KR20020043945A - Polymer electrolyte filled with titania nanoparticles and the preparation thereof - Google Patents

Abstract

PURPOSE: Provided are a polyelectrolyte filled with titania having size of nanometer, having high ion conductivity and low interfacial resistance, and a method for producing the same. CONSTITUTION: The polyelectrolyte is produced by the steps of (i) dissolving a copolymer of vinylidene fluoride and hexafluoropropylene, and titania particles of nanometer size in solvent, so as to form a polymer film; and (ii) impregnating the polymer film with electrolyte solution. The solvent is acetone or tetrahydrofuran. The electrolyte solution is a mixture of lithium salt and an organic solvent. The lithium salt is at least one selected from the group consisting of LiClO4, LiBF4, LiAsF6, LiCF3SO3 and LiN(CF3SO2)2. The organic solvent is selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, and mixture thereof.

Description

Polymer electrolyte filled with titania nanoparticles and the preparation

The present invention relates to a polymer electrolyte and a method for preparing the same, and more particularly to a copolymer of vinylidene fluoride and hexafluoropropylene and nanometer-sized titania particles (hereinafter referred to as 'Titania nanoparticles'). A polymer electrolyte and a method of manufacturing the same.

Recently, with the advent of the information age, multimedia technology is rapidly developing, and researches on small secondary batteries having high energy density and high capacity have been performed according to the trend of high performance, miniaturization, light weight, and portability in electronic information and communication devices. Doing.

A representative example thereof is a lithium ion secondary battery commercialized in the early 1990s and currently commercialized as a small battery of an information and communication device such as a mobile phone. The lithium ion secondary battery uses a liquid electrolyte in which lithium salt is dissolved in a polyethylene or polypropylene porous separator. At this time, the side reaction with the electrode material due to the use of the liquid electrolyte and the limitation of weight reduction due to the use of a metal can exterior material are caused. It has been emerging.

In order to solve this problem, a system for impregnating a liquid electrolyte into a solid polymer membrane, that is, a lithium ion polymer secondary battery has been developed, and various studies have been made to develop such a polymer electrolyte membrane for a lithium ion polymer secondary battery.

In order for such a polymer electrolyte to be applied to a small lithium secondary battery having a high energy density and a high capacity, and to exhibit excellent performance, generally,

(1) mechanical strength and stability of polymer membrane itself (long-term stability, thermal stability, chemical stability, etc.)

(2) High ionic conductivity, high cation yield and electrochemical stability should be ensured when the electrolyte is added to form the polymer electrolyte membrane.

Generally, in order to reinforce the mechanical strength and physical properties of the polymer electrolyte membrane itself, a method of adding an inorganic filler such as silica or alumina is generally adopted. In particular, US Pat. No. 5,418,091 uses a method of securing mechanical strength by including silica particles in P (VdF-HFP), a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP). In this case, the silica additive used is fumed silica (SiO ₂ ) particles, which form a crosslinked structure in the polymer matrix when the polymer film is formed, thereby securing mechanical strength and fixing the impregnated electrolyte solution. It also plays a role, contributing to improving the conductivity of lithium ions.

However, in the case of using such a polymer electrolyte membrane, the liquid is located in the internal pores of the solid polymer to take care of the transfer of lithium ions, but there is an advantage in terms of stability.However, when the electrode is actually bonded to the electrolyte and the electrolyte is leaked or the interface resistance with the electrode is In particular, there is a problem that the interfacial resistance generated when using a lithium metal electrode is considerably high, which adversely affects battery characteristics, especially battery life.

It is an object of the present invention to provide a novel polymer electrolyte having high ion conductivity and small interfacial resistance that can overcome the problems described above, and a method of manufacturing the same.

1 is a graph showing the change over time of the interface resistance between the polymer electrolyte membrane and the lithium metal interface according to the present invention.

FIG. 2 is a graph showing the change over time of interfacial resistance between a lithium metal interface filled with silica and a polymer electrolyte membrane; FIG.

In order to achieve the above object, the present invention is to dissolve the copolymer P (VdF-HFP) and titania nanoparticles of vinylidene fluoride and hexafluoropropylene in a solvent to prepare a porous polymer membrane and impregnate it in the electrolyte It provides a method for producing a polymer electrolyte comprising a. In this method, the solvent may be acetone or tetrahydrofuran, etc., in particular, it is preferable to use acetone. In addition, the electrolyte used in the present invention is preferably a mixture of a lithium salt and an organic solvent, and representative lithium salts that can be used include LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO _{2 2} ) and organic solvents include ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate DMC, diethyl carbonate, ethylmethyl carbonate ) And mixtures thereof may be used. In addition, the concentration of the lithium salt in the electrolyte is preferably 1 mol, that is, 1 M per liter of the solution, and when using an organic solvent mixture, the content ratio of each solvent is not particularly limited.

In another aspect, the present invention provides a polymer electrolyte comprising a polymer matrix consisting of a copolymer P (VdF-HFP) of vinylidene fluoride and hexafluoropropylene and a filler consisting of titania nanoparticles. At this time, the content of the titania nanoparticles is preferably 4 to 45% by weight, more preferably 10 to 25% by weight based on the mixture of the polymer matrix and the titania. In addition, it is preferable that the copolymerization content of HFP is about 12 mol%.

At this time, when the content of titania is less than 4% by weight, the impregnation amount of the polymer electrolyte is not sufficient, so that there is little effect of improving the ionic conductivity, and also it is difficult to obtain the effect of strengthening the mechanical strength. In addition, when the titania content is 45 wt% or more, the titania component dominates the overall physical properties of the polymer electrolyte, making it difficult to form a film, thereby making it impossible to secure mechanical strength.

Examples of using nanometer-sized titania particles as fillers for polymers for lithium secondary batteries include Croce et al., Nature, Vol. 394, p. 456 (1998); Journal of Physical Chemistry B, Vol. 103, p.10632 (1999)] prepared a solid polymer electrolyte in which a titania nanoparticle and a lithium salt were complexed in a polyethylene (poly (ethylene oxide)) polymer matrix. In some cases, the affinity with the lithium electrode may be greatly improved. However, the solid polymer electrolyte using polyethylene oxide is difficult to be applied to a lithium secondary battery for mobile phones, such as ionic conductivity of only 10 ^-4 S / cm or more can be obtained at a high temperature of 60 ^o C or more due to the room temperature rigidity of the polymer matrix. It is still in a distant situation.

Based on the above facts, the polymer electrolyte according to the present invention is characterized in that the P (VdF-HFP) copolymer is used as a polymer matrix and filled with titania nanoparticles, and the polymer electrolyte according to the present invention is a conventional silica particle-filled polymer. Unlike the electrolytes (Korean Patent Application No. 99-053774 and Japanese Patent Application Hei 12-43227), the slurry is prepared by dissolving the titania nanoparticles in P (VdF-HFP) copolymer powder and the titania nanoparticles in a solvent and then mixing them. After the preparation, the polymer membrane obtained by solution casting is impregnated with an organic solvent electrolyte in which lithium salt is dissolved for a predetermined time and then dried.

The production process according to the invention is described in more detail, but is not intended to limit the invention.

First, the titania nanoparticle powder is added to a predetermined amount of acetone and stirred. At this time, ultrasonic waves may be applied for 30 minutes or more for uniform dispersion of the titania nanoparticles. Into this, a small amount of copolymer powder of VdF and HFP having a HFP copolymerization content of 12 mol% was slowly added, and a zirconia ball having a diameter of 5 mm was added to be 60% or more of the liquid volume, followed by ball milling. ) For at least 24 hours. After milling is complete, the dew point is minus 40 ^o C or less second in a low-humidity atmosphere and put in a vacuum oven under reduced pressure for 5 minutes, remove the residual bubbles was poured into a predetermined amount to the slurry over a clean glass plate using a doctor blade, and the solution was cast at a constant thickness 70 ^o C Dry in oven for 12 hours. The polymer membrane thus obtained is impregnated with an electrolyte solution in which lithium salt is dissolved for 20 hours or more, and then taken out, the surface electrolyte solution is wiped off with a filter paper, and left in an ultra low humidity room for about 4 hours before the desired polymer electrolyte is obtained.

Hereinafter, the present invention will be described in more detail with reference to specific examples.

Examples 1-6

Titanium nanoparticles (PC-101 from Titan Industry Co .; anatase crystalline form; average particle diameter 20nm, specific surface area 340m ² / g) were added to a 100 ml glass sample bottle containing 20 g of acetone. The ultrasonic wave was applied for 30 minutes by using the ultrasonic generator adjusted to 35W, and then applied for 30 minutes and then stopped for 5 seconds. Subsequently, P (VdF-HFP) copolymer powder (KnarnarFlex 2801 of Elf Atochem NA Co., USA) having 12 mol% of HFP copolymer content was gradually added, and a zirconia ball having a diameter of 5 mm was added to 60% or more of the liquid phase. After sealing, ball milling was performed for 24 hours at a speed of 500 rpm. Here, the dosage and content ratio of the polymer and the titania according to each example are shown in Table 1 below.

When ball milling was completed, the sample bottle cap was loosened in an ultra-low humidity atmosphere in which the dew point was kept below minus 40 ^° C. and placed in a vacuum oven to degas for 5 minutes. A certain amount of slurry was poured onto a glass plate washed with methanol and dried, and the slurry was cast into a thin sheet using a doctor blade. At this time, the gap of the doctor blade was kept at 400 mu m. The cast thin film was attached to a glass plate and dried in a 70 ^° C. oven for 2 hours, and then peeled from the glass plate at room temperature. At this time, when the adhesion between the surface of the glass plate and the thin film is difficult to peel off, methanol was sprayed to peel off, and the resultant was placed in a 70 ^° C. oven for 12 hours to dry, thereby obtaining a polymer film having a thickness of approximately 36 to 50 μm. After cutting the dried polymer membrane to a certain size, the polymer membrane was immersed in Shaale containing an excess of 1 M LiPF ₆ electrolyte in a solution in which EC and EMC were mixed in a 1: 1 ratio by volume. Impregnation was over time. After the impregnation was completed, the polymer electrolyte membrane was taken out of the electrolyte solution, placed between two sheets of filter paper, and then lightly wiped off the surface of the electrolyte solution and left in an ultra low humidity room for about 4 hours to obtain a polymer electrolyte according to the present invention.

Comparative Example 1

A polymer electrolyte was prepared in the same manner as in the above example except that titania was not added. However, the electrolyte solution used was an electrolyte solution in which 1M LiClO ₄ was dissolved in a solution in which EC and DMC were mixed at a volume ratio of 2: 1.

Comparative Example 2

Instead of Titania, fumed silica (Carbot Co. TS-530) was added in a predetermined amount to prepare a slurry through mixing and stirring, and then a polymer electrolyte was prepared in the same manner as above. However, the electrolyte used at this time was an electrolyte in which 1M LiClO ₄ was dissolved in a solution in which EC and DMC were mixed at a volume ratio of 2: 1.

Comparison of Characteristics of Prepared Polymer Electrolyte Membranes

(1) Swelling rate and ion conductivity test of the polymer electrolyte membrane (Examples 1 to 6 and Comparative Example 1)

Before impregnating the polymer membrane, the weight of the electrolyte was measured and called W _1. After the electrolyte was impregnated, the filter paper was wiped with filter paper and the measured weight was W _{2. The} swelling rate (%) was calculated by the following equation. The results are shown in Table 1 below.

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

Composition content (g) TiO ₂ content ratio (% by weight) Swelling Rate (%) Ion Conductivity (mS / cm) Polymer TiO ₂ 2 hours later After 20 hours 0 ℃ 25 ℃ 60 ℃ Example 1 4.75 0.25 5 60.0 73.2 0.64 3.8 10.8 Example 2 4.50 0.50 10 65.2 75.1 2.9 11.0 37.5 Example 3 4.00 1.00 20 67.6 73.8 2.4 7.9 28.2 Example 4 3.50 1.50 30 62.8 65.0 1.6 6.7 27.9 Example 5 3.00 2.00 40 57.9 59.1 1.0 4.7 16.2 Example 6 2.50 2.50 50 54.5 55.6 0.32 1.2 3.5 Comparative Example 1 5.00 0.00 0 34.5 38.9 - 0.004 0.02

From the results of Table 1, the ionic conductivity tends to increase and decrease with increasing titania content, and the highest ionic conductivity is shown when the titania content is 10% by weight. It can be seen that this tendency is maintained even when the temperature is changed, and the tendency of maintaining the electrolyte solution in the polymer electrolyte membrane is also in accordance with the trend of swelling ratio after the impregnation of the electrolyte solution.

(2) Interfacial Stability Test of Lithium Metal Electrode and Polymer Electrolyte Membrane (Comparative Example 2)

Next, a test was performed on the interfacial stability with the lithium metal electrode which should be considered when applying the polymer electrolyte membrane filled with titania of the present invention to a lithium secondary battery. The lithium metal electrode was placed on both sides of the polymer electrolyte membrane. A cm cell was fabricated and impedance measured over time. As a result, the same results as in FIG. 1 were obtained.

For comparison with the interfacial properties of the polymer electrolyte filled with titania nanoparticles according to the present invention, the polymer electrolyte membrane filled with silica of Comparative Example 2 was also subjected to an impedance test using a lithium metal electrode, and the results are attached. Shown in

1 and 2, it can be seen that the polymer electrolyte membrane filled with titania nanoparticles has a low interfacial resistance with a lithium metal electrode of about 1/10. Therefore, the electrode is applied to a lithium metal polymer secondary battery. It can be seen that it is very advantageous to play its role as a mediator of the reaction. This is because titania nanoparticles have excellent adaptability in interfacing with lithium electrodes.

The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and various substitutions, modifications, and changes are possible in the art without departing from the technical spirit of the present invention. It will be apparent to those of ordinary knowledge.

As described above, the polymer electrolyte filled with the titania nanoparticles according to the present invention not only has mechanical strength and electrolyte retention of the polymer electrolyte, but also has higher ion conductivity than the polymer electrolyte membrane used in the conventional lithium ion polymer secondary battery. Due to the unique surface properties of the titania particles, the compatibility with lithium metal is maintained and the interfacial resistance is greatly reduced. Therefore, the present invention can be applied to lithium ion secondary batteries and lithium ion polymer secondary batteries. Of course, it can be provided as the most promising polymer electrolyte membrane suitable for the lithium metal polymer secondary battery which will be developed in the future as a high capacity secondary battery.

Claims (8)

A method of producing a polymer electrolyte comprising dissolving a copolymer of vinylidene fluoride and hexafluoropropylene and nanometer-sized titania particles in a solvent to prepare a porous polymer membrane and impregnating it in an electrolyte. The method of claim 1, The solvent is acetone or tetrahydrofuran method of producing a polymer electrolyte, characterized in that. The method of claim 1, The electrolyte solution is a method of producing a polymer electrolyte, characterized in that the electrolyte mixture of a lithium salt and an organic solvent. The method of claim 3, wherein The lithium salt is a method of producing a polymer electrolyte, characterized in that at least one selected from the group consisting of LiClO ₄ , LiBF ₄ , LiAsF ₆ , LiCF ₃ SO ₃ and LiN (CF ₃ SO ₂ ) ₂ . The method of claim 3, wherein The organic solvent is a method for producing a polymer electrolyte, characterized in that selected from the group consisting of ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate and mixtures thereof. A polymer electrolyte comprising a polymer matrix composed of a copolymer of vinylidene fluoride and hexafluoropropylene and a filler composed of titania nanoparticles. The method of claim 6, The content of the titania nanoparticles is 4 to 45% by weight based on the mixture of the polymer matrix and the titania. The method of claim 6, The content of the titania nanoparticles is 10 to 25% by weight based on the mixture of the polymer matrix and the titania.