JP2002305026A - Sheet-type lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a sheet-type lithium secondary battery wherein a swelling characteristic in high temperature standing (bulge characteristic) is improved. SOLUTION: This is comprised that a solid electrolyte layer which is a porous material having a salt, mutually soluble solvent and fluorine polymer wherein the main unit is composed of vinylidene fluoride as the main body component is made to be interposed between a positive electrode and a negative electrode, and that this is sheathed by a sheet-type sheathing material, and the salt is at least one kind of compound selected from LiClO4 , LiBF4 , LiPF6 , LiAsF6 , LiCF3 SO3 , LiAlCl4 and Li(CF3 SO2 )2 N. The mutually soluble solvent is a mixed solvent composed of ethylene carbonate, propylene carbonate, diethyl carbonate and ethylmethyl carbonate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a sheet-shaped lithium secondary battery.

[0002]

2. Description of the Related Art A lithium secondary battery is generally constructed by interposing an electrolyte between a positive electrode and a negative electrode. Each of the positive electrode and the negative electrode is formed by providing a layer containing an active material, a conductive material, a binder, and the like on the surface of the current collector. As the active material for the positive electrode, a Li-Mn-based composite oxide, a Li-Ni-based composite oxide, a Li-Co-based composite oxide, or the like is used.
On the other hand, a carbon material is usually used as an active material for a negative electrode.

[0003] Lithium secondary batteries can achieve higher energy density and higher voltage than nickel-cadmium batteries and the like. Therefore, in recent years, lithium secondary batteries have been rapidly adopted as drive sources for portable devices such as cellular phones and notebook computers. I have. In addition, the scope of application is expected to expand in the future. Therefore, research on lithium secondary batteries has been actively conducted in order to improve battery performance.

For example, recently, a lithium secondary battery (also referred to as a “polymer battery”) of a type in which a solid electrolyte layer is interposed between a positive electrode and a negative electrode has attracted attention and has been studied. The solid electrolyte layer is formed by impregnating a polymer substrate with an electrolyte solution (lithium salt (electrolyte) + compatible solvent) to form a gel, which itself exhibits ionic conductivity. If it is used, since the electrolyte does not exist in a liquid state (in a state where the electrolyte itself flows) in the battery, there is a great advantage that there is no fear of liquid leakage from the battery.

[0005]

However, in a lithium secondary battery using the above-mentioned solid electrolyte layer, it is known that a chemical reaction occurs at the electrode / electrolyte interface during the initial charging, and gas is generated. I have. Therefore, the lithium secondary battery is usually subjected to a degassing step to remove the above gas, and then subjected to an aging step (aging test),
The problematic battery (eg, a voltage that dropped sharply during aging) is removed and only good batteries with no problems are shipped as products, but it is still difficult to completely remove the gas. A small amount of gas remained in the battery as a product. For this reason, the lithium secondary battery is a so-called sheet using an exterior material other than a metal (aluminum, iron (chromium plating), etc.) can (for example, an aluminum laminate film, a sheet-shaped exterior material such as a plastic battery pack). In the case of a lithium secondary battery (especially a stacked type), when a sheet lithium secondary battery as a product is left in a high-temperature (for example, 90 ° C.) atmosphere for a long time, the remaining gas expands and blisters are generated. In addition, the expansion of the gas separates the positive and negative electrodes and increases the internal resistance, resulting in unstable charging and discharging. In addition, such blisters have caused the sheet-like exterior material to be damaged. An object of the present invention is to provide a sheet-shaped lithium secondary battery having improved blistering characteristics (bulge characteristics) by suppressing generation of gas when left at high temperatures.

[0006]

Means for Solving the Problems The present inventors have made intensive studies to solve the above-mentioned problems, and as a result, have completed the present invention. That is, the present invention is as follows. (1) A sheet electrolyte comprising a solid electrolyte layer mainly composed of a salt, a compatible solvent, and a porous body of a fluoropolymer having vinylidene fluoride as a main unit, between a positive electrode and a negative electrode. A sheet-shaped lithium secondary battery packaged with a package material, wherein the salt is LiClO ₄ , LiBF ₄ , Li
PF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiAlCl ₄
And at least one compound selected from Li (CF ₃ SO ₂ ) ₂ N, and the compatible solvent is a mixed solvent comprising ethylene carbonate, propylene carbonate, diethyl carbonate, and ethyl methyl carbonate. A sheet-shaped lithium secondary battery characterized by the above-mentioned. (2) In the mixed solvent, the mixing ratio of ethylene carbonate is 1% by volume to 30% by volume, the mixing ratio of propylene carbonate is 1% by volume to 30% by volume, and the mixing ratio of diethyl carbonate is 1% by volume to 30% by volume. Volume%,
The mixing ratio of ethyl methyl carbonate is 10% by volume to 97%.
The sheet-shaped lithium secondary battery according to the above (1), wherein the volume is% by volume. (3) The density of the porous body of the fluoropolymer is 0.60 g /
The sheet-shaped lithium secondary battery according to claim 1, wherein the lithium secondary battery has a density of cm ^{3 to} 1.30 g / cm ³ .

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail.
The sheet-like lithium secondary battery of the present invention has a solid electrolyte layer interposed between a positive electrode and a negative electrode, and is packaged with a sheet-like package. The solid electrolyte layer means a polymer substrate impregnated with an electrolytic solution (lithium salt (electrolyte) + compatible solvent) to be gelled and prepared to exhibit ionic conductivity. When the solid electrolyte layer is interposed between the positive electrode and the negative electrode, the electrolyte does not exist in a liquid state (in a state where the electrolyte itself flows) in the battery, so that there is an advantage that there is no fear of liquid leakage from the battery. Can be In the present invention, a porous body of a fluoropolymer having vinylidene fluoride as a main unit is used as a polymer substrate of the solid electrolyte layer. Salts (electrolytes) contained in the solid electrolyte layer include LiClO ₄ , LiBF ₄ ,
LiPF ₆ , LiAsF ₆ , LiCF ₃ SO ₃ , LiAlC
l is _4, and Li (CF ₃ SO ₂₎ one or more selected from the group consisting of ₂ N.

What is important in the present invention is that as the compatible solvent contained in the solid electrolyte layer, ethylene carbonate (EC), propylene carbonate (PC),
That is, a mixed solvent composed of diethyl carbonate (DEC) and ethyl methyl carbonate (EMC) is used. By using a solid electrolyte layer containing such a mixed solvent, the reason is not clear, but generation of gas due to a chemical reaction at the electrode / electrolyte interface when left at high temperature can be suppressed, and blistering can be achieved. Characteristics (bulge characteristics)
Can be obtained.

Further, in the present invention, in the above mixed solvent,
In ethylene carbonate, the mixing ratio is from 1% by volume to 3%.
0 vol%, preferably 15 vol% to 25 vol%
Is more preferable. In propylene carbonate, the mixing ratio is preferably from 1% by volume to 30% by volume, and more preferably from 5% by volume to 15% by volume. In diethyl carbonate, the mixing ratio is from 1% by volume to 30%.
% By volume, more preferably 5% by volume to 20% by volume. In ethyl methyl carbonate, the mixing ratio is preferably from 10% by volume to 97% by volume, and more preferably from 50% by volume to 70% by volume.

[0010] In the mixed solvent of the present invention, if the mixing ratio of ethylene carbonate is less than 1% by volume, the dielectric constant of the electrolytic solution is lowered, which tends to make it difficult to obtain a sufficient charge / discharge capacity. . If the mixing ratio of ethylene carbonate exceeds 30% by volume, the freezing temperature of the electrolytic solution increases, and the low-temperature characteristics tend to be remarkably reduced, which is not preferable.

In the mixed solvent of the present invention, if the mixing ratio of propylene carbonate is less than 1% by volume, the cycle characteristics tend to deteriorate, which is not preferable. If the mixing ratio of propylene carbonate exceeds 30% by volume, blisters due to gas generation tend to occur, which is not preferable.

In the mixed solvent of the present invention, if the mixing ratio of diethyl carbonate is less than 1% by volume, the high rate characteristic tends to be lowered due to the increase in the viscosity of the electrolytic solution, which is not preferable. If the mixing ratio of diethyl carbonate exceeds 30% by volume, swelling when left at a high temperature tends to become remarkable, which is not preferable.

In the mixed solvent of the present invention, if the mixing ratio of ethyl methyl carbonate is less than 10% by volume, the high rate characteristic tends to decrease due to the increase in the viscosity of the electrolytic solution, which is not preferable. If the mixing ratio of ethyl methyl carbonate exceeds 97% by volume, the capacity tends to decrease as the dielectric constant decreases, which is not preferable.

In the present invention, the salt (electrolyte) concentration in the electrolyte in the solid electrolytic layer is preferably from 0.5 mol / l to 1.5 mol / l, and more preferably from 0.7 mol / l to 1. More preferably, it is 3 mol / l,
It is particularly preferred that it is between 0.8 mol / l and 1.2 mol / l. If the concentration of the salt (electrolyte) is less than 0.5 mol / liter, the ion conductivity is lowered, so that a sufficient battery capacity may not be obtained or the high-rate characteristics may be deteriorated. On the other hand, if the concentration of the salt (electrolyte) exceeds 1.5 mol / liter, high-rate characteristics and low-temperature characteristics may decrease due to an increase in viscosity, which is not preferable.

The fluoropolymer having vinylidene fluoride as a main unit is a homopolymer of vinylidene fluoride (polyvinylidene fluoride (PVdF)) or a mixture of vinylidene fluoride and another vinyl monomer having a fluorine atom. It means a copolymer, each of which may be used alone or in combination. Examples of the vinyl monomer having a fluorine atom other than the vinylidene fluoride include hexafluoropropylene (HFP),
Chlorotrifluoroethylene (CTFE), tetrafluoroethylene (TFE) and the like can be mentioned. The form of the copolymer may be any of a random form and a block form. In the case of a copolymer, the proportion of (units) of vinylidene fluoride is preferably at least 70 mol%, particularly preferably at least 75 mol%.

Fluoropolymers having vinylidene fluoride as a main unit include carboxyl groups (—COO).
H) a vinyl group having a functional group consisting of a sulfonic acid group (—SO ₂ OH), a carboxylic acid ester group (—COOR), an amide group (—CONH ₂ ), or a phosphoric acid group (—PO (OH) ₂ ). A polymer of a monomer may be grafted (the substituent R in the carboxylic acid ester group (—COOR) is a lower alkyl group having 1 to 4 carbon atoms such as a methyl group, an ethyl group, and a butyl group). . When the fluoropolymer is in a polymer form in which a polymer containing such a functional group is grafted, the adhesiveness of the solid electrolyte layer to the positive electrode or the negative electrode is improved, and the resistance between the electrodes is further reduced. Battery performance is further improved. As the vinyl monomer having a functional group, a monomer composed of a compound having a carbon number of 4 or less excluding the functional group is preferable. As the carboxyl group-containing monomer, acrylic acid, methacrylic acid, crotonic acid, vinyl acetic acid,
In addition to those having one carboxyl group such as allyl acetic acid,
Those having two or more carboxyl groups such as itaconic acid and maleic acid can also be used. As the sulfonic acid group-containing monomer, styrene sulfonic acid, vinyl sulfonic acid and the like are preferable. As the carboxylic acid ester group-containing monomer, methyl acrylate, butyl acrylate, and the like are preferable. Acrylamide and the like are preferable as the amide group-containing monomer. As the phosphoric acid group-containing monomer,
Triphenyl phosphate, tricresyl phosphate and the like are preferred. Most preferred of these are acrylic acid or methacrylic acid.

As a grafting method, a radiation method is preferable. For example, the radiation is continuously or intermittently applied to the radiation in the coexistence of the polymer chain substrate (the polymer to be grafted) and the graft monomer material, or
More preferably, the polymer substrate is pre-irradiated with radiation prior to the coexistence of both. As the radiation, an electron beam, X-ray or γ-ray is used. Upon irradiation, the polymer substrate generates and activates free radicals.

The degree of grafting can be determined by several factors, but most importantly, the length of time the activated substrate is in contact with the graft monomer,
The extent of substrate preactivation by radiation, the extent to graft monomer material can penetrate the substrate, and is a temperature at which the substrate and the monomer is in contact. For example,
When the graft monomer is an acid, the degree of grafting can be observed by sampling the solution containing the monomer, titrating against the base, and measuring the residual monomer concentration. The degree of grafting is preferably from 2% to 20% of the final weight, particularly preferably from 3% to 1%.
It is 2%, particularly preferably 5% to 10%. In addition,
Grafting activates the polymer substrate (generating free radicals)
May be performed by light irradiation or heat.

In the present invention, the density of the porous body of the fluoropolymer is not particularly limited, but may be 0.60 g / cm.
^{3 to} 1.30 g / cm ³ , preferably 0.70 g
/ Cm ^{3 to} 0.80 g / cm ³ , more preferably.
By interposing such a solid electrolyte layer between the positive electrode and the negative electrode, a sheet-shaped lithium secondary battery having further improved high-rate characteristics and low-temperature characteristics can be realized. It is considered that this is because the retention of the electrolyte in the solid electrolyte layer is further improved, which leads to a dramatic increase in the amount of ion transfer.

The above fluoropolymer has a density of 0.60 g /
If it is less than cm ³ , it tends to be difficult to handle at the time of assembling the battery due to a decrease in mechanical strength. The density of the fluoropolymer is 1.30 g /
If it exceeds cm ³ , it is difficult to obtain sufficient effects of improving high-rate characteristics and low-temperature characteristics, which is not preferable.

The average pore diameter of the pores in the porous body is preferably 0.01 μm to 10 μm, particularly preferably 0.1 μm to 5 μm. This “average pore size” is
M observations were taken out of arbitrary 10 holes, and
It indicates the average value of the pore diameters of the pores.

When the average pore diameter is less than 0.01 μm, the liquid holding capacity tends to decrease, and when the average pore diameter exceeds 10 μm,
The mechanical strength of the porous body tends to decrease, and the handleability in the battery manufacturing process tends to decrease.

The fluoropolymer having vinylidene fluoride as a main unit preferably has a melt flow index at 1.0 kg / 10 min or less at 230 ° C. and 10 kg, and is preferably 0.2 g / 10 min to 0.7 g / 10 m.
More preferably, it is in. The melt flow index is a value measured by a method described in standard ASTM D1238. When the melt flow index is 1.0 g / 10 min or less, the solid electrolyte layer exhibits more excellent mechanical strength, and the ionic conductivity at room temperature is further improved.

The thickness of the solid electrolyte layer varies depending on the shape and size of the positive electrode and the negative electrode, but is generally preferably 5 μm to 100 μm, particularly preferably 8 μm.
m to 50 μm, particularly preferably 10 μm to 30 μm
It is. Here, the thickness of the solid electrolyte layer is a thickness in a state where the solid electrolyte layer is interposed between the positive electrode and the negative electrode (in a state where the battery is actually assembled), and is equal to a separation distance between the positive electrode and the negative electrode.

The method for forming the solid electrolyte layer in the present invention is not particularly limited. For example, (a) a film is prepared by subjecting a fluoropolymer having vinylidene fluoride as a main unit as a main unit to a known foam molding method such as extrusion foam molding. Into a porous film by forming into a shape, or prepare a coating liquid (paste) in which a fluoropolymer, a suitable solvent and a foaming agent are mixed, and peel the coating liquid (paste) with a suitable coater. A coating film is formed by coating on the surface of the substrate, and the coating film is heated, dried, and then peeled from the peeling substrate to form a porous film, and the obtained porous film is dissolved with a salt in a compatible solvent. (B) dissolving the salt, the compatible solvent and the foaming agent in an appropriate solvent; Sa A fluoropolymer is added to the mixture, and the polymer is dissolved while heating, if necessary, to prepare a coating solution (paste), which is applied to the surface of the substrate for peeling using a suitable coater. A method in which the coating film is heated stepwise, heated and dried to evaporate the solvent, generate bubbles, and peel off the solid electrolyte layer from the peeling substrate. The above salt, directly on at least one surface of the positive electrode and / or the negative electrode formed in a
A method of forming a coating film of a solution in which a compatible solvent, a foaming agent, and a fluoropolymer are dissolved, and evaporating the solvent and generating bubbles to form a solid electrolyte layer may be used. In the method (b), the release substrate on which the coating film is formed may be dried by passing through a heating chamber having a different temperature.

As the blowing agent used in the above methods (a) to (c), any of a decomposable blowing agent, a gas blowing agent and a volatile blowing agent can be used. Agents or volatile blowing agents are preferred, and gaseous blowing agents include nitrogen, carbon dioxide, propane, neopentane,
Methyl ether, difluoromethane dichloride, n-butane,
Isobutane and the like are preferable, and as a volatile foaming agent,
n-octanol, 1-pentanol, 3-methyl-1
-Butanol, 2-methyl-1-butanol, 2-pentanol, 3-pentanol, 2-methyl-2-butanol, 3-methyl-2-butanol and the like are preferred. In the methods (b) and (c), volatile foaming agents are preferred, and among them, n-octanol, 1-pentanol, 3-methyl-1-butanol and the like are particularly preferred.
n-Octanol is particularly preferred. Examples of the solvent in the methods (b) and (c) include, for example,
Tetrahydrofuran (THF), dimethylacetamide, dimethylformamide and the like are used.

The density of the porous body is adjusted by changing the amount of the foaming agent and various conditions during production. For example,
In the case of the method (a), the density (expansion degree) other than the amount of the foaming agent is used.
The manufacturing conditions that greatly affect the temperature include a molding temperature, a molding speed, and a molding pressure. In the case of the methods (b) and (c), the production conditions that greatly affect the density (expansion degree) other than the amount of the foaming agent include a coating speed, a drying temperature profile, a degree of exhaustion, and a molding speed. is there.

As the positive electrode, the negative electrode, and the sheet-like exterior material constituting the sheet-type lithium secondary battery of the present invention, conventionally known materials can be used. Examples of these are shown below.

For the positive electrode and the negative electrode, a positive electrode active material composition or a negative electrode active material composition in which a conductive material or a binder is mixed with a positive electrode active material or a negative electrode active material is applied on a current collector, and then dried and
Rolling is performed to form a positive electrode active material layer or a negative electrode active material layer.

As the positive electrode active material, a Li-Co-based composite oxide, a Li-Mn-based composite oxide, a Li-Ni-based composite oxide and the like can be used, and among them, it is chemically stable and easy to handle. It is preferable to use lithium cobalt oxide (LiCoO ₂ ) in view of the fact that a secondary battery having a high capacity can be manufactured. Examples of the conductive material include artificial or natural graphites, and granular carbon materials such as acetylene black, oil furnace black, and carbon black such as extra conductive furnace black ("granular" refers to flaky, spherical, pseudo spherical, Lump-like, whisker-like and the like are not particularly limited.) Can be used. Further, as the binder in the positive electrode, polytetrafluoroethylene, polyvinylidene fluoride, ethylene-propylene-diene-based polymer, or the like can be used. In the positive electrode active material composition, the conductive material is about 3 to 15 parts by weight, and the binder is 1 to 10 parts by weight based on 100 parts by weight of the total of the positive electrode active material, the conductive material, and the binder. It is preferred to be blended in about parts.

As the negative electrode active material, graphites such as fibrous graphite, flaky graphite and spherical graphite, preferably fibrous mesophase graphitized carbon can be used. As the binder for the negative electrode, the same binder as that for the positive electrode active material can be used. The amount of the negative electrode active material used may be about 80 to 96 parts by weight per 100 parts by weight of the total amount of the negative electrode active material and the binder.

As the current collector, a foil or a perforated foil formed of a conductive metal can be used, and its thickness may be about 5 μm to 100 μm. As the material of the positive electrode current collector, aluminum, stainless steel, or the like is used, and as the material of the negative electrode current collector, copper, nickel, silver, stainless steel, or the like is used.

The positive electrode and the negative electrode can be manufactured according to a known method. For example, for a positive electrode, a positive electrode active material, a binder, and a conductive material are mixed and processed, dispersed in an organic solvent such as N-methyl-2-pyrrolidone to form a paste, and the paste is applied to a positive electrode current collector and dried. Then, it can be obtained by pressing and cutting into an appropriate shape.

The form of the sheet-shaped lithium secondary battery includes a stacked type and a wound type, and the sheet-shaped lithium secondary battery of the present invention can be advantageously applied to the stacked type. . FIG. 1 is a cross-sectional view schematically showing a sheet-shaped lithium secondary battery 20 when the present invention is realized as a stacked type. As shown in the cross-sectional view of FIG. 1, the stacked type is such that the positive electrode and the negative electrode are rectangular plate-like bodies (referred to as a positive electrode plate 1 and a negative electrode plate 2, respectively. It has a positive electrode active material layer 6 and a negative electrode active material layer 7 laminated on the conductors 4 and 5), and a solid electrolyte layer 3 is sandwiched between the positive electrode plate 1 and the negative electrode plate 2 (FIG. 1). In the example of (1), the positive electrode plate 1 is accommodated in the bag-shaped solid electrolyte layer 3). It has a configuration in which it is covered with a material 8, and usually has a plate shape.

In the present invention, the sheet-like exterior material refers to a material obtained by laminating a resin layer on at least one surface of a metal foil. Examples of the metal foil include aluminum, copper, and iron. Examples of the resin include thermoplastic resins such as polyester and polypropylene. As long as it has such a thermoplastic resin layer, it can be sealed only by externally covering the laminated structure (stack) and heat-welding the periphery thereof,
The battery manufacturing operation is simple. The thickness of the metal foil is preferably 10 μm to 100 μm, more preferably 20 μm to
70 μm, and the thickness of the resin layer is preferably 5 μm to
100 μm, more preferably 10 μm to 50 μm, and the overall thickness is preferably 20 μm to 200 μm.
μm, more preferably 40 μm to 150 μm.

[0036]

The present invention will be described in detail with reference to the following examples, but the present invention is not limited to the following examples. Example 1 [Preparation of positive electrode plate] 90 parts by weight of lithium cobaltate granules as a positive electrode active material, 5 parts by weight of polyvinylidene fluoride as a binder, 5 parts by weight of artificial graphite as a conductive material, and N-methyl- A slurry was prepared by mixing 70 parts by weight of 2-pyrrolidone. This slurry is applied on both sides of an aluminum foil (thickness: 20 μm) as a positive electrode current collector, dried, and then subjected to a rolling treatment (rolling temperature: 25 ° C., rolling ratio: 3)
0%) and 20 mg / c per side of aluminum foil
A positive electrode plate (width: 30 mm, length: 50 mm) having an m ² positive electrode active material layer (thickness: 70 μm) was prepared.

[Preparation of negative electrode plate] 95 parts by weight of fibrous graphitized carbon, 5 parts by weight of polyvinylidene fluoride and N-
A slurry was prepared by mixing 60 parts by weight of methyl-2-pyrrolidone. This slurry was applied to both sides of a copper foil (thickness: 14 μm) as a negative electrode current collector, dried, and then subjected to a rolling treatment (rolling temperature: 100 ° C., rolling ratio: 20%) to give 10 mg / cm ² negative electrode active material layer (thickness: 7
Negative electrode plate having a width of 5 μm (width: 32 mm, length: 52 m)
m) was prepared.

[Preparation of porous film for solid electrolyte layer]
30 parts by weight of polyvinylidene fluoride (PVdF), 170 parts by weight of dimethylformamide (DMF),
A coating liquid (paste) is prepared by mixing 30 parts by weight of n-octanol as a foaming agent with a screw-type mixer, and the coating liquid (paste) is transferred from a transfer-type coating machine to a release substrate made of Al. After coating at a coating speed of 1 m / min, the obtained coating film was heated at 160 ° C. for 6 minutes to volatilize and foam the solvent, and the obtained film was peeled from the peeling substrate. . The thickness of the film was 25 μm, and the density was 0.85 g / cm ³ . Such a porous film of polyvinylidene fluoride (PVdF) is cut into a predetermined size, and the above film is cut using a hot press machine.
A bag having a size capable of accommodating two positive plates was produced.

[Preparation of electrolyte solution] Ethylene carbonate (EC) 20% by volume and propylene carbonate (P
C) 10% by volume and diethyl carbonate (DEC) 1
0% by volume and ethyl methyl carbonate (EMC) 60
LiPF ₆ was dissolved in a mixed solvent consisting of 10% by volume to give a concentration of 1.0 mol / liter (relative to the prepared electrolytic solution) to prepare an electrolytic solution.

[Assembly of Laminated Sheet-Like Lithium Secondary Battery] Using the positive electrode plate and the negative electrode plate prepared above and a bag of polyvinylidene fluoride (PVdF) film, one positive electrode plate was placed in each bag. The positive electrode plate, the PVdF film, and the negative electrode plate were stacked so that the unit in which the PVdF film was interposed between the positive electrode plate and the negative electrode plate was repeated eight times in a state in which was accommodated, and a laminated structure was produced. This laminated structure was immersed in the electrolyte solution prepared above for 1 hour, and PVd
The F film was gelled.

Next, the laminated structure obtained by gelling the PVdF film is accommodated in an Al-laminated film obtained by laminating a thermoplastic resin as an exterior material on one or both sides, and the laminated sheet-like lithium secondary A battery was manufactured.

Example 2 15% by volume of ethylene carbonate, 15% by volume of propylene carbonate and 35% by volume of diethyl carbonate
And 35% by volume of ethyl methyl carbonate in the same manner as in Example 1 except that a mixed solvent of
A sheet-shaped lithium secondary battery was produced.

Example 3 25% by volume of ethylene carbonate, 25% by volume of propylene carbonate and 30% by volume of diethyl carbonate
And a mixed solvent of 20% by volume of ethyl methyl carbonate for the electrolytic solution and a density of 0.75 g / cm ³
A sheet-shaped lithium secondary battery was produced in the same manner as in Example 1 except that the PVdF porous film was used.

Example 4 10% by volume of ethylene carbonate, 10% by volume of propylene carbonate, and 60% by volume of diethyl carbonate
And 20% by volume of ethyl methyl carbonate in the same manner as in Example 3 except that a mixed solvent of
A sheet-shaped lithium secondary battery was produced.

Comparative Example 1 10% by volume of ethylene carbonate, 10% by volume of propylene carbonate, and 5% by volume of diethyl carbonate
Then, a sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent of 30% by volume of ethyl methyl carbonate and 45% by volume of dimethyl carbonate (DMC) was used for the electrolytic solution.

Comparative Example 2 15% by volume of ethylene carbonate, 5% by volume of propylene carbonate, and 10% by volume of diethyl carbonate
Then, a sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent of 20% by volume of ethyl methyl carbonate and 50% by volume of dimethyl carbonate was used for the electrolytic solution.

Comparative Example 3 5% by volume of ethylene carbonate, 15% by volume of propylene carbonate and 10% by volume of diethyl carbonate
A sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent consisting of 10% by volume of ethyl methyl carbonate and 60% by volume of dimethyl carbonate was used as the electrolytic solution.

Comparative Example 4 5% by volume of ethylene carbonate, 5% by volume of propylene carbonate, 20% by volume of diethyl carbonate,
A sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent consisting of 20% by volume of ethyl methyl carbonate and 50% by volume of dimethyl carbonate was used for the electrolytic solution.

Comparative Example 5 A sheet-shaped lithium secondary battery was produced in the same manner as in Example 3 except that only ethylene carbonate was used as the electrolytic solution.

Comparative Example 6 A sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent consisting of 50% by volume of ethylene carbonate and 50% by volume of propylene carbonate was used as the electrolyte.

Comparative Example 7 30% by volume of ethylene carbonate, 20% by volume of propylene carbonate and 50% by volume of ethyl methyl carbonate
A sheet-shaped lithium secondary battery was produced in the same manner as in Example 3 except that the mixed solvent consisting of

Comparative Example 8 10% by volume of ethylene carbonate, 10% by volume of propylene carbonate and 10% by volume of diethyl carbonate
Then, a sheet-shaped lithium secondary battery was produced in the same manner as in Example 3, except that a mixed solvent of 20% by volume of ethyl methyl carbonate and 50% by volume of dimethyl carbonate was used for the electrolytic solution.

The following tests were performed on each of the samples of the laminated sheet-type lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 8 obtained above. The samples of Comparative Examples 5 and 6 could not be charged and discharged. [Bulging property (bulge property) test] Each sample of the laminated sheet-type lithium secondary battery was left in an atmosphere at 90 ° C for 3 hours, and the length (mm) swelled in the thickness direction was measured.

[High Rate Characteristics] At room temperature (20 ° C.)
2C discharge was performed, and the ratio (%) of the discharge capacity to the total capacity was calculated. Table 1 shows the results of Examples 1 to 4, and Table 2 shows the results of Comparative Examples 1 to 8.

[0055]

[Table 1]

[0056]

[Table 2]

[0057]

As is apparent from the above description, according to the present invention, there is provided a sheet-shaped lithium secondary battery having improved blistering characteristics (bulge characteristics) by suppressing generation of gas when left at high temperatures. can do.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view schematically showing a sheet-shaped lithium secondary battery 20 when realized in a stacked type according to the present invention.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Positive electrode plate 2 Negative electrode plate 3 Solid electrolyte layer 8 Sheet-shaped exterior material 10 Laminated structure 20 Sheet-shaped lithium secondary battery

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Mitsuhiro Marumoto 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Cable Industries, Ltd. Itami Works (72) Inventor Seiji Okada 4-3-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Wire In Itami Works, Industrial Co., Ltd. (72) Inventor Toshi 4-3 Ikejiri, Itami-shi, Hyogo Mitsubishi Electric Cable Co., Ltd. Itami Works F-term (reference) 5H029 AJ04 AK03 AL07 AM03 AM05 AM07 AM16 BJ04 DJ02 DJ13 EJ12 EJ14 HJ07 HJ08

Claims (3)

[Claims] 1. A solid electrolyte layer mainly composed of a salt, a compatible solvent, and a porous body of a fluoropolymer having vinylidene fluoride as a main unit interposed between a positive electrode and a negative electrode, A sheet-like lithium secondary battery packaged with a sheet-like packaging material, wherein the salt is LiClO ₄ , LiBF ₄ , LiPF ₆ , LiA
sF ₆ , LiCF ₃ SO ₃ , LiAlCl ₄ and Li (C
At least one compound selected from the group consisting of F ₃ SO ₂ ) ₂ N, and the compatible solvent is ethylene carbonate;
Propylene carbonate, diethyl carbonate,
A sheet-shaped lithium secondary battery, which is a mixed solvent comprising ethyl methyl carbonate. 2. In the mixed solvent, the mixing ratio of ethylene carbonate is 1% to 30% by volume, the mixing ratio of propylene carbonate is 1% to 30% by volume,
The mixing ratio of diethyl carbonate is 1% by volume to 30% by volume
The sheet-shaped lithium secondary battery according to claim 1, wherein the mixing ratio of ethyl methyl carbonate is 10% by volume to 97% by volume. 3. The fluoropolymer porous body has a density of 0.
Sheet lithium secondary battery according to claim 1 or 2, characterized in that a ^{60g / cm 3 ~1.30g / cm 3} .