KR20120078235A - Lewis acid complex-based polymer electrolyte for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: A Lewis acid complex-based polymer electrolyte for lithium secondary battery is provided to obtain excellent solubility to solvent by accepting Lewis acid in to a monomer, and to have excellent physical stability thereby capable of reducing thickness of a battery, and to improve degree of freedom of shape. CONSTITUTION: A Lewis acid complex-based polymer electrolyte for lithium secondary battery is formed by combination of Lewis acids and monomeric lithium salt. A singular ionic polymer electrolyte is formed from the complex and a polymer-formable monomer. The Lewis acids are selected from aprotic acid of one or more kinds selected from BF3, ZnCl2. The addition ratio of the monomer and the Lewis acid is 1:1-1:2. The lithium salt is one or more kinds selected from lithium methacrylate, lithium acrylate, and lithium styrenesulfonate. A lithium secondary battery comprises the polymer electrolyte, a negative electrode active material, and a positive electrode active material.

Description

Lewis acid complex-based polymer electrolyte for lithium secondary battery and lithium secondary battery comprising the same}

The present invention relates to a polymer electrolyte based on a Lewis acid complex for a lithium secondary battery and a lithium secondary battery including the same. More specifically, the present invention relates to a polymer electrolyte capable of simultaneously securing high electrochemical stability and ion conductivity using a Lewis acid complex. It relates to a lithium secondary battery comprising the same.

Lithium secondary batteries are most commonly used as a power source for portable electronic devices, and are being actively researched for high capacity and high output according to the development of hybrid electric vehicles and pure electric vehicles. However, the high energy density of lithium secondary batteries has been a major threat to battery safety, and various measures have been taken to solve them.

The solid polymer electrolyte has a lower ion conductivity than a liquid electrolyte but does not contain a volatile organic solvent, thereby greatly improving the safety of a lithium secondary battery. In particular, it is a next-generation battery system that can overcome the problem of ion conductivity when manufactured in a thin film.

In general, a lithium salt is added to the solid polymer electrolyte to transfer lithium ions. Active movement of anions, together with anodization, causes polarization and decreases battery performance. In order to solve this problem, a monomeric single ion conductive polymer electrolyte has been studied, but monomers capable of forming a single ion are not dissolved in a solvent, and thus are synthesized through four stages of polymer synthesis, purification, solvent exchange, and lithium substitution.

In addition, the use of Lewis acid is disclosed in Patent Publication No. 1993-7002067, which discloses a Lewis acid-promoted transition alumina catalyst, is a completely different field from the present invention.

 In order to solve the above problems, an object of the present invention is to introduce a Lewis acid to the monoionic monomer to ensure excellent solubility in the solvent, to enable the production of a monoionic polymer electrolyte in a simple polymerization process, The present invention provides a polymer electrolyte for a lithium secondary battery having excellent chemical stability and ion conductivity.

Another object of the present invention to provide a lithium secondary battery comprising the polymer electrolyte.

In order to achieve the above object, according to an embodiment of the present invention provides a polymer electrolyte for a lithium secondary battery composed of a monoionic monomer: polymer forming compound for forming a Lewis acid complex. The complex compound is preferably 5 to 50% by weight based on the weight of the polymer-forming monomer. When the content of the complex compound is 5% or less, the concentration of lithium ions is too low to show ionic conductivity. It is because the application of the battery is difficult because the physical properties become hard.

The Lewis acid may be BF ₃ , ZnCl _2, or the like, and it is preferable that the Lewis acid may form a complex with a monoionic monomer.

The monoionic monomer may be at least one selected from the group consisting of lithium methacrylate, lithium acrylate, lithium styrene sulfonate, and the like as a lithium salt. The reason why the lithium salt is used is that when the polymer is synthesized using a monomer containing a lithium salt (composed of lithium cations and a relative anion that can be polymerized), the monomer is used as a polymer chain, and thus the movement in the electrolyte is reduced. The only possible ion is lithium, which is called a monoionic polyelectrolyte. It is not possible to move anions and thus prevents anion decomposition at the anode (improving electrochemical stability) and prevents polarization due to anion migration. have. As a result, the purpose of using a monomer containing a lithium salt is to improve the battery performance by improving the electrochemical stability and suppressing the polarization phenomenon.

In addition, the monoionic monomer and the Lewis acid addition ratio are preferably 1: 1 molar ratio, and may be used up to 1: 2. To form a quantitative complex, a 1 molar ratio of Lewis acid to monomer 1 should be added and Lewis. Even if the acid is used in double excess (monomer 1: Lewis acid 2), it can be used in the range of 1: 1 ~ 1: 2 because there is no big problem in electrochemical stability and ionic conductivity.

The monoionic monomer in which the Lewis acid complex is formed may be represented by the following Chemical Formula 1.

The complex formed by combining the Lewis acid and the monoionic monomer at room temperature has a decrease in ionic binding force, so that the solubility is greatly increased. It may be more than one species. In the solvent, ethylene carbonate and die carbonate are liquid electrolytes that are frequently used in batteries, and tetrahydrofuran and methanol are general solvents for easy evaporation, and can be used as they are for polymer polymerization. It does not dissolve in the solvent, but once the complex is formed, the ionic bond strength is reduced, so that it immediately dissolves in the solvent even at room temperature, and can be further increased by simple stirring.

The monomer forming the Lewis acid complex and the polymer in the solvent is composed of acrylate, poly (ethylene glycol) methacrylate, poly (ethglycol) dimethacrylate, poly (ethylene glycol) diacrylate, and combinations thereof. It is preferably selected from the group.

The monomers acrylate, poly (ethylene glycol) dimethacrylate, poly (ethylene glycol) diacrylate, and the like are monomers commonly used in solid polymer electrolytes, and can maintain mechanical strength due to their tough molecular properties. It is used because it has a glass transition temperature and molecular chain movement is active, which is advantageous for lithium ion transfer.

A typical structure of the monoionic polymer electrolyte in which the Lewis acid complex is formed is shown in Chemical Formula 2 below.

According to another embodiment of the present invention, the polymer electrolyte; Positive electrode active material; It provides a lithium secondary battery comprising a negative electrode active material.

Since the monoionic monomer in which the Lewis acid complex of the present invention is formed is soluble in a general solvent, the polymer electrolyte can be synthesized by simple polymerization.

Since the polymer electrolyte for lithium secondary batteries of the present invention uses a monoionic lithium salt, the electrochemical stability is high and the ion dissociation degree is high, thus the ion conductivity is high.

When the polymer electrolyte for lithium secondary batteries of the present invention has excellent physical stability, it is possible to thin the battery and improve the shape freedom.

1 is a cutaway perspective view of a lithium secondary battery.
2 is a composition potential measurement graph according to an embodiment of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples. The following examples are for illustrative purposes only and are not intended to limit the scope of the invention.

Example 1

In a glove box filled with argon gas, 10 mmol of borontripulolide tetrahydrofuran complex was added to 10 mmol of lithium methacrylate and stirred at room temperature for 5 minutes to obtain a clear complex.

Example 2

Using 10 mmol of lithium methacrylate and 20 mmol of borontripulolide tetrahydrofuran complex, a transparent complex was obtained in the same manner as in Example 1.

Example 3

To 50 g of tetrahydrofuran, 2 mmol of a complex compound and 7.6 mmol of a poly (ethylene glycol) methacrylate were added, and benzoyl peroxide (BPO) was added as a radical polymerization initiator and reacted at 65 ° C. for 9 hours to react the complex with poly (ethylene Glycol) methacrylates were obtained. At this time, the number average molecular weight of the complex compound and the poly (ethylene glycol) methacrylate copolymer was 51,000 g / mol.

The mixture was vacuum dried at 80 ° C. for 72 hours to prepare a polymer composition for a lithium secondary battery.

Example 4

A polymer composition for a lithium secondary battery was prepared in the same manner as in Example 3 using 2 mmol of a complex compound and 4.6 mmol of poly (ethylene glycol) methacrylate. The number average molecular weight of the complex compound and the poly (ethylene glycol) methacrylate copolymer was 74,000 g / mol.

Example 5

A polymer composition for a lithium secondary battery was prepared in the same manner as in Example 3 using 2 mmol of a complex compound and 4.4 mmol of a poly (ethylene glycol) methacrylate. The number average molecular weight of the complex compound and the poly (ethylene glycol) methacrylate copolymer was 53,000 g / mol.

Linear sweep voltammetry

The oxidation potential of the polymer composition prepared according to Example 1 was measured by the following method. The working electrode is made of stainless steel, and the reference electrode and the counter electrode are measured by linear sweep voltammetry at 1 mV / sec scan rate and 60 ° C using lithium metal. The results are shown in FIG.

As shown in FIG. 2, in the case of the polymer composition prepared according to the embodiment, the rapid decomposition does not occur even at a voltage up to about 7.0V, even though the temperature is 60 ° C., which is superior to that of the liquid electrolyte in which the rapid decomposition occurs at about 5.7V. It can be seen that the chemical stability.

Ion Conductivity Measurement

The room temperature ion conductivity of the polymer composition prepared according to Examples 3, 4, and 5 was measured, and the results are shown in Table 1 below.

Room temperature ion conductivity
(S / cm, 25 ℃) condition Copolymer of Lewis Acid Complex with Poly (ethylene Glycol) Methacrylate Example 3 1.2 × 10 ^-6 solid Example 4 5.6 × 10 ^-6 solid Example 5 1.2 × 10 ^-5 solid

Referring to Table 1, the polymer composition prepared according to the embodiment has excellent room temperature ionic conductivity of 10 ⁻⁵ S / cm while securing physical properties of the solid form and the properties of the monoionic conductor at room temperature.

Claims (7)

Lewis acid; Soluble complex formed by bonding of monomeric lithium salts. A monoionic polymer electrolyte made of the complex compound of claim 1 and a polymer forming monomer. The complex of claim 1, wherein the Lewis acid is selected from at least one aprotic acid selected from BF ₃ and ZnCl ₂ . The complex of claim 1, wherein an addition ratio of the monomer and the Lewis acid is formed by reacting 1: 1 to 1: 2. The complexing compound of claim 1, wherein the lithium salt is at least one selected from lithium methacrylate, lithium acrylate, and lithium styrene sulfonate. The polymer electrolyte according to claim 2, wherein the complex compound is 5 to 50% by weight based on the weight of the polymer forming monomer. A polymer electrolyte according to any one of claims 1 to 6; Negative electrode active material; Lithium secondary battery comprising a positive electrode active material.

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