KR100353867B1 - Polymer Electrolyte for Rechargeable Lithium Batteries - Google Patents

Abstract

The present invention forms a polymer membrane which is a matrix of a polymer electrolyte including silica as an inorganic filler as well as a polymer, and impregnates the polymer electrolyte for lithium secondary battery by impregnating the polymer matrix with an organic solvent electrolyte of lithium salt. The polymer electrolyte of the present invention can be prepared by a simple method, has excellent mechanical strength, as well as ionic conductivity, and when used in a lithium secondary battery, interfacial resistance is quickly stabilized.

Description

Polymer Electrolyte for Rechargeable Lithium Batteries

The present invention relates to a polymer electrolyte for lithium secondary batteries, and more particularly, to a polymer electrolyte for lithium secondary batteries in which ion conductivity and mechanical properties are improved by adding an inorganic filler such as silica fine particle powder to a specific polymer material.

Recently, with the advent of the information age, as multimedia technology rapidly develops, there is a strong demand for high performance, miniaturization, and portability of electronic devices. Therefore, the research on the high energy density and the high capacity small secondary battery which can be used as a power supply of such an electronic device is actively progressing.

A representative example is a lithium ion secondary battery commercialized in the early 1990s. As an electrolyte of such a battery, a liquid electrolyte was initially used, but a metal can exterior material was used due to a problem of safety due to leakage, but this also faced problems such as weight reduction. Accordingly, a system in which an electrolyte solution is impregnated with a solid polymer membrane has been recognized as a substitute for a liquid electrolyte, and since then, research on polymer electrolytes for many polymer species has been conducted.

In order for the polymer electrolyte to be applied to a high energy density and high capacity small lithium secondary battery, the following conditions must be generally satisfied. That is, high ionic conductivity, high cation yield, mechanical strength, long time stability, thermal stability, electrochemical stability, chemical stability and the like should be satisfied. In order to satisfy these conditions, the polymer used as a polymer electrolyte is a polymer composed of a monomer having a large ion acceptor number, a polymer that can be synthesized in a uniform phase, and a polymer that is easy to form a three-dimensional crosslinked structure, and affinity with a solvent. A polymer composed of this strong monomer should be used, and a polymer electrolyte system composed of such a polymer and a high boiling point plasticizer or solvent is most frequently used.

U.S. Patent No. 5,418,091 discloses a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) as a polymer material and dibutylphthalate (DBP) as a plasticizer in acetone solvent. After making a film, the method of manufacturing a polymer electrolyte is disclosed by extracting a plasticizer from this film and then impregnating an active electrolyte solution. In this method, a film containing a plasticizer was formed and extracted again to increase the impregnation amount of the electrolyte, thereby improving ion conductivity and solving the problem of dendrite formation. However, the above method has a problem in that the production of the polymer film and then re-extracting the plasticizer causes complexity of the manufacturing process and lowers the productivity.

Japanese Patent Application Laid-open No. Hei 10-275521 discloses a method for producing a polymer solid electrolyte having excellent productivity, mass productivity, and high swelling degree such as an electrolyte even at low cost. The polymer solid electrolyte includes a quaternary ammonium salt such as a polymer, an inorganic filler, a lithium salt used in a lithium secondary battery, or boron tetraethylammonium fluoride used in an electric double layer capacitor, and an organic solvent. However, it is understood that such a polymer electrolyte in the document does not include the polymer electrolyte system for a lithium secondary battery of the present invention.

SUMMARY OF THE INVENTION An object of the present invention is to provide a polymer electrolyte for a lithium secondary battery with improved ion conductivity by containing an inorganic filler such as silica, thereby increasing the impregnation amount of the electrolyte solution in which lithium salt is dissolved, as well as mechanical strength.

It is also an object of the present invention to provide a polymer electrolyte for a lithium secondary battery in which stabilization of interfacial resistance with a lithium electrode is quickly realized.

It is also an object of the present invention to provide a method for producing a polymer electrolyte for a lithium secondary battery having a simple manufacturing process by not using a plasticizer to be extracted after formation of the polymer film.

1 is a 10,000-fold magnification SEM photograph showing the surface of a silica-free polymer electrolyte;

FIG. 2 is a 10,000-fold magnification SEM photograph showing the surface of a polymer electrolyte containing 10.3 wt% silica; FIG.

FIG. 3 is a 10,000-fold magnification SEM photograph showing the surface of a polymer electrolyte containing 35.3 wt.% Silica;

4 is a graph showing interfacial resistance between a silica-containing polymer electrolyte and a lithium electrode;

5 is a graph showing interfacial resistance between a polymer electrolyte containing 10.3 wt% silica and a lithium electrode;

6 is a graph showing interfacial resistance between a polymer electrolyte containing 35.3 wt% silica and a lithium electrode;

The polymer electrolyte for lithium secondary batteries of the present invention includes a matrix formed of a polymer and an inorganic filler and an organic solvent electrolyte solution of a lithium salt impregnated in the matrix.

The matrix used in the polymer electrolyte for lithium secondary batteries of the present invention is formed by a polymer and an inorganic filler. The polymer used is preferably a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) or a VDF polymer. At this time, when a copolymer of VDF and HFP is used, the content of HFP is preferably 5-20 mol%, more preferably about 12 mol%. The silica used is preferably fumed silica powder having an average particle diameter of several hundreds of nanometers. At this time, the amount of silica is preferably 4 to 38% by weight, more preferably 10 to 35% by weight based on the mixture of the polymer and the silica.

The polymer matrix used in the present invention is prepared as a polymer membrane by solution casting an organic solvent containing polymer powder and silica. Examples of the organic solvent used in the production of the polymer membrane include acetone or tetrahydrofuran. The organic solvent is removed during the drying of the polymer membrane. The polymer electrolyte of the present invention is prepared by impregnating such a polymer membrane in an organic solvent electrolyte in which lithium salt is dissolved. Examples of lithium salts used are LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ . Although the density | concentration of the lithium salt in electrolyte solution is not specifically limited, About 0.5-2 M is preferable. As the organic solvent used, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate and mixtures thereof can be used. When a mixture of organic solvents is used, the mixing ratio of each solvent is not particularly limited.

The present invention is described in detail below.

The present invention is to prepare a polymer electrolyte by forming a polymer membrane which is a matrix of a polymer electrolyte including silica as an inorganic filler as well as a polymer, and impregnating the polymer matrix with an organic solvent electrolyte solution of lithium salt.

For example, VDF having an HFP content of 12 mol%, HFP copolymer powder, silica powder and acetone are mixed together and stirred for a long time (at least 24 hours) to uniformly disperse the silica particles in a sealed state. However, at the beginning of mixing, the mixture is stirred for about 3 hours at 50 ° C. for rapid liquefaction, and when the liquefaction is substantially performed, the mixture is lowered to room temperature and continued stirring. After the stirring was completed, in a ultra-low humidity atmosphere where the dew point was below 40 ° C. or less, the resultant was decompressed for 5 minutes to extract the remaining bubbles. Dry in an oven at 70 ° C. for 12 hours. The polymer membrane thus obtained is impregnated with an electrolyte solution in which lithium salt is dissolved for 12 hours, then taken out, and the surface electrolyte solution is wiped off with a filter paper and left in an ultra low humidity room for about 4 hours to obtain a desired polymer electrolyte.

In the present invention, since the particles of the silica powder to be used are fine particles having a nanometer size, when the polymer electrolyte membrane of the present invention is manufactured in large quantities, a fairly harsh mixing condition may be required to improve the dispersion of the silica particles. have. In order to prepare the polymer electrolyte membrane of the present invention having excellent silica dispersion on a semi-pilot plant scale rather than a laboratory level, a large dispersion mixer or homogenizer equipped with a screw-type impeller ( homogenizer) at least 600 rpm and stirring time of 12 hours or more is required.

In the present invention, by including silica in the polymer matrix, the mechanical strength as well as the impregnation amount (swelling degree) of the electrolyte are increased to improve the ionic conductivity. The improvement of the impregnation amount of the electrolyte seems to be due to the formation of fine pores in the polymer matrix containing silica. The amount of silica contained in the polymer matrix of the present invention is preferably 4 to 38% by weight, particularly preferably 10 to 35% by weight based on the mixture of the polymer and silica. The content of silica is preferably added as long as it does not inhibit the physical properties of the polymer matrix. When the content of silica is less than 4% by weight, the electrolyte solution impregnation amount of the polymer electrolyte is not sufficient, so that the ionic conductivity is not high enough and the improvement of mechanical strength is also difficult. On the other hand, if the content of silica is more than 38% by weight, since the silica component dominates the overall physical properties of the polymer electrolyte, it is difficult to be formed into a film and thus hardly obtains mechanical strength.

In addition, the polymer electrolyte of the present invention can quickly stabilize the interfacial resistance with the lithium electrode when used in the lithium secondary battery.

On the other hand, U.S. Patent No. 5,418,091 discloses a copolymer of vinylidene fluoride (VDF) and hexafluoropropylene (HFP) as a polymer material and dibutylphthalate (DBP) as a plasticizer in acetone solvent. The method for producing a polymer electrolyte is disclosed by extracting a plasticizer from the film and then impregnating an active electrolyte solution. In particular, in Examples 9 to 12 of the above document, a polymer and a plasticizer (including propylene carbonate as a plasticizer) were dissolved in acetone or tetrahydrofuran to form a polymer film, and an active electrolyte solution (ethylene) was extracted. A polymer electrolyte was prepared by impregnating a mixture of carbonate and propylene carbonate with lithium salt dissolved therein). Here, the form in which the plasticizer is not extracted is defined as a wet form, the form in which the plasticizer is extracted is defined as a dry form, and the form that does not contain a plasticizer (which does not include silica as an inorganic filler). Except for this, the same as that of the present invention) was defined as a control film, and the impregnation rate (swelling degree) of the electrolyte solution was calculated for each of them. The impregnation rate of the electrolyte is about 27%, whereas the wet form is 47%, and the dry form is about 40% higher than the control film. In addition, it is described that an inorganic filler such as alumina or silica can be added when forming the polymer electrolyte for improving the mechanical strength, and due to the addition of the inorganic filler, in some cases, the amount of electrolyte absorption increases. However, as shown in Example 13 of the above document, the content of the electrolyte solution may be improved or decreased by the addition of the above inorganic filler. That is, in the case of alumina, both wet and dry forms result in a decrease in the impregnation rate, and in the case of silica, the wet form hardly changes, but the dry form improves the impregnation rate of the electrolyte solution. On the other hand, the change of the impregnation rate of electrolyte solution with respect to a control film is not mentioned.

In the polymer electrolyte of the present invention, no plasticizer is added during the formation of the polymer matrix, and silica, which is an inorganic filler, is used as a base material for forming the polymer matrix. In the present invention, the addition of silica is intended to improve the ionic conductivity by increasing the impregnation amount of the electrolyte as well as improving the mechanical strength. The U.S. Patent Document does not suggest or imply that the impregnation amount of the electrolyte solution is increased by the addition of silica to the control film containing no plasticizer. In addition, the amount of electrolyte impregnation of the control membrane of the above document is only 27% and the amount of dry electrolyte impregnation is about 40%. On the contrary, the electrolyte impregnation amount of the polymer matrix of the present invention increases with the addition amount of silica. When 31.8% by weight of silica was added, the impregnation amount was comparable to that of the polymer electrolyte of the US patent document as 70.2% after 20 hours of the impregnation time (see Table 1 below).

On the other hand, Japanese Patent Application Laid-open No. Hei 10-275521 discloses a method for producing a polymer solid electrolyte having excellent productivity, mass productivity, and high swelling degree such as an electrolyte even at low cost. The polymer solid electrolyte includes a quaternary ammonium salt such as a polymer, an inorganic filler, a lithium salt used in a lithium secondary battery, or boron tetraethylammonium fluoride used in an electric double layer capacitor, and an organic solvent. Here, it is described that the inorganic filler is preferably added about 5 to 70% by weight based on the polymer. However, Example 1 of the document refers to a polymer electrolyte in which an electrolyte solution of lithium salt is impregnated into a polymer matrix made of TiO ₂ as a polymer and an inorganic filler different from the polymer used in the present invention. In addition, Example 2 uses the same polymer matrix containing silica as that used in the present invention as the VDF-HFP copolymer and the inorganic filler, which is the same polymer used in the present invention, but is not intended for lithium secondary batteries. As a polymer electrolyte for a double layer capacitor, no lithium salt is included. In addition, the component ratio applied in this example is 1: 1 by weight, and the content of silica corresponds to 50% by weight based on the total weight of the polymer and silica. As mentioned in the present invention, this component ratio is a component ratio such that the silica component is difficult to form in the form of a film by dominating the overall physical properties of the polymer electrolyte and thus cannot sufficiently secure mechanical strength. In this embodiment, however, the resultant polymer electrolyte membrane was prepared by using methyl ethyl ketone as a solvent and boron tetraethylammonium fluoride [(C ₂ H ₅ ) ₄ NBF ₄ ] as an electrolyte of an electric double layer capacitor used as an electrolyte. It is believed that mechanical strength is maintained. Accordingly, the above embodiments disclose a polymer electrolyte system which is completely different from the polymer electrolyte of the present invention.

Hereinafter, the present invention will be described in detail through specific examples.

Examples 1-5

100 ml glass sample bottle with magnetic bars, VDF-HFP copolymer powder (KynarFlex 2801 from Elf Atochem) with 12 mole percent HFP content, silica powder (Cab-O-Sil TS-530 from Cabot Co., surface area 212 m ² / g, hexamethylsilazane treatment), and 30 g of acetone were added and slightly sealed with a lid, which was then placed on a hot plate adjusted to 50 DEG C and slowly starting to stir. The dosage and content ratio of the polymer powder and the silica powder are shown in Table 1.

After 3 hours from the start of stirring, the temperature of the hot plate was lowered to room temperature, the sample bottle was sealed again, and high speed stirring was performed for 24 hours. After the stirring is complete, in the ultra-low humidity atmosphere where the dew point is below 40 ° C or lower, gently unscrew the sample bottle cap and place it in a vacuum oven to depressurize for 5 minutes to extract the remaining bubbles. The slurry was cast into a thin film sheet using the rate. At this time, the gap of the doctor blade was kept at 350 micrometers. The cast thin film was placed in an oven at 70 ° C. in a state of being bonded to a glass plate, dried for 2 hours, and then peeled from the glass plate at room temperature. At this time, the higher the silica content, the stronger the adhesion between the surface of the glass plate and the thin film, which is very difficult to peel off. Thus, methanol is sprayed to weaken the adhesive force between the thin film and the glass plate. A polymer membrane having a thickness of approximately 36 to 40 μm was obtained.

The polymer membrane obtained by drying was cut to a certain size (6 cm x 5 cm), and then an excess of 1 M LiClO ₄ dissolved in a solvent in which ethylene carbonate and dimethyl carbonate were mixed at a ratio of 2: 1 by volume was excessive. The electrolyte was immersed in the saule immersed in the polymer membrane for 20 hours or more. After the impregnation was completed, the polymer electrolyte membrane was removed from the electrolyte solution, placed between two sheets of filter paper, and gently patted to wipe off the surface electrolyte solution, and then left in an ultra low humidity room for about 4 hours to obtain a desired polymer electrolyte.

Comparative Example 1

A polymer electrolyte membrane was manufactured in the same manner as in the above example except that silica was not added. The content of the polymer used is shown in Table 1.

Composition content (g) Silica Content Ratio (wt%) Swelling Rate (%) Ion Conductivity (mS / cm) Polymer Silica 2 hours later After 20 hours 0 ℃ 23 ℃ 52 ℃ Example 1 4.95 0.25 4.8 36.2 38.5 - 0.02 0.34 Example 2 5.12 0.59 10.3 40.6 48.8 0.001 0.08 1.20 Example 3 4.00 1.00 20.0 48.4 58.2 0.006 0.10 1.73 Example 4 3.99 1.86 31.8 49.7 70.2 0.027 0.32 4.36 Example 5 3.00 2.07 35.3 51.5 64.5 0.025 0.23 3.03 Comparative Example 1 5.00 0.00 0.00 34.5 38.9 - 0.004 0.015

[Characteristics of Prepared Polymer Electrolyte Membrane]

First, the swelling rate measurement of the polymer electrolyte membrane will be described. Before impregnating the polymer membrane, the weight of the electrolyte is measured and called W _1. After the electrolyte is impregnated, the filter paper is wiped off and the measured weight is W _{2. The} swelling rate (%) is expressed by the following equation. Calculations were made and the results are shown in Table 1.

Swelling Rate (%) = (w ₂ -w ₁ ) / w ₁ x 100

Next, the polymer electrolyte membrane was cut into 2 cm x 2 cm sizes between two 2 cm x 2 cm stainless steel electrode plates, and the ion conductivity was measured at various temperatures according to a conventional impedance measurement method. The results are also shown in Table 1.

In addition, in order to determine the mechanical strength of the polymer electrolyte membrane, the tensile test was conducted by increasing the force with both hands, and the polymer electrolyte membrane without silica was ruptured first. It needed strength. Of course, the results were not quantitative, but at least the mechanical strength of the silica and the silica was clearly discernible.

From the results in Table 1, it can be seen that as the silica content increases, the swelling rate and the ionic conductivity increase, because the silica can accept and fix a lot of liquid electrolyte which ensures the mobility of the ions. As can be seen in the surface photograph of the polymer electrolyte membrane shown in Figs. 1 to 3, the impregnated electrolyte solution is distributed because the finer pores are widely distributed as the content of silica becomes smaller and the roughness unit of the membrane surface becomes smaller. It is directly related to the phenomenon of stable collection around silica.

Next, as a matter to be considered when applying the polymer electrolyte membrane of the present invention to a lithium secondary battery, a test for interfacial stability with a lithium electrode was carried out, and the lithium metal electrode was placed on both sides of the polymer electrolyte membrane to have a size of 2 cm x 2 cm. Cells were fabricated to measure impedance over time and the results are shown in FIGS. 4 to 6. As a result, it can be seen that stabilization of the interfacial resistance is realized quickly when silica is added as compared to the polymer electrolyte membrane without the silica (Fig. 4). As the content of silica increases, the resistance of the coating layer formed at the interface with lithium decreases, and the resistance value is stabilized as time passes.

Example 6

In order to increase the dispersion of silica particles, a stainless steel disc-shaped impeller having a diameter of 10 cm (the circumferential portion of the disc is upper and lower) Injecting all of the above materials into a dispersion mixer (5 liter capacity, manufactured by Asada Kogyo Co., Ltd.) equipped with alternating bends and stirring, the impeller rotation speed was 1000 rpm and the stirring time was 14 The other time was performed in the same manner as in Example 4 to prepare a polymer electrolyte membrane.

Comparative Examples 2 and 3

A polymer electrolyte membrane was prepared in the same manner as in Example 6 except that the stirring speed was 300 rpm for 12 hours (Comparative Example 2) and the stirring speed was 1000 rpm for 6 hours (Comparative Example 3).

As a result, when visually observing the polymer electrolyte membrane obtained in Example 6, the silica particles were evenly distributed throughout the membrane and the residual silica did not come out even when touched by hand, compared to the surface of the polymer electrolyte membrane obtained in Comparative Examples 2 and 3. The silica particles protruded and could be easily found when touched by hand.

In the present invention, a polymer electrolyte is prepared by forming a polymer matrix with a polymer and silica without impregnating a plasticizer and impregnating the lithium electrolyte into it. Therefore, the manufacturing process is simple and the productivity can be improved. In addition, the polymer electrolyte of the present invention not only has excellent mechanical strength but also excellent ion conductivity, and when applied to a lithium secondary battery, the interfacial resistance is stabilized quickly.

Claims (14)

Matrix formed by polymer and silica, An organic solvent electrolyte of lithium salts impregnated in the matrix, The polymer is a copolymer of vinylidene fluoride and hexafluoropropylene, or a vinylidene fluoride polymer. Polymer electrolyte for lithium secondary battery. delete The method of claim 1, The content of hexafluoropropylene in the copolymer of vinylidene fluoride and hexafluoropropylene is 5-20 mol%. Polymer electrolyte for lithium secondary battery. The method of claim 1, The silica is a fumed silica powder having an average particle diameter of several hundreds of nanometers. Polymer electrolyte for lithium secondary battery. The method of claim 4, wherein The amount of silica is 4 to 38% by weight based on the mixture of the polymer and the silica. Polymer electrolyte for lithium secondary battery. The method of claim 4, wherein The amount of silica is 10 to 35% by weight based on the mixture of the polymer and the silica. Polymer electrolyte for lithium secondary battery. The method of claim 5, The lithium salt is at least one selected from the group consisting of LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ Polymer electrolyte for a lithium secondary battery. The method of claim 5, The organic solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, diethyl carbonate, and mixtures thereof. Polymer electrolyte for lithium secondary battery. Dissolving the polymer in a solvent, dispersing the silica therein to form a slurry, and casting the solution to form a polymer film serving as a matrix. Impregnating the polymer film with an organic solvent electrolyte in which lithium salt is dissolved to form a polymer electrolyte. The polymer is a copolymer of vinylidene fluoride and hexafluoropropylene or a vinylidene fluoride polymer, the silica is a fumed silica having an average particle diameter of several to several hundred nanometers, and the amount of silica is the amount of the polymer and the silica. 4 to 38% by weight relative to the mixture Method for producing a polymer electrolyte for lithium secondary battery. The method of claim 9, In the copolymer of vinylidene fluoride and hexafluoropropylene The content of hexafluoropropylene is 5-20 mol% Method for producing a polymer electrolyte for lithium secondary battery. The method of claim 9, The lithium salt is at least one selected from the group consisting of LiClO ₄ , LiBF ₄ , LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ Method for producing a polymer electrolyte for lithium secondary battery. delete delete delete