JPS62219468A - Lithium ion conductive gelled electrolyte - Google Patents

Abstract

PURPOSE:To suppress the drop in the performance of a battery, by using a high-molecular polymer having a soluble portion and an insoluble portion as to a nonaqueous solvent, as a gelation-causing agent, to improve the ion conductivity and chemical stability of the battery. CONSTITUTION:A high-molecular polymer, which is used as a gelation-causing agent, is generally a copolymer of a monomer whose homopolymer is soluble to a nonaqueous solvent and another monomer whose homopolymer is insoluble thereto. For example, the copolymer is composed of a monomer represented by a formula I and another monomer represented by a formula II, when a high-boiling-point solvent such as propylene carbonate and gamma-type butylolactone is used as the nonaqueous solvent for the electrolyte of a lithium battery. The structural portion of the copolymer, which results from the monomer represented by the formula I, is soluble to the high-boiling-point solvent, while the structural portion of the copolymer, which results from the monomer represented by the formula II, is insoluble thereto.

Description

[Detailed Description of the Invention] [Industrial Application Field] This invention relates to a non-aqueous electrolyte used in lithium batteries, etc., particularly a lithium ion electrolyte consisting of a lithium salt, a non-aqueous solvent, and a polymer as a gelling agent. It relates to conductive gel electrolytes.

[Conventional technology]

Generally, a lithium battery has a battery element consisting of a positive electrode, a negative electrode made of lithium or a lithium alloy, and a separator and a non-aqueous electrolyte interposed between these two electrodes, between a positive current collector plate and a negative current collector plate. It has a similar structure. As the non-aqueous electrolyte, a highly fluid liquid electrolyte in which a lithium salt is dissolved in a high-boiling non-aqueous solvent such as propylene carbonate or γ-butyrolactone is widely used (documents unknown).

However, in recent years, electronic devices have become smaller.

As devices become lighter and thinner, there is a demand for lithium batteries used in these devices to be compact, extremely thin, and high-performance with a total thickness of about 0.5 m+.

In such a thin battery, the positive and negative current collector plates must have a very flat shape, and the sealing means that crimp and bend the periphery of the current collector plate with a backing material in between, as in the original battery, is not necessary. Since it is difficult to adopt this method due to structural and processing technology constraints, it is necessary to use a method of sealing the opposing flat peripheral portions of the bipolar current collector plates with an adhesive.

Therefore, if the above-mentioned highly fluid liquid electrolyte is used in such a thin battery, the electrolyte will easily leak out during battery assembly, resulting in insufficient electrolyte amount or the sealing part getting wet, resulting in incomplete sealing. In addition, if the adhesive sealing material is heat-sealable, the electrolyte tends to scatter during heat sealing, and leakage is likely to occur even when the battery is in use, and in the case of secondary batteries, the negative electrode lithium Short circuits occur due to dendrite-like (resin-like) precipitation, which tends to shorten the service life.

Therefore, in order to avoid the problems caused by the high fluidity of the electrolyte as described above, it is considered effective to impart viscosity to the electrolyte to reduce its fluidity. As an electrolyte having such viscosity, a gel electrolyte using polymethyl methacrylate as a gelling agent is conventionally known (Japanese Patent Application Laid-open No. 104541/1983). For example, LiBF4 is mixed with γ-butyrolactone. By adding polymethyl methacrylate to the dissolved solution, a transparent gel-like electrolyte in which the polymer is completely dissolved can be obtained.

[Problems to be Solved by the Invention] However, as described above, the conventional gel electrolyte using polymethyl methacrylate as a gelling agent has poor ionic conductivity compared to a highly fluid electrolyte that does not use a gelling agent. The chemical stability is also insufficient. Also,
This gel-like electrolyte has the advantage that it can be added to the battery in the form of impregnating it into a separator made of porous material such as non-woven fabric using its viscosity when manufacturing the battery, but if force is applied to the separator from the outside. In some cases, the separator is pushed out to the surface, causing a shortage of electrolyte inside the separator and deteriorating battery characteristics.

[Means for Solving the Problems] As a result of intensive studies to solve the above-mentioned conventional problems, the inventors found that the conventional gel electrolyte using polymethyl methacrylate as a gelling agent It has been found that the reason for the poor conductivity and chemical stability is that the gelling agent has a certain degree of interaction with the lithium salt. Further, in continuing research based on the above findings, the inventors have found that when a specific high molecular weight polymer is used as a gelling agent instead of polymethyl methacrylate, it has good ionic conductivity and chemical stability. Moreover, when this gel electrolyte is added to a separator made of porous material in the form of impregnation during assembly of a lithium battery, the gel electrolyte does not remain on the surface even if force is applied to the separator from the outside. The inventors have discovered that the separator is less likely to be pushed out to the side, and that deterioration in performance due to lack of electrolyte inside the separator is suppressed, leading to the creation of this invention.

That is, the present invention provides a lithium ion conductive gel electrolyte comprising a lithium salt, a non-aqueous solvent, and a polymer as a gelling agent, in which the polymer has a soluble portion and an insoluble portion in the non-aqueous solvent. The present invention relates to a lithium ion conductive gel electrolyte characterized by:

[Structure and operation of the invention]

The high molecular weight polymer used as a gelling agent in the gel electrolyte of the present invention has a soluble portion and an insoluble portion in the non-aqueous solvent used as described above, and is a solution in which a lithium salt is dissolved in the non-aqueous solvent. When added to the electrolyte, it is not completely dissolved due to the presence of the insoluble portion, but becomes uniformly dispersed without layer separation due to the action of the soluble portion, and usually provides a gel electrolyte consisting of a white opaque homogeneous body.

The reason why such a gel electrolyte exhibits better ionic conductivity and chemical stability than the conventional gel electrolyte using polymethyl methacrylate as a gelling agent is not clear, but the polymethacrylic acid Because methyl is soluble in non-aqueous solvents such as propylene carbonate and γ-butyrolactone that are commonly used in this type of electrolyte, the entire polymer of the gelling agent interacts with the lithium salt in the electrolyte to increase ionic conductivity. On the other hand, in the high molecular weight gelling agent used in this invention, the insoluble part does not interact with the lithium salt.
It is presumed that the adverse effects on ionic conductivity and chemical stability are reduced accordingly.

Furthermore, when the gel electrolyte of the present invention is impregnated into a porous material such as a nonwoven fabric, the insoluble part of the high molecular weight polymer serving as the gelling agent functions as a support for the liquid phase, and the microstructure of the porous material Because the electrolyte is mechanically entangled within the battery, even if an external force is applied to the separator made of the above-mentioned material impregnated with gel electrolyte during battery manufacture or during installation into the battery, the electrolyte will be pushed out to the surface. The amount is small, and it is thought that an increase in resistance due to electrolyte shortage inside the separator is less likely to occur, and deterioration in battery performance is suppressed.

As the above-mentioned high molecular weight polymer used as the gelling agent in this invention, there are suitable polymers depending on the type of non-aqueous solvent used, but in general, homopolymers thereof are soluble in the above-mentioned non-aqueous solvent. A copolymer with a heptamer, which does not dissolve like the monomer, is preferred. Note that this copolymer may be either a block copolymer or a random copolymer.

Preferred specific examples of the above copolymer include the following general formula (I); CH2 =C-COOR2・
・(I) (R is hydrogen or methyl group, R2 is methyl group or ethyl group) and the following general formula % formula % () (R3 is hydrogen or methyl group, R4 is usually 3 or more carbon atoms) Examples include copolymers with heptamers having up to 15 alkyl groups). That is, in this copolymer, the structural part based on the monomer of the above general formula (I) becomes a soluble part in the high boiling point solvent, and the structural part based on the monomer of the above general formula (II) also becomes an insoluble part.

The number average molecular weight of the above-mentioned gelling agent polymer is preferably about 5,000 to 100,000.

Moreover, the ratio of soluble part to insoluble part in the non-aqueous solvent is 110.1 to L10.
A value of about 7 is preferable; if the soluble portion is too large, problems similar to those of the conventional gel electrolyte described above will occur; conversely, if the insoluble portion is too large, the original gelling function will be insufficient and uniform dispersion in the electrolyte will not be achieved. The condition becomes less likely to occur. Furthermore, in order to fully exhibit the gelling effect of this high molecular weight polymer, it is generally necessary to use an amount of 5 to 50 parts by weight per 100 parts by weight of the non-aqueous solvent, although there are some differences depending on the type of non-aqueous solvent. It is best to set the ratio to about 100%.

As the lithium salt, which is one of the constituent components of the gel electrolyte of the present invention, various substances conventionally known as electrolyte components for lithium batteries etc. can be used, but a particularly preferred one is LiBφ4 (φ is phenyl ), LiBF4%LiPF6, LiCF3SO3, L
Examples include iAsF6, which can also be used in the form of an adduct of a non-aqueous solvent, or two or more types can be used in combination. The amount of these lithium salts used is preferably such that the concentration in the electrolyte is about 0.3 to 2 mol/l.

The non-aqueous solvent is one that does not react with the lithium salt, can dissolve the lithium salt, partially dissolves the gelling agent polymer, and has the property of being mixed with the gelling agent to form a gel. Various types of materials can be used as long as they have the following properties. Typical examples include propylene carbonate, γ
-In addition to butyrolactone, examples include diphthoxyethane and dioxirane, and these can be used alone or in a mixture of two or more. In particular, when the gelling agent is a copolymer of heptamers of general formulas (I) and (II) described above, propylene carbonate and γ-butyrolactone may be used alone or as a mixed solvent, and dimethoxyethane using these as the main solvent may be used. A mixed solvent containing a small amount of dimethoxyethane is optimal, and the latter mixed with a small amount of dimethoxyethane is effective for improving battery characteristics for lithium batteries.

The gel electrolyte of the present invention can be used not only as an electrolyte for lithium batteries, but also as a lithium ion conductive electrolyte used in electrochromic display elements and other various electrochemical devices in which lithium ions are the conductive ion species. However, it is particularly suitable for thin lithium batteries that have a structure in which the opposing flat peripheral parts of the positive and negative current collector plates are adhesively sealed.

FIG. 1 shows an example of the above-mentioned thin lithium battery. In the figure, 1 is a rectangular flat positive electrode current collector plate made of stainless steel, and 2 is a stainless steel plate whose periphery is bent in steps toward one side to provide a flat periphery 2a in the same direction as the main surface. 3 is a bipolar current collector plate 1,
an adhesive layer that sealed between the opposing peripheral parts 1a and 2a of 2;
4 is a positive electrode arranged on the positive electrode current collector plate 1 side in the space 5 formed between the two electrode current collector plates 1 and 2, and 6 is a lithium or lithium alloy loaded in the space 5 on the negative electrode current collector plate 2 side. 7 is a separator made of a porous material such as porous polypropylene interposed between the two electrodes 4 and 6;
Reference numeral 8 denotes a rectangular ring-shaped frame made of polypropylene or the like and disposed so as to surround the positive electrode 4 .

In this case, the above-mentioned gel electrolyte of the present invention is usually added to the interior of the battery by coating and impregnating the separator 7 in advance before assembly.

At this time, the gel electrolyte will not flow out to the surrounding area even if the assembly base is slightly inclined or vibration is applied.
There is no risk of dripping when the separator 7 is transported from one position to the installation position, and the amount added can be adjusted over a wide range.

On the other hand, as the adhesive layer 3, a general paint solution type adhesive can also be used, but one made of a heat-fusible material is particularly suitable. The heat-fusible material 3 used is a molded sheet, such as an annular sheet, whose form before heat-sealing is a preset width corresponding to the width of the peripheral parts 1a and 2a of the bipolar current collector plates 1 and 2. can. That is, the sealing operation is performed on the same peripheral portion 1a.
, 2a, and press the molded sheet between them, and in this state, both local portions 1a and 2a are heated to a predetermined temperature.

In this heating process, as described above, the gel electrolyte does not scatter unlike electrolytes made of conventional general-purpose highly fluid liquids, and reliable sealing can be easily achieved.

Furthermore, as mentioned above, since the form before heat fusion is a solid molded product, handling and assembly are very easy, and the space 5 can be easily handled and assembled, unlike when using a paint solution type adhesive. There is no risk of it flowing into the electrolyte and mixing with the gel electrolyte.

Note that various materials can be used as the heat-fusible material 3, including a hot-melt adhesive and a hermetically sealable ceramic.

Further, as the positive electrode 4, a sheet formed by mixing an active material, a binder such as Teflon powder, and, if necessary, an electron conduction aid such as carbonyl nickel may be used. A viscous material obtained by kneading a gel electrolyte with an active material and, if necessary, a conductive additive, can be suitably used. In other words, the latter viscous material can be applied and formed on the positive electrode current collector plate 1 by screen printing or squeegee coating, so the forming process like the former is unnecessary, and the forming operation is extremely simple and costs can be reduced. In addition, since it can be easily made into a thin layer, it has excellent applicability to thin batteries.

The frame 8 has a function of setting the amount of the viscous material to be applied when the viscous material is used as the positive electrode 4. That is, by placing the frame 8 on the positive electrode current collector plate 1 in advance and filling the inside thereof with the viscous substance, the coating amount becomes constant.

As the active material used for the positive electrode 4, various materials conventionally known as positive electrode active materials for lithium batteries can be used, but particularly preferred ones include TiS2, MoS,...
, Va013, v205, VSe2, and N1PS3, and two or more of these may be used in combination.

Furthermore, although both lithium and lithium alloys can be used as the negative electrode 6, lithium alone may react with the gel electrolyte over a long period of time, so it is not recommended to alloy it with aluminum or the like. desirable.

In addition, in addition to making one of the current collector plates in such a thin lithium battery dish-shaped as shown in Fig. 1,
Both may be dish-shaped, or both may be plate-shaped and a spacer may be interposed between the peripheral portions. The total thickness of the battery is 1.0 mm or less, preferably 0.3 to 0.7 m.
That's about it.

[Effect of the invention] The lithium ion conductive gel electrolyte according to the present invention has the following features:
Because it uses a high-molecular polymer that has a soluble part and an insoluble part in non-aqueous solvents as a gelling agent, it has superior ionic conductivity and chemical stability compared to conventional gel electrolytes. When added to a battery as an electrolyte in a form impregnated into a separator made of a porous material, it is difficult to be pushed out to the surface even if force is applied to the separator from the outside, and the insoluble portion of the high polymer is in the liquid phase. Since the separator functions as a holder, it has the advantage of suppressing deterioration in battery performance due to electrolyte shortage inside the separator.

[Example] Examples and comparative examples of the present invention are shown below.

Example 1 A copolymer of methyl methacrylate and 2-ethylhexyl acrylate in a molar ratio of 82:18 (number average molecular weight of about 50,000 ) was added and mixed, and when the mixture was heated at 100° C. for 3 hours, a white, opaque, and uniform gel-like electrolyte was obtained. When 10y of this gel electrolyte was collected and centrifuged for 5 minutes at a rotation speed of 3,500 times/min using a centrifuge (model H-108NA2, manufactured by Kokusan Centrifuge Co., Ltd.), no layer was formed. No separation was observed.

Example 2 The copolymer of Example 1 was replaced with methyl methacrylate, ethyl methacrylate, and butyl methacrylate at a molar ratio of 1.2.
A white, opaque, and uniform gel electrolyte was obtained in the same manner as in Example 1, except that 40 parts by weight of a copolymer of : 67.1 : 31.7 (number average molecular weight approximately 30,000) was used. Even when this gel electrolyte was subjected to the same centrifugation treatment as in Example 1, no layer separation was observed.

Example 3 Except that 40 parts by weight of a copolymer of methyl acrylate and butyl methacrylate in a molar ratio of 70:30 (number average molecular weight about 50,000) was used in place of the copolymer of Example 1.
A white, opaque, and uniform gel electrolyte was obtained in the same manner as in Example 1. Even when this gel electrolyte was subjected to the same centrifugation treatment as in Example 1, no layer separation was observed.

Comparative Example Polymethyl methacrylate (
A gel electrolyte was obtained in the same manner as in Example 1, except for using 40 parts by weight of a gel having a number average molecular weight of approximately 20,000. This gel electrolyte was a colorless and transparent homogeneous body.

Regarding the gel electrolytes of the above examples and comparative examples, 2
When the ionic conductivity at 0° C. was measured, the results shown in Table 1 below were obtained.

Table 1 Next, about 100 q/c of the gel electrolytes of Examples and Comparative Examples were each applied to a 100 μ thick polypropylene nonwoven fabric.
It was coated and impregnated at a ratio of ++f, and 10 diameter and 0.1 mm thick lithium metal plates were attached to both sides of this nonwoven fabric, and 100P and 500Y were applied between both metal plates.

I after applying a pressure of IKg and releasing the pressure respectively
When the AC resistance at KHz was measured, the results shown in the following table were obtained.

Table 2 Furthermore, a thin lithium battery having the structure shown in FIG. 1 was manufactured using the gel electrolytes of the above Examples and Comparative Examples by the following method.

First, gel electrolyte and TiS2 powder are kneaded at a weight ratio of 1:1, and this mixed material is screen-printed with 15 mm on each side.
The coating was applied to the surface of a positive electrode current collector plate consisting of a stainless steel flat plate of m+++ square and 0.1 trtt thick, so that it filled the inside of a rectangular frame made of polypropylene placed on top of the positive electrode collector plate, and the negative side was 10 m. A positive electrode with a square shape and a thickness of 0.1 mm was formed. On this positive electrode, a gel electrolyte was applied to a separator made of polypropylene non-woven fabric with a thickness of 100P (product name: Duraguard 5411, manufactured by Polyplastics Co., Ltd.) for about 11 hours.
The separators are coated at a ratio of 0ff1/ffl to be uniformly impregnated, and then lithium is applied on top of this separator.
A negative electrode made of aluminum alloy and having a thickness of 80 μm and consisting of 4 square foils on each side was laminated.

Next, a layer with a thickness of 60 μm was placed on the periphery of the positive electrode current collector plate.

A rectangular annular sheet with a width of 2 rIm with a modified polyolefin hot melt adhesive placed on it weighs 15 tons on each side.
A negative electrode current collector plate made of a dish-shaped stainless steel plate with a square size of s and a thickness of 0.1 tm was covered with a negative electrode current collector plate, and the peripheral part of the current collector plate was heated to 185°C under pressure to seal by heat fusion. Battery total thickness 0.5
We fabricated a thin lithium battery between the two.

For each thin lithium battery thus produced, 25°
When the 200fiA constant current discharge characteristics of C were investigated, the results shown in FIG. 2 were obtained. In addition, curve A1 in the figure
is Example 1, curve A2 is Example 2, and curve A3 is Example 3.
, Curve B shows the characteristics of batteries using gel electrolytes of comparative examples.

From the results in Tables 1 and 2 above, the gel electrolytes of the present invention (Examples 1 to 3) have superior ionic conductivity and porous properties compared to conventional gel electrolytes (comparative examples). When impregnated into a material, it is difficult to be pushed out even if pressure is applied to the material, and is sufficiently retained inside to suppress an increase in internal resistance. Therefore, it is impregnated into a separator made of porous material during battery production. It is clear that it is suitable for addition in form. Furthermore, from the results shown in FIG.
, A3) has better discharge characteristics than the conventional battery using a gel electrolyte (curve B).

[Brief explanation of drawings]

Figure 1 is a cross-sectional view showing an example of the structure of a thin lithium battery;
The figure is a constant current discharge characteristic diagram of thin lithium batteries using gel electrolytes of Examples and Comparative Examples. Figure 1 Discharge time (hours)

Claims (2)

[Claims] (1) In a lithium ion conductive gel electrolyte consisting of a lithium salt, a nonaqueous solvent, and a polymer as a gelling agent, the polymer has a soluble portion and an insoluble portion in the nonaqueous solvent. Features a lithium ion conductive gel electrolyte. (2) The high molecular weight polymer that is the gelling agent has the following general formula (
I); ▲There are mathematical formulas, chemical formulas, tables, etc.▼...(I) The monomer represented by (R_1 is a hydrogen atom or methyl group, R_2 is a methyl group or ethyl group) and the following general formula (
II); ▲There are mathematical formulas, chemical formulas, tables, etc.▼...(II) (R_3 is a hydrogen atom or methyl group, R_4 is a carbon number of 3
The lithium ion conductive gel electrolyte according to claim 1, which is a copolymer with a monomer represented by the above alkyl group.