KR20070024886A - Polymer electrolyte containing ionic liquid - Google Patents

Abstract

Provided is a polymer electrolyte containing an ionic liquid, which has a uniform pore distribution, contains a large amount of an organic electrolyte and an ionic liquid, and shows excellent ion conductivity and electrochemical stability. The polymer electrolyte containing an ionic liquid comprises a polymer film obtained by spin-coating a mixed solution containing a polymer and a solvent via a phase transition process, wherein the polymer film is impregnated with an organic electrolyte comprising an ionic liquid and a lithium salt. The polymer film is obtained by using a mixed solution comprising 10-40 wt% of the polymer and 60-90 wt% of the solvent. The organic electrolyte contains at least one carbonate solvent. The ionic liquid includes 1-butyl-3-methylimidazolium tetrafluoroborate.

Description

Polymer Electrolyte containing Ionic Liquid

1 is an electron scanning microscope (SEM) photograph of a porous polymer film.

Figure 2 is a graph showing the thickness of the polymer film according to the mixing ratio of the polymer and the solvent and the spin coating speed.

3 is a graph showing the electrolyte solution impregnation amount of the polymer electrolyte.

The present invention relates to a gel polymer electrolyte containing an ionic liquid. Specifically, spin coating a polymer solution to prepare a polymer film having a uniform pore distribution by a phase inversion method, and then ionizing the polymer film. The present invention relates to a polymer electrolyte having excellent ion conductivity and electrochemical stability prepared by impregnating an organic electrolytic nucleus containing an ionic liquid, and the polymer electrolyte may be suitably used for a lithium polymer battery.

Currently commercialized lithium ion battery has low boiling point of organic electrolyte and is easy to volatilize and thus flammable, so there is a risk of explosion due to overcharge, overdischarge, short, etc., and it is difficult to miniaturize and thin. Therefore, the lithium polymer battery was developed to solve this problem. There are two types of lithium polymer batteries, a solid type containing no organic electrolyte and a gel type containing an organic electrolyte. Some gel electrolytes are currently used. However, these gel electrolytes have higher ionic conductivity at room temperature than lower solid polymer electrolytes, but lower levels than liquid electrolytes, and also include plasticizers and organic electrolytes, making it difficult to secure electrochemical safety.

S/cm 이하로 낮다. 따라서, 실온에서는 사용이 불가능하고 60℃ 이상의 온도에서만 사용 가능하다. 즉 고온용 전해질로는 사용할 수 있지만 일반적인 전자기기가 사용되는 상온에서는 사용이 불가능하다는 단점을 가지고 있어서 아직까지도 실용화하는데 에는 어려움이 있다.As examples of polymer solid electrolytes, US Pat. Nos. 4,758,483, 4,792,504 and 4,908,284 disclose crosslinks of polyethylene oxide (PEO). This cross-linked product is easy to manufacture and can be mass-produced. However, the polymer solid electrolyte including the polymer matrix has an ionic conductivity at room temperature.

Lower than S / cm Therefore, it cannot be used at room temperature and can be used only at a temperature of 60 ° C or higher. That is, although it can be used as a high-temperature electrolyte, it has a disadvantage in that it cannot be used at room temperature where general electronic devices are used, and there are still difficulties in practical use.

S/cm 이하이며 너무 유연하고 UV 조사 후 열 경화되어 더 이상 성형이 불가능하며 전지 조립 시 전극과 고분자 전해질간의 갭(gap)이 상대적으로 커져 전극과 전해질간에 계면 저항이 커지므로 이차전지에 실질적으로 적용하기가 어려운 단점이 있다.As an example of a polymer gel electrolyte, Japanese Patent Laid-Open No. 03-207752 discloses a polymer gel electrolyte in which ethylene glycol and dimethacrylate are mixed and then irradiated with UV to be synthesized. However, this gel polymer electrolyte has a low conductivity

It is less than S / cm, too flexible, and thermally cured after UV irradiation, so that molding is no longer possible. It is difficult to apply.

Recently, a method of first preparing a matrix polymer containing no organic solvent, laminating it with an anode and a cathode, and impregnating the obtained film with an organic solvent has been proposed (JM Tarascon et al., Solid State Ionics, 86-88, 49). , 1996, US Patent No. 5,456,000). However, vinylidene fluoride-based polymers are electrochemically stable, but have poor surface energy and low affinity with organic solvents, resulting in poor impregnation of the liquid electrolyte. Exudation or volatilization of the polymer not only reduces the ionic conductivity in the polymer matrix, but also increases the overall resistance in the battery, resulting in a continuous decrease in capacity after a long time and a high rate charge and discharge characteristic.

)이상이어야 하고 기계적 강도가 우수하여 전지제조가 용이하여야 하고, 고분자 내에 함유된 전해액이 외부로 누출되지 않아야 하며, 그리고 원하는 전압 범위 내에서 전기화학적 안정성이 있어야 한다. 또 전지제조 공정에서 전극에 대한 열 접착성을 가질 수 있어야 하고 열에 대한 안정성이 필요하며 비수계 전해액에 대한 함침성도 좋아야 하며 동시에 안정성도 있어야 한다.As such, the polymer electrolyte that can be used in a lithium secondary battery has an ion conductivity of 5 mS / cm at room temperature (m =

It should be more than), and it should be easy to manufacture the battery because of its excellent mechanical strength, and the electrolyte contained in the polymer should not leak to the outside, and it should have electrochemical stability within the desired voltage range. In addition, the battery manufacturing process should be able to have a thermal adhesiveness to the electrode, the stability to heat is required, the impregnation to the non-aqueous electrolyte solution must also be good and must also be stable.

Ionic liquids are emerging as materials capable of meeting the requirements of such polymer electrolytes. Ionic liquids refer to salts consisting of cations and anions that exist as liquids at temperatures below 100 ° C, unlike ionic salt compounds consisting of metal cations and non-metal anions, such as salts, generally melting at high temperatures above 800 ° C. Salts that exist as liquids at room temperature are called room temperature ionic liquids. Ionic liquids, which are liquids at room temperature, have no vapor pressure, are non-volatile, non-explosive, and have high thermal and chemical properties, such as wide liquid temperature ranges, high ionic conductivity, and wide electrochemical potential windows. Recently, its usefulness is widely recognized.

The present invention is to provide a polymer electrolyte containing an ionic liquid having high ionic conductivity, excellent electrochemical stability, and improved impregnation properties of the electrolyte.

In the polymer electrolyte including the ionic liquid according to the present invention, a step (a) of preparing a polymer solution by mixing a polymer and a solvent capable of dissolving the polymer solution, and spin coating the same to prepare a polymer film (b), And impregnating the film with an electrolyte containing an ionic liquid.

Hereinafter, the method of preparing the polymer electrolyte including the ionic liquid according to the present invention will be described in detail.

step a

The polymer of step a is polyurethane, polyvinylidene hexafluoropropylene copolymer (PVdF-HFP copolymer), polyvinylidene fluoride, polyvinyl chloride, polyacrylonitrile, polymethyl methacrylate, polyethylene glycol Or one or two or more polymers selected from polypropylene oxide. In the embodiment of the present invention, polyvinylidene hexafluoropropylene copolymer was used.

The solvent for dissolving the polymer is not particularly limited as long as it is suitable for dissolution of the selected organic polymer, and examples thereof include organic solvents N-methylpyrrolidone (NMP), dimethylformamide, dimethyl sulfoxide, gamma butyrolactone, Tetrahydrofuran is at least one solvent selected from the group of acetone. In the embodiment of the present invention, N-methylpyrrolidone (NMP) was used.

The mixing ratio of the polymer and the solvent is preferably 10 to 40% by weight of the polymer to 60 to 90% by weight of the solvent, and most preferably when the ratio of the polymer to the solvent (weight ratio) is 1: 5. In addition, it is recommended to use a vacuum oven to remove bubbles in the preparation of the polymer solution.

b step

Spin coating the polymer solution of step a to 100 ~ 5000rpm to produce a uniform film having a thickness of 1 ~ 1000㎛. According to the mixing ratio of the polymer and the solvent by adjusting the number of rotation (rpm) of the spin coating to prepare a film having a pore size of 1㎛ or less and 1 ~ 1000㎛ thickness. A graph showing the thickness of the film according to the mixing ratio and the rotational speed (rpm) is shown in FIG. 2.

The cast substrate was immersed in a coagulation bath containing a non-solvent to prepare a microporous polymer film by a phase inversion method between a solvent and a non-solvent in a solution. At this time, water, alcohols (methanol, ethanol, propanol, etc.) or a mixed solvent thereof can be used as the nonsolvent.

The microporous polymer film obtained through the above process was vacuum dried at about 80 ° C. for 24 hours to completely remove the residual solvent.

c step

The microporous polymer film prepared in step b was dissolved in an organic electrolyte solution with an ionic liquid and a lithium salt in a dry box, and then impregnated in the electrolyte solution to prepare a polymer electrolyte containing an ionic liquid.

The organic electrolyte is one or two or more selected from ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethylmethyl carbonate, and diethyl carbonate. Among them, cyclic carbonates such as ethylene carbonate, propylene carbonate and vinylene carbonate can be preferably used from the viewpoint of safety because they have a high flash point, a flash point and a low vapor pressure.

The lithium salt mixed with the organic electrolyte may be any lithium salt used as an electrolyte of a lithium secondary battery. Specifically, for example, lithium borofluoride (LiBF ₄ ) or lithium hexafluorophosphate (LiPF _6). ), Lithium perchlorate (LiClO ₄ ), lithium trifluoronetanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoromethanesulfonylimide (LiN (CF ₃ SO ₂ ) ₂ ) or lithium arsenylhexafluoride ( LiAsF ₆ ) may be used to select one or two or more.

At this time, the content of the lithium salt used as an electrolyte is not particularly limited, but is usually 0.05 to 3 M / L (mol / L), preferably 0.1 to 2 M / L, per 1 L of the electrolyte. If the concentration of the lithium salt is too low, the amount of lithium impregnated into the polymer electrolyte is insufficient to reduce the ionic conductivity of the electrolyte, if too high, the viscosity of the electrolyte solution increases to increase the concentration overvoltage in the electrolyte.

In addition, 1-butyl-3-methylimidazolium tetrafluoroborate (BMI-BF ₄ ) composed of a cation of Chemical Formula 1 was used as the ionic liquid contained in the electrolyte. In addition, the anionic component of the ionic liquid is tetrafluoroborate [BF ₄ ], perchlorate [ClO ₄ ], trifluoromethanesulfonide [CF ₃ SO ₃ ], arsenyl hexafluoride [AsF ₆ ], hexafluorophosphate [PF ₆ ] and trifluoromethanesulfonylimide [N (CF ₃ SO ₂ ) ₂ ].

<Formula 1>

As for content of the ionic liquid with respect to the said electrolyte solution, 5-50 weight% is suitable. If the ionic liquid is less than 5% by weight, the incombustibility of the electrolyte may be lowered, which is not preferable. If the ionic liquid is higher than 50% by weight, the viscosity of the organic electrolyte may be increased and the ion conductivity of lithium ions may be lowered.

)이상의 높은 이온 전도도를 나타내며 전기화학적으로 안정한 리튬 이차전지용 겔형 고분자 전해질이다.The gel polymer electrolyte according to the present invention contains a large amount of organic electrolyte in the matrix polymer with improved impregnation characteristics and does not leak out to the outside and does not leak to 1.0 mS / cm (m =

It is a gel type polymer electrolyte for lithium secondary battery which is electrochemically stable and shows high ion conductivity above).

Hereinafter, the content of the present invention will be described in more detail with reference to Examples.

Example 1 Preparation of Polymer Electrolyte

A polymer solution was prepared by adding PVdF-HFP and NMP vacuum-dried at 60 ° C. and NMP in a weight ratio of 1: 5 to a beaker containing a magnetic bar at room temperature for 24 hours. The polymer solution was cast to a thickness of 70 μm by spin coating, and then immersed in a non-solvent (distilled water) to extract NMP to prepare a polymer film. In order to completely remove the impurities remaining in the polymer film was dried for 24 hours at 80 ℃.

LiPF ₆ of 0.5 M (M = mole) in a solution (1: 1 volume ratio) in which the prepared microporous polymer film was mixed with ethylene carbonate (EC) and propylene carbonate (PC) in a glove box in a nitrogen atmosphere. 0.5M BMI-BF with ₄ A gel-type microporous polymer electrolyte was prepared by impregnating the mixed solution of the ionic liquid for 10 hours. Thereafter, the adhesive was sealed between two stainless steel (SUS) electrodes, sealed, and ionic conductivity was measured.

Example 2

A polymer electrolyte was prepared in the same manner as in Example 1, except that PVdF-HFP and NMP were added in a weight ratio of 1: 4.

Example 3

A polymer electrolyte was prepared in the same manner as in Example 1, except that PVdF-HFP and NMP were added in a weight ratio of 1: 6.

An electron scanning microscope (SEM) photograph of the polymer electrolyte prepared in <Example 1> is shown in FIG. 1. 1, it can be seen that the pore size has uniform pores of 1 μm or less.

In addition, the polymer electrolytes prepared in Examples 2 and 3 also had uniform pores with pore sizes of 1 µm or less.

Comparative Example 1

A polymer electrolyte was prepared in the same manner as in <Example 1>, except that the polymer solution was cast into a doctor blade.

The electrolyte solution impregnation amount according to the polymer film production method is shown in FIG. 3. The amount of electrolyte impregnation was excellent in spin coating at about 400% spin coating and 230% at doctor blade. This resulted in higher impregnation amount because the spin coating could make the pore size and distribution more uniform and maintain the overall thickness constant than the doctor blade. Due to this impregnation property, the ion conductivity of spin coating was high.

Comparative Example 2

A polymer electrolyte was prepared in the same manner as in <Example 2>, except that a solution in which 1 M LiPF ₆ was mixed with EC / PC was used.

TABLE 1

According to Table 1, the ion conductivity of the polymer electrolyte containing the ionic liquid was excellent as 1.8 mS / cm. In addition, increasing the impregnated amount of the polymer electrolyte to increase the ionic conductivity causes a decrease in the mechanical properties, so the ionic conductivity and the mechanical properties should be selected at an appropriate level. The mixing ratio (weight ratio) of the most preferable polymer and solvent was 1: 5.

Gel-type microporous polymer electrolyte containing an ionic liquid according to the present invention has a uniform pore distribution, contains a large amount of organic electrolyte in the matrix polymer, does not leak leakage to the outside, and a large amount of ionic liquid It is suitable for use in lithium polymer batteries because it shows high ionic conductivity of 1 mS / cm or more at room temperature and is electrochemically stable.

Claims (10)

A polymer electrolyte comprising an ionic liquid, comprising impregnating a polymer film prepared by a phase inversion method by spin coating a mixed solution of a polymer and a solvent into an organic electrolyte solution including an ionic liquid and a lithium salt. The method of claim 1, wherein the polymer is polyurethane, polyvinylidene hexafluoropropylene copolymer, polyvinylidene fluoride, polyvinylchloride, polyacrylonitrile, polymethyl methacrylate, polyethylene oxide or polypropylene oxide Polymer electrolyte comprising an ionic liquid, characterized in that one or two or more polymers selected from. The method of claim 1, wherein the solvent is at least one solvent selected from the group consisting of N-methylpyrrolidone, dimethylformamide, dimethyl sulfoxide, gamma butyrolactone, tetrahydrofuran, acetone Polymer electrolytes containing liquids. According to claim 1, wherein the polymer film produced by the spin coating using a mixed solution of 10 to 40% by weight of the polymer to 60 to 90% by weight of the solvent, the thickness of the spin coating to 1 ~ 1000 by setting the rpm of the spin coating 100 ~ 5000 A polymer electrolyte comprising an ionic liquid, characterized in that it is prepared to be ㎛. The polymer electrolyte of claim 1, wherein the organic electrolyte is one or two or more selected from ethylene carbonate, propylene carbonate, vinylene carbonate, dimethyl carbonate, ethyl methyl carbonate, and diethyl carbonate. . The lithium salt of claim 1, wherein the lithium salt is lithium borofluoride (LiBF ₄ ), lithium hexafluorophosphate (LiPF ₆ ), lithium trifluoronetanesulfonate (LiCF ₃ SO ₃ ), lithium trifluoromethanesulfonyl A polymer electrolyte comprising an ionic liquid, characterized in that at least one selected from imide (LiN (CF ₃ SO ₂ ) ₂ ), lithium perchlorate (LiClO ₄ ), or lithium arsenyl hexafluoride (LiAsF ₆ ). The polymer electrolyte according to claim 1, wherein the content of lithium salt is 0.05 to 3 M / L (mol / L) per liter of the electrolyte. The polymer electrolyte of claim 1, wherein the ionic liquid is 1-butyl-3-methylimidazolium tetrafluoroborate. The method of claim 8, wherein the anionic component of the ionic liquid is tetrafluoroborate [BF ₄ ] perchlorate [ClO ₄ ], trifluoromethanesulfonide [CF ₃ SO ₃ ], Arsenyl hexafluoride [AsF ₆ ], Hexafluorophosphate [PF ₆ ] and trifluoromethanesulfonylimide [N (CF ₃ SO ₂ ) ₂ ]. The polymer electrolyte of claim 1, wherein the content of the ionic liquid is 5 to 50% by weight based on the weight of the organic electrolyte.

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