KR100431966B1 - Multi-layered Gelling Separators and Rechargeable Lithium Batteries Using Same - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gelable multi-layered separator having excellent ion conductivity, mechanical properties, and electrolyte retention properties, and excellent adhesion to electrodes, and a lithium secondary battery using the same. In the present invention, a microporous membrane was used as a support to improve mechanical properties, and the polymer coated on the porous membrane was excellent in liquid retention property with respect to the electrolyte, and by using a material having excellent adhesiveness, electrolyte leakage was prevented and safety was improved. In addition, it was intended to improve long-term stability by chemically crosslinking the polymer and the porous membrane to be coated. When the gelable separator developed through the present invention is applied to a lithium secondary battery, it is possible to post-inject the electrolyte solution, and to manufacture a battery in a laminated or wound form, and the polymer to be coated is excellent in compatibility with the electrolyte solution and the electrolyte leakage. And a safe lithium secondary battery can be manufactured.

Description

Multi-layered Gelling Separators and Rechargeable Lithium Batteries Using Same}

An object of the present invention is to improve the leakage and safety of the electrolyte generated in the lithium ion battery using a liquid electrolyte. Specifically, a porous gel was used as a support to coat a polymer that can be gelled physically. At this time, the polymer coated on the porous membrane is excellent in liquid retention property with respect to the electrolyte solution, and by using a material having excellent adhesive property, electrolyte leakage after cell assembly is obtained. The present invention provides a separator having a multilayer structure and a lithium secondary battery using the same, which prevents and improves safety and improves long-term stability by chemically crosslinking a polymer to be coated with a porous membrane.

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 batteries, they are classified into primary and secondary batteries according to the reversibility of the oxidation / reduction reaction of the electrodes, and solid and liquid batteries according to the phase of the electrolyte, and cylindrical, square, coin type, etc. Can be classified. 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. The lithium polymer battery has a great advantage of eliminating the possibility of leakage and explosion risk, which are disadvantages of the lithium ion battery using a liquid electrolyte, by using a solid or gel polymer electrolyte. In addition, the polymer electrolyte is used to design various types of batteries and has no memory effect, making it a popular next-generation battery following lithium ion batteries. Recently, according to the trend of light and small, multifunctional and high performance of portable electronic devices, the same need for power supply is required. This is because the location and size of the battery act as a limiting factor in the design of electronic devices. This is because the degree of freedom in design can be increased by employing a thin battery. In accordance with this trend, research and development on materials and manufacturing 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, US Patent No. 5,681,357. A method of preparing a lithium secondary battery by coating a polyethylene porous membrane, known as Celgard, with a poly (vinylidene fluoride) solution and drying to prepare a cell, injecting an electrolyte, and gelling at a high temperature is provided. In addition, Japanese Patent Application Laid-open No. Hei 10-162802 filed by Sony Corporation 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. In addition, the polymer to be coated is mostly made of hydrophilic polymer, there is a problem that is easily peeled off from the hydrophobic porous membrane. Therefore, there is an urgent need for the development of a new material that can solve the above problems and always improve the battery performance.

The present invention is to improve the leakage and safety of the electrolyte generated in the lithium ion battery using a liquid electrolyte. To this end, in the present invention, the microporous membrane was coated with a multi-layered polymer. At this time, the polymer coated on the porous membrane was excellent in the liquid retention property with respect to the electrolyte, and by using a material having excellent adhesiveness, electrolyte leakage was prevented and safety was improved. . In addition, it was intended to improve long-term stability by chemically crosslinking the polymer and the porous membrane to be coated. When the gelable multilayer separator developed by the present invention is applied to a lithium secondary battery, post-injection of the electrolyte is possible, so that a battery can be manufactured in a stacking or winding form and coated in multiple layers. Since the polymer has excellent compatibility with the electrolyte, there is no electrolyte leakage and a safe lithium secondary battery can be manufactured.

BRIEF DESCRIPTION OF THE DRAWINGS It is a figure which shows the cross section of the gelable separator which has a multilayered structure by this invention.

FIG. 2 is a view showing a change in ion conductivity with time obtained after deposition of an electrolyte solution of the gelable separator and the pure polyethylene separator prepared in Example 1 and Comparative Example 1. FIG.

3 is a view showing a current-voltage curve measured by the linear scanning potential method using a gelable multi-layered separator according to the present invention.

4 is a view showing charge and discharge curves obtained by applying a gelable multilayer separator prepared by the present invention to a lithium secondary battery.

5 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.

According to the present invention, after coating a polymer material having excellent affinity with an electrolyte solution on a porous membrane with a thin film, the gelation proceeds easily at room temperature by post-injecting the electrolyte solution, thereby preventing leakage of the injected electrolyte solution and providing adhesion with the electrode. The present invention provides a separator for a lithium secondary battery. In this case, in order to prevent separation of the polymer having a different physicochemical property from the porous membrane, an appropriate amount of a crosslinking agent is added during the coating of the polymer, so that the microporous membrane and the gelling polymer material are integrated.

The manufacturing process of the gelable separator by this term is comprised by the process of coating the porous film with the polymer solution containing a polymer and a crosslinking agent, and the process of bridge | crosslinking these to a porous film by heat drying. First, a polymer solution composed of a polymer, a crosslinking agent, an initiator, and an organic solvent is coated on at least one surface of the microporous membrane with a uniform thickness on the microporous membrane. The organic solvent contained in the polymer solution is removed, and the porous membrane coated with the polymer solution is thermally dried to induce a chemical reaction of the crosslinking agent. At this time, the temperature needs to be appropriately adjusted according to the type of organic solvent and crosslinking agent used. When the crosslinking reaction is completed, a porous membrane coated with at least one side of the separator is obtained. When contacted with the electrolyte, they gel instantly and give ion conductivity to be used as a polymer electrolyte.As the polymer film coated on the separator is gelled by the electrolyte, adhesive properties are expressed, thereby integrating with electrodes during battery manufacturing. It's easy.

The microporous membrane 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 having excellent mechanical strength with a porosity of at least 30% and having a thickness of about 25 μm.

In addition, the polymer coated on the porous membrane in the present invention is polyethylene oxide as a material having affinity with the electrolyte solution. Polyethylene oxide is a highly polar polymer and has a function of expressing electrolyte retention property and adhesion property when infiltrating an organic solvent. It is possible to use a conventionally known material as a compound in which crosslinking agent added during the preparation of the polymer solution includes at least two or more double bonds in the molecule, and thus crosslinking may proceed by heat. These compounds include divinylbenzene, diacrylate or triacrylate, dimethacrylate or trimethacrylate, diallyl ester or triallyl ester, diglycidyl ester, polyethylene glycol dimethacrylate, and the like. Two or more compounds can also be mixed and used. Preferably, polyethylene glycol dimethacrylate having a polyethylene oxide unit is preferably used as a crosslinking agent. Any initiator material added to induce a chemical reaction of the crosslinking agent may be any material in which free radicals are generated by heat. These pyrolysis initiators include azo compounds such as azobisisobutylonitrile and peroxide compounds such as benzoyl peroxide. The organic solvent used to dissolve the polymer and the crosslinking agent should have excellent dissolution properties for the polymer and the crosslinking agent. 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 instantaneously gel the polymer film coated on the porous membrane and impart ionic conductivity is a mixture consisting 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 diethoxy ethane. Cyclic ethers such as tetrahydrofuran, It is possible to use lactones such as butyrolactone, and these can be used alone or in combination of two or more thereof. Examples of lithium salts include lithium hexafluorophosphate (LiPF ₆ ), lithium perchloroate (LiClO ₄ ), lithium tetrafluoroborate (LiBF ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), and lithium hexafluoroacenate (LiAsF ₆ ), lithium trifluoromethanesulfonylimide (LiN (CF ₃ SO ₂ ) ₂ ), etc. can be used, and not only these lithium salts, but also lithium salts combined with various other anions can be used. .

When manufacturing a lithium secondary battery using a gelable multilayer separator manufactured by 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)

A separator having a multilayer structure according to the method of the present invention was prepared as follows. Polyethylene oxide having an average molecular weight of 600,000 is dissolved in 1% by weight of acetonitrile solvent. A polymer solution is prepared by adding a polyethyleneglycol dimethacrylate (molecular weight: 350) as a crosslinking agent and azobisisobutylonitrile as an initiator. The weight ratio of the crosslinking agent added at this time was 0.1%. The polyethylene porous membrane 1 having a thickness of 25 µm and a porosity of 40% serving as a substrate is immersed in the polymer solution prepared above for 1 hour. After removing these and evaporating the organic solvent, the free radical reaction of the crosslinking agent was advanced by drying for 24 hours in a vacuum oven at 80 ℃. The film obtained after drying was a separator having a three-layer structure as shown in FIG. 1, and the total film thickness was about 28 µm, and the thickness of the coated polymer 2 was 1.5 µm on a cross-sectional basis. 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 solution was used by dissolving a lithium tetrafluoroborate salt in 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 contained in the separator was 73% by weight, and the ion conductivity measured was 1.0 X 10 ^-3 S / cm at room temperature. The change in ionic conductivity over time is shown in Figure 2 (A curve). Little decrease in ionic conductivity with time was observed, indicating no increase in resistance due to electrolyte leakage. The electrochemical stability was investigated through a linear scanning potential experiment, which was found to be electrochemically stable up to 4.7 V using lithium as a reference electrode (see FIG. 3).

(Comparative Example 1)

A 25 μm polyethylene separator not coated with polyethylene oxide was deposited on the electrolyte to diffuse the electrolyte into the porous membrane. The electrolyte solution used at this time was the same as that in Example 1, and the lithium tetrafluoroborate salt was dissolved in a mixed solvent of ethylene carbonate / dimethyl carbonate at a concentration of 1 M. The electrolyte content of the microporous membrane was calculated from the weight change before and after the electrolyte deposition was 51%. As a result of measuring their ionic conductivity, it was 1.7 × 10 ⁻⁴ S / cm at room temperature, and the change in ionic conductivity with time is shown in FIG. 2 (B curve). It is observed that the decrease in ionic conductivity with time is large, indicating that the electrolyte gradually escapes from the separator.

(Comparative Example 2)

A polymer-coated separator was prepared in the same manner as in Example 1 except that polyacrylonitrile was used as the polymer coated on the polyethylene porous membrane. They were easily brittle under the influence of polyacrylonitrile having a high glass transition temperature, so that the coated polymer material was broken when the separator was wound. These were deposited in the same electrolyte solution used in Example 1 to measure the electrolyte retention amount, it was 58%, the ion conductivity was very low 9.0 x 10 ^-6 S / cm.

(Example 2)

A polymer-coated separator was prepared in the same manner as in Example 1 except that the polyethylene oxide film thickness coated on the porous polyethylene film was 12 µm (excluding the polyethylene film thickness). These were deposited in the same electrolyte solution used in Example 1, and the amount of the electrolyte solution was measured. As a result, it was 90%, and the ionic conductivity was 1.4 X 10 ^-3 S / cm.

(Example 3)

A lithium _secondary battery was manufactured using a multilayer separator, a carbon cathode, and lithium cobalt oxide (LiCoO ₂ ) prepared by the method of Example 1, using a cathode. The negative electrode is composed of 92% by weight of the active material mesocarbon microbead (mesocarbon microbead (MCMB)) and 8% by weight of polyvinylidene fluoride as a binder, it was used by coating a single side on a copper foil. 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 Example 1, 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 carried out using a charge and discharger from Toyo. 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 to obtain a charge and discharge curve as in the fourth degree. The discharge capacity of the obtained secondary battery is a value corresponding to a specific amount of 145 mAh / g when converted based on the mass of LiCoO ₂ active material of the positive electrode. The change of the discharge capacity according to the number of cycles obtained at this time is shown in FIG. 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 polyethylene oxide polymer coated on both sides of the polyethylene porous film effectively maintains the electrolyte solution and improves the interfacial adhesion between the electrode and the separator.

When the gelable separator of the multilayer structure devised by the present invention is applied to a lithium secondary battery, 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. 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 (3)

A gelable separator of a multi-layered structure prepared by coating a solution composed of polyethylene oxide, a crosslinking agent, an initiator and an organic solvent on at least one side of the porous membrane, and then drying the organic solvent to remove the organic solvent and chemically crosslinking the porous membrane and the polymer. And a lithium secondary battery prepared from them. The method of claim 1, wherein the component used as the crosslinking agent is divinylbenzene, diacrylate or triacrylate, dimethacrylate or trimethacrylate, diallyl ester or triallyl ester, diglycidyl ester, polyethylene Gelable separator of a multi-layer structure, characterized in that consisting of one or at least two or more mixtures selected from the group consisting of glycol dimethacrylate The polymer used in the porous membrane is an olefin resin such as polyethylene or polypropylene, a fluorine resin such as polyvinylidene fluoride or polytetrafluoroethylene, or an ester resin such as polyethylene terephthalate. And a single component or two or more components selected from the group consisting of cellulose-based nonwoven fabrics.