of electric charges in the solution to form ultrafine fibers and collected by the collector. By disposing a porous film 6 made of a thermoplastic resin between the nozzle 3 and the collector 5, it is possible to obtain a multilayer porous film made of a thermoplastic resin and a porous layer made of polyphenylene ether, which is preferable.
In this method, an electrostatic field may be generated between the nozzle 3 and the collector 5, and a high voltage may be applied to the collector and the nozzle 4 may be grounded. The magnitude of the applied voltage is preferably 5 kv or more and 100 kV or less. When the applied voltage is 5 kV or more, the repulsive force of the charge in the solution is sufficient to cause fiber formation, and when it is 100 kV or less, it is preferable that dielectric breakdown of air hardly occurs. A more preferable range is 8 to 50 kV.

The distance between the nozzle 4 and the collector 5 is preferably 1 to 20 cm. When the porous film 6 made of a thermoplastic resin made of a thermoplastic resin is disposed on the collector 5 to directly obtain a multilayer porous film, a range of 1 cm to 10 cm is preferable. When the distance between the nozzle and the collector is in the range of 1 cm to 10 cm, the solvent may not completely volatilize before the solution reaches the porous film, which is preferable because the adhesion with the porous film is improved. In the case where a fiber assembly of polyphenylene ether alone is prepared in the collector 5, a range of 5 to 20 cm is preferable. Shorter than 20 cm is preferable because it can be achieved with a relatively small applied voltage to form an electrostatic field that can be fiberized.
The inner diameter of the nozzle 4 is preferably 0.1 to 2 mm. In consideration of the balance between productivity and the obtained fiber diameter, 0.5 to 1.2 mm is more preferable.
Further, it is preferable to use a multi-nozzle for increasing productivity.
The temperature of the solution holding tank 1 and the nozzle 2 depends on the combination of the solvents, but can be arbitrarily selected as long as the temperature is in a range where the polymer is uniformly dissolved, and is preferably in the range of 20 to 80 ° C.

Examples of the present invention will be described below.
Each value in the examples was determined by the following method.
(1) Film thickness It measured at room temperature (23 degreeC) with the dial gauge (Ozaki Seisakusho: PEACOCK No.25).
(2) Air permeability It measured with the Gurley type air permeability meter based on JIS P-8117.
When the porous layer made of polyphenylene ether was peelable from the multilayer porous film, the air permeability of the porous layer made of polyphenylene ether was measured after peeling. If the porous layer made of polyphenylene ether cannot be removed from the multilayer porous membrane, use a solvent that is a good solvent for polyphenylene ether and does not dissolve the thermoplastic resin of the multilayer porous membrane, and dissolve and remove the porous layer made of polyphenylene ether Then, the air permeability was measured, and the air permeability subtracted from the air permeability of the multilayer porous membrane was defined as the air permeability of the porous layer made of polyphenylene ether.

(3) Maximum pore diameter Based on ASTM F-316-86, it was measured using ethanol.
When the porous layer made of polyphenylene ether was peelable from the multilayer porous film, the maximum pore size of the porous layer made of polyphenylene ether was measured after peeling. When the porous layer made of polyphenylene ether could not be peeled off from the multilayer porous film, the surface was photographed using a scanning electron microscope and measured. When the porous layer made of polyphenylene ether is a microporous membrane, the major axis of the pore is defined as the pore size. Among the measured hole diameters, the maximum hole diameter is defined as the maximum hole diameter. When the porous layer made of polyphenylene ether is an aggregate of fibers, images are taken at a magnification at which several tens of fibers are observed in one field of view, and a quadrangle composed of four fibers is regarded as a hole. When a rectangle is further formed inside the rectangle, the rectangle inside is regarded as a hole. The square of the smallest unit is a hole, and the major axis is the hole diameter. Among the measured hole diameters, the maximum hole diameter is defined as the maximum hole diameter.

(4) Average fiber diameter Photographed at a magnification at which several tens of fibers were observed in one field of view using a scanning electron microscope, measured the diameters of 10 arbitrary fibers, and the average value was defined as the average fiber diameter Da.
(5) Puncture strength Using a Kato-Tech KES-G5 handy compression tester, a puncture test was conducted under the conditions of a radius of curvature of the needle tip of 0.5 mm and a puncture speed of 2 mm / sec. It was.
(6) Reduced viscosity Polyphenylene ether was dissolved in chloroform to a concentration of 0.5 g / dl, and the viscosity at 30 ° C. was measured using an Ubbelohde viscosity tube.

(7) Evaluation of oxidation resistance a. Fabrication of positive electrode
92.2% by mass of lithium cobalt composite oxide (LiCoO ₂ ) as a positive electrode active material, 2.3% by mass of flake graphite and acetylene black as a conductive material, and 3.2% of polyvinylidene fluoride (PVDF) as a binder, respectively. % Is dispersed in N-methylpyrrolidone (NMP) to prepare a slurry. This slurry is applied to one side of a 20 μm thick aluminum foil serving as a positive electrode current collector with a die coater, dried at 130 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material coating amount of the positive electrode is 250 g / m ² , and the active material bulk density is 3.00 g / cm ³ .

b. Production of negative electrode
A slurry is prepared by dispersing 96.6% by mass of artificial graphite as a negative electrode active material, 1.4% by mass of ammonium salt of carboxymethyl cellulose and 1.7% by mass of styrene-butadiene copolymer latex as binders in purified water. This slurry is applied to one side of a 12 μm-thick copper foil serving as a negative electrode current collector with a die coater, dried at 120 ° C. for 3 minutes, and then compression molded with a roll press. At this time, the active material application amount of the negative electrode is set to 106 g / m ² , and the active material bulk density is set to 1.35 g / cm ³ .
c. Preparation of Nonaqueous Electrolyte Solution Prepared by dissolving LiPF ₆ as a solute in a mixed solvent of ethylene carbonate: ethyl methyl carbonate = 1: 2 (volume ratio) to a concentration of 1.0 mol / L.

d. Battery assembly
The multilayer porous membrane is cut into a circle of 30 mmφ, and the positive electrode and the negative electrode are cut into a circle of 16 mmφ. The container and the lid are insulated, the container is in contact with the negative electrode copper foil, and the lid is in contact with the positive electrode aluminum foil. The non-aqueous electrolyte described above is injected into this container and sealed.
After standing at room temperature for 1 day, the battery voltage was charged to 4.2V with a current value of 2mA (0.33C) in a 25 ° C atmosphere, and the current value was reduced from 2mA so as to maintain 4.2V after reaching the battery voltage. The first charge after making the battery for a total of 8 hours is performed by the method of starting.
Subsequently, the battery is discharged to a battery voltage of 3.0 V at a current value of 2 mA (0.33 C).

e. Evaluation In a 25 ° C atmosphere, the battery voltage was charged to 4.2 V at a current value of 6 mA (1.0 C), and after reaching the voltage value, the current value was started to be reduced from 6 mA so as to hold 4.2 V, for a total of 3 hours. Charge the battery. Next, the battery is stored for 7 days in an atmosphere of 70 ° C. while being charged so as to hold 4.4V. Thereafter, the cell is taken out and discharged to a battery voltage of 3.0 V at a current value of 6 mA (1.0 C) in a 25 ° C. atmosphere.
In addition, the multilayer porous membrane is taken out from the battery, and ultrasonic cleaning is performed in dimethoxyethane, ethanol, and 1 N hydrochloric acid for 15 minutes each in order to remove deposits.
Then, it dries in the air, visually observes the black discoloration on the positive electrode contact surface side of the multilayer porous membrane, and evaluates oxidation resistance. Here, when the ratio of black discoloration is 3% or less per area, it is determined that there is no black discoloration (A), and when it is 10% or less, it is determined that there is almost no black discoloration (O), and the ratio exceeds 10%. It was determined that there was a black discoloration (×).

(8) Output characteristic evaluation In heat resistance evaluation, a. Making a positive electrode, b. Preparation of negative electrode, c. Preparation of a non-aqueous electrolyte, d. The same contents as the battery assembly were carried out to produce a simple battery.
e. Evaluation In a 25 ° C atmosphere, the battery voltage was charged to 4.2 V at a current value of 6 mA (1.0 C), and after reaching the voltage value, the current value was started to be reduced from 6 mA so as to hold 4.2 V, for a total of 3 hours. Charge the battery. Next, the battery is discharged to a battery voltage of 3.0 V at a current value of 6 mA (1.0 C). The discharge capacity at that time is A (mAh). Next, the battery is charged to a battery voltage of 4.2 V with a current value of 6 mA (1.0 C), and after reaching the voltage, the current value starts to be reduced from 6 mA so as to hold 4.2 V, and charging is performed for a total of 3 hours. . Subsequently, the battery is discharged to a battery voltage of 3.0 V at a current value of 18 mA (3.0 C). Let the discharge capacity at that time be B (mAh). The output characteristics were compared based on the ratio (B / A) of the discharge capacity at the time of 1.0 C discharge and 3.0 C discharge.
Output characteristics (%) = B / A × 100

[Example 1]
a. Preparation of microporous polyolefin membrane 12% by weight of ultrahigh molecular weight polyethylene with a viscosity average molecular weight of 2 million, 12% by weight of high density polyethylene with a viscosity average molecular weight of 280,000, 16% by weight of linear low density polyethylene with a viscosity average molecular weight of 150,000, phthalate After mixing and granulating 42.4% by weight of dioctyl acid (DOP) and 17.6% by weight of fine silica, it was kneaded and extruded into a sheet having a thickness of 90 μm by a twin screw extruder equipped with a T-die. DOP and fine silica were extracted and removed from the molded product to form a microporous membrane.
Two microporous membranes are stacked and guided to a roll stretching machine, stretched 5.0 times at 117 ° C in the longitudinal direction, then stretched 1.9 times at 115 ° C in the transverse direction and 5% at 128 ° C. A polyolefin microporous membrane was obtained by heat relaxation.

b. Preparation of multilayer porous membrane 10 parts by weight of poly (2,6-dimethyl-1,4-phenylene ether) having a reduced viscosity of 0.515 g / dl, 89 parts by weight of 1,1,2,2-tetrachloroethane as a solvent, An electrospinning solution was prepared by stirring 1 part by weight of tetrabutylammonium bromide as an additive salt until a uniform solution was obtained. a. The polyolefin microporous film obtained in the above is placed on the collector 5, the applied voltage is 11 kV, the distance between the nozzle 3 and the collector 5 is 6 cm, the discharge amount is 0.7 ml / hr, and the voltage application time is 3 seconds. Spinning was performed to form a porous layer made of an aggregate of fibers made of poly (2,6-dimethyl-1,4-phenylene ether) on the polyolefin porous film. Thereafter, it was sufficiently immersed in water and ethanol to remove tetraammonium bromide as an added salt and dried to obtain a multilayer porous membrane.

[Example 2]
In Example 1, instead of poly (2,6-dimethyl-1,4-phenylene ether) having a reduced viscosity of 0.515 g / dl, poly (2,6-dimethyl-) having a reduced viscosity of 0.54 g / dl was used. A multilayer porous membrane was obtained in the same manner as in Example 1 except that 1,4-phenylene ether was used.

[Example 3]
In Example 1, a multilayer porous membrane was obtained in the same manner as in Example 1 except that the voltage application time was 10 seconds.

[Example 4]
12 parts by mass of ultra high molecular weight polyethylene having a viscosity average molecular weight of 950,000, 28 parts by mass of high density polyethylene having a viscosity average molecular weight of 270,000, and pentaerythrityl-tetrakis- [3- (3,5-di-t-butyl as an antioxidant What added 0.3 mass part of -4-hydroxyphenyl) propionate] was premixed with a Henschel mixer. The obtained mixture was supplied to the feed port of the twin-screw co-directional screw extruder by a feeder. Further, the liquid paraffin was side-fed to the twin-screw extruder cylinder so that the liquid paraffin content ratio in the total mixture (100 parts by mass) melt-kneaded and extruded was 60 parts by mass. Subsequently, the melt-kneaded material was extruded through a T-die between cooling rolls controlled at a surface temperature of 25 ° C. to obtain a sheet-like polyolefin composition having a thickness of 1200 μm. Next, it was continuously led to a simultaneous biaxial tenter, and simultaneous biaxial stretching was performed 7 times in the longitudinal direction and 7 times in the transverse direction. At this time, the set temperature of the simultaneous biaxial tenter is 120 ° C. Next, it was led to a methylene chloride bath and sufficiently immersed in methylene chloride to extract and remove liquid paraffin. Thereafter, methylene chloride was dried. Further, the film was led to a transverse tenter and stretched 1.6 times in the transverse direction at 125 ° C., and then thermally relaxed at 130 ° C. for 12% to obtain a microporous polyolefin membrane.
The resulting polyolefin microporous membrane was multilayered with polyphenylene ether in the same manner as in Example 1.

[Comparative Example 1]
The polyolefin microporous membrane used in Example 1 was used alone without being multilayered with poly (2,6-dimethyl-1,4-phenylene ether).

[Comparative Example 2]
A coating solution was prepared by stirring 25 parts by weight of polyphenylene ether (manufactured by GE Plastics, NOLYL PPO534) and 75 parts by weight of N-methyl-2-pyrrolidone until a uniform solution was obtained. Using a bar coater on the polyolefin microporous film used in Example 1, the solution was applied to a thickness of 10 μm to form a multilayer. The multilayer body was immersed in isopropyl alcohol for 3 minutes and then dried to remove isopropyl alcohol to obtain a multilayer porous membrane.

[Comparative Example 3]
N, N-dimethylformamide was added to a solvent-soluble polyimide (manufactured by Shin Nippon Rika Co., Ltd., Rika Coat SN-20) to prepare an electrostatic spinning solution so that the solid concentration was 17 parts by weight. The polyolefin microporous film used in Example 1 was placed on the collector 5, the applied voltage was 13 kV, the distance between the nozzle 3 and the collector 5 was 20 cm, the discharge amount was 1.0 ml / hr, and the voltage application time was 10 seconds. Electrospinning was carried out to obtain a multilayer porous membrane comprising an aggregate of fibers made of polyimide on the polyolefin porous membrane.
Table 1 shows the characteristics of the multilayer porous membranes obtained in Examples 1 to 4 and Comparative Examples 1 to 3.

  The multilayer porous membrane of the present invention can be suitably used as a separator used in a non-aqueous electrolyte secondary battery having high safety and output characteristics.

It is an example of the apparatus used with the electrospinning method of this invention. 2 is a scanning electron micrograph (magnification 10,000 times) of the surface of the multilayer porous membrane obtained in Example 1 on the polyphenylene ether side.

Explanation of symbols

1. 1. Solution holding tank 2. Metering pump Metal nozzle 4. 4. High voltage generator Collector 6. Porous membrane made of thermoplastic resin

Claims (7)

A multilayer porous membrane comprising a porous layer made of a thermoplastic resin and a porous layer made of polyphenylene ether, wherein the air permeability of the porous layer made of polyphenylene ether is less than 60 seconds / 100 ml. The multilayer porous membrane according to claim 1, wherein the air permeability of the porous layer made of polyphenylene ether is smaller than the air permeability of the porous layer made of thermoplastic resin. The multilayer porous membrane according to claim 1 or 2, wherein the thermoplastic resin is a porous layer made of a polyolefin resin. The multilayer porous membrane according to any one of claims 1 to 3, wherein the maximum pore size of the porous layer made of polyphenylene ether is larger than 0.1 µm and not larger than 20 µm. The multilayer porous membrane according to any one of claims 1 to 3, wherein the porous layer made of polyphenylene ether is a porous layer made of an ultrafine fiber aggregate having an average fiber diameter of 0.01 µm or more and 10 µm or less. The separator for non-aqueous electrolyte secondary batteries which consists of a multilayer porous membrane in any one of Claims 1-5. A non-aqueous electrolyte secondary battery using the separator according to claim 6.

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