JPH01107471A - Lithium ion conductive polymer electrolyte - Google Patents

Abstract

PURPOSE:To obtain a polymer electrolyte which is solid at room temperature and has good lithium ion conductivity by using an organic polymer prepared by the reaction of ethylene oxide-propylene oxide copolymer, an organic substance having two functional groups, and glycerine as the organic polymer for forming a composite with a lithium salt. CONSTITUTION:As an organic polymer for forming a polymer electrolyte, a polymer prepared by crosslinking ethylene oxide-propylene oxide copolymer and glycerine with an organic substance having two functional groups is used. The end OH group of ethylene oxide-propylene oxide copolymer reacts with the organic substance having two functional groups such as diisocyanate, the functional group is introduced into the end group and the functional group reacts with glycerine to form a crosslinked polymer having three dimensional structure. By using this polymer as a polymer component, a polymer electrolyte which is solid at room temperature and has good lithium ion conductivity can be obtained.

Description

[Detailed Description of the Invention] [Industrial Application Field] The present invention relates to lithium ion conductive materials used in electrolytes such as lithium batteries and electrochromic displays, lithium ion concentration sensors, and lithium ion separation membranes. Concerning polymer electrolytes.

[Conventional technology]

Lithium ion conductive electrolytes for lithium batteries include liquid electrolytes such as LiClO4/propylene carbonate, Li5N, Lit-Attos, etc.
Solid electrolytes such as those represented by
Recently, attempts have been made to use polymer electrolytes based on organic polymers that can be easily formed into flexible films.

If this type of polymer electrolyte is applied to lithium batteries, which require ultra-thin films and miniaturization, it will be advantageous in terms of workability and sealing reliability for battery production, and will also reduce costs. There are also some useful benefits. In addition, due to its flexibility, it can be used as a lithium ion separation membrane, and is also useful as an electrolyte for electrochromic displays and as a lithium ion concentration sensor.

Conventionally, one known polymer electrolyte is a composite of polyethylene oxide as an organic polymer and lithium salt (F).
ast Ion transport in 5oli
dP, 131 (1979)).

[Problem that the invention seeks to solve]

However, although the above-mentioned polyethylene oxide-lithium salt-based polymer electrolyte melts at high temperatures of 60°C or higher and exhibits relatively good lithium ion conductivity,
Lithium ion conductivity is low at room temperature of about 25℃,
When applied to most lithium batteries and the various uses mentioned above, there is a problem in that the performance cannot be fully satisfied when used at room temperature.

Therefore, the present invention aims to provide a polymer electrolyte that is solid at room temperature and exhibits good lithium ion conductivity by using a polymer different from the conventional lithium ion conductive polymer electrolyte as its polymer component. purpose.

[Means for solving problems]

As a result of intensive research to achieve the above object, the inventors have developed a polymer made by crosslinking ethylene oxide-propylene oxide copolymer and glycerin with a bifunctional organic material as an organic polymer constituting the polymer electrolyte. It was discovered that when using lithium ion conductive polymer electrolyte, it is possible to obtain a lithium ion conductive polymer electrolyte that is solid at room temperature and has very good lithium ion conductivity, and can be suitably applied to various uses even at room temperature, and completed this invention. It arrived.

That is, the present invention provides a lithium ion conductive polymer electrolyte consisting of a composite of a lithium salt and an organic polymer, in which the organic polymer contains an ethylene oxide-propylene oxide copolymer, a difunctional organic substance, and glycerin. The present invention relates to a lithium ion conductive polymer electrolyte characterized in that it is a reacted polymer.

The organic polymer used as the polymer component of the polymer electrolyte in this invention is one obtained by reacting an ethylene oxide-propylene oxide copolymer, a bifunctional organic substance, and glycerin as described above.
The ethylene oxide-propylene oxide copolymer used as the base polymer is a block or random copolymer of ethylene oxide and propylene oxide, and the copolymerization ratio is 20% of ethylene oxide.
-65% by weight, preferably 80-35% by weight of propylene oxide. A particularly preferred copolymerization ratio is 20 to 45% by weight of ethylene oxide and 80 to 55% by weight of propylene oxide. The reason why such a copolymerization ratio is preferable is that the ionic conductivity becomes particularly high within such a copolymerization ratio.

In addition, the molecular weight of this ethylene oxide-propylene oxide copolymer is 200 to 1 in terms of number average molecular weight.
The number average molecular weight of the copolymer is preferably in the range of 0.000, particularly 1.000 to s, ooo,
ooo to 8,000 is particularly preferable because the distance between the crosslinking points when the copolymer is crosslinked with a bifunctional organic substance and glycerin is suitable for lithium ion migration.

The reaction of this ethylene oxide-propylene oxide copolymer, the bifunctional organic substance, and glycerin is carried out as follows.

The terminal OH group of the ethylene oxide-propylene oxide copolymer reacts with the bifunctional organic substances shown in Table 1 (diisocyanate, diamine, dicarboxylic acid, dicarboxylic acid chloride, methylol compound, epichlorohydrin) to form ethylene oxide. - A functional group is introduced at the end of the propylene oxide copolymer.Next, the end functional group of the ethylene oxide-propylene oxide copolymer reacts with glycerin to obtain a three-dimensionally crosslinked polymer. For example, as an organic substance having two functions, succinel chloride (CIOC-CI(tCHx C0
The reaction when using CI) is as follows.

Hs HO4CHt CHzOlCHzCHO Door H filtration polymer No. 1 Table 1 A polymer electrolyte using a polymer obtained by reacting the above ethylene oxide-propylene oxide copolymer with a bifunctional functional group and glycerin is a polymer electrolyte that uses conventional polyethylene oxide, that is, ethylene oxide alone. The reason why the lithium ion conductivity at room temperature is better than that using a polymer is not necessarily clear at present, but it is thought to be as follows.

First, considering the ethylene oxide-propylene oxide copolymer, this copolymer, like conventional polyethylene oxide, has ether bonds with high inductivity in its main chain. When the constituent oxygen and lithium salt form a complex, lithium ion conductivity is imparted to the polymer.

Conventional polyethylene oxide is a crystalline polymer with a melting point around 60°C, so it melts above the above temperature and exhibits relatively good lithium ion conductivity, but due to its high crystallinity, There is a tendency for lithium ion conductivity to decrease rapidly at temperatures lower than 60°C.

On the other hand, since ethylene oxide-propylene oxide copolymer has a methyl group based on propylene oxide in its side chain, the crystallinity of the polymer decreases due to steric hindrance, and the polyethylene oxide It exhibits good lithium ion conductivity even at lower temperatures.

The above-mentioned effect of improving lithium ion conductivity on the low-temperature side becomes more noticeable as the copolymerization ratio of propylene oxide increases (as the copolymerization ratio of propylene oxide increases, but if it becomes too high, for example, in extreme cases, the copolymerization ratio of propylene oxide When it comes to polypropylene oxide, the lithium ion conductivity itself is not so high, so the above-mentioned improvement effect is reduced.The preferred copolymerization ratio of ethylene oxide and propylene oxide is based on the above-mentioned reason, and the above-mentioned As shown, ethylene oxide is 20-65% by weight and propylene oxide is 80-3% by weight.
Particularly good lithium ion conductivity can be exhibited by setting the content within the range of 5% by weight.

In other words, if the ethylene oxide content is less than the above range, the properties of polypropylene oxide will appear strongly, resulting in low lithium ion conductivity, and if the ethylene oxide content is more than the above range, the crystallinity will become strong, resulting in poor ion conductivity at room temperature. This makes it worse.

On the other hand, the higher the molecular weight of the ethylene oxide-propylene oxide copolymer, the stronger the tendency for it to solidify, and when the number average molecular weight exceeds 10.000, it will solidify by itself even at room temperature. Accordingly, lithium ion conductivity decreases. In addition, when the molecular weight is low, the distance between the crosslinking points when crosslinked is short, and the steric hindrance makes it difficult for ions to pass through, resulting in poor ionic conductivity.
,oo.

The ion movement of lithium is greatest in the range of 1,000 to 8,000.

The organic substance having difunctionality is used to introduce a functional group to the end of the ethylene oxide-propylene oxide copolymer, and the glycerin is used to introduce a functional group into the terminal of the ethylene oxide-propylene oxide copolymer.
The ethylene oxide-propylene oxide copolymer is crosslinked into three dimensions by reacting with the terminal functional groups of the group-based copolymer, allowing it to remain solid at room temperature. Therefore, by using an ethylene oxide-propylene oxide copolymer, which has a relatively low molecular weight and good ionic conductivity but is liquid at room temperature, and crosslinking it with a difunctional organic substance and glycerin, However, by making use of the good ionic conductivity of the liquid ethylene oxide-propylene oxide copolymer, a solid polymer electrolyte can be obtained at room temperature. The above-mentioned glycerin makes the ethylene oxide-propylene oxide copolymer have a three-dimensional structure, suppresses the linear chain extension that occurs when reacting only with a crosslinking agent, and creates a three-dimensional structure based on the linear chain extension. (Suppresses the decline in ionic conductivity.

In addition, in the reaction of the above-mentioned ethylene oxide-propylene oxide copolymer, the organic substance having difunctionality, and glycerin, as illustrated, the ethylene oxide-propylene oxide copolymer and the organic substance having difunctionality are reacted first. This is followed by reaction with glycerin, but it is thought that these reactions proceed almost simultaneously.

The reaction is carried out in a stream of inert gas such as argon at 60 to 1
Under such heating conditions, which are preferably heated to about 00°C, the reaction usually takes about 2 to 10 hours.

In this invention, the above ethylene oxide-propylene oxide copolymer, an organic substance having bifunctionality,
Any of the lithium salts used in conventional polymer electrolytes can be used as the lithium salt that constitutes the polymer electrolyte together with the polymer (crosslinked polymer) reacted with glycerin.
r, LI I, Li5CN, LiBFa, LiAsF
*, LiCl0n, LiCFiSOs, LiC4F
+3S01 , L icFssOg , L iHg 1
Examples include s. The amount of this lithium salt used is usually 1 to 30% by weight, particularly 3 to 20% by weight, based on the crosslinked polymer.
The reason why the amount of lithium salt used is preferably in the range of 3 to 20% by weight based on the crosslinked polymer is that the lithium salt increases the lithium ion carrier concentration and the lithium salt increases. This is because the lithium ion conductivity is maximized within the above range.

The polymer electrolyte of the present invention is a composite of the above-mentioned cross-linked polymer and lithium salt, but a general method for obtaining this composite is to mix the above-mentioned cross-linked polymer with an organic compound in which lithium salt is dissolved. There is a method in which the crosslinked polymer is immersed in a solvent solution, a lithium salt solution is permeated into the crosslinked polymer, and then the organic solvent is evaporated off. In this method, the lithium salt forms a complex and binds to the ether oxygen in the crosslinked polymer, and the above bond is maintained even after the solvent is removed, yielding a composite of the crosslinked polymer and the lithium salt.

The form of this composite is determined depending on the purpose of use.6 For example, the polymer electrolyte of the present invention is used as an electrolyte for lithium batteries, and this polymer electrolyte also functions as a separator between positive and negative electrodes. In this case, the polymer electrolyte may be formed into a sheet.

To obtain this sheet-like polymer electrolyte, for example, an ethylene oxide-propylene oxide copolymer, a bifunctional organic substance, and glycerin are mixed in a predetermined ratio,
The obtained viscous solution-like mixture is cast onto a suitable substrate to cause a crosslinking reaction to obtain a crosslinked polymer in the form of a sheet, and this sheet-like crosslinked polymer is immersed in an organic solvent solution of lithium salt. Thereafter, the organic solvent can be removed by evaporation, and the above-mentioned sheet can be made into an extremely thin sheet generally on the order of microns, generally referred to as a film. The organic solvent for dissolving the lithium salt may be an organic solvent that sufficiently dissolves the lithium salt and does not react with the polymer, such as acetone, tetrahydrofuran, dimethoxyethane, dioxolane, propylene carbonate, acetonitrile, dimethylformamide, etc. A solvent is used.

In addition, when the polymer electrolyte of the present invention is applied to a positive electrode in a lithium battery, after adding ethylene oxide-propylene oxide copolymer, a bifunctional organic substance, glycerin, a positive electrode active material, etc. in a predetermined ratio, the above reaction components are added. are reacted, and after the reaction, it is molded into a desired shape such as a sheet, the obtained molded body is immersed in an organic solvent solution in which a lithium salt is dissolved, and after the immersion, the organic solvent is removed by evaporation.

By doing so, a rod in which the polymer electrolyte and the positive electrode active material are mixed can be obtained.

FIG. 1 shows an example of a lithium battery using the polymer electrolyte of the present invention described above. a shallow rectangular dish-shaped negative electrode current collector plate made of stainless steel with a flat peripheral part 2a facing the same direction as the curved main surface;
Reference numeral 3 denotes an adhesive layer that seals between the opposing peripheral parts 1a and 2a of the bipolar current collector plate 1.2.

4 is a polymer electrolyte of the present invention, a positive electrode active material, etc. arranged on the side of the positive electrode current collector plate 1 in a space 5 formed between the two electrode current collector plates 1.2, and formed into a sheet shape by the method described above. 6 is a negative electrode made of lithium or lithium alloy loaded on the negative electrode current collector plate 2 side in the space 5; 7
is a separator formed by molding the polymer electrolyte of the present invention interposed between a positive electrode 4 and a negative electrode 6 into a sheet shape.

In addition, the positive electrode 4 may be formed by mixing a positive electrode active material with a binder such as polytetrafluoroethylene powder, or an electron conduction aid, and forming the mixture into a sheet shape. As the positive electrode active material, for example, T t
S zSM o S z,V,O,,,V,OS,VS
One or more of the following are used: e, NtPS3, polyaniline, polypyrrole, polythiophene, and the like.

In the lithium battery constructed in this way, the separator 7 is a sheet-like material made of the polymer electrolyte, and the positive electrode 4 is a similar sheet-like material containing the polymer electrolyte, so that the battery can be made thinner. The above-mentioned electrolyte has various advantages such as contributing to improvements in workability and sealing performance for battery work, and essentially no worries about leakage unlike liquid electrolytes. Due to its excellent ionic conductivity, it has excellent discharge characteristics as a secondary battery and excellent charge/discharge cycle characteristics as a secondary battery.

〔Effect of the invention〕

As described above, according to the present invention, an organic polymer obtained by reacting an ethylene oxide-propylene oxide copolymer, an organic substance having bifunctionality, and glycerin is used as the organic polymer constituting the complex with the lithium salt. Accordingly, it is possible to provide a lithium ion conductive polymer electrolyte that is solid at room temperature and has excellent lithium ion conductivity, and can be suitably applied to lithium batteries and various other uses.

〔Example〕

Examples of the present invention will be described below in comparison with comparative examples.

Example 1 Number average molecular weight 3. Goo's ethylene oxide-propylene oxide copolymer (ethylene oxide content 44
% by weight), 0.60 g of 2.4-)lylene diisocyanate, and 0.103 g of glycerin were placed in an Erlenmeyer flask and sufficiently stirred with a magnetic stirrer. The resulting viscous solution mixture was poured onto an aluminum plate, heated to 100° C. in an argon gas flow, and reacted for 4 hours to obtain a crosslinked product (hereinafter referred to as a crosslinked polymer). After peeling off the obtained crosslinked polymer from the aluminum plate, it was added to a LiCFsSOs acetone solution with a concentration of 3%.
Soak for an hour, LiCF35O! After the acetone solution was permeated into the crosslinked polymer, it was heated to 60° C. to evaporate and remove the acetone, thereby obtaining a sheet-like polymer electrolyte with a thickness of 20 gm.

Example 2 An ethylene oxide-propylene oxide copolymer with a number average molecular weight of 400 was used instead of an ethylene oxide-propylene oxide copolymer with a number average molecular weight of 3.000 (
A polymer electrolyte was obtained in the same manner as in Example 1, except that 0.689 g of ethylene oxide (containing 144% by weight) was used.

Example 3 In place of the ethylene oxide-propylene oxide copolymer with a number average molecular weight of 3,000, a number average molecular weight of 1,0
00 ethylene oxide-propylene oxide copolymer (ethylene oxide content 44% by weight) 1.72
A polymer electrolyte was obtained in the same manner as in Example 1 except that g was used.

Example 4 In place of the ethylene oxide-propylene oxide copolymer with a number average molecular weight of 3.000, number average molecular weights s, oo
8.60 g of ethylene oxide-propylene oxide copolymer (144% by weight of ethylene oxide)
A polymer electrolyte was obtained in the same manner as in Example 1 except that .

Comparative Example 1 Polyethylene oxide Ig having a number average molecular weight of 600.000 and 236 g of Li CF ss Os O were dissolved in 5 ml of acetonitrile and uniformly stirred with a magnetic stirrer. Next, this viscous solution was dropped onto a glass substrate, left for 5 hours in an argon gas flow under normal pressure, and then reduced to a vacuum degree of I Xl0-'torr.

The acetonitrile was evaporated off by heat treatment at 130° C. for 10 hours to obtain a sheet-like polymer electrolyte with a thickness of 20 IIm.

Comparative Example 2 Ethylene oxide-propylene oxide copolymer with number average molecular weight 3.000 (ethylene oxide content 44
Weight%) Ig and LiCFiSOio, 207g were placed in an Erlenmeyer flask, heated to 60°C and stirred with a stirrer,
A viscous liquid polymer electrolyte was obtained by uniformly mixing the mixture.

In order to examine the properties of the polymer electrolytes of Examples 1 to 4 and Comparative Examples 1 to 2, the following ionic conductivity tests and discharge characteristic tests were conducted.

<Ionic conductivity test> The polymer electrolytes of Examples 1 to 4 were sandwiched between lithium foils in the form of a sandwich, and the polymer electrolyte of Comparative Example 1 was coated with an Au<L type electrode by a vapor deposition method. Since the polymer electrolyte in step 2 is in a liquid state, it is placed in a box-shaped measurement cell, and gold (
Au) plates were placed on both sides to serve as electrodes, the AC impedance between the electrodes was measured, and complex impedance analysis (
Cole-Cole plot) was performed at room temperature (25
The ionic conductivity at (°C) was determined. The results are shown in Table 2.

Table 2 Also, the results of measuring the ionic conductivity under various temperature conditions in the same manner as above are shown in FIG. Second
In the figure, the vertical axis is ionic conductivity (S/cm),
The horizontal axis is the reciprocal of absolute temperature 10”/T (-1),
Also, curve 2a is the result of Example 1, and curve 2b is the result of Example 2.
As a result, curve 2c is the result of Example 3, curve 2d is the result of Example 4, curve 2e is the result of Comparative Example 1, and curve 2f is the result of Comparative Example 2.

<Discharge characteristic test) Total thickness of 0.5-
A square thin lithium battery with m+ and - side lengths of 15 mm was produced.

The negative electrode was an alloy of lithium and aluminum, and the positive electrode was a sheet-shaped molded product containing an electrolyte with the same components as the polymer electrolytes of Examples 1 to 4 and Comparative Examples 1 to 2 and titanium disulfide (TIS*). Using. The discharge characteristics of these lithium batteries when discharged at a constant current of 200 μA at 25° C. are shown in FIG. In FIG. 3, curve 3a is the result of Example 1, curve 3b is the result of Example 2,
Curve 3c is the result of Example 3, curve 3d is the result of Example 4, and curve 3e is the result of Comparative Example 1. In addition, in the case of the polymer electrolyte of Comparative Example 2, since the polymer electrolyte was in a liquid state, the positive electrode and the negative electrode were stuck together and the battery could not be formed.

As is clear from the above test results, the polymer electrolytes of Examples 1 to 4 according to the present invention have a 1.0×10
The polymer electrolyte of Comparative Example 1, whose polymer component was polyethylene oxide with a number average molecular weight of 600,000, had an ionic conductivity of 1.0 to 3.0X10-''S/cm at room temperature. 0X10-”S/C11
The ionic conductivity was lower than that of the polymer electrolytes of Examples 1 to 4 according to the present invention.

Furthermore, as is clear from the results shown in Figure 3, the lithium batteries using the polymer electrolytes of Examples 1 to 4 according to the present invention have a high utilization rate of titanium disulfide, which is the active material, and have excellent discharge characteristics. However, the lithium battery using the polymer electrolyte of Comparative Example 1 was inferior in the above characteristics.

In addition, ethylene oxide with a number average molecular weight of 3.000
The propylene oxide copolymer was not crosslinked (the polymer electrolyte of Comparative Example 2 used as a polymer component had excellent ionic conductivity at room temperature because it was liquid, but it could not be used as it was as a battery electrolyte). 'A separate separator was required.

[Brief explanation of the drawing]

FIG. 1 is a vertical cross-sectional view showing an example of a lithium battery using the lithium ion conductive polymer electrolyte of the present invention, and FIG. Diagram showing relationships, 3rd
The figure is a discharge characteristic diagram of a lithium battery having the configuration shown in FIG. 1 using a lithium ion conductive polymer electrolyte of the present invention and a comparative lithium ion conductive polymer electrolyte. 7... Separator (polymer electrolyte) Patent applicant Hitachi Maxell, Ltd. Figure 1 Figure 2 Figure 103/T (K-') Figure 3 Utilization rate of titanium disulfide (%)

Claims (4)

[Claims] (1) In a lithium ion conductive polymer electrolyte consisting of a composite of a lithium salt and an organic polymer, the above organic polymer is reacted with an ethylene oxide-propylene oxide copolymer, an organic substance having bifunctionality, and glycerin. A lithium ion conductive polymer electrolyte characterized in that it is a polymer. (2) The lithium ion conductive polymer electrolyte according to claim 1, wherein the ethylene oxide-propylene oxide copolymer has a number average molecular weight of 200 to 10,000. (3) The lithium ion conductive polymer electrolyte according to claim 1, wherein the ethylene oxide-propylene oxide copolymer is a copolymer of 20 to 65% by weight of ethylene oxide and 80 to 35% by weight of propylene oxide. (4) The lithium ion according to claim 1, wherein the bifunctional organic substance is at least one selected from the group consisting of diisocyanates, dicarboxylic acids, diamines, dicarboxylic acid chlorides, methylol compounds, and epichlorohydrin. Conductive polymer electrolyte.