JPH0831319B2 - Lithium ion conductive polymer electrolyte - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial field of application] The present invention relates to a lithium ion conductive polymer used for electrolytes such as lithium batteries and electrochromic displays, lithium ion concentration sensors, and lithium ion decomposition membranes. Regarding electrolytes.

[Conventional technology]

Attempts have been made to use a polyethylene oxide-based polymer electrolyte that is flexible and can be easily formed into a film as a lithium ion conductive solid electrolyte such as a lithium battery [Fast Ion Transport in Solid P. 13
1 (1979)].

This polyethylene oxide-based polymer electrolyte is composed of a complex of polyethylene oxide and a lithium salt, and it is possible to make it thin and miniaturized by taking advantage of its characteristics that it is flexible and can be easily formed into a film. When applied to the demanded lithium battery, there is an advantage in that it is advantageous in terms of workability for battery production and sealing, and can be useful for cost reduction. In addition, due to its flexibility, it is considered to be useful as an electrolyte for electrochromic displays and as a lithium ion concentration sensor.

[Problems to be Solved by the Invention]

However, the polyethylene oxide-based polymer electrolyte, although melts at a high temperature of 60 ℃ or more and shows relatively good lithium ion conductivity, at room temperature of about 25 ℃, because of high crystallinity lithium ion conductivity is low, However, there is a problem that when used at room temperature, most lithium batteries and the above-mentioned various applications cannot be fully satisfied in terms of performance.

Therefore, an object of the present invention is to provide a polymer electrolyte that is solid at room temperature and exhibits good lithium ion conductivity by using a polymer different from the above-mentioned polyethylene oxide as a polymer component of the polymer electrolyte. .

[Means for solving the problem]

The inventors of the present invention have conducted extensive studies in order to achieve the above object, and as a result of using a crosslinked polymer obtained by crosslinking a triol ether of an ethylene oxide-propylene oxide copolymer as an organic polymer constituting a polymer electrolyte, at room temperature. It was found that a polymer electrolyte which is solid and shows good lithium ion conductivity can be obtained, and has completed the present invention.

That is, the present invention relates to a lithium ion conductive polymer electrolyte comprising a composite of a lithium salt and an organic polymer, wherein the organic polymer is a triol ether of an ethylene oxide-propylene oxide copolymer represented by the following formula (I). The present invention relates to a lithium ion conductive polymer electrolyte, which is a crosslinked crosslinked polymer.

(In the formula, R is H or CH ₃ , χ is 0.1 to 0.5, and n is 1 to 20).

The use of a crosslinked polymer obtained by crosslinking the triol ether of the ethylene oxide-propylene oxide copolymer represented by the above formula (I) as a polymer component makes it possible to exhibit good lithium ion conductivity even at room temperature. It is thought that it is due to the following reasons.

First, the main constituent part of the polymer component is an ethylene oxide-propylene oxide copolymer, and since propylene oxide in this ethylene oxide-propylene oxide copolymer has a methyl group in its side chain, its steric hindrance is As a result, the crystallinity of the polymer becomes low, and good lithium ion conductivity is exhibited even at a lower temperature as compared with a conventional polymer electrolyte in which polyethylene oxide having high crystallinity is used as a polymer component. Further, since crosslinking is carried out using a crosslinking agent, a liquid compound can be made into a solid state by this crosslinking, and lithium ion conductivity can be obtained as a triol ether of the ethylene oxide-propylene oxide copolymer represented by the formula (I). It is also possible to use a liquid having a good low molecular weight and a low molecular weight. Furthermore, since the ethylene oxide-propylene oxide copolymer is reacted with triol (glycerin) and etherified, a three-dimensional structure can be obtained by crosslinking, and the ethylene oxide-propylene oxide copolymer can be crosslinked only by a crosslinking agent. As in the case of, the linear chain extension does not occur, and therefore, the decrease in the lithium ion conductivity due to the chain extension can be prevented. As described above, the main component of the polymer component contains propylene oxide, high ionic conductivity, it is possible to use a small molecular weight,
It is considered that the fact that a three-dimensional structure can be obtained synergistically works to obtain high lithium ion conductivity.

Here, the reason why the complex of the crosslinked polymer obtained by crosslinking the triol ether of the ethylene oxide-propylene oxide copolymer and the lithium salt has lithium ion conductivity is described. The ethylene oxide-propylene oxide copolymer in the polymer component is described. But,
This is because the main chain has an ether bond with a high dielectric constant as in the case of the conventionally used polyethylene oxide, and thus oxygen forming the ether bond and a lithium salt form a complex.

In the formula (I) showing a triol ether of an ethylene oxide-propylene oxide copolymer, χ represents a molar ratio of ethylene oxide and propylene oxide, and in the present invention, χ is set to 0.1 to 0.5. This is because when χ is larger than 0.5, that is, when ethylene oxide exceeds 50 mol%, the crystallinity increases and the lithium ion conductivity near room temperature decreases,
Further, when χ is smaller than 0.1, that is, when ethylene oxide is less than 10 mol%, the proportion of propylene oxide is increased, so that the amount of carrier ions is reduced and the ionic conductivity is lowered.

Further, n in the formula (I) indicates the degree of polymerization of the ethylene oxide-propylene oxide copolymer, and in the present invention, n is set to 1 to 20, but when n exceeds 20, the molecular weight is This is because the lithium ion conductivity increases and the lithium ion conductivity decreases.

When the triol ether of the ethylene oxide-propylene oxide copolymer represented by the above formula (I) is used as the polymer component of the lithium ion conductive polymer electrolyte, it is crosslinked with a crosslinking agent to form a crosslinked polymer. In the case of using a bifunctional organic substance or organic peroxide. However, the bifunctional organic compound can be used only when R in the formula (I) is H, and the organic peroxide can be used when R in the formula (I) is H or CH ₃ .

The bifunctional organic substance forms a crosslinked product (that is, a crosslinked polymer) by addition and condensation reaction with the OH group of the triol ether of the ethylene oxide-propylene oxide copolymer represented by the formula (I). 2 like this
Examples of functional organic compounds include diisocyanates (for example, hexamethylene diisocyanate, tolylene-2,4
-Diisocyanate, methylenebis (4-phenylisocyanate), xylylenediisocyanate, etc.], diamine (for example, ethylenediamine, putrescine, etc.), dicarboxylic acid (for example, oxalic acid, malonic acid, succinic acid, phthalic acid, isophthalic acid) , Terephthalic acid, etc.), dicarboxylic acid chlorides (eg, succinyl chloride), methylol compounds (eg, dimethylurea), epichlorohydrin and the like.

On the other hand, the organic peroxide generates a radical when heated to an appropriate temperature, and this radical abstracts hydrogen of the ethylene oxide-propylene oxide copolymer represented by the formula (I) to generate a radical in the polymer. Let Then, the radicals generated in the polymer bond with each other to cause crosslinking of the polymer. Examples of such organic peroxides include cumene hydroperoxide, benzoyl peroxide, lauroyl peroxide, potassium peroxide, t-butyl hydroperoxide, dicumyl peroxide, di-t.
-Butyl peroxide and the like.

In the present invention, when the triol ether of the ethylene oxide-propylene oxide copolymer represented by the formula (I) is used as the polymer component of the lithium ion conductive polymer electrolyte, the crosslinking with the crosslinking agent is relatively This is because a polymer having a small molecular weight can be solidified at room temperature, and it becomes easy to obtain a polymer electrolyte having high lithium ion conductivity. That is, when the ethylene oxide-propylene oxide copolymer portion of the ethylene oxide-propylene oxide copolymer represented by the formula (I) has a high degree of polymerization at the ethylene oxide-propylene oxide copolymer portion and a high molecular weight, high lithium ion conductivity can be obtained. It becomes difficult, but by making it solid by cross-linking, one with a low degree of polymerization of the ethylene oxide-propylene oxide copolymer part, that is, high lithium ion conductivity can be expected, but at room temperature because of its small molecular weight. This is because it becomes possible to use a material that cannot maintain a solid state.

In the present invention, a lithium salt that constitutes a lithium ion conductive polymer electrolyte together with a triol ether crosslinked polymer of an ethylene oxide-propylene oxide copolymer represented by the formula (I) is used in a conventional polymer electrolyte. Any of the above can be used, and specific examples thereof include LiBr,
LiI, LISCN, LiBF ₄ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiC ₆ F
₁₂ SO ₃ , LiCF ₃ SO ₂ , LiHgI _{3 and the} like. The amount of these lithium salts used is usually in the range of 1 to 30% by weight, preferably 3 to 20% by weight, based on the crosslinked polymer.

The lithium ion conductive polymer electrolyte of the present invention comprises a composite of the above-mentioned crosslinked polymer and a lithium salt, and this composite is, for example, an organic solvent solution prepared by dissolving the above-mentioned crosslinked polymer in a lithium salt. It can be obtained by dipping, infiltrating the above lithium salt solution into the crosslinked polymer, and then removing the organic solvent by evaporation.

By immersing the crosslinked polymer in the lithium salt solution as described above, the lithium salt forms a complex with ether oxygen in the crosslinked polymer and is bonded, and the above bond is maintained even after the solvent is removed. A complex with is obtained.

The form of the polymer electrolyte is appropriately determined depending on the purpose of use. For example, when the polymer electrolyte is used as an electrolyte for a lithium battery and also functions as a separator between the positive and negative electrodes, the polymer electrolyte may be formed into a sheet shape. To obtain this sheet-shaped polymer electrolyte, the cross-linked polymer is formed into a sheet,
The sheet-like crosslinked polymer may be immersed in a solution of a lithium salt in an organic solvent, and then the organic solvent may be removed by evaporation. As the above-mentioned sheet, an extremely thin sheet of micron order, which is generally called a film, can be prepared.

Further, when the polymer electrolyte of the present invention is applied to a positive electrode of a lithium battery, a triol ether of an ethylene oxide-propylene oxide copolymer represented by the general formula (I), a cross-linking agent, a positive electrode active material and the like are added at a predetermined ratio. ,
The triol ether of the ethylene oxide-propylene oxide copolymer may be crosslinked and then molded, and the obtained molded product may be immersed in a solution of a lithium salt in an organic solvent, and after the immersion, the organic solvent may be removed by evaporation. By doing so, a polymer electrolyte and a positive electrode active material and the like are mixed and integrated.

In obtaining the polymer electrolyte, as the organic solvent for dissolving the lithium salt, sufficiently dissolve the lithium salt,
An organic solvent that does not react with the polymer, such as acetone, tetrahydrofuran, dimethoxyethane, dioxolane, propylene carbonate, acetonitrile, dimethylformamide, etc., is used.

FIG. 1 shows an example of a lithium battery using the above-described polymer electrolyte of the present invention. In the figure, 1 is a square flat plate-shaped positive electrode current collector plate made of stainless steel, and 2 is a stepped peripheral surface toward one side. A shallow rectangular dish-shaped negative electrode current collector plate made of stainless steel provided with a flat peripheral part 2a in the same direction as the bent main surface, and 3 are opposite peripheral parts 1a, 2a of the bipolar current collector plates 1, 2. It is an adhesive layer for sealing the space.

4 is formed into a sheet by the method described above for the polymer electrolyte of the present invention and the positive electrode active material, which are arranged on the side of the positive electrode current collector plate 1 in the space 5 formed between the two electrode current collector plates 1 and 2. And 6 is a negative electrode made of lithium or a lithium alloy loaded in the space 5 on the negative electrode current collector 2 side, 7
Is a separator formed by molding the lithium ion conductive polymer electrolyte of the present invention interposed between the positive electrode 4 and the negative electrode 6 into a sheet.

The positive electrode 4 may be formed into a sheet by mixing a positive electrode active material with a binder such as polytetrafluoroethylene powder or an electron conduction aid as the case may be. Examples of the positive electrode active material used for the positive electrode 4 include TiS ₂ and
One or more of MoS ₂ , V ₆ O ₁₃ , V ₂ O ₅ , VSe, NiPS ₃ , polyaniline, polypyrrole, polythiophene and the like are used.

The lithium battery configured as described above has a separator 7
Is a sheet-like product made of the lithium ion conductive polymer electrolyte, and the positive electrode 4 is a similar sheet-like product containing the lithium ion conductive polymer electrolyte. In addition to having various advantages that it can contribute to improvement of workability, reliability of sealing, etc. and that there is essentially no concern about liquid leakage unlike liquid electrolyte, the polymer electrolyte described above has a lithium ion Due to its excellent conductivity, it has excellent discharge characteristics as a primary battery and charge / discharge cycle characteristics as a secondary battery.

〔Example〕

Hereinafter, examples of the present invention will be described in comparison with comparative examples.

Example 1 Unilube 50TG-32 (trade name, manufactured by NOF CORPORATION, a polymer belonging to the range of the triol ether of the ethylene oxide-propylene oxide copolymer represented by the formula (I), and the mole of the ethylene oxide portion thereof is The ratio is 0.5 and the number average molecular weight is 2,800. Displaying this in accordance with the formula (I), χ = 0.5 and n = about 9) 4g
And 20 mg of benzoyl peroxide were placed in an Erlenmeyer flask, stirred with a magnetic stirrer, the resulting viscous solution was dropped on a nickel plate, and heated to 70 ° C. on a hot plate in an argon gas flow to react for 8 hours. And crosslinked to obtain a crosslinked polymer. The obtained crosslinked polymer was peeled off from the nickel plate and immersed in acetone to remove unreacted substances by dissolving in acetone. Subsequently, the crosslinked polymer was immersed in a 3 wt% LiBF ₄ acetone solution for 8 hours, the above LiBF ₄ acetone solution was allowed to penetrate into the crosslinked polymer, and then acetone was removed by evaporation to form a sheet having a thickness of 0.1 mm. A polymer electrolyte was obtained.

Example 2 H-4540-4 (trade name, manufactured by Dai-ichi Kogyo Kabushiki Kaisha, a polymer belonging to the range of the triol ether of the ethylene oxide-propylene oxide copolymer represented by the formula (I), and its ethylene oxide The molar ratio of the moieties is 0.4, and the number average molecular weight is 3,000. When this is expressed in accordance with the formula (I), χ = 0.4 and n = about 9) 4 g
And 0.3 g of hexamethylene diisocyanate were placed in an Erlenmeyer flask, stirred with a magnetic stirrer, the resulting viscous solution was dropped on a nickel plate, and heated to 100 ° C. on a hot plate in an argon gas flow to 6 Cross-linking was carried out by reacting for a time to obtain a cross-linked polymer. Thereafter, in the same manner as in Example 1, a sheet-shaped polymer electrolyte having a thickness of 0.1 mm was obtained.

Comparative Example 1 Polyethylene oxide having a number average molecular weight of 600,000 1 g and L
iBF _{4 (} 0.326 g) was dissolved in acetonitrile (5 ml) and stirred with a magnetic stirrer to uniformly dissolve. The viscous solution obtained was dropped on a glass substrate and left in an argon flow at room temperature for 5 hours, then the degree of vacuum was 1 × 10 ^-2 torr and the temperature was 120 ° C.
And evaporate off the acetonitrile for 10 hours.
A 0.1 mm sheet-shaped polymer electrolyte was obtained.

In order to investigate the performance of the polymer electrolytes of Examples 1 to 2 and Comparative Example 1, the following ionic conductivity test and battery internal resistance test were performed.

<Ion Conductivity Test> The polymer electrolytes of Examples 1 and 2 were sandwiched between Au (gold) plates, and the polymer electrolyte of Comparative Example 1 was formed by depositing an Au comb-shaped electrode thereon by an evaporation method. AC impedance analysis was performed to measure ionic conductivity at 25 ° C.
The results are shown in Table 1.

The results of measuring the ionic conductivity under various temperature conditions in the same manner as above are as shown in FIG.
In FIG. 2, the vertical axis is the ionic conductivity (S / cm),
The horizontal axis is the reciprocal of absolute temperature 10 ³ / T (K ^-1 ).

<Battery Internal Resistance Test> Using the polymer electrolytes of Examples 1 and 2 and Comparative Example 1, a square thin lithium battery having a structure shown in FIG. 1 and having a total thickness of 0.5 m and a side length of 1 cm was produced. An alloy of lithium and aluminum was used for the negative electrode, and a sheet-like molded product containing TiS ₂ and a polymer electrolyte having the same composition as in Examples 1 and 2 and Comparative Example 1 was used for the positive electrode.

For these lithium batteries, 25 ℃, 60 ℃, 100 ℃
Table 2 shows the results of measuring the internal resistance at.

As is clear from the above test results, the polymer electrolytes of Examples 1 and 2 of the present invention were 2 × 10 ⁻⁵ to 4 × 10 ⁻⁵ at 25 ° C.
Although it showed a high ionic conductivity of S / cm, the polymer electrolyte of Comparative Example 1 using polyethylene oxide as a polymer component had a low ionic conductivity of 1 × 10 ⁻⁷ S / cm at 25 ° C.,
The ionic conductivity was inferior to the polymer electrolytes of Examples 1 and 2 of the present invention. Further, as shown in Table 2, the internal resistance at 25 ° C. of the lithium batteries using the polymer electrolytes of Examples 1 and 2 of the present invention was 250 to 500Ω, but Comparative Example 1
The internal resistance at 25 ℃ of the lithium battery using the polymer electrolyte was 100,000Ω, which was very large. In addition, as shown in FIG. 2, the polymer electrolyte of Comparative Example 1 using polyethylene oxide as a polymer component is more preferable than the polymer electrolytes of Examples 1 and 2 in the high temperature region (region where the value of the horizontal axis is small). Although the ionic conductivity was high and the ionic conductivity was excellent, when the temperature became low and the temperature was around room temperature (the value on the horizontal axis in FIG. 2 was around 3.35), the ionic conductivity was significantly reduced, and Example 1 Much lower than the polymer electrolyte of ~ 2.

〔The invention's effect〕

As described above, according to the present invention, it is possible to provide a lithium conductive polymer electrolyte which is solid at room temperature and exhibits high ionic conductivity.

[Brief description of drawings]

FIG. 1 is a vertical sectional view showing an example of a lithium battery using the lithium ion conductive polymer electrolyte of the present invention. FIG. 2 is a graph showing the relationship between ionic conductivity and temperature of the present invention and comparative lithium ion conductive polymer electrolytes. 7: Separator made of polymer electrolyte

Continuation of the front page (51) Int.Cl. ⁶ Identification number Reference number in the agency FI Technical indication location H01M 10/36 A (72) Inventor Kozo Kajita 1-88, Tora-ta, Ibaraki-shi, Osaka Hitachi Maxell Co. (56) Reference JP-A 63-55811 (JP, A) JP-A 63-136407 (JP, A) JP-A 60-154248 (JP, A) JP-A 60-217263 (JP, A)

Claims (3)

[Claims] 1. A lithium ion conductive polymer electrolyte comprising a composite of a lithium salt and an organic polymer, wherein the organic polymer cross-links a triol ether of an ethylene oxide-propylene oxide copolymer represented by the following formula (I). A lithium ion conductive polymer electrolyte, which is a crosslinked polymer. (In the formula, R is H or CH ₃ , χ is 0.1 to 0.5, and n is 1 to 20). 2. An ethylene oxide represented by the formula (I):
The crosslinking agent for crosslinking the triol ether of propylene oxide copolymer (wherein R in the formula (I) is H) is a diisocyanate, diamine, dicarboxylic acid, dicarboxylic acid chloride, methyl roll compound, or The lithium ion conductive polymer electrolyte according to claim 1, which is at least one kind of bifunctional organic material selected from the group consisting of epiclohydrin. 3. An ethylene oxide represented by the formula (I)
Crosslinking agents for crosslinking the triol ethers of propylene oxide include cumene hydroperoxide, benzoyl peroxide, lauroyl peroxide, potassium peroxide, t-
The lithium ion conductive polymer electrolyte according to claim 1, which is at least one organic peroxide selected from the group consisting of butyl hydroperoxide, dicumyl peroxide and di-t-butyl peroxide.