JP4832430B2 - Lithium ion secondary battery separator and lithium ion secondary battery - Google Patents

Description

  The present invention relates to a separator used for a lithium ion secondary battery. In particular, the present invention relates to separator technology for the purpose of improving the safety of lithium ion secondary batteries.

A lithium ion secondary battery that obtains an electromotive force by doping and dedoping of lithium ions has a high energy density, and is widely used as a power source for portable electronic devices such as mobile phones and laptop computers. In addition, the output has been increased and application to power applications such as electric tools has begun.
In recent years, interest in hybrid electric vehicles (HEV) has increased due to global environmental problems. Currently, nickel-metal hydride batteries are generally used as batteries, but lithium-ion secondary batteries have less memory effect than nickel-metal hydride batteries. It is considered as a power source for HEV because it is easy to use and has the advantage of being able to be miniaturized because of its high energy and high output density. When considering a battery used for HEV, the requirement is naturally different from that of a portable electronic device. For example, in HEV, there is a high possibility that a battery is exposed to a high temperature, and ensuring safety in a high temperature environment is one of the very important characteristics. As described above, since the characteristics required for the battery are different between the portable electronic device and the HEV, the characteristics required for the constituent members of the battery are naturally different.
In the current lithium ion secondary battery, a polyethylene microporous membrane is used as a separator. This separator has a shutdown function and contributes to ensuring the safety of the battery. This shutdown function utilizes the fact that the microporous film melts and closes the pores due to heat, and is characterized by the thermal fuse temperature and the short circuit temperature. The thermal fuse temperature is a temperature at which the separator resistance starts to increase due to hole closure, and the short-circuit temperature is a temperature at which the separator resistance is abruptly lowered because the separator is broken. Between this thermal fuse temperature and the short circuit temperature, the separator resistance is very high and the current can be shut out. This function is said to be effective for ensuring safety such as external short circuit.
In the case of HEV, a case where it is actively heated from the outside is assumed. When considering a shutdown function in such applications, thermal fuses can lead to performance degradation when exposed to high temperatures, and film breakage can lead to ignition due to an internal short circuit between the positive and negative electrodes. Therefore, in HEV, a separator having a shutdown function is not always effective, and it is considered that a separator having sufficient heat resistance is more suitable.
Several proposals have already been made for the construction of a separator having high heat resistance. For example, a nonwoven fabric sheet made of highly heat-resistant fibers such as aromatic polyamide fibers has been proposed in Japanese Patent No. 3142893. However, the shape of the nonwoven fabric has a large opening and prevents the short circuit between the positive and negative electrodes and retains the electrolyte, and the characteristics related to the essence of the separator are insufficient. To reduce the opening, thin fibers are used. However, at the current technical level, it is very difficult to obtain a suitable separator for a lithium ion secondary battery, and it has not been put into practical use.
A porous film made of a polymer having high heat resistance such as aromatic polyamide has also been proposed in WO01 / 019906. This system has sufficient heat resistance in terms of short-circuiting due to membrane breakage. However, in cases where heat is actively generated from the inside, such as overcharging, the battery may ignite due to a runaway reaction. However, in such cases, safety is not sufficiently secured, and other safety measures are complicated. Has the problem of becoming. In addition, a porous membrane made of an aromatic polyamide has a problem that productivity is low from the viewpoint of strength.
A technique for providing a shutdown function to a porous film made of a polymer having high heat resistance as described above has also been proposed. For example, Japanese Patent Application Laid-Open No. 2001-23602 discloses a technique for coating a porous film having a shutdown function with a porous layer made of a high heat resistant resin. However, the resistance after the thermal fuse has increased only to about 10 times the resistance before the thermal fuse, and it is difficult to say that the shutdown function is sufficient for ensuring the safety of the battery. Japanese Patent Laid-Open No. 10-6453 proposes a structure in which fine particles made of polyethylene are mixed in a porous film made of a heat resistant resin. Similarly, this system does not have a sufficient shutdown function.
WO 01/067536 proposes a configuration in which a non-woven fabric made of highly heat-resistant fibers is coated with a porous layer made of a polyvinylidene fluoride copolymer. This specification describes a technical element that enables prevention of an overcharge prevention function by appropriately controlling the morphology of the separator. Moreover, a manufacturing method suitable for this morphology control is disclosed in Japanese Patent Application Laid-Open No. 2003-171495. However, the heat resistance of the polyvinylidene fluoride copolymer swollen in the electrolyte is not high at all, and it melts at high temperatures, so it is considered that the heat resistance of this system is ensured by the nonwoven fabric. Similar to the discussion of Japanese Patent No. 3142893, there is a problem that the prevention of short circuit between the positive and negative electrodes in the non-woven fabric lacks certainty.
Japanese Patent Application Laid-Open No. 10-324758 discloses a separator in which the surface and voids of a substrate made of fibers or pulp are covered with a porous para-aramid polymer. Japanese Patent No. 3175730 is a system in which ceramic is dispersed in a porous layer in addition to the system disclosed in JP-A-10-324758. The manufacturing method of the separator described in these patent documents is obtained by placing a non-woven fabric on a carrier sheet, applying para-aramid polymer dope from above, and depositing para-aramid polymer in an appropriate humidity and temperature environment to obtain a porous film. ing. In this method, the porous para-aramid polymer cannot be substantially coated on both sides of the nonwoven fabric, and is obviously a single-sided coating. In addition, since the deposition rate differs between the front and back surfaces, the morphology of the porous layer on the front and back surfaces is greatly asymmetric. Such a separator having a significant difference between the front and the back is not practical because it is difficult to form an appropriate electrode / separator interface and there are problems in battery performance. Another problem is curling and poor handling. Furthermore, the para-aramid polymer has a problem that it is difficult to adjust the dope and to be molded. Specifically, since it is difficult to form holes continuously and only very small holes are formed, sufficient ion permeability cannot be obtained. In addition, this technique has a problem that the manufacturing method is complicated.

As mentioned above, separators with high heat resistance using heat-resistant resins such as aromatic polyamides have been proposed, but this heat resistance is a feature that significantly improves the safety of lithium ion secondary batteries and produces them. At present, a separator structure with good properties has not been found and has not been put into practical use. In the sufficiently high heat-resistant separator, the present inventors lacked from the viewpoint of ensuring the safety of the lithium ion secondary battery is an overcharge countermeasure that leads to a runaway reaction due to self-heating from the inside. Thought. Accordingly, it is an object of the present invention to provide a separator having sufficiently high heat resistance, effective for overcharge countermeasures, and good handling properties.
As a result of intensive studies on the above problems by the present inventors, it is confirmed that a separator in which a porous layer mainly made of an aromatic polyamide is coated on both front and back surfaces of a nonwoven fabric is effective for ensuring the safety of a lithium ion secondary battery. The headline has led to the present invention. That is, the present invention provides a separator for a lithium ion secondary battery, in which a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric. The present invention also provides the following inventions.
1. The separator for a lithium ion secondary battery according to the invention, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc.
2. A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. A separator for a lithium ion secondary battery according to any one of the above inventions or 1.
3. 3. The separator for a lithium ion secondary battery according to 2, wherein the surfactant is attached in an amount of 0.005 to 0.750 g / m ² .
4). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.
5). 5. The separator for a lithium ion secondary battery according to 4, wherein the weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m ² .
6). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of metaaromatic polyamide.
7). The separator for a lithium ion secondary battery according to the above invention or any one of claims 1 to 3, wherein the nonwoven fabric is composed of a meta-aromatic polyamide short fiber and a para-aromatic polyamide pulp.
8). The separator for a lithium ion secondary battery according to the above invention or any one of 1 to 5, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.
9. The above invention, wherein the porous layer contains ceramic fine particles having an average particle size of 0.05 to 2 μm, and the ceramic fine particles are 30 to 80% by weight based on the weight of the porous material layer or The separator for lithium ion secondary batteries in any one of 1-8.
10. A polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both the front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is poor against the metaaromatic polyamide. A method for producing a separator for a lithium ion secondary battery, comprising coagulating in a mixed liquid mainly composed of a solvent as a solvent and a solvent as a good solvent, followed by washing with water and drying.
11. In a lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, and a separator, and obtaining an electromotive force by doping and dedoping lithium ions, the separator is a porous layer mainly composed of metaaromatic polyamide on both front and back surfaces of the nonwoven fabric. Is formed, a lithium ion secondary battery.
12 11. The lithium ion secondary battery according to 11, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc.
13. A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The lithium ion secondary battery according to any one of 11 and 12, wherein:
14 14. The separator for a lithium ion secondary battery according to 13, wherein the surfactant is attached in an amount of 0.005 to 0.750 g / m ² .
15. The lithium ion secondary battery according to any one of 11 to 14, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.
16. 16. The lithium ion secondary battery according to 15, wherein the weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m ² .
17. 16. The lithium ion secondary battery according to 15, wherein the electrolyte of the electrolytic solution is mainly composed of LiPF ₆ and contains 0.5 to 5% by weight of vinylene carbonate or vinyl acetate with respect to the total weight of the solvent.
18. 15. The lithium ion secondary battery according to any one of 11 to 14, wherein the non-woven fabric is a non-woven fabric mainly composed of a metaaromatic polyamide.
19. The lithium ion secondary battery according to any one of 11 to 18, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.

  FIG. 1 shows the result of the overcharge test.

Hereinafter, embodiments of the present invention will be described.
[Separator]
The separator for a lithium ion secondary battery of the present invention is characterized in that a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric. By using a non-woven fabric, the strength and dimensional stability of the porous membrane made of aromatic polyamide can be improved, and handling properties and productivity can be improved.
Here, the porous layer mainly made of meta-aromatic polyamide is formed on the front and back surfaces of the nonwoven fabric, when the front and back surfaces of the separator are observed, the entire front and back surfaces are covered with the porous layer made of aromatic polyamide. The fibers constituting the nonwoven fabric are not visible. This can be easily observed with a scanning electron microscope (SEM). In the separator in which the fibers constituting the nonwoven fabric are exposed on one or both of the front and back surfaces, the bonding interface between the electrode and the separator becomes non-uniform, resulting in a problem in battery performance. This is due to insufficient electrolyte retention at the electrode / separator interface, and as the cycle progresses, the electrolyte present at the electrode / separator interface is depleted, resulting in poor cycle characteristics and cycle discharge characteristics. Become.
In the separator of the present invention, the layer formed on the nonwoven fabric surface is a porous layer made of metaaromatic polyamide. In the porous layer, it is necessary that pores are continuously formed, and the structure of the porous layer can use the Gurley value (JIS P8117) as an index. The Gurley value of the separator of the present invention is preferably 10 to 50 seconds / 100 cc. If the Gurley value is lower than 10 seconds / 100 cc, the probability that a non-woven fabric-forming fiber is exposed or has a pinhole is high, which is not preferable. On the other hand, if it is greater than 50 seconds / 100 cc, the ion permeability is not sufficient and the rate characteristics are deteriorated. Furthermore, it becomes difficult to obtain the overcharge prevention function described in WO01 / 067536 well, which is not preferable from the viewpoint of ensuring safety during overcharge.
In the separator of the present invention, the film thickness is preferably 15 to 40 μm. If the film thickness is less than 15 μm, the separator's original function of preventing short circuit becomes insufficient. On the other hand, if it is thicker than 40 μm, the resistance associated with ion conduction is increased, resulting in problems that the rate characteristics are not sufficient and the energy density of the battery is not increased.
In order to realize the separator thickness as described above, the nonwoven fabric thickness is 10 to 39 μm, and the total thickness of the porous layer made of the metaaromatic polyamide is preferably about 1 to 10 μm. The substantial strength of the separator of the present invention is determined by the nonwoven fabric, but if the thickness of the nonwoven fabric is less than 10 μm, it is difficult to ensure sufficient strength as a separator for a lithium ion battery. On the other hand, when the thickness is larger than 39 μm, it becomes substantially difficult to make the separator thickness 40 μm or less. In addition, if the thickness of the porous layer is thinner than 1 μm in total, it is difficult to substantially cover the entire surface of the nonwoven fabric. If the porous layer has a rate limiting ion conduction resistance of the separator and is thicker than 10 μm, it is not preferable from the viewpoint of ensuring sufficient discharge performance.
The non-woven fabric used in the separator of the present invention is preferably as fine as possible, and in order to obtain such a non-woven fabric, the fiber diameter is preferably fine. From such a viewpoint, the fiber diameter of the fibers constituting the nonwoven fabric is preferably 10 μm or less, and more preferably 5 μm or less.
When forming a nonwoven fabric, a binder for binding the main fiber to the main fiber is required. The binder for forming the nonwoven fabric is preferably fiber or pulp.
A known method can be applied to the method for producing the nonwoven fabric. Specific examples include a dry method, a water needle method, a wet papermaking method, a spunbond method, a melt blow method, and an electrospinning method. The wet papermaking method is particularly suitable in view of the thinning and uniformity of the openings.
The material constituting the nonwoven fabric is not particularly limited as long as it has sufficient heat resistance and resistance to an electrolytic solution. Specifically, polyesters represented by polyethylene terephthalate (PET), aromatic polyamide, polysulfone, polyethersulfone, Examples include polyphenylene sulfide and polyimide. In particular, PET is preferable from the viewpoint that it is easy to form thinner fibers and has high heat resistance. Further, aromatic polyamides, particularly metaaromatic polyamides are preferable from the viewpoint of high strength and high heat resistance, and polymetaphenylene isophthalamide is particularly preferable from the viewpoint of moldability. In the non-woven fabric, there is no problem even if polyolefin fibers such as polyethylene and polypropylene are mixed within a range that does not impair the heat resistance of the non-woven fabric, and if these materials are mixed in a small amount, the safety may be improved. In this case, the amount of the polyolefin fiber added is preferably 30% by weight or less based on the weight of the nonwoven fabric.
In particular, from the viewpoint of thin film and high strength, short fibers made of meta-aromatic polyamide and non-woven fabric using para-aromatic polyamide pulp are suitable. The nonwoven fabric has a high affinity with the metaaromatic polyamide that forms the porous layer, and a high strength can be obtained by combining the nonwoven fabric. In addition, the nonwoven fabric having the above structure can be easily thinned.
In the separator of the present invention, the material that forms the porous layer is preferably metaaromatic polyamide. Aromatic polyamides include para-aromatic polyamides typified by polyparaphenylene terephthalamide and meta-aromatic polyamides typified by polymetaphenylene isophthalamide. In the separator of the present invention, meta-aromatic polyamides, particularly polymetaphenylene isophthalate are used. Amides are preferred. Para-aromatic polyamide has low solubility in a solvent and high viscosity even at a low concentration, so that it is very difficult to form the porous layer with sufficient strength and ion permeability. Specifically, only small pores are formed and these are discontinuous, resulting in insufficient ion permeability. On the other hand, the metaaromatic polyamide is sufficiently dissolved in a solvent, and it is easy to prepare an appropriate polymer solution in terms of concentration and viscosity. Further, the pore diameter can be easily controlled and sufficient ion permeability can be ensured. In particular, in order to develop the overcharge prevention function of WO01 / 067536, it is necessary to appropriately control the pore diameter of the porous layer. From the viewpoint of adding such a function, metaaromatic polyamides are para-aromatic. It is preferable compared to the group polyamide.
The material for forming the porous layer is mainly preferably a metaaromatic polyamide, and other materials may be mixed in a category that does not impair the heat resistance, which is a characteristic of the metaaromatic polyamide, and the structure control of the porous layer. Absent. Specific examples include para-aromatic polyamide, polysulfone, polyethersulfone, polyvinylidene fluoride, polyvinylidene fluoride copolymer, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide, polypropylene oxide, polyvinylpyrrolidone, and the like. Is preferably 30% by weight or less based on the metaaromatic polyamide forming the porous layer.
When PET is used for the nonwoven fabric, the weight of the metaaromatic polyamide forming the porous layer is preferably 4 to 10 g / m ² . When PET is used as a material for a lithium ion secondary battery separator, there is a problem in durability in a special environment, but this durability can be remarkably improved by combining with a metaaromatic polyamide. If the weight of the metaaromatic polyamide is less than 4 g / m ² , the durability is not sufficiently improved, which is not preferable. When the weight is more than 10 g / m ² , there arises a problem that the ion permeability is deteriorated.
When the metaaromatic polyamide in the present invention is dissolved in N-methyl-2-pyrrolidone, it is represented by the logarithmic viscosity of the following formula (1), and is 0.8 to 2.5 dl / g, preferably 1.0 to A polymer in the range of 2.2 dl / g is preferred. If the logarithmic viscosity is lower than 0.8 dl / g, sufficient mechanical strength cannot be obtained. If the logarithmic viscosity exceeds 2.5 dl / g, it becomes difficult to obtain a stable polymer solution, and a uniform porous layer is formed. Therefore, it is not preferable.
Logarithmic viscosity (unit: dl / g) = ln (T / TO) / C (1)
T: Flow time of capillary viscometer at 30 ° C. in solution of 0.5 g of metaaromatic polyamide in 100 ml of N-methyl-2-pyrrolidone TO: Capillary viscometer of N-methyl-2-pyrrolidone at 30 ° C. Flow time C: Polymer concentration (g / dl) in the polymer solution
In the separator of the present invention, the one obtained by attaching a surfactant to the porous layer is also preferable from the viewpoint of antistatic. The surfactant is not particularly limited, and for example, a cationic, anionic, zwitterionic or nonionic surfactant can be used. Examples of the cationic surfactant include higher amine halogenates, alkylpyridinium halides, quaternary ammonium salts and the like. Anionic surfactants include higher fatty acid alkali salts, polyoxyethylene alkyl ether sulfonate esters, polyoxyethylene alkyl ether phosphonates, alkyl sulfates, alkyl sulfonates, alkyl aryl sulfonates, sulfosuccinic acid. Examples include ester salts. Examples of zwitterionic surfactants include alkylbetaine compounds, imidazoline compounds, alkylamine oxides, and bisoxyborate compounds. Nonionic surfactants include polyoxyethylene alkyl ethers, polyoxyethylene alkyl phenyl ethers, polyoxyethylene alkyl allyl ethers, glycerin fatty acid esters, polyoxyethylene sorbitan fatty acid esters, sorbitan fatty acid esters, and the like. .
In particular, cationic surfactants, anionic surfactants, and zwitterionic surfactants have a strong antistatic effect, so that the amount used can be kept low, and it is desirable to use them. Also, by mixing these, the affinity between the aromatic amide and the surfactant can be increased, and the antistatic effect can be improved.
The amount of the surfactant is preferably 0.005 to 0.750 / m ² . 0.005 g / ^m less than ² and a sufficient antistatic effect can not be obtained, may be adversely affected the performance of the battery is more than 0.750 g / ^{m 2.} The amount of the surfactant applied is that of the battery separator vacuum-dried at 90 ° C. for 10 hours after the surfactant coating, and the battery separator vacuum-dried at 90 ° C. for 10 hours after being immersed in the surfactant-soluble solvent. It can be determined by analyzing the mass difference or the extracted components obtained by drying the solution extracted with the surfactant-soluble solvent by H-NMR or gas chromatography and measuring the mass.
As a method for evaluating static electricity, the friction band voltage measurement method of JIS L 1084 is used. It is preferable that the half-life of static electricity is 30 seconds or less by the frictional voltage measurement method. If the half-life is 30 seconds or longer, the static electricity decay is slow, so that the antistatic effect is not sufficient.
[Separator manufacturing method]
The separator for a lithium ion secondary battery of the present invention can be produced by various methods, and is not limited by the production method. For example, a method of pressing a meta-aromatic polyamide porous film on both surfaces of a nonwoven fabric by press working, a polymer solution composed of a meta-aromatic polyamide is coated on both surfaces of a nonwoven fabric, and the coating layer is dry-processed or dry-wet-processed And a method of microphase separation by any of the wet methods.
In particular, a polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is a poor solvent for the metaaromatic polyamide. A production method (wet microphase separation method) in which coagulation is carried out in a mixed liquid (coagulation liquid) mainly comprising a solvent and a good solvent, followed by washing with water and drying is preferred. By adopting this production method, the structure control of the porous layer of the metaaromatic polyamide porous layer becomes easy. Moreover, it becomes possible to manufacture a high-quality separator by eliminating the exposure of the non-woven fabric constituting fiber due to poor impregnation of the polymer solution by coating on both front and back surfaces.
Specifically, the manufacturing apparatus and concept described in Patent Document 6 can be applied to the above manufacturing method. Specifically, as a coating method in which a polymer solution is applied to both the front and back surfaces of a nonwoven fabric, an excess polymer solution is supplied from both sides of the nonwoven fabric, and the nonwoven fabric is passed between a pair of opposed Meyer bars and dies. In addition, a non-woven fabric coated with this polymer solution is immersed in a coagulating liquid so that both the front and back surfaces are in contact with the coagulating liquid. Is done.
In the production method, the good solvent is suitably an amide solvent, and for example, dimethylacetamide, N-methyl-2-pyrrolidone and the like are preferable. Specific examples of the poor solvent include alcohols and water, and water is particularly preferable.
The polymer concentration of the polymer solution cannot be determined unconditionally because it largely depends on the type and degree of polymerization of the metaaromatic polyamide to be used. For example, when polymetaphenylene isophthalamide is used, it is 5 to 20% by weight. % Range is preferred.
When the polymer concentration is low, a separator having particularly excellent ion permeability can be obtained. However, since the viscosity of the polymer solution is low, productivity is not sufficient from the viewpoint of pinholes and the like when applied to a nonwoven fabric. In order to obtain a polymer solution having a particularly excellent ion permeability by lowering the polymer concentration, a method of adding ceramic fine particles to the polymer solution is suitable. The addition of the ceramic fine particles increases the viscosity of the polymer solution, so that it can be easily applied to the nonwoven fabric even at a low polymer concentration.
Specific examples of the ceramic fine particles include silica, alumina, zirconia, magnesia, titania, barium titanate, aluminum nitride, calcium oxide, calcium carbonate, lithium fluoride, lithium oxide and the like. In particular, alumina, zirconia, and magnesia are suitable.
The average particle size of the ceramic fine particles is preferably 0.05 to 2 μm, particularly preferably 0.1 to 1 μm. When the ceramic fine particles are 0.05 μm or less, handling properties are difficult from the viewpoint of aggregation and the like. On the other hand, when the thickness is 2 μm or more, die streaks are likely to occur during coating, which is not preferable. Here, the average particle diameter can be measured by a laser diffraction measurement method, and means the central particle diameter (D50) in the volume particle size distribution. Moreover, the average particle diameter in this invention is an average particle diameter of a primary particle.
The amount of the ceramic fine particles to be added is suitably determined in view of the polymer concentration of the polymer solution, but is generally a metaaromatic polyamide that forms the porous material (if other organic polymers are included, this is not the case). 30 to 80% by weight based on the weight of the ceramic fine particles. If the amount of ceramic fine particles is less than 30% by weight, a sufficient thickening effect cannot be obtained and the effect is insufficient. On the other hand, if the amount of the ceramic fine particles is larger than 80% by weight, problems such as powder falling when the separator is molded are not preferable.
The effect of adding ceramic fine particles to this production method is particularly suitable when the polymer concentration in the polymer solution is 10% by weight or less.
Further, the effect of adding ceramic fine particles is not only the effect of the above manufacturing method, but also has structural features as described below. Ceramic fine particles generally have higher heat resistance than metaaromatic polyamide, and function as a separator in the configuration in which ceramic fine particles are added even when the battery temperature rises to around 400 ° C. where the metaaromatic polyamide is thermally decomposed. Furthermore, the ceramic fine particles also function as a lubricant and contribute to an antistatic effect or improved handling properties when static electricity is generated.
A phase separation agent may be added to the polymer solution for the purpose of controlling the porous structure. The phase separation agent is a poor solvent for the metaaromatic polyamide and can be used as long as it is compatible with the coagulating liquid. Specifically, water and alcohols are preferred, and in particular, propylene glycol, ethylene glycol, diethylene glycol, tripropylene glycol, 1,3-butanediol, 1,4-butanediol, polyethylene glycol monoethyl ether containing a polymer. Polyhydric alcohols such as methanol, ethanol and glycerin are preferably selected. The concentration of the phase separation agent in the polymer solution is suitably selected in the range of 0 to 40% by weight with respect to the mixture of the good solvent and the phase separation agent.
The coagulation liquid is preferably a mixed liquid of the aforementioned good solvent and poor solvent. Here, when the phase separation agent is applied to the polymer solution, it is preferable in terms of process management to mix the phase separation agent into the coagulation liquid at an appropriate ratio. Specifically, it is preferable to match the ratio of the coagulating liquid with the ratio of the good solvent and the phase separation agent in the polymer solution.
The ratio of the poor solvent in the coagulation bath is suitably selected from the range of 10 to 80% by weight when water is applied to the poor solvent.
The non-woven fabric on which the solidified porous layer is formed is then transferred to a water washing step, and then water is dried in the drying step to obtain the separator of the present invention. Here, the drying process is preferably selected to be brought into contact with a heating roll and dried.
In the production method of the separator of the present invention, when the surfactant is attached, the method is not particularly limited, but the surfactant is dissolved in a solvent and sprayed onto the porous film to dry, or the porous film is immersed and dried. The method etc. are mentioned.
[Lithium ion secondary battery]
Generally, a lithium ion secondary battery has a configuration in which a battery element in which a positive electrode and a negative electrode are opposed to each other and bonded via a separator is impregnated with an electrolytic solution and enclosed in an exterior. The lithium ion secondary battery of the present invention is characterized by the use of the separator of the present invention described above, and a known technique can be applied to other components, which are not essentially limited to other components. Absent.
A negative electrode is generally used in which a layer formed from a negative electrode active material, a binder, and a conductive additive is coated on a current collector. This is prepared by adding a solvent to a negative electrode active material, a binder, and a conductive additive to knead to prepare a slurry, which is coated on a current collector, dried and pressed. When the total weight of the negative electrode active material, the binder, and the conductive additive is 100%, the negative electrode active material has a weight of 80 to 98% by weight, the binder is 2 to 20% by weight, and the conductive auxiliary is in the range of 0 to 10% by weight. Is preferred. Examples of the negative electrode active material include carbon materials, silicon, and tin. Examples of the carbon material include those obtained by using, as a precursor, pitches that are easily graphitized such as mesocarbon microbeads and microcarbon fibers, and those that are difficult to graphitize such as phenol resins. Examples of the binder include polyvinylidene fluoride and carboxymethyl cellulose. As the conductive additive, graphite powder, acetylene black, ketjen black, vapor grown carbon fiber, and the like are preferably used. The current collector is preferably copper foil, stainless steel, or the like.
As in the case of the negative electrode, a positive electrode in which a layer formed of a positive electrode active material, a binder, and a conductive additive is coated on a current collector is generally used. This is prepared by adding a solvent to a positive electrode active material, a binder, and a conductive additive to prepare a slurry, applying the slurry onto a current collector, drying and pressing. When the total weight of the positive electrode active material, the binder, and the conductive additive is 100%, the positive electrode active material is in the range of 80 to 98% by weight, the binder is in the range of 2 to 20% by weight, and the conductive auxiliary is in the range of 0 to 10% by weight. Is preferred. Examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , spinel-type LiMn ₂ O ₄ , olivine-type LiFePO _{4, and the} like, and those obtained by solidifying different elements therein, and these may be used in combination. As the binder, polyvinylidene fluoride is preferably used. As the conductive additive, graphite powder, acetylene black, ketjen black, vapor grown carbon fiber, and the like are preferably used. For the current collector, aluminum foil, stainless steel or the like is suitable.
As the electrolytic solution, a non-aqueous electrolytic solution in which a lithium salt is dissolved in a non-aqueous solvent is used. As the lithium salt, LiPF ₆ , LiBF ₄ , LiClO _{4 and the} like are preferably used. Nonaqueous solvents include propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), and the like. These lithium salts and non-aqueous solvents may be used alone or in combination of two or more. The concentration of the lithium salt is preferably in the range of 0.5 to 2.0M. In addition, it is preferable from the viewpoint of durability that vinylene carbonate (VC) or vinyl acetate (VA) is added to the electrolytic solution.
In the lithium ion secondary battery of the present invention, the battery element composed of the positive electrode, the negative electrode, and the separator is wound into a cylindrical or flat shape or a laminated structure and enclosed in an exterior. From the viewpoint of satisfactorily expressing the overcharge prevention function, it is better to have a folded structure, and it is particularly preferable to have a wound and flat configuration.
The exterior can be implemented in any form such as a metal case or an aluminum laminated film case.

Hereinafter, the present invention will be described in detail by way of examples. However, the present invention is not limited to the following examples.
[Example 1]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 12.6 g / m ² by a wet papermaking method, and calendered at 140 ° C. to obtain a PET nonwoven fabric with a film thickness of 18 μm. It was.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Co., Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 85: 15 (weight ratio) to be 9% by weight, A polymer solution was prepared. This polymer solution was applied to both sides of the PET nonwoven fabric, and the coated product was immersed in a coagulation bath at 30 ° C. having a composition of dimethylacetamide: water = 55: 45 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 1 of the present invention. The separator 1 of the present invention had a film thickness of 21 μm, a basis weight of 14.7 g / m ² , and a Gurley value (JIS P8117) of 23 seconds / 100 cc.
As a result of observing the front and back surfaces of this sample with a scanning electron microscope (SEM), exposure of fibers constituting the nonwoven fabric was not recognized.
[Example 2]
Using a polymetaphenylene isophthalamide short fiber having a fineness of 0.9 dtex (average fiber diameter of about 10 μm), a dry method is used to form a film with a basis weight of 15.0 g / m ² , a calender is applied at 320 ° C., and a film thickness is 30 μm. A nonwoven fabric was obtained.
Using the nonwoven fabric, a polymer solution was used in the same manner as in Example 1 to obtain a lithium ion secondary battery separator of the present invention in the same manner. This is the separator 2 of the present invention. The separator 2 of the present invention had a film thickness of 34 μm, a basis weight of 18.1 g / m ² , and a Gurley value (JIS P8117) of 35 seconds / 100 cc.
[Comparative Example 1]
The same non-woven fabric as in Example 1 was fixed on a PET fill bag, and the polymer solution used in Example 1 was applied thereon. The film was immersed in the same coagulating liquid as in Example 1 to obtain a coagulated film. The coagulated film was washed with water in a 50 ° C. water bath for 10 minutes and then dried. Thereafter, the PET film was peeled off to obtain a comparative separator 1. The comparative separator 1 had a film thickness of 20 μm, a basis weight of 13.9 g / m ² , and a Gurley value (JIS P8117) of 15 seconds / 100 cc.
As a result of observing the front and back surfaces of this sample with a scanning electron microscope (SEM), exposure of fibers constituting the nonwoven fabric was significantly observed on the surface in contact with the PET film.
[Comparative Example 2]
A PET short fiber having a fineness of 0.33 dtex (average fiber diameter of 5.5 μm) and a PET fiber for binder having a fineness of 0.22 dtex (average fiber diameter of 4.5 μm) are blended at a weight ratio of 6/4, and the basis weight is obtained by a wet papermaking method. A film was formed at 12.0 g / m ² and calendered at 200 ° C. to obtain a PET nonwoven fabric having a film thickness of 18 μm.
A polyvinylidene fluoride copolymer of vinylidene fluoride: hexafluoropropylene: chlorotrifluoroethylene = 95.5: 2.3: 2.2 (weight ratio) was converted into dimethylacetamide: tripropylene glycol = 70: 30 (weight). The polymer solution was prepared by dissolving in a mixed solvent having a ratio of 12% by weight.
This polymer solution was applied to both sides of the PET nonwoven fabric, and the coated product was immersed in a coagulation bath at 30 ° C. having a composition of dimethylacetamide: water = 55: 45 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a comparative separator 2. The comparative separator 2 had a film thickness of 24 μm, a basis weight of 17.3 g / m ² , and a Gurley value (JIS P8117) of 12 seconds / 100 cc.
[Heat resistance evaluation 1]
The above-mentioned separators 1 and 2 of the present invention, comparative separators 1 and 2, and a commercially available polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), each 10 cm × 10 cm. Cut out. The electrolyte was taken in a 70 cc sample bottle. Here, 1 M LiBF ₄ PC / EC (1/1 weight ratio) was used as the electrolytic solution. The cut separator was placed in the sample bottle and treated at 150 ° C. for 2 hours. Then, the separator was taken out from the sample bottle and the separator was observed. The results are shown in Table 1.
[Heat resistance evaluation 2]
The above-mentioned separators 1 and 2 of the present invention, comparative separators 1 and 2, and a commercially available polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), each 10 cm × 10 cm. Cut out. This was fixed to a frame that can be fixed in four directions, and heat-treated at 200 ° C. for 30 minutes. The form and dimensions after heat treatment were measured. The results are shown in Table 1.
[Example 3]
Lithium cobaltate (LiCoO ₂ ; manufactured by Nippon Chemical Industry Co., Ltd.) powder 89.5 parts by weight, acetylene black 4.5 parts by weight, polyvinylidene fluoride 6 parts by weight using N-methyl-2pyrrolidone solvent It knead | mixed and produced the slurry. The obtained slurry was applied onto an aluminum foil having a thickness of 20 μm, dried and pressed to obtain a positive electrode having a thickness of 100 μm.
These are kneaded using N-methyl-2pyrrolidone solvent so as to be 87 parts by weight of mesophase carbon microbeads (MCMB: Osaka Gas Chemical Co., Ltd.), 3 parts by weight of acetylene black and 10 parts by weight of polyvinylidene fluoride, and slurry Was made. The obtained slurry was applied onto a copper foil having a thickness of 18 μm, dried and pressed to obtain a negative electrode having a thickness of 90 μm.
The positive electrode and the negative electrode were opposed to each other through the separator 1 of the present invention produced in Example 1. This was impregnated with an electrolytic solution and sealed in an exterior made of an aluminum laminate film to produce a lithium ion secondary battery of the present invention. Here, 1M LiPF ₆ EC / EMC (3/7 weight ratio) was used as the electrolytic solution. This battery is referred to as a battery 1 of the present invention.
[Comparative Example 3]
A lithium ion secondary battery was produced in the same manner as in Example 1 using the comparative separator 1 as the separator. This lithium ion secondary battery is referred to as comparative battery 1.
[Comparative Example 4]
Using a polypropylene microporous membrane (manufactured by Celgard; trade name “Celgard # 2400”), which is a commercially available separator for lithium ion secondary batteries, as the separator, a lithium ion secondary battery was produced in the same manner as in Example 1. did. The secondary battery is referred to as a comparative battery 2 for this lithium ion.
[Cycle test]
The battery 1 of the present invention and the comparative batteries 1 and 2 were subjected to 100 cycles of 1C, 4.2V, 2 hours constant current and constant voltage charge, 1C and 2.75V constant current discharge. For each battery, (capacity maintenance ratio) = (discharge capacity at the 100th cycle) / (discharge capacity at the first cycle) was determined. This is shown in Table 2. Only the comparative battery 1 using the comparative separator 1 in which the exposure of the nonwoven fabric was confirmed has significantly poor cycle characteristics, and the inventive battery 1 using the inventive separator 1 coated on both sides has the same characteristics as a commercially available separator. is there.
[Overcharge test]
The battery 1 of the present invention and the comparative battery 2 were subjected to constant current charging at 1 C for 10 hours (overcharge test in which charging was 10 times the original charging). However, when the voltage reached 6V, the power reception was forcibly terminated. The case where charging was forcibly terminated 10 hours before the voltage reached 6 V was regarded as insufficient overcharge characteristics, and the case where it was not satisfied was regarded as sufficient overcharge characteristics. The result of the overcharge test is shown in FIG. 1. The present battery 1 using the separator of the present invention has sufficient overcharge characteristics, but the comparative battery 2 using a commercially available separator is insufficient.
[Example 4]
Emulgen 120 (manufactured by Kao; nonionic surfactant) was dissolved in methanol to prepare a 1% by weight solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, thereby obtaining the separator 3 of the present invention. Here, the amount of the surfactant adhered to the separator 3 of the present invention was 0.15 g / m ² .
[Example 5]
Electro stripper AC (manufactured by Kao; amphoteric surfactant) was dissolved in methanol to prepare a 1 wt% solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, whereby the separator 4 of the present invention was obtained. Here, the amount of the surfactant adhered to the separator 4 of the present invention was 0.02 g / m ² .
[Example 6]
Coatamine 60W (manufactured by Kao; cationic surfactant) was dissolved in methanol to prepare a 1% by weight solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, thereby obtaining the separator 5 of the present invention. Here, the amount of the surfactant adhered to the separator 5 of the present invention was 0.04 g / m ² .
[Example 7]
Electro stripper F (manufactured by Kao; anionic surfactant) was dissolved in methanol to prepare a 1 wt% solution. The separator 1 of the present invention produced in Example 1 was immersed in the surfactant methanol solution and dried to attach the surfactant, whereby the separator 6 of the present invention was obtained. Here, the amount of the surfactant adhered to the separator 6 of the present invention was 0.10 g / m ² .
[Electrostatic voltage measurement]
The charged voltage half-lives of the separators 1 and 3 to 6 of the present invention were measured using a Static Honestometer H-0110 (manufactured by Sisid Electric). The results are shown in Table 3. From Table 3, it can be seen that the attachment of the surfactant is effective for antistatic.
[Example 8]
A polymetaphenylene isophthalamide short fiber having a fineness of 0.9 dtex (average fiber diameter of about 10 μm) is used as a main fiber, pulp made of para-aramid is used as a binder, and these are mixed at a main fiber / binder = 1/1 (weight ratio). A wet papermaking method was used to form a film with a basis weight of 29.9 g / m ² and calendering to obtain an aramid nonwoven fabric (aramid paper) having a thickness of 30 μm.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Co., Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 85: 15 (weight ratio) to be 9% by weight, A polymer solution was prepared. This polymer solution was coated on both sides of the aramid nonwoven fabric, and this coated material was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 60: 40 (weight ratio) for 60 seconds to obtain a coagulated film. It was. This solidified film was washed with water in a 30 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 7 of the present invention. The separator 7 of the present invention had a film thickness of 39 μm, a basis weight of 34.5 g / m ² , and a Gurley value (JIS P8117) of 40 seconds / 100 cc. The weight of the porous layer made of polymetaphenylene isophthalamide was 4.6 g / m ² .
[Example 9]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 13.0 g / m ² by a wet papermaking method, and calendered to obtain a PET nonwoven fabric having a film thickness of 18 μm.
A separator for a lithium ion secondary battery of the present invention was obtained in the same manner as in Example 8 using the PET nonwoven fabric. This is the separator 8 of the present invention. The separator 8 of the present invention had a film thickness of 29 μm, a basis weight of 18.3 g / m ² , and a Gurley value (JIS P8117) of 28 seconds / 100 cc. The weight of the porous layer made of polymetaphenylene isophthalamide was 5.3 g / m ² .
[Example 10]
In the same manner as in Example 9, the coating clearance was changed to obtain the present separator 9 having a film thickness of 37 μm, a basis weight of 22.1 g / m ² and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of metaphenylene isophthalamide was 9.1 g / m ² .
[Example 11]
In the same manner as in Example 9, the coating clearance was changed to obtain the separator 10 of the present invention having a film thickness of 22 μm, a basis weight of 163 g / m ² and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of isophthalamide was 3.3 g / m ² .
[Example 12]
In the same manner as in Example 9, the coating clearance was changed to obtain the present separator 11 having a film thickness of 24 μm, a basis weight of 173 g / m ² and a Gurley value (JIS P8117) of 32 seconds / 100 cc. The weight of the porous layer made of isophthalamide was 4.3 g / m ² .
[Puncture strength measurement]
The puncture strength of the present separators 2 and 7 to 11 was measured. The puncture strength was measured by setting a separator on a fixed frame of 11.3 mmφ, pushing a needle with a tip radius of 0.5 mm perpendicularly to the center of the separator, and pushing the needle at a constant speed of 2 mm / second. The maximum load applied to the separator during the movement of the needle by 5 mm was defined as the puncture strength. The results are shown in Table 4. When an aramid material is applied as the material of the nonwoven fabric, the amount of increase in puncture strength before and after coating is large. Nonwoven fabric made of an aramid material has a high affinity with polymetaphenylene isophthalamide forming the porous layer and is effective in terms of obtaining a high-strength separator because a high reinforcing effect is obtained.
[Examples 13 to 16]
Inventive batteries 2 to 5 were obtained in the same manner as in Example 3, using the inventive separators 8 to 11 obtained in Examples 9 to 12.
[Examples 17 to 20]
Similar to Example 3 except that the separators 8 to 11 of the present invention obtained in Examples 9 to 12 were used and 1 M LiPF ₆ EC / EMC / VC (29/70/1 weight ratio) was used as the electrolyte. Invention batteries 6 to 9 were obtained.
[Examples 21 to 24]
Similar to Example 3 except that the separators 8 to 11 of the present invention obtained in Examples 9 to 12 were used, and 1 M LiPF ₆ EC / EMC / VA (29/70/1 weight ratio) was used as the electrolyte. Invention batteries 10 to 13 were obtained.
[Comparative Example 5]
A battery was produced in the same manner as in Example 3 except that the PET nonwoven fabric produced in Example 9 was used as the separator. This battery is referred to as a comparative battery 3.
[Battery durability evaluation]
Invention batteries 2 to 13 and Comparative battery 3 prepared in Examples 13 to 24 were charged to 4.2 V and stored at 80 ° C. for 4 days. Thereafter, the battery was disassembled and the separator inside was taken out and observed. The results are shown in Table 5. From Table 5, it can be seen that there is an effect of sufficiently suppressing the deterioration of PET when the amount of polymetaphenylene isophthalamide applied is appropriate. It can also be seen that it is effective to add VC or VA to the electrolytic solution in order to prevent this PET deterioration.
[Example 25]
PET short fibers having a fineness of 0.11 dtex (average fiber diameter of about 3.2 μm), PET short fibers having a fineness of 0.33 dtex (average fiber diameter of about 5.5 μm), and fineness of 0.22 dtex (average fiber diameter of about 4.5 μm) The PET short fibers for binder were blended at a weight ratio of 3/2/5, formed into a film with a basis weight of 12.6 g / m ² by a wet papermaking method, and calendered at 140 ° C. to obtain a PET nonwoven fabric with a film thickness of 18 μm. It was.
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 60: 30 (weight ratio) so as to be 6% by weight, A polymer solution was prepared. In this polymer solution, α-alumina fine particles (manufactured by Iwatani Chemical Industry Co., Ltd .; SA-1) having an average particle diameter of 0.8 μm were dispersed in the same weight part as polymetaphenylene isophthalamide to prepare a coating slurry. This coating slurry was applied to both sides of the PET nonwoven fabric, and this coated product was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 50: 50 (weight ratio) for 60 seconds to form a coagulated film. Obtained. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to obtain a separator for a lithium ion secondary battery of the present invention. This is the separator 12 of the present invention. The separator 1 of the present invention had a film thickness of 23 μm, a basis weight of 16.6 g / m ² , and a Gurley value (JIS P8117) of 20 seconds / 100 cc. In addition, as a result of visually observing the separator 12 of the present invention, no pinhole was observed.
[Reference Example 1]
Polymetaphenylene isophthalamide (manufactured by Teijin Techno Products Ltd .; trade name “Conex”) was dissolved in a mixed solvent of dimethylacetamide: tripropylene glycol = 60: 30 (weight ratio) so as to be 6% by weight, A polymer solution was prepared.
This polymer solution was applied to both sides of the PET nonwoven fabric obtained in Example 25, and this coated product was immersed in a coagulation bath at 40 ° C. having a composition of dimethylacetamide: water = 50: 50 (weight ratio) for 60 seconds. A coagulated film was obtained. This solidified film was washed with water in a 50 ° C. water bath for 10 minutes and then dried to produce the separator of the present invention. However, pinholes frequently occurred, and a sufficient product could not be obtained.
[Reference Example 2]
A separator of the present invention was produced in the same manner as in Example 25 except that the amount of α-alumina fine particles was changed to 10% by weight with respect to polymetaphenylene isophthalamide. However, pinholes occurred frequently and a sufficient product could not be obtained. It was.
[Reference Example 3]
A separator of the present invention was prepared in the same manner as in Example 25 except that the amount of α-alumina fine particles was 900% by weight with respect to polymetaphenylene isophthalamide. A sufficient separator could not be obtained.
[Membrane resistance measurement]
A measurement cell was prepared by impregnating an electrolyte with a separator, sandwiching the separator between a pair of electrodes (size: 2 cm × 1.4 cm = 2.8 cm ² ) made of aluminum foil, and enclosing it in an aluminum laminate film. Here, 1M LiBF ₄ PC / EC (1/1 weight ratio) was used as the electrolytic solution. The resistance of one, two, and three separators in the cell was measured by the AC impedance method, and the resistance of one separator was obtained from the slope when this resistance was plotted against the number of separators. It was. The AC impedance measurement was performed by the 4-terminal method, and the amplitude was 10 mV and the frequency was 100 kHz. The measurement temperature was 20 ° C.
Results performed above the membrane resistance measurement to the present invention the separator 1 and 12, respectively, membrane resistance 6.026ohm ^· cm 2, was 5.211ohm · cm ^2.
From the measurement results of the film resistance, it can be seen that it is effective to reduce the polymetaphenylene isophthalamide concentration in the polymer solution. However, when coating is performed using such a polymer solution, as in Reference Examples 1 and 2, the viscosity is too low and pinholes occur frequently, making it difficult to obtain a sufficient product. As described in Example 25, this problem can be solved by appropriately adding ceramic fine particles. However, if the ceramic fine particles are added too much as in Reference Example 3, it is difficult to obtain a sufficient coating film due to the problem of powder falling.

  Since the separator for a lithium ion secondary battery of the present invention has high heat resistance and is effective for overcharge countermeasures, the use of this separator can improve the safety of the lithium ion secondary battery. The lithium ion secondary battery of the present invention using the separator of the present invention is suitable for HEV applications that require safety and performance at high temperatures.

Claims (17)

A separator for a lithium ion secondary battery in which a porous layer mainly composed of a metaaromatic polyamide is formed on both front and back surfaces of a nonwoven fabric ,
A separator for a lithium ion secondary battery, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc . A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The separator for a lithium ion secondary battery according to claim 1.   Adhesion amount of the surfactant is 0.005 to 0.750 g / m ² The separator for a lithium ion secondary battery according to claim 2, wherein:   The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.   The weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m. ² The lithium ion secondary battery separator according to claim 4, wherein the separator is a lithium ion secondary battery separator.   The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the nonwoven fabric is a nonwoven fabric mainly composed of a metaaromatic polyamide.   The separator for a lithium ion secondary battery according to any one of claims 1 to 3, wherein the non-woven fabric is composed of a meta-aromatic polyamide short fiber and a para-aromatic polyamide pulp.   The separator for a lithium ion secondary battery according to any one of claims 1 to 7, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.   2. The ceramic fine particles having an average particle diameter of 0.05 to 2 [mu] m are contained in the porous layer, and the ceramic fine particles are 30 to 80% by weight based on the weight of the porous layer. The separator for lithium ion secondary batteries of any one of 1-8.   A polymer solution mainly composed of a metaaromatic polyamide and a solvent that is a good solvent for the metaaromatic polyamide is applied to both the front and back surfaces of the nonwoven fabric, and then the coated nonwoven fabric is poor against the metaaromatic polyamide. The separator for a lithium ion secondary battery according to any one of claims 1 to 9, wherein the separator is solidified in a mixed liquid mainly composed of a solvent which is a solvent and a solvent which is a good solvent, and then washed with water and dried. Manufacturing method.   In a lithium ion secondary battery comprising a positive electrode, a negative electrode, a non-aqueous electrolyte, a separator, and obtaining an electromotive force by doping and dedoping of lithium ions,
  The separator is formed with a porous layer mainly made of a metaaromatic polyamide on both front and back surfaces of the nonwoven fabric,
  A lithium ion secondary battery, wherein the separator has a thickness of 15 to 40 μm and a Gurley value (JIS P8117) of 10 to 50 seconds / 100 cc.   A surfactant containing at least one selected from the group consisting of a cationic surfactant, an anionic surfactant, an amphoteric surfactant and a nonionic surfactant is attached to the porous layer. The lithium ion secondary battery according to claim 11. 12. The lithium ion secondary battery according to the amount deposited of the surfactant is characterized by a 0.005~0.750g / ^{m 2.}   The lithium ion secondary battery according to any one of claims 11 to 13, wherein the nonwoven fabric is a nonwoven fabric mainly composed of polyethylene terephthalate.   The weight of the porous layer mainly composed of metaaromatic polyamide is 4 to 10 g / m. ² The lithium ion secondary battery according to claim 14, wherein:   The lithium ion secondary battery according to any one of claims 11 to 13, wherein the nonwoven fabric is a nonwoven fabric mainly composed of a metaaromatic polyamide.   The lithium ion secondary battery according to any one of claims 11 to 16, wherein the metaaromatic polyamide is polymetaphenylene isophthalamide.

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