KR100403806B1 - Lithium secondary battery - Google Patents

Abstract

PURPOSE: A lithium secondary battery is provided to minimize the interfacial resistance between a collector and an electrode, to reduce the internal impedance of a battery, to increase the battery life, and to prevent the reduction of a battery capacity. CONSTITUTION: The lithium secondary battery comprises a collector, an adhesive layer, a composite electrode layer and a polymer electrolyte layer, wherein the adhesive layer is composed of a composition containing polyvinyl butyral, a conductive agent and a solvent, and the content of polyvinyl butyral is 3-7 wt% based on the total weight of the composition. Each of the composite electrode layer and the polymer electrolyte layer comprises a copolymer of a vinyl monomer selected from the group consisting of N-isopropylacryl amide, acryloyl morpholine and N,N-dimethylaminopropyl acrylamide with polyethyleneglycol diacrylate or polyethyleneglycol dimethacrylate, an ionic lithium salt and an organic solvent.

Description

Lithium secondary battery

The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery capable of preventing an interface from being separated as charging and discharging is repeated by improving adhesion between the composite electrode and the current collector.

Lithium secondary batteries are attracting the most attention as next-generation power sources due to their excellent characteristics such as long life and high capacity. They can be subdivided into primary batteries such as Li / MnO2 and lithium secondary batteries such as lithium ion batteries and plastic lithium ion batteries. .

Lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), lithium manganese oxide (LiMn2O4), etc. are used for the positive electrode active material of a lithium secondary battery, and lithium metal, its alloy, carbon material, etc. are used for a negative electrode active material. As the electrolyte, a liquid electrolyte or a solid electrolyte is used. However, when the liquid electrolyte is used as the electrolyte, many problems related to safety, such as the risk of fire due to leakage and breakage of the battery due to vaporization, are generated. In order to solve this problem, a method of using a solid electrolyte instead of a liquid electrolyte has been proposed. Solid electrolytes are generally studied with great interest because there is no risk of leakage of electrolytes and they are easy to process.

Since the use of a composite of lithium salt and polyethylene oxide as a solid electrolyte, research into a polymer electrolyte having high conductivity at room temperature has been actively conducted. This study aims to find a solid polymer electrolyte that can maintain a discharge rate almost equal to that of a lithium battery using a liquid electrolyte in which lithium salt is dissolved in an organic solvent. Requirements as a polymer electrolyte that can meet this purpose are as follows.

First, the liquid electrolyte used in the rechargeable lithium battery should have a conductivity of about 10-3 to 10-2 S / cm.

Second, the chemical, electrochemical and mechanical stability should be excellent.

Third, since the ionic conductivity is high when the polymer has almost no aromaticity in the amorphous state, the polymer should be an amorphous polymer having a low glass transition temperature (Tg).

When manufacturing a lithium secondary battery using a polymer electrolyte which satisfies the above requirements, it is necessary to use a composite electrode. That is, since the polymer electrolyte is not a solution electrolyte, the positive electrode, the negative electrode, and the electrolyte cannot be used independently, so that the polymer electrolyte composition is added to make the movement path of the ions in addition to the main component of the electrode active material during electrode formation. At this time, LiNiO2, LiCoO2, V6O13, LiMnO4 and the like, which are often used as the electrode active material, have a low electrical conductivity, and therefore, it is preferable to add a conductive agent such as carbon black. At this time, it is important to harmonize the porosity of the current collector, the active material component and its particle size, etc., because these factors have a very important influence on the internal impedance, the adhesion between the current collector and the active material.

In the lithium secondary battery including the composite electrode and the polymer electrolyte, the battery capacity gradually decreases as charging and discharging are repeated. This phenomenon stems from morphological changes in the anode due to repeated charge and discharge. That is, in the discharge process, lithium from the negative electrode penetrates into the positive electrode, thereby expanding the volume of the positive electrode. This causes the polymer electrolyte to have elastic and viscous imbalance movement toward the adjacent empty space. In contrast, the charging process reduces the volume of the positive electrode. At this time, the elastic shrinkage of the polymer electrolyte is recovered, but the imbalance due to the viscous flow is not recovered. In this case, an interface other than the interface between the anode / electrolyte and the electrolyte / cathode is generated to increase the interface resistance as a whole.

In order to solve the above problems, a method of reducing the resistance of the interface by stacking the positive electrode, the electrolyte and the negative electrode in a sheet state, and then heat-sealing them at a temperature of about 135 ° C has been proposed. However, this method is very limited because it is possible only when using a thermopolymer.

As another method, a sheet-like anode, an electrolyte, and a cathode are stacked and then pressed using pressure. According to this method, since the interface between the anode / electrolyte and the electrolyte / cathode is clear, a large impedance component exists between the interfaces. If the battery is continuously charged and discharged, the adhesion between the current collector and the composite electrode gradually decreases, resulting in a phenomenon of separation. In view of these problems, a method of forming a separate adhesive layer between the current collector and the composite electrode is used.

Accordingly, in the present invention, the composition for forming the adhesive layer capable of minimizing the interfacial resistance in forming the adhesive layer for improving the adhesion between the current collector and the composite electrode has been completed.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide a lithium secondary battery in which a battery can exhibit full capacity by minimizing interfacial resistance between a current collector and an electrode.

In order to achieve the above object, in the present invention, in the lithium secondary battery comprising a current collector, an adhesive layer, a composite electrode layer, and a polymer electrolyte layer, the adhesive layer is made of a composition containing polyvinyl butyral, a solvent, and a conductive agent. Provided is a lithium secondary battery.

The polyvinyl butyral in the adhesive layer-forming composition is preferably 3 to 7% by weight in the total composition. If the polyvinyl butyral exceeds 7% by weight of the total composition, the original resistance component of the polyvinyl butyral is remarkable, so that a double layer is formed between the oxide layer and the current collector and the deterioration of the battery during charging and discharging is severe. If it is less than%, the effect of adhering the current collector to the composite electrode layer is insignificant, which is not preferable. And the conductive agent is selected from carbon black, acetylene black, super-P-carbon, the content is 3 to 7% by weight of the total composition. The solvent is at least one selected from tetrahydrofuran, ethyl alcohol, acetone and ethyl ether, the content of which is the remaining content except the polyvinyl butyral and the conductive agent in the total composition.

The composition for forming an adhesive layer according to the present invention has excellent adhesion between the current collector and the electrode layer due to the hydrogen bond between the oxygen atom of polyvinyl butyral and the hydrogen atom between the metal hydroxide formed on the current collector surface such as an aluminum thin film and a copper thin film. It can be improved (wherein the metal hydroxide is produced in a pretreatment process using sodium hydroxide or nitric acid before using the current collector).

On the other hand, since the polyvinyl butyral is low in conductivity, it is preferable to use a conductive agent such as carbon black, and when the conductive agent is added, the conductivity is greatly improved, so that the addition of polyvinyl butyral to the current collector capacity of the current collector. Negative effects can be ruled out.

The polymer solid electrolyte used in the present invention is a vinyl monomer such as N-isopropylacrylamide, acryloyl morpholine, N, N-dimethyl aminopropylacrylamide, polyethylene glycol diacrylic acid ester or polyethylene glycol dimethacrylic acid ester. It has a structure using a copolymer of, containing an ionic lithium salt and an organic solvent in the copolymer.

The organic solvent is at least one solvent selected from ethylene carbonate, propylene carbonate, γ-butyrolactone, dimethoxyethane, dimethyl carbonate, diethyl carbonate and the like. And at least one selected from lithium perchlorate (PClO4), lithium tetrafluoroborate (LiBF4), lithium hexafluorophosphate (LiPF6), and meta trifluoride imide (LiN (CF3SO2) 2). do.

Hereinafter, a method of manufacturing a lithium secondary battery according to the present invention will be described.

A composition comprising 3 to 7 wt% of polyvinyl butyral, 3 to 7 wt% of a conductive agent and the remaining amount of solvent is sufficiently mixed, and the composition is cast on a positive electrode and a negative electrode current collector and dried to form an adhesive layer, respectively. . Subsequently, a composite anode layer and a cathode layer are formed on the adhesive layer of the resultant, respectively. A polymer electrolyte layer is formed on the composite cathode layer, the composite anode layer is compressed on the polymer electrolyte layer, and heat treated to complete the lithium secondary battery according to the present invention.

Hereinafter, the present invention will be described in detail with reference to Examples, but the present invention is not limited to the following non-limiting Examples.

<Example>

First, the composition for forming an adhesive layer was prepared as follows.

5% by weight of polyvinyl butyral and 5% by weight of carbon black were added to a mixed solvent of 90% by weight of THF and 10% by weight of ethyl alcohol, followed by stirring for about 1 hour.

An aluminum thin film and a copper thin film each having a thickness of about 30 μm were attached to two glass substrates, and then the composition for forming the adhesive layer was poured and cast into a thin thickness of about 20 μm using a coating machine. Then, drying was performed at 50 ° C. for about 30 minutes to form an adhesive layer. In order to examine the adhesion of the adhesive layer to the current collector obtained by using the scotch tape was evaluated. As a result of observing using an optical microscope, no peeling phenomenon between the aluminum and copper thin films and the adhesive layer was observed.

The mixture of lithium manganese dioxide oxide and acetylene black was mixed in a 90:10 weight ratio, and then the mixture was placed in a high speed mixer to perform eight times of one minute at a speed of 3000 rpm per minute to uniform the conducting agent around the lithium manganese dioxide having a manganese spinel structure. It can be distributed. Thereafter, the polymer electrolyte composition was added thereto and mixed in a ball mill for 24 hours. At this time, the polymer electrolyte composition was prepared by mixing N-isopropylacrylamide with (EO) 23DMA having a number of repeating units (polyethylene glycol) in a 15: 5 weight ratio, and adding propylene carbonate and lithium tetrafluoroborate thereto. Then, AIBN which is a thermal polymerization initiator was added, and it stirred well, and made the composition for composite anode formation.

The composite anode forming composition was poured onto an aluminum current collector on which an adhesive layer was formed, a composite film having a thickness of about 200 μm was made using a casting machine, and thermal polymerization was performed at 70 ° C. for about 30 minutes.

On the other hand, carbon powder, acetylene and the polymer electrolyte composition were mixed to obtain a composition for forming a composite negative electrode. Then, the composition was applied onto the copper thin film on which the adhesive layer was formed, and then a composite negative electrode was prepared in the same manner as the positive electrode manufacturing method.

Subsequently, the polymer electrolyte composition was coated on the composite anode layer and photopolymerized by irradiation with ultraviolet rays to form a polymer electrolyte layer. Then, the composite negative electrode layer was stacked on the polymer electrolyte layer, and then heated at 70 ° C. for 30 minutes while applying a pressure of 500 g / cm 2 to complete the battery.

In order to examine the interfacial junction state of the lithium secondary battery obtained by the above method, a part of the battery was cut out using a punch having a diameter of 13 mm, and then visually observed or peeled off with tweezers. As a result, it was found that the layers of the positive electrode, the electrolyte, and the negative electrode were effectively bonded by photopolymerization and thermal polymerization, so that a separate interface did not exist.

Meanwhile, to examine the electrochemical state of the lithium secondary battery, a part of the battery was removed using a punch having a diameter of 13 mm, and the battery was assembled using aluminum foil as a lead wire. Thereafter, the battery was placed in a sealed container in a dry box, and then the change in ion conductivity over time was observed using an impedance analyzer (Yokohama Hewlett Packard 4192A). Here, the inside of the dry ice box is made into a vacuum state, and then, argon gas is injected to control oxygen gas and moisture to 1 ppm or less to remove decomposition factors or suppression factors of the electrolyte due to the presence of moisture and oxygen gas in advance. It was. In addition, the compound reagents used in the present invention are products having a purity of at least 99%, and are used after drying for 30 minutes using a vacuum dryer before use.

As a result of the experiment, even after 72 hours, the conductivity was 1.3 × 10 −3 S / cm (error: ± 10%).

Comparative Example

The same procedure as in Example was carried out except that no adhesive layer was formed between the current collector and the composite anode and between the current collector and the composite cathode.

In the battery manufactured according to the above method, the Scotch tape was evaluated to examine the adhesive force between the current collector and the composite electrode. Observing the results using an optical microscope, it was possible to observe a fine peeling phenomenon between the aluminum and copper thin film and the composite electrode.

A part of the battery was removed using a punch having a diameter of 13 mm, and the battery was assembled using an aluminum foil as a lead wire. Thereafter, the battery was placed in an airtight container in a dry box, and then a change in conductivity over time was observed using an impedance analyzer (Yokohama Hewlett Packard 419A). As a result, the conductivity was 1.34 × 10-3S / cm (error: ± 10%) at the beginning, 1.25 × 10-3S / cm after 24 hours, 0.97 × 10-3S / cm after 48 hours, 72 After elapse of time, the result was 0.75 × 10 −3 S / cm.

As can be seen from the above, when the adhesive layer was formed according to the present invention, it was found that not only the adhesion between the current collector and the composite electrode was excellent but also the conductivity was excellent.

According to the present invention, the following effects are obtained.

First, since the adhesion between the current collector and the composite electrode is improved and the interface is not peeled off, the internal impedance of the battery is lowered. As a result, a battery having a high efficiency and having an extended life can be manufactured.

Second, when the electrolyte layer and the composite electrode layer are bonded to each other, the electrolyte layer and the composite electrode layer are bonded under relatively low temperature conditions of about 70 ° C., so there is no fear of evaporation of the electrolyte and there is almost no capacity reduction due to the lack of electrolyte. Can maintain conductivity.

Third, the polymer electrolyte layer is made by photopolymerization. The photopolymerization is possible at room temperature, and can be made at a relatively long wavelength of 365 to 400 nm, thereby making it possible to use an inexpensive ultraviolet fluorescent lamp, thereby lowering the manufacturing cost.

Claims (4)

In a lithium secondary battery comprising a current collector, an adhesive layer, a composite electrode layer and a polymer electrolyte layer, The adhesive layer is formed using a composition comprising polyvinyl butyral, a conductive agent and a solvent, The content of the polyvinyl butyral is 3 to 7% by weight based on the composition, The composite electrode layer and the polymer electrolyte layer are each, Copolymers of N-isopropylacrylamide with a vinyl monomer selected from acryloyl morpholine, N, N-dimethyl aminopropylacrylamide, and polyethylene glycol diacrylic acid ester or polyethylene glycol dimethacrylic acid ester; A lithium secondary battery comprising an ionic lithium salt and an organic solvent. According to claim 1, wherein the content of the conductive agent is a lithium secondary battery, characterized in that 3 to 7% by weight in the total composition. The lithium secondary battery according to claim 1, wherein the conductive agent is selected from the group consisting of carbon black, acetylene black and super-P-black. The lithium secondary battery according to claim 1, wherein the solvent is at least one solvent selected from the group consisting of ethyl alcohol, tetrahydrofuran, acetone, and ethyl ether.