JP2005317494A - Porous membrane, its manufacturing method, and secondary battery using this - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a separator for a lithium-ion secondary battery superior in balance of a shut-down characteristic and a melt-down characteristic. <P>SOLUTION: This is related to a polyether sulfone porous film of film thickness of 5 to 100 μm and a composite film of the porous film and a polyolefin porous film. Furthermore, this is related to the lithium-ion secondary battery composed by interposing the porous film as the separator between a positive electrode and a negative electrode storing and releasing the lithium ions. <P>COPYRIGHT: (C)2006,JPO&NCIPI

Description

  The present invention relates to a polyethersulfone porous membrane exhibiting excellent shutdown temperature characteristics and high meltdown temperature characteristics as a separator for secondary batteries, particularly lithium ion secondary batteries, for which improvement in safety is required, and a method for producing the same And a secondary battery using the same.

  In recent years, with the development of electronic portable devices, batteries with high energy density and high electromotive force have been developed. Among them, nonaqueous electrolyte batteries, particularly lithium ion secondary batteries, have been vigorously developed from the viewpoint of high electromotive force. One of the problems with such non-aqueous electrolyte batteries is the danger of using flammable organic solvents. When both electrodes of the battery are short-circuited and a decomposition reaction of the battery contents occurs, the contents are ejected due to a rapid temperature rise inside the battery. In order to solve such problems, there are currently attachment of a safety valve and provision of a shutdown function by a separator containing a meltable component.

  However, the safety valve is not an essential protective measure against a short circuit, but only relieves a sudden pressure increase inside the battery.

  On the other hand, the shutdown function of the separator uses a porous film made of a heat-meltable material. When the temperature inside the battery reaches a certain temperature due to a short circuit or the like, By blocking, the ionic conductivity is hindered and the battery reaction that causes heat generation is suppressed. As such a separator, a porous membrane of an olefin polymer material disclosed in Patent Documents 1 to 3 is disclosed. However, when such a heat-meltable material is used, even if the shutdown function works due to heat rise, if there is a further temperature rise, the film itself melts and the isolation between the electrodes, which is the original function, is impaired. End up. This is a phenomenon called meltdown, which is not preferable for a battery. It has been proposed to widen the range of the shutdown temperature as a remedy for such problems. For example, as disclosed in Patent Documents 4 to 7 and the like, it is a technique of laminating and coating a heat-meltable material on a porous film or a nonwoven fabric substrate. However, these preparation methods may be complicated and an insulation property at the time of shutdown is not necessarily obtained.

Japanese Patent No. 2642206 JP-A-6-212006 Japanese Patent Laid-Open No. 8-138643 Japanese Patent Publication No. 4-1692 JP-A-60-52 JP 61-232560 A Japanese Patent Laid-Open No. 10-6453

  The present invention has been made in view of such circumstances, and an object of the present invention is to provide an inexpensive separator that has excellent shutdown characteristics and melt-down characteristics, which are superior to conventional porous membrane separators, and is excellent in insulation. .

  In order to achieve the above-mentioned object, the present invention has been intensively studied. It has been found that an excellent lithium ion secondary battery can be obtained. That is, this invention is the following porous membrane, its manufacturing method, and a secondary battery using the same.

(1) A secondary battery using a polyethersulfone resin porous film having a film thickness of 5 to 100 μm as a separator.

(2) A secondary battery using as a separator a composite porous membrane having a thickness of 5 to 100 μm, which is a combination of a polyethersulfone resin porous membrane and a polyolefin-based porous membrane.

(3) The secondary battery according to (1) or (2), wherein the air permeability of the porous membrane is 1 to 2000 sec / 100 cc Air.

(4) The porous membrane described in any one of (1) to (3).

(5) After applying or immersing the polyethersulfone resin solution on the substrate, it is mixed with the solvent in which the polyethersulfone resin is dissolved, but for the polyethersulfone resin, it is put into a poor solvent. A method for producing a porous film to be solidified.

(6) After applying or immersing the polyethersulfone resin solution on one or both surfaces of the polyolefin-based porous membrane, it is mixed with a solvent in which the polyethersulfone resin is dissolved, but it is a poor solvent for the polyethersulfone resin. A method for producing a composite porous membrane that is charged into a solution and solidified.

  The present invention provides a secondary battery excellent in balance between shutdown characteristics and meltdown characteristics by using a porous film of polyethersulfone resin or a composite porous film in which a porous film of polyethersulfone resin and a polyolefin film are laminated. Separator can be provided.

The present invention will be described in detail below. The polyethersulfone resin used in the present invention is usually
Those synthesized by a self-condensation reaction involving dehydrochlorination of dichlorophenyl sulfone and manufactured by Sumitomo Chemical, BASF Japan, etc. can be used. Polyethersulfone resin dissolves in amide solvents such as N-methyl-2-pyrrolidone and N, N'-dimethylformamide, and polar solvents such as pyridine, but as a separator to withstand the electrolyte for Li ion secondary batteries. It is a useful material.

  Next, the manufacturing method of a polyether sulfone resin porous membrane is demonstrated. The production of the porous membrane of the present invention is not particularly limited, but the above polyethersulfone resin can be used as dimethyl acetate, N, N′-dimethylformamide, N, N′-dimethylacetamide, N-methyl-2-pyrrolidone, pyridine. After being dissolved in a polar solvent such as this and coating this solution on a substrate such as a polyester film to a predetermined thickness or by extruding these solutions from a slit die into a film, the polyethersulfone resin is dissolved. Although it is miscible with the solvent, it is preferable that the polyethersulfone resin is poured into a poor solvent and solidified. In addition, the poor solvent said here shall not be able to melt | dissolve this polyethersulfone resin at 25 weightC and a 5-weight% density | concentration.

  Solvents that dissolve the polyethersulfone resin are mainly amide solvents such as N-methyl-2-pyrrolidone, dimethylacetamide, and N, N′-dimethylformamide as described above. In order to adjust the pore size and pore size distribution, it is possible to add alcohols such as methanol, ethanol, propyl alcohol, ethylene glycol, diethylene glycol, polyethylene glycol and polypropylene glycol, and ketones such as acetone and methyl ethyl ketone. Yes. These additives are 5 to 300 parts, preferably 10 to 200 parts, and more preferably 20 to 100 parts with respect to 100 parts of the polyethersulfone resin solution.

  The coagulation bath used when producing the porous material of the present invention is preferably a solution mainly composed of water. This coagulation bath can be mixed with other solvents miscible with water in order to adjust the coagulation rate, the pore size of the porous membrane and its distribution. Such solvents include alcohols such as methanol, ethanol, propyl alcohol, ethylene glycol, propylene glycol, diethylene glycol and polyethylene glycol, ketones such as acetone and methyl ethyl ketone, N, N′-dimethylformamide, N, N′-dimethyl. Examples include amide solvents such as acetamide and N-methyl-2-pyrrolidone. Among these, glycols such as ethylene glycol and polyethylene glycol and N-methyl-2- Amide solvents such as pyrrolidone, N, N′-dimethylacetamide, and N, N′-dimethylformamide are preferred. The amount of these solvents added is 5 to 500 parts, preferably 10 to 400 parts, and more preferably 20 to 300 parts with respect to 100 parts of water.

  The thickness of the polyethersulfone porous membrane is 5 to 100 μm, preferably 10 to 70 μm, and more preferably 15 to 50 μm. If the film thickness is 5 μm or less, the film becomes weak and may be broken. Conversely, when the film thickness exceeds 100 μm, the cycle characteristics may deteriorate. The porosity of the polyethersulfone porous membrane is preferably 30 to 90%. More preferably, it is 40 to 70%. When the porosity is 30% or less, the electric resistance of the film becomes high and it becomes difficult to flow a large current. On the other hand, if it is 90% or more, the film strength becomes weak. The air permeability, which is a measure of the pore diameter, is preferably 1 to 2000 sec / 100 cc Air measured by a method based on JIS-P8117. More preferably, it is 50-1500 sec / 100 cc Air, More preferably, it is 100-1000 sec / 100 cc Air. If the air permeability is less than 1 sec / 100 cc Air, the film strength becomes weak, and if it exceeds 2000 sec / 100 cc Air, the cycle characteristics may be deteriorated.

  The polyether sulfone resin porous membrane thus produced exhibits excellent shutdown characteristics and meltdown characteristics even when used alone as a separator.

  Another feature of the present invention is that a polyethersulfone resin porous membrane can be used in combination with a porous polyolefin membrane. The polyolefin-based porous membrane is a polyethylene or polypropylene film produced by, for example, the stretch opening method or the phase separation method described in 7th Polymer Material Forum (1998) Abstract 1 BIL09. When the polyethersulfone porous membrane and the polyolefin porous membrane are laminated, when the polyethersulfone porous membrane is A and the polyolefin-based porous membrane is B, A / B, A / B / A or B / A / B.

The production of these composite porous membranes is not particularly limited, but the following method is preferred.
(1) A polyethersulfone porous membrane and a polyolefin porous membrane are simply stacked.
(2) Using a polyolefin porous membrane as a support, impregnating or coating one or both sides with a polyethersulfone resin solution, and putting it in a coagulation bath in the same manner as described above to coagulate it.
(3) Combine (1) and (2) above.

  In the case of these composite porous membranes, the thickness is 5 to 100 μm, preferably 10 to 70 μm. The porosity is preferably 30 to 80% and the air permeability is preferably 1 to 2000 sec / 100 cc Air.

  The lithium ion secondary battery using the thus configured polyethersulfone resin porous membrane of the present invention as a separator exhibits the same battery performance as the conventional one, and is a safe battery excellent in shutdown characteristics and meltdown characteristics. Can be obtained. The lithium ion secondary battery according to the present invention can be produced according to a conventional method except that the porous membrane of the present invention is used as a separator.

  That is, a material containing lithium can be used as the positive electrode active material, a material that can store and release lithium as ions can be used as the negative electrode, and an organic solvent solution of an electrolyte composed of a compound containing lithium and fluorine can be used as the electrolytic solution.

  Specifically, lithium metal oxides such as lithium cobaltate and lithium manganate capable of inserting and removing lithium ions can be used as the positive electrode active material. A known active carbon, various cokes, carbon black, a binder, a solvent, and the like are blended into the positive electrode active material as a conductive agent, and this dispersion is applied to a current collector such as aluminum and dried to form a positive electrode material. Can do.

  Coke, graphite, amorphous carbon, etc. are used as the negative electrode active material, and these are applied to a current collector such as a copper foil with a dispersion composed of a binder and an organic solvent, and dried to form a negative electrode material. it can.

Examples of the electrolyte used in the electrolytic solution include LiClO ₄ , LiAsF ₆ , LiPF ₄ , LiBF ₄ , LiBr, LiCF ₃ SO ₃ , and the like, and examples of the organic solvent include propylene carbonate, ethylene carbonate, γ-butyrolactone, dimethyl One or more of carbonate, ethyl methyl carbonate, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran and the like are used.

  EXAMPLES Hereinafter, although an Example demonstrates this invention further in detail, this invention is not restrict | limited by these Examples.

Film thickness: A laminated film of a polyethersulfone porous film and a polyolefin was measured with a SONY μmeter.

Shutdown temperature characteristics: A porous membrane filled with 1 mol / l of lithium tetrafluoroborate dissolved in propylene carbonate was used and measured at an AC frequency of 1 kHz, an AC amplitude of 100 mV, and a temperature increase rate of 2 ° C./min. The temperature at which the rise of the impedance value due to the temperature rise once becomes 100 Ωcm ² is the shutdown start temperature, the temperature at which the impedance value exceeds 1 kΩcm ² , further increases and then decreases to 1 kΩcm ² again is the meltdown temperature It was.

[Example 1]
A commercially available separator (polyolefin manufactured by Tonen Chemical Co., Ltd.) was prepared by mixing 10 parts of Ultrazone E1010 (polyethersulfone manufactured by BASF Japan) in 90 parts of N-methyl-2-pyrrolidone with 10 parts of polyethylene glycol # 400. The film was applied to a porous film (25 μm) so as to have a film thickness of 1 μm, immersed in a 70/30 coagulation bath of water / N-methyl-2-pyrrolidone, washed with water, and dried. The obtained composite porous membrane had a thickness of 26 μm and an air permeability of 580 sec / 100 cc Air. The shutdown temperature of this film was 120 ° C., and the meltdown temperature was 200 ° C. or higher. Using this porous membrane as a separator, a positive electrode using lithium cobaltate as a positive electrode active material, acetylene black as a conductive agent, polyvinylidene fluoride as a binder, a negative electrode active material mixed with graphite and amorphous carbon, and polyvinylidene fluoride A coin-type battery was prepared using a negative electrode as a binder and Sollite (manufactured by Mitsubishi Chemical) as an electrolytic solution, and the battery characteristics were evaluated. Compared to the battery using the above commercially available separator, the discharge capacity and cycle characteristics were almost the same.

[Example 2]
Using a solution obtained by dissolving 10 parts of Sumika Excel 4100G (polyethersulfone manufactured by Sumitomo Chemical Co., Ltd.) in 90 parts of N-methyl-2-pyrrolidone, a composite porous film having a thickness of 27 μm was prepared in the same manner as in Example 1. did. The composite porous membrane had an air permeability of 670 sec / 100 cc Air, a shutdown temperature of 120 ° C., and a meltdown temperature of 200 ° C. or higher.

[Example 3]
The polyethersulfone solution used in Example 1 was coated on a 100 μm polyester film, immersed in a water / N-methyl-2-pyrrolidone 70/30 coagulation bath, washed with water and dried to obtain a polyethersulfone having a film thickness of 25 μm. A porous membrane was created. This porous membrane had a porosity of 71%, an air permeability of 9.3 sec / 100 cc Air, a shutdown temperature of 185 ° C., and a meltdown temperature of 200 ° C. or higher. Using this porous membrane as a separator, a coin-type battery was prepared by the same method as in Example 1 and the battery characteristics were evaluated. As a result, the discharge capacity and cycle durability were almost the same as the polyolefin porous membrane that is a commercially available separator. It was.

[Example 4]
After immersing a polyolefin porous membrane (25 μm) manufactured by Tonen Chemical in the polyethersulfone resin solution of Example 1, scraping with a squeeze roll so that the dry film thickness becomes 1 μm on each side of the polyolefin porous membrane. / Polyethylene glycol (molecular weight 400) ratio was put into a coagulation bath of 70/30 to coagulate, washed and dried to obtain a composite porous membrane of 3 layers having a thickness of 27 μm. The composite porous membrane had a shutdown temperature of 120 ° C. and a meltdown temperature of 200 ° C. or higher. The battery performance such as discharge capacity and cycle durability of the coin battery prepared by using the composite porous membrane as a separator and having the same configuration as in Example 1 showed the same characteristics as the polyolefin porous membrane single separator.

[Example 5]
Discharge capacity of coin-type battery prepared under the same conditions as in Example 1 using a composite membrane in which a polyolefin porous membrane was superimposed on the polyethersulfone porous membrane side of the polyethersulfone composite porous membrane prepared in Example 1 The battery performance, such as cycle durability, was almost the same as that of the polyolefin porous membrane single separator.

[Comparative Example 1]
The shutdown temperature of the polyolefin porous membrane to which polyethersulfone was not applied in Example 1 was 120 ° C., and the melted down state was caused by shrinkage and melting at 150 ° C.

  The present invention provides a lithium ion secondary that has a good balance between shutdown characteristics and meltdown characteristics by using a porous film of polyethersulfone resin or a composite porous film in which a porous film of polyethersulfone resin and a polyolefin film are laminated. A battery separator can be provided.

Claims (6)

  A secondary battery using a polyethersulfone resin porous membrane having a thickness of 5 to 100 μm as a separator.   A secondary battery using as a separator a composite porous membrane having a thickness of 5 to 100 μm, which is a combination of a polyethersulfone resin porous membrane and a polyolefin-based porous membrane.   The secondary battery according to claim 1, wherein the air permeability of the porous membrane is 1 to 2000 sec / 100 cc Air.   The porous membrane as described in any one of Claims 1-3.   After applying or immersing the polyethersulfone resin solution on the base material, it is mixed with the solvent in which the polyethersulfone resin is dissolved. A method for producing a membrane.   After applying or immersing the polyethersulfone resin solution on one or both surfaces of the polyolefin-based porous membrane, it is mixed with the solvent in which the polyethersulfone resin is dissolved. A method for producing a composite porous membrane that is charged and solidified.