KR20010046626A - Polymer electrolyte prepared from terploymer ionomer - Google Patents

Abstract

PURPOSE: A polymer electrolyte is provided which minimizes the leakage of a plasticizer and has improved interfacial stability and ion conductivity between electrode by introducing an alkali metal ion group to an acrylonitrile-methyl methacrylate-methacrylic acid terpolymer. CONSTITUTION: In the polymer electrolyte containing a host polymer, a plasticizer and a lithium salt, the host polymer is an ionomer of acrylonitrile-methyl methacrylate-alkali metal methacrylate terpolymer having a repeating unit represented by the following chemical formula (1) wherein, each x, y and z is independently 0 or a positive integer, and M is Li, Na or K.

Description

Polymer electrolyte prepared from terpolymer ionomer

The present invention relates to a polymer electrolyte, and more particularly, a polymer having an alkali metal ion introduced into a terpolymer containing an acrylonitrile repeating unit as a host polymer, and has excellent ion conductivity and interface stability with an electrode. An improved polymer electrolyte.

Recently, with the rapid development of the electric, electronic, communication and computer industries, the demand for high performance and high stability secondary batteries is increasing. In particular, the miniaturization, thinning, and lightening of electronic devices are being rapidly made. Especially in the field of office automation, the portable electronic devices such as camcorders and cellular phones are rapidly being reduced in size from desktop computers to laptop and notebook computers. Is spreading.

In accordance with the trend toward miniaturization, light weight, and thinness of such electronic devices, high performance is also required for secondary batteries that supply power to them. That is, it is possible to replace the existing lead acid battery or nickel-cadmium battery, etc., the development of a lithium secondary battery that can be charged and discharged repeatedly with a high energy density while being compact and lightweight.

Lithium secondary batteries include a cathode, an anode, and an organic electrolyte or polymer electrolyte in which lithium ions can move between the cathode and the anode using a material capable of intercalation and deintercalation of lithium ions as an active material. As a battery manufactured by charging, electric energy is generated by oxidation and reduction reactions when lithium ions are intercalated / deintercalated at the cathode and the anode.

As a cathode of a lithium secondary battery, a complex oxide of lithium and a transition metal capable of intercalating / deintercalating lithium ions and having a potential of about 3-4.5 V higher than the electrode potential of Li / Li ⁺ is mainly used. Examples thereof include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), and the like.

In addition, the anode is a lithium metal or lithium alloy capable of reversibly accepting or supplying lithium ions while maintaining structural and electrical properties, or the chemical potential at the time of intercalation / deintercalation of lithium ions is almost similar to that of metallic lithium. Carbon-based materials are mainly used.

The use of lithium metal or an alloy thereof as an anode active material is called a lithium metal battery, and the use of a carbon material is called a lithium ion battery. A lithium metal battery using lithium metal or an alloy as an anode has a short battery life and low stability, such as a change in volume of the lithium metal during charging and discharging, and a short circuit due to local deposition of lithium on the surface of the lithium metal. In order to solve this problem, a lithium ion battery using a carbon material as an anode active material has been developed. Lithium ion batteries have only a movement of lithium ions during charge and discharge, and thus the electrode active material remains intact, thereby improving battery life and stability compared to lithium metal batteries.

In addition, lithium secondary batteries are distinguished according to the type of electrolyte, and a polymer electrolyte is used differently from the conventional one using a liquid electrolyte. Lithium secondary batteries using a liquid electrolyte have been raised as a problem of stability, and the electrode material and a method of mounting a safety device have been proposed as an alternative, but there is a problem that the manufacturing cost is expensive and it is difficult to increase the capacity.

In contrast, lithium polymer batteries can be manufactured at a lower cost, can be sized and shaped as desired, have a high energy density per unit weight, and can have a high voltage and a large capacity by lamination. It is attracting attention as an advanced battery.

In order for the lithium polymer battery to be commercialized, the polymer electrolyte must first have excellent ion conduction properties, mechanical properties, and interface stability with a lithium electrode. In view of this, the most popular material as a polymer electrolyte of a lithium polymer battery is a polymer electrolyte prepared by impregnating ethylene carbonate, propylene carbonate, and lithium salt as a main component as polyacrylonitrile or polyvinylidene fluoride as a host polymer. to be.

Specifically, U. S. Patent No. 5,219, 679 includes polyacrylonitrile and poly-N-vinyl-2-pyrrolidone, which contain an electron donor element oxygen or nitrogen element in a molecular structure and can be combined with lithium ions as a host polymer. It is suggested to use.

U. S. Patent No. 5,296, 318 also discloses a copolymer of 8-25 weight hexafluoropropylene with vinylidene fluoride as a host polymer.

However, the polymer electrolytes have excellent ion conducting properties and mechanical properties, but plasticizer leakage is caused due to incompatibility between the host polymer and the plasticizer. This is known to have a problem of deterioration in performance due to long-term use by deteriorating the ionic conduction characteristics of the polymer electrolyte as well as increasing the interfacial resistance between the lithium electrode and the polymer electrolyte by corroding the lithium electrode.

In addition, in the case of polyacrylonitrile, its boiling point is soluble only in a high boiling point solvent such as dimethylformamide, which reaches about 153 ° C., and thus, it takes a long time in the process of volatilizing the solvent to obtain a polymer electrolyte film.

Accordingly, the technical problem to be achieved by the present invention is to solve the above problems of the polymer electrolyte based on polyacrylonitrile, thereby increasing the compatibility of the host polymer and the plasticizer, thereby minimizing the leakage of the plasticizer and improving the interface stability with the electrode. It is possible to provide a polymer electrolyte which can be improved and can be dissolved in a low boiling point solvent to improve manufacturing processability.

Another object of the present invention is to provide a method for producing the polymer electrolyte.

1 is a graph showing a change in ion conductivity with a temperature change of the polymer electrolyte according to Example 4 of the present invention.

2 is a graph showing a change in the interface resistance of the polymer electrolyte according to Example 4 and Comparative Example according to the present invention over time.

The present invention to achieve the above technical problem,

A polymer electrolyte comprising a host polymer, a plasticizer and a lithium salt. The host polymer is an acrylonitrile-methyl methacrylate-alkali metal methacrylate terpolymer ionomer having a repeating unit represented by the following formula (1) provides a polymer electrolyte.

[Formula 1]

Wherein x, y, z are independently 0 or a positive integer, and M is Li, Na or K.

In the terpolymer ionomer, the mole fraction of each repeating unit is not particularly limited. That is, 1 to 98 moles of acrylonitrile repeating units, 1 to 98 moles of methyl methacrylate repeating units, and 1 to 98 moles of alkali metal methacrylate repeating units can be used.

The molecular weight of the terpolymer ionomer is not particularly limited, but is preferably between 10,000 and 1,000,000.

The weight ratio of the host polymer and the plasticizer is 1: 1 to 1: 9, and the weight ratio of the lithium salt and the plasticizer is preferably 1: 5 to 1:30.

According to one embodiment of the invention, the polymer electrolyte preferably further comprises less than 30 silica based on the total weight of the polymer electrolyte.

Hereinafter, the present invention will be described in more detail.

The polymer electrolyte according to the present invention is an acrylonitrile-methyl methacrylate-alkali metal methacrylate terpolymer synthesized using acrylonitrile, methyl methacrylate and alkali metal methacrylate having ion groups introduced therein as comonomers. It is characterized by using an ionomer as a host polymer.

Accordingly, the polymer electrolyte according to the present invention has improved ion conductivity characteristics due to alkali metal ions introduced into the molecular structure of the host polymer, increased compatibility with the plasticizer, thereby preventing leakage of the plasticizer, and upon contact with the lithium electrode. An increase in interfacial resistance with time of contact can be suppressed.

In addition, it can be dissolved well in low boiling point solvents such as tetrahydrofuran having a boiling point of only about 66 ℃, which is very advantageous in the manufacturing process.

A method of preparing a polymer electrolyte according to the present invention will be described below. Acrylonitrile, methyl methacrylate and methacrylic acid monomers are copolymerized in a proportion to prepare an acrylonitrile-methyl methacrylate-methacrylic terpolymer having a repeating unit represented by the following formula (2).

Wherein x, y and z are independently 0 or a positive integer.

Thereafter, the molar ratio of methacrylic acid present in the terpolymer is measured by a titration method, and then the terpolymer is neutralized with an aqueous solution of an alkali metal (lithium, sodium or potassium) hydroxide at a predetermined concentration. Acrylonitrile-methylmethacrylate-alkalimetal methacrylate terpolymer ionomer having the same repeating unit is obtained.

[Formula 1]

Wherein x, y and z are independently 0 or a positive integer and M is Li, Na or K.

Using the terpolymer ionomer synthesized as described above as a host polymer, the organic solvent and the lithium salt, which are plasticizers, are quantified in desired ratios and dissolved in a cosolvent. In some cases Inorganic materials may be added to increase the mechanical properties of the polymer electrolyte film. When a homogeneous solution is obtained, it is cast in a Teflon container to volatilize the cosolvent to obtain a polymer electrolyte in the form of a film.

The organic solvent used as the plasticizer may be any one widely known in the lithium battery manufacturing field. Specifically, for example, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, methylformate, ethylformate, gamma-butyrolactone or mixtures thereof and the like can be used.

Examples of the lithium salt include lithium perchlorate (LiClO ₄ ), lithium triplate (LiCF ₃ SO ₃ ), lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ) or lithium trifluoromethanesulfonyl Mid (LiN (CF ₃ SO ₂ ) ₂ ) may be mentioned.

Examples of the inorganic substance include silica, alumina (Al ₂ O ₃ ), γ-LiAlO ₂ , LiI, or zeolite.

The content of each component is a weight ratio of the host polymer: plasticizer is 1: 1 to 1: 9, preferably 1: 3 to 1: 6, the weight ratio of lithium salt: plasticizer is 1: 5 to 1:30, preferably Is from 1:10 to 1:15. It is not preferable that out of this weight ratio range leads to a decrease in ionic conductivity and a decrease in mechanical properties of the polymer electrolyte film.

In addition, the amount of ?? of the inorganic substance is preferably 0 to 30 weight, more preferably 5 to 15 weight based on the total weight of the polymer electrolyte.

Hereinafter, an embodiment of the present invention will be described in more detail. However, the following examples are merely illustrative to aid the understanding of the present invention, and the scope of the present invention should not be construed as being limited thereto.

<Production example>

Using tertiary distilled water as a solvent, a predetermined amount of acrylonitrile, methyl methacrylate and methacrylic acid were put into the reactor as monomers and stirred for 30 minutes under a nitrogen atmosphere. After the completion of stirring, 0.5 mol of potassium persulfate and sodium bisulfite as an initiator were injected based on the number of moles of the monomer and reacted at 50 ° C. for 6 hours. After completion of the reaction, the reaction product was filtered to obtain a white powdery reaction product, and filtered three times using tertiary distilled water at 80 ° C. to remove impurities such as an initiator. The white powder obtained by filtration was dried in a hood at room temperature for 24 hours, and then dried for 48 hours in a vacuum oven to completely remove moisture to obtain an acrylonitrile-methyl methacrylate-methacrylic terpolymer.

The molar ratio of methacrylic acid present in the terpolymer by titration was measured, and then the terpolymer was neutralized with an aqueous lithium hydroxide solution of 0.1 normal concentration to acrylonitrile-methyl methacrylate-lithium methacrylate. A terpolymer ionomer was obtained.

<Example 1>

A terpolymer ionomer of 25 mol of acrylonitrile, 70 mol of methyl methacrylate, and 5 mol of lithium methacrylate was prepared according to the method disclosed in the above preparation.

A mixture of 16.7 weight of the terpolymer ionomer as a host polymer, 66.9 weight of ethylene carbonate (EC) as a plasticizer, 9.4 weight of LiClO ₄ as lithium salt and 7.0 weight of silica was added to a tetrahydrofuran solvent and stirred at 60 ° C. for 2 hours. A homogeneous solution was obtained in which all contents were completely dissolved.

The solution was cast in a Teflon container to gastric solvent to prepare a polymer electrolyte in the form of a film. This was made into a disk-shaped specimen and bonded with a stainless steel electrode to measure the ionic conductivity at room temperature (25 ℃), the results are shown in Table 1.

<Example 2>

A polymer electrolyte was prepared in the same manner as in Example 1 except that 53.4 weight of ethylene carbonate (EC) and 13.5 weight of dimethyl carbonate (DMC) were used as a plasticizer, and the ion conductivity at room temperature was measured. The results are shown in Table 1. .

<Example 3>

The polymer electrolyte was prepared in the same manner as in Example 1 except that 46.8 weight of ethylene carbonate (EC) and 20.1 weight of dimethyl carbonate (DMC) were used as the plasticizer, and the ion conductivity at room temperature was measured. The results are shown in Table 1. .

<Example 4>

Except that 33.45 weight of ethylene carbonate (EC) and 33.45 weight of dimethyl carbonate (DMC) were used as a plasticizer, polymer electrolytes were prepared in the same manner as in Example 1, and the ionic conductivity according to the temperature change over -20 to 60 ° C was measured. It measured, and the result is shown in Table 1 and FIG.

Example 5

Except that 26.8 weight of ethylene carbonate (EC) and 40.1 weight of dimethyl carbonate (DMC) were used as the plasticizer, the polymer electrolyte was prepared in the same manner as in Example 1, and the ion conductivity was measured, and the results are shown in Table 1.

<Example 6>

The polymer electrolyte was prepared in the same manner as in Example 1 except that 13.5 weight of ethylene carbonate (EC) and 63.4 weight of dimethyl carbonate (DMC) were used as the plasticizer, and ionic conductivity was measured. The results are shown in Table 1 below.

Example Plasticizer Mechanical properties Ion Conductivity (S / cm), 25 ℃ EC: DMC weight ratio Example 1 10: 0 Freestanding 2.0 × 10 ^-4 Example 2 8: 2 Freestanding 1.3 × 10 ^-4 Example 3 7: 3 Freestanding 1.3 × 10 ^-4 Example 4 5: 5 Freestanding 1.1 × 10 ^-3 Example 5 4: 6 Freestanding 1.0 × 10 ^-3 Example 6 2: 8 Freestanding 3.1 × 10 ^-4

As can be seen from Table 1, the polymer electrolyte according to the present invention is generally excellent in ion conductivity at room temperature of 2.0 × 10 ⁻⁴ S / cm or more. In addition, referring to FIG. 1, which shows the ion conductivity characteristic according to temperature, the polymer electrolyte according to Example 4 is 1.3 × 10 ⁻⁴ S / cm at a low temperature of −15 ° C., and 1.1 × 10 ⁻³ S / at 25 ° C. at room temperature. It can be seen that the ion conductivity characteristic is very excellent at more than cm.

Comparative Example

As a comparative example, a polyacrylonitrile-based polymer electrolyte (18 weight of Aldrich's polyacrylonitrile, 71.9 weight of ethylene carbonate, 10.1 weight of LiClO ₄ ) and the polymer electrolyte of Example 4 were used for the lithium electrode. The interfacial resistance was measured and the results are shown in FIG. 2.

Referring to FIG. 2, the conventional polyacrylonitrile-based polymer electrolyte rapidly increases the interfacial resistance with respect to the lithium electrode over time, whereas the polymer electrolyte according to the present invention (Example 4) has an interfacial resistance over time. It can be seen that there is almost no increase in the interfacial stability.

The polymer electrolyte according to the present invention increases the compatibility between the host polymer and the plasticizer by introducing an alkali metal ion group into the acrylonitrile-methyl methacrylate-methacrylic acid terpolymer, thus minimizing the leakage of the plasticizer. And the interfacial stability with the electrode and the ion conductivity characteristics can be improved. In addition, since the acrylate-based comonomer forms a polymer soluble in a low boiling point solvent such as tetrahydrofuran, it is also expected to improve the polymer electrolyte manufacturing processability.

Claims (8)

A polymer electrolyte comprising a host polymer, a plasticizer and a lithium salt. The polymer electrolyte, characterized in that the host polymer is an acrylonitrile-methyl methacrylate-alkali metal methacrylate terpolymer ionomer having a repeating unit represented by the following formula (1). [Formula 1] Wherein x, y, z are independently 0 or a positive integer, and M is Li, Na or K. According to claim 1, wherein the terpolymer ionomer is 1 to 98 moles of acrylonitrile repeating unit, 1 to 98 moles of methyl methacrylate repeating unit, 1 to 98 moles of alkali metal methacrylate repeating unit, characterized in that Polymer electrolyte. The polymer electrolyte of claim 1, wherein the terpolymer ionomer has a molecular weight of 10,000 to 1,000,000. The polymer electrolyte of claim 1, wherein the weight ratio of the host polymer and the plasticizer is 1: 1 to 1: 9, and the weight ratio of the lithium salt and the plasticizer is 1: 5 to 1:30. The polymer electrolyte of claim 1, further comprising up to 30 inorganic materials based on the total weight of the polymer electrolyte. The method of claim 1, wherein the plasticizer is ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, dimethoxyethane, methyl formate, ethyl formate, gamma A polymer electrolyte selected from the group consisting of butyrolactone and mixtures thereof. The method of claim 1, wherein the lithium salt is lithium perchlorate (LiClO ₄ ), lithium triplate (LiCF ₃ SO ₃ ), lithium hexafluoro phosphate (LiPF ₆ ), lithium tetrafluoro borate (LiBF ₄ ) or lithium trifluor Polymer electrolyte, characterized in that at least one lithium salt selected from the group consisting of romethanesulfonylimide (LiN (CF ₃ SO ₂ ) ₂ ). The polymer electrolyte of claim 5, wherein the inorganic material is at least one selected from the group consisting of silica, alumina (Al ₂ O ₃ ), γ-LiAlO ₂ , LiI, and zeolite.