JPH09293519A - Electrolyte thin film and battery using this electrolyte thin film - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an electrolyte thin film with high ionic conductivity and no electrolyte leak by filling an electrolyte in a cavity whose maximum major axis is equal to or smaller than the film thickness of a solid polymer porous thin film having a dense skin on the surface and the specified coefficient of water permeability. SOLUTION: In a solid polymer porous thin film having a dense skin layer on the surface, the coefficient of water permeability is limited to 5-1000l/ m<2> .hr.atm, and the major diameter of a cavity in the film is limited so as not to exceed the film thickness. An electrolyte is filled in the cavity to obtain an electrolyte thin film. The solid polymer porous thin film is made of an ion conductive solid polymer such as polyvinylidene fluoride or a copolymer containing vinylidene fluoride, and the porosity is preferable to be 10-95%. The electrolyte thin film has suitable mechanical strength, high reliability, high working capability, and high softness, and when electrodes are bonded through this thin film, the contact efficiency with the electrode is enhanced, and a solid battery with light weight and high energy density is obtained.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to an electrolyte thin film and a battery using the electrolyte thin film.

[0002]

2. Description of the Related Art Recently, miniaturization of mobile phones, personal computers, and the like,
A battery having a high energy density is required for weight reduction, and a lithium ion battery has been developed and industrialized to meet the demand. As the ion transfer medium between the positive electrode and the negative electrode of this battery, a form in which a porous polymer separator having a through hole is impregnated with an electrolytic solution is used, and in order to prevent leakage of the electrolytic solution impregnated in the separator, A product in which the entire battery structure is packaged in a heavy metal container is commercialized.

On the other hand, a solid-state battery in which a solid electrolyte is used as an ion transfer medium has no liquid leakage, and thus has high reliability and reliability.
Along with improving safety, it is expected to reduce the thickness, form a laminate, and simplify and lighten the package. In particular, a solid polymer electrolyte using an ion-conductive polymer has favorable properties such as forming a laminated structure with a battery because of its processing flexibility, and being able to maintain an interface following a volume change due to ion occlusion and release of an electrode. Expected.

As an attempt for such a polymer solid electrolyte, an alkali metal salt complex of polyethylene oxide by Wright has been proposed by British Polymer.
Journal, 7 p. 319 (1975), a polymer solid electrolyte material having a skeleton of polyacrylonitrile, polyphosphazene, polysiloxane or the like, as well as polyalkylene ether-based materials such as polyethylene glycol and polypropylene oxide, has been actively studied. Such a polymer solid electrolyte usually takes a form in which an electrolyte is uniformly solid-dissolved in a polymer and is known as a dry polymer solid electrolyte, but its ionic conductivity is remarkably higher than that of an electrolytic solution. The battery is low, and the battery configured by using it has problems such as limited charge / discharge current density and high battery resistance.

Therefore, various attempts have been made to improve the ionic conductivity by forming a state closer to the electrolyte. For example, a gel-type polymer solid electrolyte in which a plasticizer such as an electrolyte solvent is added for the purpose of improving the dissociation degree of the electrolyte or promoting the molecular motion of the polymer to improve the ionic conductivity is known ( Kaisho 56-1433
No. 56, etc.). In particular, a polymer containing a large amount of a solvent has a property intermediate between liquid and solid as a hybrid electrolyte, and high ionic conductivity is obtained (for example, Gozdz et al., US Pat. No. 5,296,318). ).

On the other hand, an attempt has been proposed to prevent leakage of electrolyte by filling the pores of a porous membrane with a liquid ionic conductor and retaining it by utilizing capillary action. An electrically insulative net impregnated with an electrolyte (Hope et al., US Pat. No. 5,102,752)
There is. Conventionally, a thin film-like porous material has been widely used as a separator for batteries, and it has been proposed to use an ion-conductive polymer such as vinylidene fluoride as a material (for example, Japanese Patent Laid-Open No. Hei 4- 239041
In Japanese Patent Laid-Open Publication No. 2003-242242, the separator was originally intended to prevent a short circuit between the electrodes, and in such an example, the pore size was too large to prevent the electrolytic solution from leaking to the outside. Therefore, a method of preventing electrolyte leakage by reducing the through-hole diameter of the porous membrane to 0.1 μm or less has been proposed. Ion conduction is achieved in a microporous membrane made of a high-strength material widely used as a separator such as polyolefin. An electrolyte thin film is prepared by filling the body (Japanese Patent Laid-Open No. 1-158051).
Issue).

Further, in such a porous membrane, a large number of pores are complicatedly interlocked with each other, and ions pass through a labyrinthine electrolytic solution phase, so that the ionic conductivity is lowered, but an object thereof is to improve this. Then, an electrolyte membrane using a membrane having a hole penetrating the membrane surface almost linearly is also proposed (JP-A 3-1).
45005 publication). Various attempts have been known to obtain high conductivity by uniformly and non-uniformly combining a polymer and an electrolytic solution. However, in the case of a gel-type polymer solid electrolyte, which is an example of a homogeneous system, a considerable amount of solvent is required to obtain high conductivity, while on the other hand, the mechanical strength is remarkably reduced as the solvent content increases, It is extremely difficult to satisfy high conductivity and sufficient mechanical strength. In particular, the method of impregnating the electrolyte solvent afterwards is generally not easy, and the process is considerably complicated.

Further, in the case of the non-uniform system, it is difficult to form a thin film when a net is used, it is difficult to control the pore size to be small, and it is difficult to prevent leakage of the electrolytic solution. Even in the example using a polyolefin porous membrane as disclosed in Japanese Patent Laid-Open No. 1-158051, ions cannot pass through the polymer layer. Therefore, as described above, the ions pass through the complicated route of the porous layer. There is no choice but to obtain high conductivity. Furthermore, an ion transfer medium having a large molecular weight is used in order to prevent leakage of the electrolytic solution, and the actually obtained ionic conductivity is as low as 10 ⁻⁵ to 10 ⁻⁶ S / cm. In addition, since a highly porous polyolefin porous film with high solvent resistance is used to obtain high breaking strength, the effective contact area with the electrode becomes low, which is also a cause of high conductivity not being obtained. ing.

In the example of Japanese Patent Application Laid-Open No. 3-145005, the material is limited, and the technique of forming the holes penetrating the film surface substantially linearly with high density is not yet known, and in reality, High ionic conductivity has not been obtained. Therefore, no electrolyte material having high ionic conductivity, leakage of electrolyte solution, and sufficient strength to form a battery has been reported yet.

[0010]

DISCLOSURE OF THE INVENTION The present invention has a high ionic conductivity, does not leak an electrolyte solution, and has a suitable mechanical strength even under the conditions of use as a solid electrolyte such as a sheet battery. It is an object of the present invention to obtain an electrolyte thin film which has high contact efficiency with an electrode and is excellent in reliability, processability and flexibility.

[0011]

SUMMARY OF THE INVENTION The inventors of the present invention have made extensive studies in view of the problems of the prior art described above, and arrived at the present invention. That is, the present invention is as follows. (1) It has a dense skin layer on the surface of the membrane and has a water permeability of 5 to 5.
The range is 1000 liters / m ² · hr · atm,
An electrolyte thin film obtained by filling an electrolyte solution into the voids of a solid polymer porous thin film in which the maximum major axis of the voids in the film does not exceed the length of the film thickness. (2) The electrolyte thin film according to the above 1, wherein the solid polymer porous thin film is made of an ion-conductive solid polymer and has a porosity in the range of 10 to 95%. (3) The electrolyte thin film as described in 2 above, wherein the ion conductive solid polymer is made of polyvinylidene fluoride or a copolymer containing vinylidene fluoride. (4) After filling the voids of a porous thin film having a dense skin layer only on one surface with an electrolyte solution, two porous thin films are directly or impregnated with the electrolyte solution. A method for producing an electrolyte thin film, which is characterized in that it is sandwiched and laminated so that the skin layer is on the outside. (5) A battery characterized in that electrodes are joined via the electrolyte thin film of 1, 2 or 3 above.

Hereinafter, the present invention will be described in detail. First, the electrolyte thin film will be described. The electrolyte thin film of the present invention is formed by filling the voids of a solid polymer porous membrane having a dense skin layer on the surface of the membrane with an electrolytic solution. That is, like the conventional separator, the inside of the electrolyte thin film is a porous layer impregnated with an electrolytic solution, but this has a structure covered with a dense skin layer, and this dense skin layer prevents the leakage of the electrolytic solution. Will be responsible for That is, the dense skin layer is difficult for liquid to pass through, and when expressed as a water permeability when a hydrostatic pressure of 1 atm is applied to the membrane, it is 5 to 1000 liters / m ^2.
It is in the range of hr · atm, preferably in the range of 5 to 800 liters / m ² · hr · atm, and more preferably in the range of 5 to 500 liters / m ² · hr · atm. Permeability is 1000 liter / m ² · hr · at
If it exceeds m, it will be difficult to prevent the leakage of the electrolytic solution.
If it is less than liter / m ² · hr · atm, the resistance of the skin layer becomes too large and the ionic conductivity becomes low. Here, the water permeability is measured by the following method. That is, a membrane punched out to a diameter of 25 mm is immersed in ethanol for 15 minutes or more, then incorporated into a membrane filter holder having an effective area of 3.5 cm ² , filled with water at 25 ° C., and 1 att.
The permeation amount of water when a static pressure of m is applied is measured. In the present invention, the dense skin layer refers to a skin layer having a uniform surface without voids which can be seen on the surface of the porous film by SEM observation. The presence of this skin layer can be confirmed from the increase in the amount of water permeation when the surface of the membrane is scraped by rubbing with a razor blade or the like.

The average through-hole diameter of the porous membrane is 0.1.
It is preferably less than μm. If the thickness is 0.1 μm or more, it is difficult to prevent the electrolyte from leaking. The shape of the voids of the porous membrane is not particularly limited, but the maximum major axis of the voids in the membrane must be within the range not exceeding the thickness of the membrane, and voids having a size exceeding this range. The presence of the electrolyte makes it impossible to retain the electrolytic solution when there is external pressure. In the present invention, the maximum major axis of the voids in the porous membrane is the maximum length by which the size of the voids in the porous membrane can be measured linearly, and is actually a scanning electron microscope (S
It is determined as the maximum major axis of the voids in the porous film when observing 10 or more cross sections with EM). Further, the porous portion preferably has substantially continuous pores, and may have closed cells as long as it is less than 5%. The continuous hole here means that void portions are three-dimensionally communicated with each other.

The porosity of the porous membrane is preferably in the range of 10 to 95%, more preferably 20 to 95.
%, And more preferably 40 to 95%. If it is less than 10%, the ionic conductivity of the electrolyte thin film is not sufficiently high, and if it exceeds 95%, it is difficult to obtain sufficient strength after impregnation with the electrolytic solution. The porosity is measured by the following method, for example. That is, a sample of the porous membrane is immersed in ethanol to make it hydrophilic, then immersed in water for 2 hours or more to sufficiently replace water, and the surface is wiped off, and then the weight (A) is measured. This sample is vacuum dried at 60 ° C. or higher for 4 hours or longer, and the weight (B) after drying is measured. It is calculated from these weights and the true specific gravity (d) of the material of the membrane. Porosity = (A−B) / (B / d + A−B) The thickness of the electrolyte thin film is not particularly limited because the suitability range varies depending on the intended use, but generally 0.1 to 0.1.
The thickness is preferably 500 μm, preferably 1 to 300 μm, and more preferably 10 to 100 μm. If the thickness is less than 0.1 μm, the strength is insufficient, and when assembled as an electrochemical element such as a battery, the electrodes are likely to be short-circuited. Also 500μ
When it is more than m, the effective electric resistance of the whole film becomes too high, and when assembled as a battery, the energy density per volume becomes small.

The thickness of the dense skin layer of the porous membrane is
Since this portion becomes an electrical resistance, it is desirable that the thickness be as thin as possible. Generally, it is preferably in the range of 0.01 to 5 μm. Further, a definite interface does not necessarily exist between the dense skin layer and the porous layer inside. The respective layers may change continuously in the interfacial region or may be intricate with each other.

The material of the porous membrane forming the electrolyte thin film of the present invention may be a solid polymer from the viewpoint of workability, but among them, an ion conductive solid polymer is preferable because it enhances the ionic conductivity, Those having a structure that mainly enhances the dissociation of ions due to a high dielectric constant are preferable. For example, in the presence of a salt containing an ion to be transferred, when the ionic conductivity is measured in a system to which a plasticizer is added if necessary, 10 ⁻¹⁰ S /
cm or more, preferably 10 ^-9 S / cm or more, more preferably 10 ^-8 S / cm or more. Specific examples thereof include polyethylene oxide,
Ions in molecules such as polypropylene oxide, polyvinylidene fluoride, polyacrylonitrile, poly (meth) acrylic acid oligoethylene oxide, polyethyleneimine, polyalkylene sulfide, polyphosphazene and polysiloxane having side chains of oligoethylene oxide, Nafion and Flemion. Examples thereof include polymers having a functional group. Also, a copolymer mainly containing these or a cross-linked cured product can be used. Among these, polyvinylidene fluoride or a copolymer containing vinylidene fluoride is preferable from the viewpoint of ionic conductivity and processability, and in the case of a copolymer, a copolymer with hexafluoropropylene or the like is preferable. When a polymer having an ionic group in the molecule is used in a lithium battery, the ionic group is preferably a lithium salt.

The solid polymer forming the porous membrane may contain an electrolyte compound in advance. In particular,
It can be contained by a method of forming a porous film in a state of mixing an electrolyte compound, a method of swelling with an electrolyte compound after film formation, and the like. The method for producing the solid polymer porous thin film of the present invention is not particularly limited as long as it satisfies the conditions of the present invention. For example, a usual method for producing a microfilter or an ultrafilter can be used. That is, as a method of producing these filters, a method of producing a porous thin film having a dense skin layer on the surface by extruding a solution of a polymer in a thin film into a solvent that solidifies the polymer is known. The skin layer can be controlled by controlling the temperature conditions and the surfactant. In this case, if the film is extruded directly into the coagulation solvent, a so-called double-skin film having skin layers on both sides can be produced, and if one that is cast into a substrate such as glass is poured into the coagulation solvent, only one side will be produced. A single-skin film with a skin layer is formed on. The double-skin membrane can be used as the electrolyte thin film of the present invention by directly impregnating it with an electrolytic solution.

On the other hand, a single skin membrane can also be used as the electrolyte thin film of the present invention. That is, the single-skin membrane is also impregnated with the electrolyte solution, and the skin layer is placed outside.
By sticking the sheets together, an electrolyte thin film that does not leak can be obtained. In this case, another layer may be provided between the single-skin films impregnated with the electrolytic solution. For example, it is also possible to stick two sheets with a skin layer on the outside on both sides of a porous membrane such as a separator impregnated with an electrolytic solution.

The electrolyte thin film of the present invention formed by these methods can be selected according to the purpose of use such as high ionic conductivity and strength. When particularly high strength is required, it is possible to increase the strength by cross-linking with an electron beam or the like after forming the porous film. Next, the electrolytic solution used for the electrolyte thin film of the present invention will be described.

First, as the electrolyte compound filling the voids of the continuous pores, any of inorganic salts, organic salts, inorganic acids and organic acids can be used. As this example, for example, tetrafluoroboric acid, hexafluorophosphoric acid, perchloric acid, hexafluoroarsenic acid, nitric acid, sulfuric acid, phosphoric acid, hydrofluoric acid, hydrochloric acid, hydrobromic acid, inorganic acids such as hydrogen iodide oxygen, Trifluoromethanesulfonic acid, heptafluoropropylsulfonic acid, bis (trifluoromethanesulfonyl) imidic acid,
Organic acids such as acetic acid, trifluoroacetic acid, propionic acid,
And salts of these inorganic acids and organic acids. Further, mixtures of these inorganic acids, organic acids, and salts thereof can also be used. As the cation of this salt-type electrolyte compound, an alkali metal, an alkaline earth metal, a transition metal, a rare earth metal or the like can be used alone or in a mixed state. The preferred species of this cationic species differ depending on the intended use. For example, when using the electrolyte thin film of the present invention as a lithium battery, it is preferable to use a lithium salt as an electrolyte compound to be added. In particular, when used as a lithium secondary battery, an electrochemically stable lithium salt is preferable as an electrolyte compound in order to use a wide potential region. For example, CF ₃ SO ₃ Li,
Lithium fluoroalkylsulfonic acid salts such as C ₄ F ₉ SO ₃ Li, lithium sulfonylimide salts such as (CF ₃ SO ₂ ) ₂ NLi, LiBF ₄ , LiPF ₆ , LiCl
O ₄ and LiAsF ₆ can be exemplified.

As a solvent for dissolving these electrolyte compounds, water, alcohol or the like may be used as long as it is chemically stable and can dissolve the electrolyte compounds, and particularly when it is used as a non-aqueous electrolyte solution such as a lithium battery. In addition, ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, carbonate compounds such as methyl ethyl carbonate, tetrahydrofuran, dimethoxyethane, diglyme, tetraglyme, ether compounds such as oligoethylene oxide, butyrolactone, lactone compounds such as propyrolactone, Examples thereof include nitrile compounds such as acetonitrile and propionitrile. It is preferable that these solvents are swollen in a small amount in the polymer phase as well, since higher ionic conductivity can be obtained.

The electrolyte thin film of the present invention is formed by filling the voids of the solid polymer porous thin film with an electrolytic solution, that is, a solution in which the above-mentioned electrolyte compound is dissolved in the above-mentioned solvent. As a method of filling, generally, the polymer solid porous thin film may be impregnated with an electrolytic solution at room temperature or under heating, and if necessary, the pressure is reduced to eliminate the air in the voids. Impregnation may be difficult depending on the material and structure of the porous thin film. In this case, a solvent that promotes swelling into the polymer phase may be used together during impregnation to facilitate diffusion into the pores. it can. Here, when the swelling solvent has a lower boiling point than the solvent of the electrolyte compound, the swelling solvent can be removed by air drying or reduced pressure treatment.

Next, a battery using the electrolyte thin film of the present invention will be described. The battery of the present invention has a structure in which a positive electrode and a negative electrode are joined via the electrolyte thin film, and can be used as a primary battery and a secondary battery. For example, when the battery is a lithium battery, it is preferable that the electrolyte thin film contains a lithium salt, and it is preferable to use a lithium salt as the electrolyte. At this time, a material capable of inserting and extracting lithium is used for the positive electrode and the negative electrode of the battery. As the positive electrode material, a material having a higher potential than the negative electrode is selected. As an example of this, Li _1-x CoO ₂ , Li _1-x Ni
O ₂ , Li _1-x Mn ₂ 0 ₄ , Li _1-x MO ₂ (0 <x <
1, M is a mixture of Co, Ni, Mn, and Fe), Li _{2-y
Mn 2 O 4 (0 <y} <2), Li 1-x V 2 0 5, Li
_2-y V ₂ O ₅ (0 <y <2), Li _{1.2-x ′} Nb ₂ O
₅ Oxides such as (0 <x '<1.2), Li _1-x TiS
₂ , Li _1-x MoS ₂ , Li _3-z NbSe ₃ (0 <z <
3) and other metal chalcogenides, organic compounds such as polypyrrole, polythiophene, polyaniline, polyacene derivatives, polyacetylene, polythienylenevinylene, polyarylenevinylene, dithiol derivatives, and disulfide derivatives.

As the negative electrode, a material having a lower potential than the positive electrode is used. Examples of this are metallic lithium, metallic lithium such as aluminum-lithium alloy, magnesium-aluminum-lithium alloy, ALSb, Mg ₂ G
e, intermetallic compounds such as NiSi ₂ , carbon-based materials such as graphite, coke, and low-temperature baked polymers, SnM-based oxides (M represents Si, Ge, Pb), Si _1-y M ′.
_y O _z (M 'represents W, Sn, Pb, B, etc.) composite oxide, lithium solid solution of metal oxide such as titanium oxide, iron oxide, Li ₇ MnN ₄ , Li ₃ FeN ₂ , Li
_3-x Co _x N, Li _{3- x} NiN, Li _3-x Cu _x N, L
Ceramics such as nitrides of i ₃ BN ₂ , Li ₃ AlN ₂ , and Li ₃ SiN ₃ can be cited. However, when lithium ions are reduced at the negative electrode and used as metallic lithium, the material is not limited to the above, as long as the material has conductivity.

The positive electrode and the negative electrode used in the battery of the present invention are formed by processing the above materials into a predetermined shape. Either a continuous body or a binder dispersion of powder material can be used as the form of the electrode. As the former continuous body molding method, electrolysis, vapor deposition, sputtering, CVD, melt processing, sintering,
Compression or the like is used. In the latter method, a powdery electrode material is mixed with a binder and molded. As the binder material, ion conductive polymers such as polyvinylidene fluoride, non-ion conductive polymers such as styrene-butadiene latex and Teflon latex, metals, etc. are used. It is also possible to add a polymerizable monomer or a cross-linking agent and polymerize and cross-link after molding. Furthermore, electron beam, γ
It is also possible to irradiate radiant energy such as rays and ultraviolet rays. Further, in order to perform electron transfer of the positive electrode or the negative electrode material, a current collector can be provided on the electrode with a material having low electric resistance, and the current collector can be an electrode formed on the substrate by the above method.

The form of the battery has a structure in which a positive electrode and a negative electrode are joined together via an electrolyte thin film. For example, a sheet-shaped or roll-shaped structure can be formed with a unit of positive electrode / electrolyte thin film / negative electrode in which sheet-shaped constituent elements are sequentially laminated. Also,
It is also possible to make an assembled battery in which electrodes of battery units are connected in parallel or in series. In particular, it has a feature that the voltage can be increased depending on the number of series connections.

Since the electrolyte thin film of the present invention has a high ionic conductivity and does not leak an electrolyte solution, it is applied not only to the above lithium battery but also to various electrochemical elements and devices such as photoelectrochemical devices and electrochemical sensors. Because it is possible, it is industrially useful.

[0028]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention will be described in more detail with reference to the following examples. The measurement was performed as follows.・ Water permeability is measured by immersing a membrane punched to a diameter of 25 mm in ethanol for 15 minutes or more, then incorporating it in a membrane filter holder with an effective area of 3.5 cm ² , filling it with water at 25 ° C., and applying a static pressure of 1 atm. Measure the amount of water permeation. The porosity is measured by immersing a sample of the porous membrane in ethanol to make it hydrophilic, then immersing it in water for 2 hours or more to sufficiently replace it with water, wiping the surface, and then measuring the weight (A). This sample is vacuum dried at 60 ° C. or higher for 4 hours or longer, and the weight (B) after drying is measured. It is calculated from these weights and the true specific gravity (d) of the material of the membrane. Porosity = (A−B) / (B / d + A−B) ・ The maximum major axis of the void is 1 with a scanning electron microscope (SEM).
The size of voids in the porous film when observing cross sections at 0 or more places is determined as the maximum length that can be linearly measured.・ Ionic conductivity is determined by sandwiching an electrolyte thin film between metal electrodes to form an electrochemical cell, and applying an alternating current between the electrodes to measure the resistance component using an AC impedance method.
It was calculated from the real impedance intercept of the Cole-Cole plot.

[0029]

Example 1 A vinylidene fluoride-hexafluoropropylene copolymer (1.4% by weight of hexafluoropropylene) in dimethylacetamide solution (22.5% by weight of polymer) was applied at room temperature to form a liquid film having a thickness of 100 μm on a glass plate. Cast so as to be immediately immersed in water at 17 ℃,
It was washed with water and alcohol and then dried to prepare a porous sheet having a skin layer on the water contact surface side and having a film thickness of 38 μm and a porosity of 48%. The water permeability of the sheet is 20 liter / m ² · hr
・ Atm, and it was confirmed that when the surface layer side was rubbed with a razor blade to scrape the surface, the amount of water permeation increased significantly. The sheet was treated with 1 mol of an ethylene carbonate / propylene carbonate 1: 1 mixed solvent of LiBF _4.
The transparent sheet impregnated with the solution was obtained by immersing it in the 1 / liter solution at room temperature for 10 minutes. Excess solution that was not impregnated was wiped off. Two sheets were stacked so that the skin layer was on the outside (film thickness: 80 μm), sandwiched between stainless sheets, and impedance measurement (Model 389 impedance meter manufactured by EG & G) was performed. As a result, the ion conductivity at room temperature was 1.0. It was × 10 ^-3 S / cm. The maximum major axis of the voids observed by SEM was maximum major axis / film thickness = 0.6.

[0030]

Example 2 A porous sheet having a skin layer was prepared in the same manner as in Example 1 except that the temperature of water soaked after casting was 70 ° C. (thickness 31 μm, porosity 60%).
The water permeability of the sheet was 15 l / m ² · hr · atm. When the ionic conductivity at room temperature was measured in the same manner as in Example 1, it was 3.6 × 10 ⁻⁴ S / cm (two-layer film thickness 67 μm). The maximum major axis of the voids observed by SEM was maximum major axis / film thickness = 0.3.

[0031]

[Example 3] Setting of the liquid film when cast is 50 μm
A porous sheet having a skin layer was prepared in the same manner as in Example 1 except that the above was used (film thickness 21 μm, porosity 54).
%). The water permeability of the sheet was 8 l / m ² · hr · atm. When the ionic conductivity at room temperature was measured in the same manner as in Example 1, it was 1.1 × 10 ⁻⁴ S / cm (thickness of two stacked layers: 36 μm). The maximum major axis of the voids observed by SEM did not exceed the film thickness.

[0032]

Example 4 An electron beam was applied to the porous sheet prepared in Example 1.
Irradiate (irradiation dose 15 Mrad) and create a crosslinked sheet
did. The water permeability of the sheet is 79 l / m^Two・ Hr ・ atm
Met. The sheet was impregnated with the electrolytic solution in the same manner as in Example 1.
The impregnated sheets so that the skin layer is on the outside.
Ne (thickness is 76 μm), sandwiched between stainless sheets,
As a result of impedance measurement,
Conductivity is 7.3 × 10 ^-FourIt was S / cm. Electron beam
The sheet before firing was completely dissolved in the electrolyte at 120 ° C.
However, the sheet after electron beam irradiation retains its shape even at 120 ° C.
Was.

[0033]

Example 5 An electron beam was applied to the porous sheet prepared in Example 2.
Irradiate (irradiation dose 15 Mrad) and create a crosslinked sheet
did. The water permeability of the sheet is 16 l / m^Two・ Hr ・ atm
Met. The sheet was impregnated with the electrolytic solution in the same manner as in Example 1.
The impregnated sheets so that the skin layer is on the outside.
Ne (the film thickness is 42 μm), sandwiched between stainless sheets,
As a result of impedance measurement,
Conductivity is 1.0 × 10 ^-FourIt was S / cm.

[0034]

Examples 6 to 7 Vinylidene fluoride-hexafluoropropylene copolymer (hexafluoropropylene 1.4% by weight) 17.3 parts by weight, polyethylene glycol having an average molecular weight of 200 11.5 parts by weight, dimethylacetamide 7
A solution of 1.2 parts by weight was prepared, and 100 g of this solution was mixed with polyoxyethylene sorbitan monooleate (trade name: Tween 80, Kao Atlas Co., Ltd.).
8 ml was added to make a uniform solution. The solution was cast at a temperature setting shown in Table 1 so that the liquid film was 100 μm on a glass plate, immediately immersed in water, washed with water and alcohol, and then dried to form a skin layer on the water contact surface side. A porous sheet having The dry film thickness and water permeability of the sheet are also shown in Table 1. Further, the sheet was impregnated with the electrolytic solution in the same manner as in Example 1, two impregnated sheets were stacked so that the skin layer was on the outer side, and sandwiched between stainless sheets, and impedance measurement was performed. The measurement results of the ionic conductivity at room temperature and the film thickness of the electrolyte at that time are collectively shown in Table 1. Further, the maximum major axis of the voids observed from the SEM is the maximum major axis / film thickness = 0.8 in the film of Example 6,
In Example 7, the maximum major axis / film thickness was 0.3.

[0035]

[Table 1]

[0036]

Comparative Examples 1-2 Pores having a skin layer were prepared in the same manner as in Examples 6 and 7, except that the temperature of the polymer solution at the time of casting and the temperature of water soaked after casting were set to the temperatures shown in Table 2. Created a quality sheet. The measurement results of water permeability and ionic conductivity of the sheet at room temperature are shown in Table 2. The sheet had voids having a major axis equal to or longer than the length of the film thickness in the film. This porous sheet was impregnated with a 1: 1 mixed solution of ethylene carbonate / propylene carbonate at room temperature, and two sheets having the same size (2 cm × 2 cm) were placed so that the skin layers were on the outside. These sheets were sandwiched between two pieces of filter paper (No. 3 manufactured by Kiriyama Seisakusho Co., Ltd.) cut into 2.5 cm x 2.5 cm, and then sandwiched with slide glass, a 1 kg weight was placed, and the weight change when held at room temperature for 1 hour The rate was measured. Table 3 shows the measurement results.

[0037]

Examples 8 to 10 The porous sheets prepared in Examples 1, 6 and 7 were impregnated with a 1: 1 mixture of ethylene carbonate / propylene carbonate at room temperature to obtain the same size (2c).
m × 2 cm) was prepared by stacking two epidermal layers on the outside. 2.5 cm x 2.5 for each sheet
It was sandwiched between two pieces of filter paper (No. 3 manufactured by Kiriyama Seisakusho Co., Ltd.) cut into cm, and further sandwiched between slide glasses, and a weight of 1 kg was placed thereon, and the rate of change in weight when kept at room temperature for 1 hour was measured. Table 3 shows the measurement results.

[0038]

[Table 2]

[0039]

[Table 3]

[0040]

[Embodiment 11] A predetermined amount of lithium hydroxide and cobalt oxide are mixed and then heated at 750 ° C. for 5 hours to obtain an average particle diameter of 10 μm.
m of LiCoO ₂ powder was synthesized. The powder and carbon black were mixed with polyvinylidene fluoride (Kureha Chemical Industry Co., Ltd.).
KF1100) (manufactured by KF1100) was mixed and dispersed in an N-methylpyrrolidone solution (5% by weight) to prepare a slurry. The solid composition by weight of the slurry was LiCoO ₂ (85%),
Carbon black (8%) and polymer (7%) were used.
This slurry was applied on an aluminum foil by a doctor blade method and dried to prepare a sheet having a film thickness of 110 μm. Next, a needle coke powder having an average particle size of 10 μm was mixed with the same N-methylpyrrolidone solution of polyvinylidene fluoride (5% by weight) as described above to prepare a slurry (dry weight composition:
Needle coke (92%), polymer (8%)). The slurry was applied to a metal copper sheet by a doctor blade method to form a film (electrode layer) with a dry film thickness of 120 μm. The LiCoO ₂ electrode sheet and the needle coke electrode sheet were each cut into 2 cm square, the electrolyte thin film sheet prepared in Example 1 was cut into 2.3 cm square, and the two electrode sheets were laminated so as to sandwich the electrolyte sheet. To form a battery joined by coke (negative electrode) / electrolyte thin film / LiCoO ₂ (positive electrode). Then, stainless steel terminals were attached to the positive electrode and the negative electrode of the battery, respectively connected to the terminals of the glass cell and sealed in an argon atmosphere. The battery was charged and discharged using a battery (Hokuto Denko 101SM6) with a current density of 3 mA /
Charging / discharging was performed at a current density of cm ² . The inter-electrode potential after charging was 4.2 V (4.2 V constant potential charging after constant current), and charging was confirmed. Discharge is 2.7V cut-off voltage.
As a result of constant current discharge, the initial charge / discharge efficiency was 80%, the charge / discharge efficiency after the second time was 99% or more, and it was found that repeated charge / discharge was possible and the battery operated as a secondary battery.

[0041]

Example 12 A battery was prepared in the same manner as in Example 11 except that the crosslinked electrolyte thin film sheet prepared in Example 4 was used. As a result of charging and discharging the battery in the same manner as in Example 11, the initial charging / discharging efficiency was 80%, the charging / discharging efficiency after the second time was 99% or more, the charging / discharging was possible repeatedly, and the battery operated as a secondary battery. I understood.

[0042]

EFFECT OF THE INVENTION The electrolyte thin film of the present invention has a high ionic conductivity, does not leak an electrolyte solution, has an appropriate mechanical strength even under the conditions of use as a solid electrolyte such as a sheet battery, Since it has high contact efficiency, excellent flexibility, processability and mechanical strength, and has characteristics equivalent to those of a solid electrolyte, it is possible to provide a solid battery using the same as an ion transfer medium.

[Brief description of drawings]

FIG. 1 is a cross-sectional SEM photograph of a porous membrane prepared in Example 6.

FIG. 2 is a surface SEM photograph of a dense skin layer side of the porous membrane prepared in Example 6. (Note that the white object in the upper center of the photo is dust.)

FIG. 3 is a cross-sectional SEM photograph of the porous membrane prepared in Comparative Example 1.

FIG. 4 is a surface SEM photograph of a dense skin layer side of the porous membrane prepared in Comparative Example 1.

FIG. 5 is a cross-sectional SEM photograph of the porous membrane prepared in Comparative Example 2.

FIG. 6 is a SEM photograph of the surface of the porous membrane prepared in Comparative Example 2 on the side of a dense skin layer.

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[Submission date] May 9, 1996

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Continuation of front page (72) Inventor Sho Minamikata 1 of 2 Samejima, Fuji City, Shizuoka Prefecture Asahi Kasei Corporation

Claims (5)

[Claims] 1. A membrane having a dense skin layer on the surface thereof, a water permeability of 5 to 1000 liters / m ² · hr · atm, and a maximum major axis of voids in the membrane is a length of the membrane. An electrolyte thin film obtained by filling the voids of a solid polymer porous thin film that does not exceed the size with an electrolyte solution. 2. The electrolyte thin film according to claim 1, wherein the solid polymer porous thin film is made of an ion conductive solid polymer and has a porosity in the range of 10 to 95%. 3. The electrolyte thin film according to claim 2, wherein the ion conductive solid polymer comprises polyvinylidene fluoride or a copolymer containing vinylidene fluoride. 4. A porous thin film having a skin layer on only one surface thereof is filled with an electrolytic solution, and then the two porous thin films are sandwiched directly or by impregnating with the electrolytic solution. A method for producing an electrolyte thin film, which comprises laminating the outer skin layer to the outer side. 5. A battery characterized in that electrodes are bonded via the electrolyte thin film according to claim 1, 2 or 3.