JPH0732022B2 - Lithium ion conductive gel electrolyte - Google Patents

Description

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte used in a lithium battery or the like, particularly a lithium ion composed of a lithium salt, a non-aqueous solvent, and a polymer as a gelling agent. The present invention relates to a conductive gel electrolyte.

[Conventional technology]

In general, a lithium battery encloses, between a positive electrode current collector plate and a negative electrode current collector plate, a battery element composed of a positive electrode, a negative electrode made of lithium or a lithium alloy, a separator interposed between these electrodes, and a non-aqueous electrolyte. It has a structure. As the non-aqueous electrolyte, a high-fluidity liquid electrolyte in which a lithium salt is dissolved in a high-boiling-point non-aqueous solvent such as propylene carbonate or γ-butyrolactone is widely used (literature unknown).

However, in recent years, electronic devices have become smaller and lighter,
Along with the reduction in thickness, there is a demand for a lithium battery to be used in this, which is small and has a total thickness of about 0.5 mm and is extremely thin and has high performance. In such a thin battery,
There is no choice but to make the positive and negative electrode current collector plates extremely flat, and the sealing means that crimps and bends the periphery of the both electrode current collector plates with a packing material between them, like thick batteries, is structurally and processing technically. Since it is difficult to adopt due to the restriction of No. 2, there is no choice but to adopt the method of sealing with an adhesive at the flat peripheral portions of the bipolar current collector plates facing each other.

Therefore, when using the above-mentioned high-fluidity liquid electrolyte in such a thin battery, the electrolyte is likely to flow out to the outside during battery assembly, which causes insufficient electrolytic mass or sealing of the sealing part due to wetting. If the adhesive sealing material is incomplete, and the adhesive material is a heat-sealing material, the electrolyte may be scattered during heat-sealing. In that case, there is a tendency that a short circuit occurs due to dendrite-like (resin-like) deposition of the negative electrode lithium and the life is shortened.

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 the fluidity. As an electrolyte having such a viscosity, a gel electrolyte using polymethylmethacrylate as a gelling agent is conventionally known (Japanese Patent Laid-Open No. 54-104541), for example LiBF ₄
By adding polymethylmethacrylate to a solution of γ-butyrolactone, a transparent gel 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 polymethylmethacrylate as a gelling agent has a large decrease in ionic conductivity and is chemically stable as compared with a high fluidity electrolyte which does not use a gelling agent. The sex is also insufficient. Also,
This gel electrolyte has the advantage of being able to be added to the inside of the battery in the form of being impregnated into a separator made of a porous material such as a non-woven fabric by utilizing its viscosity at the time of manufacturing the battery, but a force is applied to the separator from the outside. In this case, it is extruded to the surface, which causes a shortage of electrolyte inside the separator, which causes deterioration of battery characteristics.

[Means for solving problems]

As a result of intensive studies to solve the above-mentioned conventional problems, the present inventors found that the conventional gel electrolyte using polymethyl methacrylate as a gelling agent is inferior in ion conductivity and chemical stability. Has been found to be due to the gelling agent having some interaction with the lithium salt. Furthermore, the present inventors have found that, in a continuous study based on the above findings, they have good ion conductivity and chemical stability when a specific polymer is used instead of polymethylmethacrylate as a gelling agent. A gel electrolyte is obtained, and when the gel electrolyte is added in the form of being impregnated in a separator made of a porous material at the time of assembling a lithium battery, the gel electrolyte has a surface even if external force is applied to the separator. The present invention has been accomplished by finding that it is difficult to be pushed out to the side and deterioration of performance due to insufficient electrolyte inside the separator is suppressed.

That is, the present invention is a lithium ion conductive gel electrolyte comprising a lithium salt, a non-aqueous solvent and a high molecular polymer which is a gelling agent, wherein 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:

[Constitution / Operation of Invention]

The polymer used as the gelling agent in the gel electrolyte of the present invention has a soluble portion, an insoluble portion and an insoluble portion for the non-aqueous solvent used as described above, and the lithium salt is used as the non-aqueous solvent. When added to a dissolved solution, it is not completely dissolved due to the presence of the insoluble portion, but due to the action of the soluble portion, a uniform dispersion state without layer separation is obtained, and a gel electrolyte composed of a white opaque homogeneous body is usually formed. give.

The reason why such a gel electrolyte exhibits good ionic conductivity and chemical stability as compared with the conventional gel electrolyte using polymethyl methacrylate as a gelling agent is not clear, but the above polymethacrylic acid is used. Methyl is generally soluble in non-aqueous solvents such as propylene carbonate and γ-butyrolactone used for this type of electrolyte, so the entire polymer of the gelling agent interacts with the lithium salt in the electrolyte to improve ionic conductivity. While it lowers the chemical stability as well as lowers, in the high molecular polymer of the gelling agent used in the present invention, its insoluble portion does not interact with the lithium salt,
It is presumed that the adverse effects on the ionic conductivity and chemical stability are reduced.

Further, when the gel electrolyte of the present invention is impregnated into a porous material such as a non-woven fabric, the insoluble portion of the high molecular polymer which is the gelling agent functions as a liquid phase retainer and has a fine structure of the porous material. Since it is mechanically entangled in the inside, the electrolyte that is extruded to the surface side even when external force is applied during handling of the separator made of the above material impregnated with gel electrolyte during battery fabrication and during assembly into the battery. The amount is small, and it is considered that the increase in resistance due to insufficient electrolyte inside the separator is unlikely to occur and the deterioration in battery performance is suppressed.

As the above-mentioned polymer used for the gelling agent in the present invention, there are suitable ones depending on the kind of the non-aqueous solvent used, but generally, the homopolymer can be dissolved in the non-aqueous solvent. Copolymers with monomers that are insoluble as well as monomers are preferred. The copolymer may be either a block copolymer or a random copolymer.

As a preferred specific example of the above-mentioned copolymer, for example, when a high boiling point solvent such as propylene carbonate or γ-butyrolactone, which is a typical electrolyte for a lithium battery, is used as a non-aqueous solvent, the following general formula (I); (R ₁ is hydrogen or a methyl group, R ₂ is a methyl group or an ethyl group), and the following general formula (II); (R ₃ is hydrogen or a methyl group, R ₄ is an alkyl group having 3 or more and usually 15 carbon atoms) and a copolymer with a monomer. That is, in this copolymer, the structural part based on the monomer of the general formula (I) becomes a soluble part in the high boiling point solvent, and the structural part based on the monomer of the general formula (II) also becomes an insoluble part.

The number average molecular weight of the high molecular weight polymer as the gelling agent as described above is preferably about 5,000 to 100,000. Further, the ratio of the soluble portion / insoluble portion to the non-aqueous solvent is preferably about 1 / 0.1 to 1 / 0.7 in terms of the molar ratio of the constituent monomers, and if the soluble portion is too much, the same problem as the conventional gel electrolyte occurs. On the contrary, if the insoluble portion is too much, the original gelling function becomes insufficient and it becomes difficult to exhibit a uniform dispersed state in the electrolyte. Further, in order to sufficiently exert the gelling action by using this high molecular polymer, there is a certain degree of difference depending on the kind of the non-aqueous solvent, but generally the amount used is the non-aqueous solvent.
It is preferable that the ratio is about 5 to 50 parts by weight with respect to 100 parts by weight.

As the lithium salt which is one of the constituents of the gel electrolyte of the present invention, various ones conventionally known as electrolyte components for lithium batteries and the like can be used, but LiBφ ₄ (φ Means a phenyl group), LiBF ₄ , LiPF ₆ , LiCF ₃ So ₃ , LiAsF _{6 and the} like,
These may be used in the form of an adduct of a non-aqueous solvent in advance, and two or more kinds may 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 has a property that it does not react with the lithium salt, can dissolve the lithium salt, and partially dissolves the high molecular weight polymer of the gelling agent and mixes with the gelling agent to form a gel. Various materials can be used as long as they have them. Typical examples thereof include propylene carbonate and γ described above.
In addition to butyrolactone, dimethoxyethane, dioxirane and the like can be mentioned, and these can be used alone or in a mixed form of two or more kinds. In particular, when the gelling agent is a copolymer of the monomers of the above-mentioned general formulas (I) and (II), propylene carbonate and γ-butyrolactone are used alone or as a mixed solvent, and a small amount of dimethoxyethane is used as the main solvent. The mixed solvent added is most suitable, and the latter mixed with a small amount of dimethoxyethane has the effect of 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 various electrochemical elements in which an electrochromic display element or other lithium ion is a conductive ion species. However, it is most suitable for a thin lithium battery having a structure in which the positive and negative bipolar current collector plates are bonded and sealed at opposite flat peripheral portions.

FIG. 1 shows an example of the thin lithium battery. In the figure, 1 is a square flat plate positive electrode current collector plate made of stainless steel, 2 is a stainless steel plate having a flat peripheral portion 2a bent in the same direction as the main surface by bending the periphery in one step Narrow shallow dish-shaped negative electrode current collector plates 3 are bipolar electrode current collector plates 1 and 2
Adhesive layer sealing between the opposing peripheral portions 1a, 2a of the positive electrode, 4 is a positive electrode arranged on the side of the positive electrode current collector plate 1 in the space 5 formed between the two current collector plates 1, 2, and 6 is a space A negative electrode made of lithium or a lithium alloy loaded on the negative electrode current collector plate 2 side in 5, a separator 7 made of a porous material such as porous polypropylene interposed between both electrodes 4 and 6, and a positive electrode 4
It is a rectangular annular frame body made of polypropylene or the like and arranged so as to surround it.

In this case, the above-mentioned gel electrolyte of the present invention is usually added to the inside of the battery by pre-coating and impregnating the separator 7 before incorporation. At this time, the gel electrolyte does not flow out to the surroundings even if the assembly base surface is slightly inclined or vibration is applied, and there is a fear that the gel electrolyte may be dropped when the separator 7 is transported from the application position to the installation position. Moreover, it is possible to adjust the addition amount over a wide range.

On the other hand, as the adhesive layer 3, a general coating solution type adhesive can be used, but one made of a heat-fusible material is particularly preferable. As the heat-fusible material 3, use is made of a molded sheet such as an annular shape whose shape before heat-sealing is preset to a width corresponding to the width of the peripheral portions 1a, 2a of the bipolar current collector plates 1, 2. it can. That is, in the sealing operation, the molded sheet is sandwiched between both the peripheral portions 1a and 2a and pressed, and in this state, the peripheral portions 1a and 2a are pressed.
It suffices to heat the part a to a predetermined temperature. In the heating process, as described above, the gel electrolyte does not scatter like an electrolyte made of a conventionally used high fluidity liquid, and reliable sealing can be easily achieved. Further, as described above, since the form before heat fusion is a solid molded product, the handling operation and the assembling operation are very easy, and, as in the case of using a coating solution type adhesive, There is no fear that it will flow in and mix with the gel electrolyte.

As the heat-fusible material 3, various materials such as a hot-melt adhesive and a ceramic capable of hermetic sealing can be used.

Further, as the positive electrode 4, a material formed by mixing an active material, a binder such as Teflon powder, and an electron conduction aid such as carbonyl nickel, if necessary, and forming it into a sheet may be used. A viscous material obtained by kneading a gel electrolyte with an active material and, if necessary, a conductive additive can be preferably used. That is, since the latter viscous material can be applied and formed on the positive electrode current collector plate 1 by screen printing or a squeegee application method, the former molding step is unnecessary, and the forming operation is extremely simple and the cost can be reduced. In addition to being able to achieve a thin layer, it can be easily applied to a thin battery.

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

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 TiS ₂ , MoS ₂ , V ₆ O ₁₃ , and V ₂ O ₅ are particularly preferable. , V
Se ₂ and NiPS ₃ are mentioned, and these may be used in combination of two or more kinds.

Further, both lithium and lithium alloy can be used as the negative electrode 6, but lithium alone may react with the gel electrolyte for a long period of time, so it is desirable to alloy with aluminum or the like. .

The bipolar current collector plate in such a thin lithium battery has one of the dish-shaped ones as shown in FIG.
Both may be plate-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 about 0.3 to 0.7 mm.

〔The invention's effect〕

The lithium ion conductive gel electrolyte according to the present invention,
Since a high molecular polymer having a soluble portion and an insoluble portion in a non-aqueous solvent is used as a gelling agent,
It has superior ionic conductivity and chemical stability compared to conventional gel electrolytes, and when it is added to the battery as a lithium battery electrolyte impregnated in a separator made of a porous material, the separator is It is difficult to be extruded to the surface even if a force is applied from the outside, and since the insoluble part of the high molecular polymer functions as a liquid phase holder,
There is an advantage that deterioration of battery performance due to insufficient electrolyte inside the separator is suppressed.

〔Example〕

 Examples and comparative examples of the present invention will be shown below.

Example 1 100 parts by weight of a solution of LiCF ₃ So ₃ dissolved in propylene carbonate at a concentration of 1 mol / l was added with methyl methacrylate and acrylic acid 2-
40 parts by weight of a copolymer (number average molecular weight: about 50,000) with a molar ratio of 82:18 with ethylhexyl was added and mixed, and the mixture was allowed to stand at 100 ° C. for 3 hours to obtain a white opaque and uniform gel electrolyte. . Collect 10g of this gel electrolyte,
With a centrifuge (model H-108NA2 manufactured by domestic centrifuge) 3,
When centrifugation was performed for 5 minutes at a rotation speed of 500 times / minute, no layer separation was observed.

Example 2 Instead of the copolymer of Example 1, the molar ratio of methyl methacrylate, ethyl methacrylate and butyl methacrylate was 1.2: 6.
A white opaque and uniform gel electrolyte was obtained in the same manner as in Example 1 except that 40 parts by weight of the 7.1: 31.7 copolymer (number average molecular weight of about 30,000) was used. No layer separation was observed in this gel electrolyte even after the centrifugal separation treatment as in Example 1.

Example 3 Example 1 was repeated except that 40 parts by weight of a copolymer of methyl acrylate and butyl methacrylate at a molar ratio of 70:30 (number average molecular weight of 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. No layer separation was observed in this gel electrolyte even after the centrifugal separation treatment as in Example 1.

Comparative Example Except that 40 parts by weight of polymethylmethacrylate (number average molecular weight of about 20,000) was used instead of the copolymer of Example 1,
A gel electrolyte was obtained in the same manner as in Example 1. This gel electrolyte was a colorless and transparent uniform body.

For the gel electrolytes of the above Examples and Comparative Examples, 20
When the ionic conductivity at ℃ was measured,
The results shown in the table were obtained.

Next, the gel electrolytes of Examples and Comparative Examples are respectively
A 100 μm thick polypropylene non-woven fabric is coated and impregnated at a rate of about 100 mg / cm ² , and both sides of this non-woven fabric have a diameter of 10 mm and a thickness of 10 μm.
Laminate a 0.1 mm lithium metal plate and put 100 between both metal plates.
After applying pressures of g, 500 g and 1 Kg and releasing the respective pressures, the AC resistance of 1 KHz was measured, and the results shown in the following table were obtained.

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

First, the gel electrolyte and TiS ₂ powder were kneaded at a weight ratio of 1: 1 and the kneaded product was screen-printed on the surface of a positive electrode current collector plate made of a stainless steel flat plate having a square shape of 15 mm on a side and a thickness of 0.1 mm. Then, it was applied so as to fill the inside of a square frame made of polypropylene placed thereon, and a square having a side of 10 mm and a positive electrode having a thickness of 0.1 mm was formed. On this positive electrode, thickness 1
A separator made of polypropylene non-woven fabric of 00 μm (trade name: JYURAGARD 5411 manufactured by Polyplastics Co., Ltd.) was coated with a gel electrolyte at a rate of about 100 mg / cm ² and uniformly impregnated, and laminated on the separator. -A negative electrode having a thickness of 80 µm made of a square foil made of an aluminum alloy and having a side of 4 mm was laminated.

Next, a modified polyolefin-type hot melt adhesive consisting of a square annular sheet having a thickness of 60 μm and a width of 2 mm was placed on the peripheral portion of the positive electrode current collector plate, and a square having a side of 15 mm and a thickness of 0.
A negative electrode current collector made of a 1 mm dish-shaped stainless steel plate is capped, and the peripheral parts of both current collectors are heated to 185 ° C. under pressure and heat-sealed to form a thin lithium battery with a total thickness of 0.5 mm. A battery was made.

The 200 μA constant current discharge characteristics of each thin lithium battery thus produced were examined at 25 ° C., and the results shown in FIG. 2 were obtained. In the figure, a curve A ₁ shows the characteristics of a battery using the gel electrolyte of Example 1, a curve A ₂ of Example 2, a curve A ₃ of Example 3, and a curve B of Comparative Example.

From the results shown in Tables 1 and 2 above, the gel electrolytes of the present invention (Examples 1 to 3) were superior in ion conductivity to the conventional gel electrolytes (Comparative Examples) and were porous. When the material is impregnated, even if pressure is applied to the material, it is difficult to be pushed out to the outside and is sufficiently retained inside and the increase in the internal resistance is suppressed, so that the separator made of a porous material is impregnated at the time of manufacturing the battery. It is clear that it is suitable for addition in the form. From the results shown in FIG. 2, the batteries (curves A ₁ , A ₂ , A ₃ ) using the gel electrolyte of the present invention are
It can be seen that the discharge characteristics are better than those of the battery using the conventional gel electrolyte (curve B).

[Brief description of drawings]

FIG. 1 is a sectional view showing a structural example of a thin lithium battery, FIG.
The figure is a constant current discharge characteristic diagram of thin lithium batteries using the gel electrolytes of Examples and Comparative Examples.

Claims (2)

[Claims] 1. A lithium ion conductive gel electrolyte comprising a lithium salt, a non-aqueous solvent and a polymer as a gelling agent, wherein the polymer has a soluble portion and an insoluble portion in the non-aqueous solvent. A lithium ion conductive gel electrolyte characterized by the above. 2. A high molecular polymer which is a gelling agent has the following general formula (I): (R ₁ is a hydrogen atom or a methyl group, R ₂ is a methyl group or an ethyl group), and the following general formula (II); The lithium ion conductive gel electrolyte according to claim 1, which is a copolymer with a monomer represented by (R ₃ is a hydrogen atom or a methyl group, and R ₄ is an alkyl group having 3 or more carbon atoms). .