KR20040042749A - Porous Polymer-Coated Gelling Separators and Electrochemical Cells Using the Same - Google Patents

Abstract

PURPOSE: A gelled separator and an electrochemical cell using the separator are provided, to improve ion conductivity, electrochemical stability, mechanical properties, electrolyte solution maintenance and adhesive strength to an electrode. CONSTITUTION: The gelled separator is prepared by coating a porous polymer which can be gelled by an electrolyte solution, on at least one surface of a separator porously. Preferably the porous polymer is at least one selected from the group consisting of polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, poly(methyl methacrylate), polystyrene, poly(vinyl pyrrolidone), poly(vinyl chloride) and polybutadiene. The polymer used as a separator is at least one selected from the group consisting of an olefin-based resin such as polyethylene and polypropylene, a fluoride-based resin such as polyvinylidene fluoride and polytetrafluoroethylene, an ester-based resin such as polyethylene terephthalate, and cellulose-based nonwoven.

Description

Porous Polymer-Coated Gelling Separators and Electrochemical Cells Using the Same}

An object of the present invention is to solve the leakage and safety problems of the electrolyte solution generated in the secondary battery using the liquid electrolyte, the weak physical properties of the gel polymer electrolyte generated in the polymer battery using the gel polymer electrolyte. Specifically, a polyolefin membrane was used as a support to coat a polymer that can be physically gelled with a porous material. At this time, a polymer coated on the membrane is a polymer that is excellent in liquid retention property and does not dissolve in the electrolyte solution and exhibits adhesiveness when gelled. The present invention provides a porous gelation separator and an electrochemical cell using the same, which prevent electrolyte leakage after battery assembly, improve safety, and improve cycle life and long-term stability due to electrode / separation membrane integration.

Secondary batteries are the three key components of the future information industry, and their demands are exploding along with semiconductors and displays. This is because secondary batteries, which are energy sources, are essential for the portability, high performance, and light and small size of future electronic devices closely related to human life in the 21st century. The secondary battery is composed of a negative electrode, a positive electrode and an electrolyte. In the classification of the battery, it is classified into primary and secondary batteries according to the reversibility of the oxidation / reduction reaction of the electrode, and may be classified into a solid, a liquid battery, and a cylindrical, square, coin type, etc. according to the appearance of the electrolyte. Currently used secondary batteries include lead acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, lithium batteries, etc., which are gradually changing to lithium secondary batteries in small electronic devices. Most of the lithium secondary batteries currently used in portable devices are lithium ion batteries using a liquid electrolyte. Lithium polymer battery has the advantage of eliminating the possibility of leakage and explosion risk, which is a disadvantage of lithium ion battery using liquid electrolyte, by using solid or gel polymer electrolyte, and also using polymer electrolyte. It is possible and does not have a memory effect, so it is attracting attention as the next generation battery that will succeed the lithium ion battery. In accordance with this trend, research and development of materials and manufacturing processes of lithium polymer batteries are being actively conducted. Until now, many inventors have developed gel polymer electrolytes having excellent conductivity at room temperature. These gel polymer electrolytes are prepared by adding a large amount of liquid electrolyte to a polymer matrix. Representative examples of the polymer used as the polymer matrix include polyacrylonitrile, polyvinylidene fluoride, polyethylene oxide, polymethyl methacrylate, polyvinyl chloride, and the like. However, in the case of using a gel polymer electrolyte as an electrolyte of a lithium polymer battery, since a large amount of organic electrolyte is added to the matrix polymer and the mechanical properties become weak, there is a possibility of internal short-circuit, and the film thickness for molding is thickened, so that at a high rate. There is a disadvantage that the battery characteristics are sharply dropped. In addition, the organic electrolyte is volatilized during the preparation of the gel polymer electrolyte, thereby making it difficult to accurately adjust the electrolyte content.

As a means for solving the above-mentioned problems, a number of inventions for preparing a gel polymer electrolyte using a porous membrane as a support have been published. Motorola of the United States has applied for a number of patents in the field of using a porous membrane as a support, a typical US Patent No. 5,681,357, a polyethylene porous membrane known as Celgard by coating and drying a poly (vinylidene fluoride) solution in the cell After the preparation, an electrolyte solution is injected, and a method of producing a lithium secondary battery by gelling at a high temperature is provided. In addition, Japanese Patent Application Laid-open No. Hei 10-162802 filed by Sony, discloses a separator produced by applying or impregnating a gel polymer electrolyte such as polyacrylonitrile to an insulating porous membrane. In order to improve the processability of the gel polymer electrolyte as described above, many researches and developments have been conducted to use a porous membrane as a support, but in most inventions including the above invention, the polymer coated on the porous membrane is incompatible with the electrolyte solution, so that the coating is performed at high temperature. The process of gelling the polymer is essential, and also, the problems of electrolyte leakage and safety from the battery are always inherent when the battery is manufactured using them. Therefore, there is an urgent need for the development of new materials that can solve the above problems and improve battery performance.

The present invention is to improve the leakage and safety of the electrolyte generated in the secondary battery using a liquid electrolyte, and to solve the weak physical properties of the gel polymer electrolyte from the polymer battery point of view. To this end, in the present invention, the gelable polymer in the porous polyethylene separator is coated with porous on both sides of the separator. At this time, the polymer coated on the separator is excellent in liquid retention property with respect to the electrolyte, and the polymer is expressed by physical gelation by the electrolyte. By using the material, it was intended to prevent electrolyte leakage and to integrate the electrode / electrolyte. When the gelling separator developed by the present invention is applied to a lithium secondary battery, post-injection of the electrolyte solution is possible, so that the battery can be manufactured in a laminated or wound form, and the porous coated polymer has excellent compatibility with the electrolyte solution. A lithium secondary battery having no leakage and excellent in cycle characteristics can be manufactured by electrode / electrolyte integration.

1 is a photograph of the surface of the gelling separator coated with a porous polymer according to the present invention observed with an electron microscope.

2 is a graph showing a current-voltage curve measured by a linear scanning potential method after gelling a gelling separator coated with a porous polymer according to the present invention using an electrolyte solution.

3 is a view showing the discharge capacity according to the number of cycles obtained by charging and discharging at a constant current of the voltage range of 2.8 to 4.2 V, 0.2C of the lithium secondary battery manufactured by the present invention.

4 is a view showing a change in the discharge curve of the discharge current of the lithium secondary battery manufactured by the present invention.

According to the present invention, after coating a polymer material having excellent affinity with electrolyte on both sides of the separator with a porous material, the gelation proceeds easily even at room temperature by post-injecting the electrolyte solution, thereby preventing leakage of the injected electrolyte solution and providing adhesion with the electrode. It is to provide a gelling separator. In this case, the polymer to be coated is manufactured into a porous membrane having fine pores for rapid absorption and gelation of the electrolyte during post-injection of the electrolyte.

The manufacturing process of the gelling separator coated with the porous polymer according to the present invention is largely composed of two steps of coating the polymer solution on the membrane and making them porous by a phase inversion process. First, a polymer solution composed of a polymer and an organic solvent is coated with a uniform thickness on both sides of a polyolefin separator. The phase transition is advanced by soaking them in a container containing water for 5 hours or more. During the phase transition process, a change occurs between the organic solvent and the nonsolvent water, and numerous pores are formed in the coated polymer layer. After washing several times with pure distilled water and drying in a vacuum oven for more than 24 hours, a separator coated with a porous polymer on both sides is obtained. These porosities are about 30 to 80% and show a large difference in mechanical properties depending on the porosity. In addition, it is possible to control the thickness of the porous polymer to be coated by adjusting the concentration of the polymer solution, in order to apply them to the secondary battery, a thickness in the range of 10 to 20 ㎛ is preferred. When the separator coated with the porous polymer obtained in this manner is brought into contact with the electrolyte solution, the gelation proceeds instantaneously and the ion conductivity is imparted, thereby being used as the polymer electrolyte. At this time, as the gelation progresses due to the polymer membrane electrolyte coated on the separator, the adhesive property is expressed, and thus, the battery is easily integrated with the electrode.

The separator used in the present invention is not particularly limited, and conventionally known ones can be used. For example, olefin resins such as polyethylene and polypropylene, fluorine resins such as polyvinylidene fluoride and polytetrafluoroethylene, polyester resins such as polyethylene terephthalate, and nonwoven fabrics such as cellulose Do. These are microporous membranes excellent in mechanical strength having a porosity of at least 30% or more and having a thickness of about 25 μm. In addition, in the present invention, the polymer coated on the separator is preferably a substance which does not dissolve in the electrolyte while having a proper affinity with the electrolyte. For example, acrylonitrile-methyl methacrylate copolymer is representative. The relative ratio of acrylonitrile and methyl methacrylate in the copolymer is an important parameter that determines the amount of their absorption and adhesion to the electrode when the electrolyte is injected. The organic solvent used for dissolving the polymer should have excellent dissolution properties for the polymer to be coated and have non-solvent properties for the separator used as a support. Such organic solvents include acetone, tetrahydrofuran, acetonitrile, dimethylformamide, dimethyl sulfoxide, N-methylpyrrolidone, dimethelcarbonate, and the like, and two or more of these organic solvents may be used in combination. The electrolytic solution used to gel the porous polymer-coated separator and impart ionic conductivity is a mixture composed of an aprotic solvent and a lithium salt. Specifically, examples of the aprotic solvent used in the electrolyte include cyclic esters such as ethylene carbonate and propylene carbonate, linear esters such as dimethyl carbonate, diethyl carbonate, and ethyl methyl carbonate, and linear ethers such as dimethoxyethane and diethoxyethane. And cyclic ethers such as tetrahydrofuran, lactones such as γ-butyrolactone, and the like can be used, and these can be used alone or in combination of two or more thereof. As the lithium salt, lithium hexafluorophosphate (LiPF ₆ ), lithium perchloroate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), or the like can be used. It is not limited only to these lithium salts, The lithium salt couple | bonded with various other anions can also be used.

When the lithium secondary battery is manufactured using the gelable separator coated with the porous polymer prepared according to the present invention, the following materials may be used as the positive electrode and the negative electrode. Lithium metal oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganese oxide are mainly used as the positive electrode. In addition, reactive organic materials such as sulfur and materials such as titanium sulfide and vanadium oxide may be used. In addition, as the negative electrode, it is possible to use lithium metal, lithium alloy, amorphous carbon and graphite carbon.

The present invention will be further illustrated by the following examples, which are only specific examples of the present invention and are not intended to limit or limit the protection scope of the present invention.

EXAMPLE

(Example 1)

The gelling separator coated with the porous polymer according to the method of the present invention was prepared as follows. A polymer solution is prepared by dissolving an acrylonitrile-methyl methacrylate copolymer having an acrylonitrile composition of 85 mol% in a dimethylformamide solvent at 1% by weight. After coating the polymer solution prepared above on a polyethylene membrane having a thickness of 25 μm and a porosity of 40% to be used as a support, they are immersed in a container containing water for 12 hours. They are taken out, washed thoroughly with distilled water and dried in a vacuum oven for 24 hours. An electron micrograph of the surface of the separator coated with the obtained porous polymer is shown in FIG. It can be observed that many pores were formed in the coated polymer layer, and the thickness of the coated porous polymer layer was 2.5 μm based on the cross section, and the total thickness of the separator was 30 μm. When these are deposited in the electrolyte solution, the electrolyte solution diffuses and penetrates into the porous membrane, and the polymer on the surface of the porous membrane gels to have ion conductivity. At this time, the electrolyte was used by dissolving a lithium perchloroate salt in a ethylene carbonate / dimethyl carbonate (volume ratio 1/1) mixed solvent at a concentration of 1 M. From the weight change before and after electrolyte deposition, the amount of electrolyte solution contained in the separator was 85% by weight, and the ionic conductivity was measured to be 7.7 × 10 ⁻⁴ S / cm at room temperature. The electrochemical stability was investigated through a linear scanning potential experiment, which was found to be electrochemically stable until 5.0 V using lithium as a reference electrode (see FIG. 2).

(Example 2)

The thickness of the porous polymer layer coated on the separator was controlled by changing the concentration of the polymer solution coated on the polyethylene separator. At this time, the electrolyte absorption amount and the ion conductivity value according to the thickness change (double side basis) of the porous polymer layer are shown in Table 1.

As a result of the electrochemical stability measurement, it was found to be stable up to 5.0 V regardless of the thickness of the porous polymer layer (see FIG. 2).

(Example 3)

A lithium _secondary battery was manufactured by using a porous polymer-coated gelation separator and a carbon anode and lithium cobalt oxide (LiCoO ₂ ) prepared by the methods of Examples 1 and ₂ using a cathode. The negative electrode was composed of 92% by weight of artificial graphite as an active material and 8% by weight of polyvinylidene fluoride as a binder. The positive electrode was composed of 94 wt% of lithium cobalt oxide (LiCoO ₂ ) as an active material, 3 wt% of Super-P carbon as a conductive material, and 3 wt% of polyvinylidene fluoride as a binder. In Examples 1 and 2, the separator gelled by the electrolytic solution was placed on the positive electrode, the carbon negative electrode was placed thereon, and vacuum-packed with an aluminum blue bag to prepare a cell. Charge and discharge experiments were performed using Toyo's charger and discharger. The prepared lithium secondary battery was charged and discharged at a charge and discharge rate of 0.2 C within a range of 2.8 to 4.2 V. The change of the discharge capacity according to the number of cycles obtained at this time is shown in FIG. Discharge capacity of the resultant secondary battery is a value converted based on the mass of the positive electrode active material is LiCoO _2. It can be seen that the capacity reduction due to the cycle repetition is small and thus shows relatively good cycle characteristics. This is because the porous polymer coated on both sides of the polyethylene separator not only effectively holds the electrolyte solution but also improves the interfacial adhesion between the electrode and the separator. The discharge curve of the discharge rate of the manufactured lithium secondary battery is shown in FIG. Even at 2C, which is a high rate, it shows a specific capacity of 138 mAh / g, which shows excellent high rate discharge characteristics.

When the gelling separator coated with the porous polymer devised by the present invention is applied to an electrochemical cell, the following features and effects are expected. Less leakage of electrolyte solution improves safety and light aluminum pouch can be used as a packaging material. By preparing the polymer coated on the membrane in a porous manner, it is possible to increase the electrolyte absorption and at the same time improve the absorption rate of the electrolyte during the battery manufacturing process. The electrode / electrolyte binding force can be improved by coating an adhesive gel polymer electrolyte on the separator surface. With excellent mechanical properties, it is possible to manufacture a thin film electrolyte film that can be handled, high yield when assembling batteries, and less possibility of internal short circuit when using.

Claims (4)

A separator produced by coating a polymer that can be gelled by an electrolyte on at least one surface of the separator with a porous membrane and an electrochemical cell prepared therefrom. The electrochemical cell of claim 1, wherein the gelation process is performed by an electrolyte solution injected after battery assembly. The method of claim 1, wherein the gelable porous polymer coated on the separator is polyethylene oxide, polyacrylonitrile, polyvinylidene fluoride, polymethyl methacrylate, polystyrene, polyvinylpyrrolidone, polyvinyl chloride, A gelling separator characterized by using a copolymer or a blend consisting of a single component or two or more components selected from polybutadiene. The polymer used in the separator is an olefin resin such as polyethylene, polypropylene, fluorine resin such as polyvinylidene fluoride, polytetrafluoroethylene, ester resin such as polyethylene terephthalate, and the like. A porous gelling separator, comprising a single component or two or more components selected from the group consisting of cellulosic nonwovens.