JP2003257486A - Gel electrolyte formable sheet, gel electrolyte supporting sheet and lithium secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a thickness reducible gel electrolyte formable sheet having superior mechanical strength, less possible internal short-circuit, high electrolytic solution retaining ability and low internal resistance, a gel electrolyte supporting sheet, and a lithium secondary battery using the same. <P>SOLUTION: The gel electrolyte formable sheet is formed of an non-woven cloth consisting dominantly of polyvinylidene fluoride microfibers having an average diameter of 2 μm or less. It desirably contains fibers which cannot be gelled by a non-aqueous solvent for gelling the polyvinylidene fluoride microfibers. It also desirably has a thickness of 20 μm or less. The gel electrolyte supporting sheet is formed by gelling the polyvinylidene fluoride microfibers for the gel electrolyte formable sheet with the non-aqueous solvent. The lithium secondary battery uses the gel electrolyte supporting sheet. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a gel electrolyte formable sheet, a gel electrolyte supporting sheet, and a lithium secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, as electronic devices have become smaller and lighter and have higher functionality, development of thinner and higher capacity secondary batteries (especially lithium secondary batteries among them) as power sources thereof has been actively conducted. Is being advanced to.

Among them, recently, a lithium secondary battery using a gel electrolyte sheet in place of the conventional separator has been attracting attention. Even though various gel electrolytes have been investigated, many studies have been conducted because polyvinylidene fluoride-based resins are excellent in electrochemical stability in batteries and low-temperature characteristics of batteries. As one of them, a separator / gel electrolyte sheet has been proposed in which an olefin-based microporous membrane material is used as a support, and a polyvinylidene fluoride-based resin is supported on this support. However, since the olefin microporous membrane has a low porosity and the amount of polyvinylidene fluoride resin supported is limited, the amount of electrolyte retained when gelled by an electrolyte (non-aqueous solvent) is low and the internal resistance is low. There was a problem that the battery capacity was large and the battery capacity after storage at a high temperature was large. Further, according to this method, the thickness of the separator / gel electrolyte sheet is restricted by the thickness of the microporous membrane, so that the lower limit is about 20 μm at the present time, and the battery can be made thinner by making the separator / gel electrolyte sheet thinner. It was difficult to increase the capacity.

As another method, a method of using a porous film of polyvinylidene fluoride resin as a separator / gel electrolyte sheet has been proposed. In this method, since the amount of polyvinylidene fluoride resin per unit volume is larger than that of the above method, the electrolyte retention after gel formation is high,
It has an advantage that the capacity of the battery does not easily decrease. However, since the mechanical strength of the porous film itself is low, breakage in the battery assembly process or cutting at the electrode plate end easily occurs,
It has a drawback that internal short circuits are likely to occur.

[0005]

SUMMARY OF THE INVENTION An object of the present invention is to solve the above-mentioned problems, which is excellent in mechanical strength, less likely to cause an internal short circuit, and has high electrolyte retention and low internal resistance. An object of the present invention is to provide a gel electrolyte formable sheet that can be made thin, a gel electrolyte supporting sheet, and a lithium secondary battery using the same.

[0006]

The gel electrolyte formable sheet of the present invention according to claim 1 has an average fiber diameter of 2 μm.
The following non-woven fabric is mainly composed of polyvinylidene fluoride fine fibers. In addition to the retention of non-aqueous solvent (sometimes referred to as "electrolyte solution") due to the swelling of polyvinylidene fluoride resin itself in non-aqueous solvent, it is composed mainly of extremely thin fibers with an average fiber diameter of 2 μm or less Since it is a nonwoven fabric having a high porosity, the non-aqueous solvent (electrolytic solution) is more excellent in retention. Therefore, it is possible to form a battery having a reduced internal resistance and excellent cycle characteristics. In addition, since it can be a thin non-woven fabric mainly composed of extremely fine fibers having an average fiber diameter of 2 μm or less, it can contribute to the thinning and high capacity of the battery. Further, since the average fiber diameter is small, the number of fibers is large and the number of intersections between fibers is large, so that the mechanical strength is also high. Therefore, the gel electrolyte-formable sheet is excellent in mass productivity, and is unlikely to cause internal short circuit due to breakage in the battery assembly process or disconnection at the end of the electrode plate.

The gel electrolyte formable sheet of the present invention according to claim 2 is characterized in that it contains fibers that cannot be gelled by a non-aqueous solvent that gels polyvinylidene fluoride fine fibers. In the gel electrolyte-forming sheet of the present invention, by containing a non-gelling fiber by a non-aqueous solvent for gelling polyvinylidene fluoride-based fine fibers, polyvinylidene fluoride-based fine fibers gelled gel electrolyte-supporting sheet Since the mechanical strength can be kept high, it is possible to form a gel electrolyte-supporting sheet having excellent internal short-circuit prevention properties.

The gel electrolyte formable sheet of the present invention according to claim 3 has a thickness of 20 μm or less. Therefore, the electrical resistance of the gel electrolyte formable sheet itself can be lowered, and the internal resistance can be lowered. Also,
By making it thinner, the area of the electrode plate per the same volume can be increased, which can also contribute to higher capacity of the battery.

The gel electrolyte-supporting sheet of the present invention according to claim 4 is a gel electrolyte-forming sheet according to any one of claims 1 to 3, wherein the polyvinylidene fluoride-based fine fibers are gelled with a non-aqueous solvent. It has been made into. Therefore, an internal short circuit is unlikely to occur and a battery having a low internal resistance can be manufactured. Further, since the gel electrolyte formable sheet is thin and has excellent mechanical strength, it is possible to manufacture a thin battery or a high capacity battery.

The lithium secondary battery of the present invention as defined in claim 5 is a lithium secondary battery using the gel electrolyte supporting sheet as defined in claim 4. Therefore, an internal short circuit is unlikely to occur and the internal resistance is low. In addition, since the gel electrolyte formable sheet is thin and has excellent mechanical strength, it can be a thinner lithium secondary battery or a higher capacity lithium secondary battery than ever before.

[0011]

BEST MODE FOR CARRYING OUT THE INVENTION The gel electrolyte formable sheet of the present invention comprises a non-woven fabric mainly composed of polyvinylidene fluoride fine fibers having an average fiber diameter of 2 μm or less, and has excellent non-aqueous solvent (electrolyte) retention. Therefore, the internal resistance can be reduced and the nonwoven fabric can be thin, so that the electrical resistance of the nonwoven fabric itself is low, and the battery can be thinned or the capacity can be increased. Further, since the average fiber diameter is small, the number of fibers is large and the number of intersections between fibers is large, so that the mechanical strength is also high. Therefore, the gel electrolyte-formable sheet is excellent in mass productivity, and is unlikely to cause internal short circuit due to breakage in the battery assembly process or disconnection at the end of the electrode plate.

The average fiber diameter of the polyvinylidene fluoride-based fine fibers constituting the gel electrolyte formable sheet of the present invention is such that the non-aqueous solvent (electrolyte) retention is excellent and the thickness of the nonwoven fabric is small. It is necessary to be 2 μm or less so that the thinner the average fiber diameter, the better the retention of the non-aqueous solvent (electrolytic solution). Therefore, the average fiber diameter of the polyvinylidene fluoride fine fibers is 1 μm. It is preferably not more than 0.5 μm, more preferably not more than 0.5 μm. The lower limit of the average fiber diameter of the polyvinylidene fluoride fine fibers is not particularly limited as long as it does not significantly increase the electric resistance of the gel electrolyte-supporting sheet formed into a gel, but is about 0.01 μm. Appropriate.

The average fiber diameter of the polyvinylidene fluoride fine fibers is 2 μm or less, but at any point of the polyvinylidene fluoride fine fibers, the fiber diameter is preferably 2 μm or less, preferably 1 μm or less. Is more preferable,
It is more preferably 0.5 μm or less.

The "fiber diameter" in the present invention refers to the diameter when the fiber cross-sectional shape is circular, and the circle having the same area as the cross-sectional area when the fiber cross-sectional shape is non-circular. The diameter is considered to be the fiber diameter. Also, "average fiber diameter"
Is the average value of the fiber diameter at 100 points of the fiber.

Examples of the resin that constitutes the polyvinylidene fluoride fine fibers that constitute the nonwoven fabric of the present invention include polyvinylidene fluoride (PVDF) and vinylidene fluoride-6.
Propylene Fluoride (VDF-HFP) Copolymer, Vinylidene Fluoride-Trifluorochloroethylene (VDF-CTF)
E) Copolymer, vinylidene fluoride-4 fluorinated ethylene-
A propylene hexafluoride (VDF-TFE-HFP) copolymer etc. can be mentioned, and it can be comprised from 1 type, or 2 or more types of these. It should be noted that the polyvinylidene fluoride-based fine fibers may include the above-mentioned polyvinylidene fluoride-based resin as a resin constituting the fiber surface, and may contain a resin other than the polyvinylidene fluoride-based resin. For example, a resin that does not gel with a non-aqueous solvent may be included. In this case, it is possible to form a gel electrolyte supporting sheet having more excellent mechanical strength.

The polyvinylidene fluoride fine fibers constituting the gel electrolyte formable sheet of the present invention can swell and form a gel when contacted with a non-aqueous solvent.
The non-woven fabric mainly composed of polyvinylidene fluoride fine fibers, that is, the gel electrolyte formable sheet has a high non-woven fabric structure in addition to the retention of the non-aqueous solvent (electrolytic solution) due to the swelling of the polyvinylidene fluoride fine fibers. Due to the porosity, it has an extremely high non-aqueous solvent (electrolytic solution) retention property.

The solvent for gelling such polyvinylidene fluoride fine fibers differs depending on the intended use of the gel electrolyte formable sheet, and is not particularly limited. For example, the gel electrolyte formable sheet may be a lithium secondary When used in a gel electrolyte carrier sheet for a battery, the non-aqueous solvent (electrolyte) is preferably one that does not decompose even when a high voltage is applied, for example, carbonates (for example, ethylene carbonate , Propylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, ethylmethyl carbonate, etc.), tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-
Dioxolane, γ-butyrolactone, sulfolane, 3-
A nonaqueous solvent in which methylsulfolane, dimethoxyethane and the like are used alone or in a mixture is preferable.

The gel electrolyte formable sheet of the present invention preferably contains fibers that cannot be gelled by a non-aqueous solvent that gels polyvinylidene fluoride fine fibers in the sheet. This non-gelling fiber is a fiber whose mechanical and chemical properties do not deteriorate due to swelling or dissolution even when it comes into contact with a non-aqueous solvent that gels polyvinylidene fluoride fine fibers. Such fibers can exist while maintaining the fiber shape even when gelling polyvinylidene fluoride fine fibers with a non-aqueous solvent, and high mechanical strength can be maintained, further improving separation performance. It is possible to form a gel electrolyte-supporting sheet having excellent internal short-circuit prevention properties.

The non-gelling fiber is not particularly limited, but as the non-gelling fiber that can be used in the lithium secondary battery as described above, for example, a polyester resin ( Fibers made of polyethylene terephthalate, polypropylene terephthalate, polybutylene terephthalate, polyethylene naphthalate, etc.), polyolefin resins (polyethylene, polypropylene, polymethylpentene etc.), polyphenylene sulfide resin, etc. can be mentioned.

The non-gelling fiber may be composed of the above resin component alone, or may be a mixture or a mixture of two or more kinds of resin components. The cross-sectional shape of such non-gelling fiber is not particularly limited, but when it is composed of the former single resin component, it can be, for example, a perfect circle type or an elliptic type, and the latter two types. When the above resin components are used, for example, a core-sheath type, an eccentric type,
It can be sea-island type. When two or more kinds of resin components are compounded, the resin components forming the fiber surface can be fused, so that it is possible to produce a gel electrolyte formable sheet having more excellent mechanical strength.

The fiber diameter of such non-gelling fibers is not particularly limited, but it is averaged from the viewpoint of satisfying the retention of the non-aqueous solvent (electrolyte) and the separation performance such as prevention of internal short circuit. The fiber diameter is preferably 5 μm or less, and 2 μm
It is more preferably m or less, still more preferably 1 μm or less. The lower limit is not particularly limited,
About 0.01 μm is suitable.

Such non-gelling fibers may be compounded at the time of producing the above-mentioned non-woven fabric mainly composed of polyvinylidene fluoride fine fibers, or a sheet containing non-gelling fibers may be formed. After that, it may be combined with a non-woven fabric mainly composed of polyvinylidene fluoride fine fibers.

The gel electrolyte formable sheet of the present invention is mainly composed of the above polyvinylidene fluoride fine fibers (50 m).
However, the internal resistance is low when incorporated in a battery, and the decrease in battery capacity after storage at high temperature is small. , Polyvinylidene fluoride fine fiber 60mass
s% or more, preferably 70 mass% or more, more preferably 80 mass% or more, still more preferably 90 mass% or more, still more preferably 100 mass% polyvinylidene fluoride-based fine Most preferably it consists of fibers. As mentioned above, in order to maintain high mechanical strength,
When non-gelling fibers are included, the mass ratio is preferably (polyvinylidene fluoride fine fibers) :( non-gelling fibers) = 50 to 95:50 to 5, and (polyfluoride). Vinylidene fluoride fine fiber): (non-gelling fiber) = 60 to 90:40 to 10 is more preferable, and (polyvinylidene fluoride fine fiber): (non-gelling fiber) = 70 to It is more preferably 90:30 to 10.

The thickness of the gel electrolyte formable sheet of the present invention is such that the electric resistance of the gel electrolyte formable sheet itself can be lowered and the internal resistance can be lowered.
In addition, the thickness is preferably 20 μm or less, and more preferably 18 μm or less so that the battery can have a higher capacity. The lower limit of the thickness is 5 from the standpoint of separation performance.
About μm is appropriate. The "thickness" in the present invention
Indicates the thickness measured by an outer micrometer specified in JIS B7502.

Further, the gel electrolyte formable sheet of the present invention
The areal weight and apparent density of are not particularly limited,
The unit weight is 3 to 15g so that it has some mechanical strength.
/ M ^TwoIt is preferable that the apparent density is 0.3 to 1.0.
g / cm³Is preferred. In addition, "weight" is JI
Specified in S P 8124 (Paper and board-Basis weight measurement method)
"Apparent Density" means the basis weight
It means the value divided by the thickness.

The gel electrolyte formable sheet of the present invention can be manufactured, for example, by the following method. (1)
A step of forming a gel solution mainly composed of polyvinylidene fluoride resin, (2) the gel solution is extruded from a nozzle, and the extruded gel solution is thinned by applying an electric field to the polyvinylidene fluoride gel thin A step of forming fibers and accumulating the polyvinylidene fluoride gel thin fibers on a support; (3) drying the accumulated polyvinylidene fluoride gel thin fibers to obtain polyvinylidene fluoride thin fibers And a step of forming a non-woven fabric mainly composed of. According to this method, it is possible to manufacture a non-woven fabric mainly composed of polyvinylidene fluoride fine fibers having an average fiber diameter of 2 μm or less as described above.

More specifically, first, (1) a gel solution containing a polyvinylidene fluoride resin as a main component is formed. This gel solution can be obtained, for example, by dissolving a predetermined amount in a solvent capable of dissolving the above-mentioned polyvinylidene fluoride resin (for example, N-methylpyrrolidone, acetone, methyl ethyl ketone, dimethylformamide, etc., alone or mixed). it can. The polyvinylidene fluoride resin to be dissolved does not have to be one type, and may be two or more types.

Next, (2) the gel solution is extruded from a nozzle, and the extruded gel solution is thinned by applying an electric field to form polyvinylidene fluoride gel-like fine fibers, and polyfluorinated on the support. Vinylidene-based gel fine fibers are accumulated. By intermittently extruding the gel solution from the nozzle, a polyvinylidene fluoride gel thin fiber having a short fiber length is formed, and a short fiber polyvinylidene fluoride gel thin fiber is formed on the support. It can also be accumulated. If a nozzle that continuously extrudes the gel solution and a nozzle that intermittently extrudes the gel solution are installed together, long-fiber-like polyvinylidene fluoride gel-like fine fibers and short-fiber-like polyvinylidene fluoride gel-like fine fibers can be obtained. It can be accumulated in a state where fibers are mixed. Furthermore, if a non-gelling fiber is supplied (for example, sprayed) to the flow of the polyvinylidene fluoride gel thin fiber extruded from the nozzle, the polyvinylidene fluoride gel thin fiber and the gelling non-gelable It can be collected in a mixed state with possible fibers.

The diameter of the nozzle for extruding this sol solution is
The average fiber diameter of polyvinylidene fluoride gel-like fine fibers is 2
The thickness is preferably 0.1 to 3 mm so that the thickness may easily be less than or equal to μm. In the drying step described below, the polyvinylidene fluoride gel-like fine fibers contract to some extent, so that the average fiber diameter of the polyvinylidene fluoride gel-like fine fibers is 2 μm.
It does not need to be below, and if the average fiber diameter is 3 μm or less, a polyvinylidene fluoride fine fiber having an average fiber diameter of 2 μm or less can be obtained.

The nozzle may be made of metal or non-metal. If the nozzle is made of metal, the nozzle can be used as one electrode, and if the nozzle is made of non-metal, by installing the electrode inside the nozzle,
An electric field can be applied to the polyvinylidene fluoride gel thin fibers.

By applying an electric field to the gel solution extruded from such a nozzle, the gel solution is thinned to form polyvinylidene fluoride gel thin fibers. This electric field is generated by the polyvinylidene fluoride gel thin fibers. An electric field is applied to make the average fiber diameter 2 μm or less. The electric field varies depending on the distance between the nozzle and the support, the solvent of the raw material solution, the viscosity of the gel solution, etc., and is not particularly limited, but is preferably 0.5 to 5 kV / cm.

Such an electric field can be caused to act by providing a potential difference between the nozzle (the nozzle itself in the case of metal, the electrode inside the nozzle in the case of non-metal) and the support. You can

Although the polyvinylidene fluoride gel-like fine fibers thus thinned are accumulated on a support, this support may be simply the polyvinylidene fluoride-based gel fine fibers. Alternatively, it may be one capable of being composite-integrated with the polyvinylidene fluoride-based gel fine fibers. In the former case, examples of the support include a porous roll, a non-perforated roll, a perforated sheet or belt, a non-perforated sheet or belt (for example, a film), and the like. In the latter case, when a fiber sheet containing fibers that cannot be gelled by a non-aqueous solvent that gels polyvinylidene fluoride fine fibers is used, the fiber sheet containing the non-gelling fibers and the polyvinylidene fluoride fine fibers are used. It is possible to form a composite with an aggregate including. In this way, in the case of composite integration with a fiber sheet containing non-gelling fibers,
It is preferable to accumulate polyvinylidene fluoride-based gel fine fibers on both the front and back surfaces of a fiber sheet containing non-gelling fibers. This is because the interface resistance at the contact interface with both electrodes can be reduced when a battery is manufactured.

When the support is used as one of the electrodes as described above, the support is preferably made of a conductive material (for example, metal) having a volume resistance of 10 ⁹ Ω or less. In addition,
A conductive material may be arranged as a counter electrode below the support. In this case, the support does not necessarily have to be a conductive material. When the counter electrode is arranged below the support, the support and the counter electrode may be in contact with each other or may be separated from each other.

Next, (3) the accumulated polyvinylidene fluoride gel fine fibers are dried and the solvent is removed to produce a nonwoven fabric containing polyvinylidene fluoride fine fibers, that is, a gel electrolyte formable sheet. be able to. This drying can be performed by an oven, freeze drying, or supercritical drying. In addition, in order to improve the mechanical strength of the gel electrolyte formable sheet and adjust the thickness, it may be further heated and pressed.

The gel electrolyte-supporting sheet of the present invention is obtained by gelling the polyvinylidene fluoride fiber of the above-mentioned gel electrolyte-forming sheet with a non-aqueous solvent.
Therefore, an internal short circuit is unlikely to occur and a battery having a low internal resistance can be manufactured. Further, since the gel electrolyte formable sheet is thin and has excellent mechanical strength, it is possible to manufacture a thin battery or a high capacity battery.

The non-aqueous solvent varies depending on its use and is not particularly limited. For example, when the non-aqueous solvent is used for a lithium secondary battery, the above-mentioned non-aqueous solvent can be used. That is, for example, carbonates (for example, ethylene carbonate, propylene carbonate, butylene carbonate, dimethyl carbonate,
Diethyl carbonate, ethylmethyl carbonate, etc.), tetrahydrofuran, 2-methyltetrahydrofuran, 1,3-dioxolane, 4-methyl-1,3-dioxolane, γ-butyrolactone, sulfolane, 3-methylsulfolane, dimethoxyethane, etc. Alternatively, a mixed non-aqueous solvent can be used.

Various substances are added in addition to the non-aqueous solvent.
You may. For example, when using for lithium secondary batteries
In order to improve ionic conductivity, it is necessary to add electrolytic salt.
Is preferred. As this electrolytic salt, for example, LiPF₆,
LiClO_Four, LiBF_Four, LiAsF₆, LiCF_Three
SO_Three, LiN (CF_ThreeSO_Two)_Three, LiN (C_TwoF ₅
SO_Two)_ThreeIt includes one or more types such as
preferable. Such electrolytic salt is a non-aqueous solvent gel electrolyte
Electrolytic salt may be added when contacting the formable sheet.
An electrolytic salt prepared by dissolving an electrolytic salt in a non-aqueous solvent in advance.
Bring the dissolved non-aqueous solution into contact with the gel electrolyte formable sheet.
May be. As in the latter case, a non-aqueous solution containing electrolytic salt is
If it is a method of contacting with a degradable sheet, electrolytic salt
Is more preferable because it is uniformly dissolved in the non-aqueous solution.
The concentration of the electrolytic salt in the electrolytic salt-dissolved non-aqueous solution is
It is preferably 0.5 to 3 mol / L.

The lithium secondary battery of the present invention uses the gel electrolyte supporting sheet as described above, and is combined with the negative electrode and the positive electrode to form a lithium secondary battery. Therefore, an internal short circuit is unlikely to occur and the internal resistance is low. In addition, since the gel electrolyte formable sheet is thin and has excellent mechanical strength, it can be a thinner lithium secondary battery or a higher capacity lithium secondary battery than ever before.

The negative electrode is composed of, for example, a negative electrode active material, a binder polymer, and a current collector. The negative electrode active material therein may be any material capable of inserting and extracting lithium ions and may be used without particular limitation, but a carbon-based material is particularly preferably used. More specifically, for example, a graphite-based carbon material,
A coke-based carbon material or a fibrous carbon material can be used.

As the binder polymer, a non-aqueous solvent (particularly an electrolytic salt-dissolving non-aqueous solution) can be retained,
Moreover, it is not particularly limited as long as it can bind the active material, and it is not particularly limited. -HF
P) copolymer, vinylidene fluoride-trifluoroethylene chloride (VDF-CTFE) copolymer, vinylidene fluoride-
Tetrafluoroethylene-6-fluoropropylene (VDF-TF
E-HFP) copolymer, polytetrafluoroethylene,
Fluorine-based rubber) and hydrocarbon-based polymers (for example,
Styrene-butadiene copolymer, styrene-acrylonitrile copolymer, etc.) can be used. In addition, these binder polymers can be used individually or in mixture of 2 or more types.

The amount of the binder polymer added is preferably in the range of 5 to 30 parts by weight with respect to 100 parts by weight of the active material. If the added amount of the binder polymer is less than 5 parts by weight, the active material cannot be sufficiently bound,
This is because the active material is likely to come off, and when the amount of the binder polymer added exceeds 30 parts by weight,
This is because the density of the active material in the negative electrode is reduced, and the energy density is likely to be reduced.

The negative electrode current collector is not particularly limited, but a material having excellent reduction stability is preferably used. More specifically, for example, copper foil, stainless steel, nickel, etc. can be mentioned. Among them, copper foil, which is excellent in reduction stability, is particularly suitable.

On the other hand, the positive electrode can be composed of a positive electrode active material, a binder polymer, a conductive auxiliary agent, and a current collector. Examples of the positive electrode active material in this include various lithium-containing metal oxides and chalcogen compounds. Among these, transition metal oxides containing lithium are preferable because they have high energy density and excellent reversibility. More specifically, for example, LiCoO ₂
Lithium cobalt oxide such as LiMn ₂ O ₄ , lithium manganese oxide such as LiMn ₂ O _4, and lithium nickel oxide such as LiNiO ₂ are preferably used. These may be used alone or in combination of two or more.

As the binder polymer, the same binder polymer as that used in the above-mentioned negative electrode composition can be used. For the same reason, the addition amount is 5 to 30 parts by weight per 100 parts by weight of the positive electrode active material. It is preferably in the range of parts.

The conductive auxiliary agent is not particularly limited as long as it can increase the conductivity between the positive electrode active materials and the utilization efficiency of the positive electrode active materials, and is not particularly limited.
For example, graphite, acetylene black, Ketjen black, etc. can be used. The amount of the conductive additive added is preferably in the range of 3 to 15 parts by weight with respect to 100 parts by weight of the active material. If the amount of the conductive additive added is less than 3 parts by weight, the conductivity between the active materials becomes insufficient, and the utilization efficiency of the active materials is likely to decrease. The addition amount of the conductive additive is 1
This is because if the amount is more than 5 parts by weight, the density of the active material in the positive electrode decreases, and the energy density of the battery tends to decrease.

As the positive electrode current collector, a material having excellent oxidation stability is preferably used. Specifically, aluminum,
Mention may be made of stainless steel, nickel and the like. Among them, aluminum foil, which is particularly excellent in oxidation stability, is most suitable.

The positive electrode and the negative electrode can be manufactured in the same manner, and the manufacturing method is not particularly limited, but for example, they can be manufactured by the following method. First, an active material, a binder polymer, and a volatile solvent capable of dissolving the binder polymer are mixed in a predetermined amount to prepare a paste of the active material. Then, after applying the obtained paste on the current collector with a uniform thickness by a blade coater or the like,
It can be produced by drying and removing the volatile solvent.

The method for producing the lithium secondary battery is not particularly limited, but a method for laminating the gel electrolyte supporting sheet between the positive electrode and the negative electrode holding the electrolytic salt-dissolving non-aqueous solution and laminating It is common. Alternatively, a gel electrolyte formable sheet is laminated between a positive electrode and a negative electrode, integrated by pressure bonding or heat fusion, and the integrated product is dipped in a non-aqueous solution in which an electrolytic salt is dissolved to obtain polyvinylidene fluoride fine fibers. And a method of holding the electrolytic salt-dissolved non-aqueous solution at the same time as gelling. According to the latter manufacturing method, the step of immersing in the electrolytic salt-dissolving non-aqueous solution immerses the integrated product 1
Since the process can be completed in a single time and can be simplified, there is an advantage that battery manufacturing efficiency can be improved. Also, in terms of battery performance, since the gel electrolyte formable sheet and both electrodes are integrated in advance, the bondability between the gel electrolyte and each electrode interface is improved, and the interface resistance can be reduced. There is.

Examples of the present invention will be described below, but the present invention is not limited to the following examples.

[0051]

EXAMPLES Example 1 (1) 20 parts of polyvinylidene fluoride as a polyvinylidene fluoride resin was dissolved in 80 parts of methyl ethyl ketone as a non-aqueous solvent to prepare a gel solution having a viscosity of about 30 poises.

Next, (2) a stainless steel nozzle having an inner diameter of 0.5 mm was attached to one nozzle by a pump,
The gel solution was supplied at a rate of 1.2 cc / hour, the gel solution was continuously extruded from a nozzle, and a voltage (25 kV) was applied to the nozzle to ground the stainless non-porous roll as a support to extrude the gel solution. The gel solution was thinned by applying an electric field (2.5 kV / cm) to form polyvinylidene fluoride gel-like fine fibers, which were accumulated on a rotating stainless non-perforated roll. In addition,
The distance between the nozzle and the stainless non-perforated roll was 10 cm.

Next, (3) the accumulated polyvinylidene fluoride gel-like fine fibers are dried in an oven set at a temperature of 90 ° C. for 30 minutes, the solvent is removed, and the average fiber diameter is 2 μm.
A gel electrolyte formable sheet having a non-woven structure composed only of polyvinylidene fluoride fine fibers (having a fiber diameter at each point of about 2 μm) was produced. The physical properties of the obtained gel electrolyte formable sheet are as follows: weight is 7.8 g / m ² and thickness is 1
At 8 μm, the apparent density was 0.43 g / cm ³ .

(Comparative Example 1) A porous sheet of polyvinylidene fluoride resin was used as a sheet capable of forming a gel electrolyte. The physical properties of this gel electrolyte formable sheet are as follows:
m ² , thickness 20 μm, apparent density 0.78 g / c
It was m ³ .

(Preparation of Electrolytic Salt-Dissolved Non-Aqueous Solution) Ethylene carbonate / γ-butyrolactone mixed solvent (weight ratio 1 /
In 1), LiPF ₆ was dissolved as an electrolytic salt to a concentration of 2 mol / L to prepare an electrolytic salt-dissolving non-aqueous solution.

Example 2 The gel electrolyte-formable sheet produced in Example 1 was dipped in the electrolyte salt-dissolving non-aqueous solution prepared as described above, and then surplus electrolysis was performed by a pair of rolls having a linear pressure of 100 N / cm. By removing the salt-dissolved non-aqueous solution, a gel electrolyte-supporting sheet having a retention of the electrolytic salt-dissolved non-aqueous solution of 250% was produced. Since the gel electrolyte-supporting sheet of the present invention has a high retention rate, it could be predicted that a battery having a low internal resistance could be manufactured.

The retention rate of the electrolytic salt-dissolving non-aqueous solution is defined as Mb is the mass before the electrolytic salt-dissolving non-aqueous solution is retained, and Ma is the mass after the electrolytic salt-dissolving non-aqueous solution is retained.
A value calculated from the following formula. Retention rate of electrolytic salt-dissolved non-aqueous solution (%) = {(Ma-Mb)
/ Ma} × 100

(Comparative Example 2) The gel electrolyte-forming sheet of Comparative Example 1 was treated in the same manner as in Example 2 to obtain a gel electrolyte-supporting sheet having a retention rate of the electrolytic salt-dissolving non-aqueous solution of 120%. Manufactured.

(Evaluation of Gel Electrolyte Formable Sheet) (Measurement of Porosity) The basis weight (M) and the thickness (T) of each gel electrolyte formable sheet of Example 1 and Comparative Example 1 were measured.

Next, the porosity was calculated by the following formula from the specific gravity (d) of the fibers or resins constituting the gel electrolyte formable sheet, the above basis weight, and the thickness. Porosity (%) = {1-M / (T × d)} × 100

The results are shown in Table 1. As is clear from Table 1, the gel electrolyte formable sheet of the present invention has a high porosity, and thus has excellent non-aqueous solvent retention,
It was predicted that it would be effective in reducing the internal resistance.

(Measurement of Tensile Strength of Gel Electrolyte Formable Sheet) Test pieces (width: 15 mm, length: 2) were obtained from the gel electrolyte formable sheets of Example 1 and Comparative Example 1, respectively.
00 mm) was sampled, and the tensile strength was measured according to the method specified in JIS P-8113. The results are shown in Table 1. The tensile strength is 15 N /
When the width is 15 mm or more, the mechanical strength is excellent, and for example, it can be actually used without breaking during battery assembly.

[0063]

[Table 1]

(Measurement Method of Ionic Conductivity) The gel electrolyte-supporting sheets of Example 2 and Comparative Example 2 were sandwiched between platinum electrodes having a diameter of 20 mm, and 1 mH was measured by an AC impedance method.
The frequency dependence of the impedance was analyzed in the range of z to 100 kHz, and the ionic conductivity was calculated from the value of the bulk resistance of the obtained cole-col plot. The results are shown in Table 2. This value is 5 × 10 ⁻⁴ S / c
When it is higher than m, the impedance does not become high, the capacity decrease in the high-rate charge / discharge characteristics is small, and the battery having good discharge characteristics at low temperature can be manufactured.

[0065]

[Table 2]

As is clear from Table 1 and Table 2 above,
It has been found that the gel electrolyte formable sheet of the present invention is excellent in mechanical strength and also excellent in retention of a non-aqueous solvent, and therefore can produce a battery having low internal resistance.

[0067]

The gel electrolyte formable sheet of the present invention according to claim 1 is excellent in retention of a non-aqueous solvent (electrolyte solution), and is therefore excellent in reduction of internal resistance in a battery and cycle characteristics. A battery can be formed. Further, it can contribute to making the battery thinner and increasing the capacity. Furthermore, since the mechanical strength is high, internal short circuit due to breakage in the battery assembly process or disconnection at the end of the electrode plate is unlikely to occur, resulting in excellent mass productivity.

The sheet capable of forming a gel electrolyte according to the second aspect of the present invention contains fibers that cannot be gelled by a non-aqueous solvent that gels polyvinylidene fluoride fine fibers. Since the mechanical strength of the gel electrolyte-supporting sheet in which the fine fibers are gelled can be kept high, it is possible to form the gel electrolyte-supporting sheet excellent in the internal short-circuit prevention property.

Since the gel electrolyte formable sheet of the present invention according to claim 3 has a thickness of 20 μm or less, the internal resistance can be reduced. In addition, it can also contribute to increasing the capacity of the battery.

The gel electrolyte-supporting sheet of the present invention according to claim 4 is one in which the polyvinylidene fluoride-based fine fibers of the above-mentioned gel electrolyte-formable sheet are gelled with a non-aqueous solvent, so that an internal short circuit occurs. It is possible to manufacture a battery that does not easily occur and has a low internal resistance. In addition, a thin battery or a high capacity battery can be manufactured.

Since the lithium secondary battery of the present invention according to claim 5 is a lithium secondary battery using the above-mentioned gel electrolyte supporting sheet, it is a lithium secondary battery which is unlikely to cause an internal short circuit and has a low internal resistance. is there. In addition, since the gel electrolyte formable sheet is thin and has excellent mechanical strength, it can be a thinner lithium secondary battery or a higher capacity lithium secondary battery than ever before.

Claims (5)

[Claims] 1. A sheet capable of forming a gel electrolyte, comprising a non-woven fabric mainly composed of polyvinylidene fluoride fine fibers having an average fiber diameter of 2 μm or less. 2. The sheet capable of forming a gel electrolyte according to claim 1, characterized in that it contains fibers that cannot be gelled by a non-aqueous solvent that gels polyvinylidene fluoride fine fibers. 3. The gel electrolyte formable sheet according to claim 1, which has a thickness of 20 μm or less. 4. A gel electrolyte support, characterized in that the polyvinylidene fluoride fine fibers of the gel electrolyte formable sheet according to any one of claims 1 to 3 are gelled with a non-aqueous solvent. Sheet. 5. A lithium secondary battery using the gel electrolyte supporting sheet according to claim 4.