JPS63136407A - Lithium ion conducting polymer electrolyte - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a lithium ion conductive polymer electrolyte used for electrolytes such as lithium batteries and electrochromic displays, lithium ion concentration sensors, and lithium ion separation membranes. .

[Conventional technology]

As lithium ion conductive electrolytes for lithium batteries, liquid electrolytes such as LiC104-propylene carbonate and solid electrolytes such as Li3N and Li1-Al oz are known, but recently Attempts have been made to use polymer electrolytes based on organic polymers that can be easily formed into flexible films.

This type of polymer electrolyte has the structural advantages of being relatively easy to make thinner and larger in area than liquid electrolytes and solid electrolytes, and has excellent flexibility. It is attracting particular attention as an electrolyte for lithium batteries because it also has the advantage of excellent chemical stability against Li at the Li/electrolyte interface, which tends to cause problems in terms of performance. It is also expected to be used as an electrolyte in electrochromic displays. Furthermore, there are attempts to apply this type of polymer electrolyte to lithium ion concentration sensors and lithium ion separation membranes.

Conventionally, one known polymer electrolyte is a composite of polyethylene oxide as an organic polymer and lithium salt (F).
ast (on Transport in
Soj! id P131 (1979)].

[Problem that the invention seeks to solve]

However, although the above-mentioned conventional polymer electrolyte exhibits relatively good lithium ion conductivity at high temperatures of 60°C or higher, it is considerably inferior to the above characteristics at room temperature of about 25°C, and most lithium ion conductivity is used at room temperature. When applied to batteries and various other uses such as those mentioned above, there was a problem in that the performance was not fully satisfactory.

Therefore, the present invention provides a polymer that exhibits good lithium ion conductivity even at room temperature and can be suitably used for lithium batteries and various other applications as described above, by using a specific polymer different from the conventional organic polymers mentioned above. Its purpose is to provide electrolytes.

[Means for Solving the Problems] As a result of intensive studies to achieve the above object, the present inventors have developed a combination of ethylene oxide and propylene oxide in a specific ratio as a stepped polymer constituting the polymer electrolyte. He completed this invention after learning that when a polymer was used, it was possible to obtain a polymer electrolyte that had excellent lithium ion conductivity, and in particular, showed good ion conductivity even at room temperature and could be applied to a variety of applications. I ended up doing it.

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 is ethylene oxide 20 to 20%.
The present invention relates to a lithium ion conductive polymer electrolyte characterized by comprising a copolymer of 65% by weight of propylene oxide and 80 to 35% by weight of propylene oxide.

[Structure and operation of the invention]

The organic polymer used in this invention is a block or random copolymer of ethylene oxide and propylene oxide, with a copolymerization ratio of 20 to 65% by weight of ethylene oxide and 80 to 35% by weight of propylene oxide. is used. In particular, a suitable copolymerization ratio is 20 to 45% by weight of ethylene oxide and 80 to 55% by weight of propylene oxide. The number average molecular weight of this copolymer is 3°00.
It is preferably 0 or more, and usually in the range of 10.000 to soo, ooo.

The reason why the use of such a copolymer significantly improves lithium ion conductivity at room temperature compared to the use of conventional polyethylene oxide, or ethylene oxide homopolymer (hereinafter referred to as PEO). Although it is not necessarily clear, it can be considered as follows. First, like conventional PEO, the above copolymer has an ether bond with a high dielectric constant in its main chain, so a complex is formed between oxygen and lithium salt that constitute this ether bond. This imparts lithium ion conductivity.

Conventional PEO is a crystalline polymer with a melting point around 60°C, so it exhibits very good lithium ion conductivity above the above temperature, but at temperatures lower than 60°C. lithium ion conductivity tends to decrease rapidly. On the other hand, since the copolymer according to the present invention has a methyl group based on propylene oxide in its side chain, the crystallinity of the polymer decreases due to steric hindrance, and the fecal PEO It is thought that good lithium ion conductivity can be achieved even at lower temperatures.

The above-mentioned effect of improving lithium ion conductivity at low temperatures becomes more pronounced as the copolymerization ratio of propylene oxide increases, but if the copolymerization ratio of propylene oxide becomes too high, for example, in an extreme case, a homopolymer of propylene oxide, that is, polypropylene When it comes to oxide (hereinafter referred to as PPO), the lithium ion conductivity itself is not so high, so the above-mentioned improvement effect is reduced. That is, the setting of the copolymerization ratio of ethylene oxide and propylene oxide described above is based on the above reasons,
By setting such a specific range, conventional PE
The lithium ion conductivity at room temperature can be made very good compared to not only O but also the above-mentioned PPO.

In this invention, any of the lithium salts used in conventional polymer electrolytes can be used as the lithium salt constituting the polymer electrolyte together with the above copolymer.
Specific examples include LiBr, LiI, L 1scN,
, L 1BF4, L 1AsF b , L i C10
a , L I CF s S Owl , L x Ch
F+3SOs, LicF:+COz, Lf
Examples include Hg Is. The amount of this lithium salt used is usually 0.02 to 0.2 mol, particularly preferably 0.05 to 0.2 mol, per mol of the mixed monomer of ethylene oxide and propylene oxide constituting the copolymer.
It is preferably in the range of 1 mole.

The polymer electrolyte of the present invention is a composite of the above-mentioned copolymer and lithium salt, but the general method for obtaining this composite is to dissolve both of the above-mentioned components in an appropriate organic solvent. There is a method in which the organic solvent is removed by volatilization after preparing a solution.

In this method, the lithium salt is bonded to the molecular chains of the copolymer in a complex manner in the solution state, and a complex is obtained in which the bond is maintained even after the solvent is removed.

The form of this composite is determined as appropriate depending on the purpose of use and the properties of the copolymer. For example, when using a polymer electrolyte for lithium batteries to serve as a separator between the positive and negative electrodes, a sheet-like material consisting of the above composite alone or a sheet-like material containing a filler such as alumina may be used. Alternatively, the above-mentioned composite may be made into a sheet-like product held on a support such as a non-woven fabric. All of these sheet-like materials have flexibility.

Among the above-mentioned sheet-like products, those made of the composite alone can be obtained by casting the solution to an appropriate thickness and then removing the solvent by volatilization. Further, a sheet-like material containing a filler can be obtained in the same manner as described above except that the filler is mixed into the solution. Further, a sheet-like article with a support can be obtained by impregnating the support with the solution and then removing the solvent. The latter two sheet-like materials are generally employed when the copolymer is semi-solid and has poor shape retention.

In addition, when applied to a positive electrode in a lithium battery, the composite material can be formed by adding a positive electrode active material or the like to the solution in a predetermined ratio, molding the solution, and then removing the solvent by volatilization, or by removing the solvent by volatilization before molding and then molding. It is possible to form a molded positive electrode in which the body, positive electrode active material, etc. are mixed and integrated.

Similarly, for other uses such as lithium ion separation membranes, various forms can be taken according to the respective uses in accordance with the above.

The organic solvent used to obtain such a complex is
There is no particular limitation as long as the copolymer and lithium salt can be dissolved and finally removed by volatilization. However, it is desirable to select an appropriate solvent depending on the purpose of use of the composite so that residual trace components do not cause deterioration of battery performance.

For example, in polymer electrolytes for lithium batteries, solvents such as dimethoxyethane, dioxolane, acetonitrile, and dimethylformamide are preferably used as the solvent.

FIG. 1 shows an example of a lithium battery using the polymer electrolyte of the present invention described above. In the figure, 1 is a rectangular flat positive electrode current collector plate made of stainless steel; A flat peripheral portion 2a that is bent into a shape and facing the same direction as the main surface.
3 is a shallow rectangular dish-shaped negative electrode current collector plate made of stainless steel provided with a bipolar current collector plate 1. 2 facing peripheral parts 1a, 2
．． This is an adhesive layer that seals the space between a.

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
This is a separator formed by forming the polymer electrolyte of the present invention, which is interposed between the two electrodes 4.6, alone or together with a filler, a nonwoven fabric, etc. into a sheet shape.

The positive electrode 4 may be formed into a sheet by mixing a positive electrode active material with a binder such as Teflon powder or an electron conduction aid, as the case may be. As the positive electrode active material used for the positive electrode 4, TiS2, Mo Sz, V60
One or more of +z, V 20 s , V S e 5NiPs3, etc. is used.

The lithium battery constructed in this manner has the advantage that the separator 7 is a sheet-like material made of the above polymer electrolyte, and the positive electrode 4 is a similar sheet-like material containing the above-mentioned electrolyte. In addition to having various advantages such as being able to contribute to improvements in workability and reliability of sealing, and essentially not having to worry about leakage unlike liquid electrolytes, the above-mentioned 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, by using a copolymer of ethylene oxide and propylene oxide in a specific ratio as an organic polymer for forming a complex with a lithium salt, lithium ions can be absorbed at room temperature. It is possible to provide a lithium ion conductive polymer electrolyte that has excellent conductivity and can be advantageously 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 Copolymer of 24.8% by weight of ethylene oxide and 75.2% by weight of propylene oxide (number average molecular weight 30.
000) 4g and L i CFil SO:l 0.
778 g was dissolved in 5 ml of acetonitrile and stirred uniformly with a Stark stirrer. Next, this solution was dropped onto a glass substrate, left for 5 hours in an argon gas stream under normal pressure, and then vacuumed at a vacuum level of I x 10''Torr and a temperature of 130mm.
Heat-treated at °C for 10 hours to remove acetonitrile by evaporation,
A polymer electrolyte with a thickness of 20 μm was obtained.

Example 2 Copolymer of 15% by weight of ethylene oxide and 85% by weight of propylene oxide (number average molecular weight 30.000)
A polymer electrolyte was obtained in the same manner as in Example 1, except that 4 g of L i CF3 SO3 and 744 g of L i CF3 SO3 were used.

Example 3 Copolymer of 44.4% by weight of ethylene oxide and 55.6% by weight of propylene oxide (number average molecular weight 30.
000) 4g and L i CF2 SC) + 0.808
A polymer electrolyte was obtained in the same manner as in Example 1 except that G was used.

Comparative example l PE0 (number average molecule 1600.000) Ig and Li
Example 1 except that CF3SO30,236g was used.
A polymer electrolyte was obtained in the same manner as above.

Comparative Example 2 PPO (number average molecule 11,500,000) Ig and Li
Except for using CFz SOs O,7L 7 g.
A polymer electrolyte was obtained in the same manner as in Example 1.

Comparative Example 3 Copolymer of 70% by weight of ethylene oxide and 30% by weight of propylene oxide (number average molecular weight 20, i) 00
A polymer electrolyte was obtained in the same manner as in Example 1 except that 4 g of ) and 864 g of Li CF:l SO3 were used.

In order to investigate the performance of the polymer electrolytes according to Examples 1 to 3 and Comparative Examples 1 to 3, the following ionic conductivity test and discharge characteristic test were conducted.

<Ionic conductivity test> Au comb-shaped electrodes were formed by vapor deposition on each polymer electrolyte according to Examples 1 to 3 and Comparative Examples 1 to 3, AC impedance between the electrodes was measured, and complex impedance analysis (
Co 1 e-Co I e plot) was performed to measure the ionic conductivity at room temperature (25° C.). The results were as shown in the table below.

Further, the ionic conductivity was measured under various temperature conditions in the same manner as above, and the results were as shown in FIG. In the figure, the vertical axis is ionic conductivity (SzcI11), and the horizontal axis (
The reciprocal of the absolute temperature (1/T) xl, 000 (K-'), and the straight line -2a is the result of Example 1, the line 2b is the result of Example 2, the line 2C is the result of Example 3, 2d is the result of Comparative Example 1, 2e is the result of Comparative Example 2, and 2f is the result of Comparative Example 3.
This is the result.

<Discharge characteristic test> The total thickness of the structure shown in FIG. 1 using each polymer electrolyte according to Examples 1 to 3 and Comparative Examples 1 to 3 as a separator was 0.
．． A thin rectangular lithium battery with a length of 5 m and a side length of 15 mm was fabricated. An alloy of lithium and aluminum was used as the negative electrode, and a sheet-shaped molded product containing T i Sz and an electrolyte having the same components as the polymer electrolytes of Examples 1 to 3 and Comparative Examples 1 to 3 was used as the positive electrode. The discharge characteristics of these lithium batteries when subjected to constant current discharge of 270 IIA at 25° C. are shown in FIG. In the figure, curve 3a is the result of Example 1, curve 3b is the result of Example 2,
3C is the result of Example 3, 3D is the result of Comparative Example 1, 3e is the result of Comparative Example 2, and 3F is the result of Comparative Example 3.

As is clear from the above test results, the polymer electrolytes of Examples 1 to 3 according to the present invention have approximately 0.2 ~4
It can be seen that, while a high ionic conductivity of about X10-'S/cm is obtained, the polymer electrolytes of Comparative Examples 1 to 3 are inferior to the above characteristics at around room temperature. For this reason, as is clear from the results in FIG. 3, the lithium batteries using the polymer electrolytes of Examples 1 to 3 according to the present invention exhibit excellent discharge characteristics, but the polymer electrolytes of Comparative Examples 1 to 3 Lithium batteries using electrolytes are considerably inferior to the above characteristics.

[Brief explanation of the drawing]

Figure 1 is a longitudinal cross-sectional view showing an example of a lithium battery using the lithium ion conductive polymer electrolyte of the present invention, and Figure 2 is the correlation between ionic conductivity and temperature of the lithium ion conductive polymer electrolyte of the present invention and a comparative lithium ion conductive polymer electrolyte. 3 are discharge characteristic diagrams of lithium batteries having the configuration shown in FIG. 1 using the lithium ion conductive polymer electrolyte of the present invention and a comparative lithium ion conductive polymer electrolyte. 7...Polymer electrolyte (separator) patent applicant
Hitachi Maxell, Ltd.'' 1st Figure 1/T X100O(K')

Claims (1)

[Claims] (1) In a lithium ion conductive polymer electrolyte made of a composite of a lithium salt and an organic polymer, the above organic polymer is made of a copolymer of 20 to 65% by weight of ethylene oxide and 80 to 35% by weight of propylene oxide. Features a lithium ion conductive polymer electrolyte.