JPH11214039A - N0naqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a polymeric battery having low short-circuit occurrence ratio and superior temperature characteristic even when it is exposed to a high temperature. SOLUTION: In this secondary battery having a positive electrode, a negative electrode, a diaphragm, and a nonaqueous electrolyte, the diaphragm consists of (A) a vinylidene fluoride based resin, consisting of vinylidene fluoride based homopolymer and copolymer and 90 wt.% to 98 wt.% of the entire composed of a vinylidene fluoride monomer units, and (B) an organic solvent containing a lithium salt. Thereby, a battery less causing internal short-circuit during battery assembling and having its superior low-temperature characteristics and high-temperature storage properties can be provided, and a polymeric battery with safety superior to a conventional nonaqueous secondary battery can be provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous secondary battery such as a lithium ion battery, and more particularly, to a non-aqueous secondary battery using an ion conductive polymer thin film as a diaphragm.

[0002]

2. Description of the Related Art Recently, miniaturization of mobile phones, personal computers, and the like,
A battery with a high energy density is required for weight reduction, and a non-aqueous lithium-ion battery has been developed as a corresponding battery. A porous polyolefin diaphragm, which does not swell in the electrolytic solution, is disposed between the positive and negative electrodes of the battery. When the polyolefin diaphragm is used, leakage of the electrolyte is likely to occur. Therefore, the entire battery structure is packaged in a heavy metal container to prevent leakage of the electrolyte.

On the other hand, recently, there has been no electrolyte leakage,
A so-called “polymer battery” has been developed which can adopt a non-metallic package and is excellent in terms of reducing the thickness and weight of the battery. As such a battery, a battery using a lithium ion conductive polymer instead of a polyolefin diaphragm has been proposed. (Hereinafter, in this specification, a polymer battery means a "battery using a lithium ion conductive polymer for a diaphragm.") For example, in Japanese Patent Application Laid-Open No. 8-195220, polyacrylonitrile contains an electrolytic solution. Further, by using the porous membrane having a porosity of 10% to 80% for the diaphragm portion,
It is disclosed that a battery having excellent charge / discharge efficiency can be obtained. As a method for producing the porous lithium ion conductive polymer membrane, a method has been proposed in which a porous polymer membrane is prepared in advance and immersed in a non-aqueous electrolyte containing a lithium salt to hold the electrolyte in pores. Have been. Japanese Patent Application Laid-Open No. 8-250127 discloses that a battery can be formed by using a membrane in which a porous membrane made of a vinylidene fluoride-based resin is impregnated with an electrolytic solution for a diaphragm portion.

Further, in Japanese Patent Application Laid-Open No. 9-259923, an ion-conductive organic polymer membrane obtained by wetting or swelling a porous membrane obtained by a so-called "wet film forming method" with an electrolytic solution is used for a diaphragm portion. It is disclosed that a battery having excellent low-temperature characteristics in which the charge-discharge capacity at low temperature is almost equal to the charge-discharge capacity at room temperature can be obtained. (Hereinafter, the ratio of the capacity at low temperature to the capacity at room temperature is referred to as low-temperature characteristics.) As the organic polymer material, polyvinylidene fluoride, polyacrylonitrile, and polyvinyl chloride are particularly preferred.

However, in the case of a film using a vinylidene fluoride-based homopolymer, only a brittle film is obtained, so that it has a disadvantage that an internal short-circuit is likely to occur during battery assembly. On the other hand, when the copolymer is used for the purpose of increasing the flexibility of the film, the copolymer having sufficient reduction in the content of the vinylidene fluoride monomer unit exhibits sufficient mechanical strength characteristics. (For example, 60
C.), the discharge characteristics at low temperatures (hereinafter referred to as high-temperature storage characteristics) are reduced. Further, in the case of polyacrylonitrile or polyvinyl chloride, the mechanical strength characteristics of the film itself cannot be said to be sufficient, and there are drawbacks in that the assembly yield of the battery is low and the high-temperature storage characteristics are deteriorated.

[0006]

SUMMARY OF THE INVENTION An object of the present invention is to provide a polymer battery having a low short-circuit occurrence rate and excellent low-temperature characteristics even when exposed to high temperatures.

[0007]

Means for Solving the Problems The present inventors have examined the film physical properties of porous films having different compositions and the battery characteristics using the same. As a result, even if the vinylidene fluoride resin has the same composition, The inventors have found that a better film can be obtained when a homopolymer and a copolymer are blended than when only a copolymer is used alone, and the present invention has been achieved.

That is, the present invention provides (1) a positive electrode, a negative electrode,
In a secondary battery having a diaphragm and a non-aqueous electrolyte, the diaphragm is composed of (A) a vinylidene fluoride-based homopolymer and a copolymer, and 90 wt% or more of the whole.
A non-aqueous secondary battery comprising 98% by weight of a vinylidene fluoride resin composed of vinylidene fluoride monomer units and (B) a lithium salt-containing organic solvent;
(2) The non-aqueous secondary battery according to the above (1), wherein the vinylidene fluoride copolymer is a vinylidene fluoride-hexafluoropropylene copolymer, and (3) the vinylidene fluoride copolymer is a vinylidene fluoride-hexafluorofluoropolymer. (1) The propylene copolymer, wherein the content of the vinylidene fluoride monomer unit is from 80 wt% to 90 wt%.
(4) The nonaqueous secondary battery according to (1), wherein the vinylidene fluoride resin is crosslinked.

Hereinafter, the present invention will be described in detail. The polymer species of the component (A) constituting the diaphragm in the present invention is a vinylidene fluoride resin, and needs to be composed of both a homopolymer and a copolymer. In the case of using only a homopolymer, only a brittle film having a low tensile elongation at break is obtained, so that an internal short circuit occurrence rate during battery assembly increases. In addition, in order to obtain a practically sufficient film strength using only the copolymer, it is necessary that the copolymer has a vinylidene fluoride monomer unit content of less than 90 wt%. In this case, however, the heat resistance and the solvent swelling resistance of the film are required. , The high-temperature storage characteristics are lowered.

The term "homopolymer" as used herein means a resin having a vinylidene fluoride monomer unit content of more than 98.5% by weight. The copolymer used in the present invention is a copolymer of vinylidene fluoride and a copolymerizable monomer, specifically, vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride-
Trifluoropropylene copolymer, vinylidene fluoride
Tetrafluoroethylene copolymer, vinylidene fluoride
Trifluoroethylene copolymer, vinylidene fluoride-fluoroethylene copolymer, vinylidene fluoride-propylene copolymer, vinylidene fluoride-ethylene copolymer, vinylidene fluoride-hexafluoroacetone copolymer, vinylidene fluoride- Examples thereof include a perfluorovinyl ether copolymer, a vinylidene fluoride-ethylene-tetrafluoroethylene copolymer, a vinylidene fluoride-tetrafluoroethylene-hexafluoropropylene copolymer, and the like. Among these polymer types, a vinylidene fluoride-hexafluoropropylene copolymer is particularly preferable because of a good balance between the mechanical strength of the film, heat resistance and solvent swelling resistance. Further, vinylidene fluoride
In the case of a hexafluoropropylene copolymer, the content of vinylidene fluoride is preferably from 80 wt% to 90 wt%. If the amount is less than 80 wt%, the heat resistance and solvent swelling resistance of the film will be reduced, so that the high-temperature storage characteristics tend to be deteriorated. If it exceeds 90 wt%, the mechanical strength of the film will be reduced.

In the present invention, the polymer constituting the diaphragm is 90 wt% of the whole vinylidene fluoride resin.
９８98 wt% must be vinylidene fluoride monomer units. If the content is less than 90 wt%, the mechanical strength characteristics of the film are good, but the heat resistance and the solvent swelling resistance are significantly reduced, so that the high-temperature storage characteristics tend to deteriorate. Also,
If the amount exceeds 98 wt%, the tensile elongation at break is extremely low, resulting in a brittle film. The above ranges can be adjusted by setting the amount of each compound according to the composition of the homopolymer or copolymer.

In addition, a polymer other than the above-mentioned vinylidene fluoride resin may be contained as a constituent component within a range that does not impair its properties. Although the range of the allowable amount cannot be unconditionally determined by the type of the polymer, it is preferably 10% by weight or less of the total amount of the constituent polymers, and more preferably 5% by weight.
The following is more preferred. The polymer composition of the diaphragm in the present invention can be easily confirmed by NMR measurement or the like. In addition, the fact that the constituent polymer is a blend is confirmed by a method of comparing the composition of the outermost surface and the inner portion of the film by FT-IR or XPS, or a method of analyzing and comparing the polymer separated by a solution fractionation method. can do.

[0013] The polymer of the diaphragm in the present invention further improves the balance between the mechanical strength characteristics and the heat resistance and solvent swelling resistance by crosslinking the constituent vinylidene fluoride resin, thereby improving the battery characteristics. Can be done.
Generally, a vinylidene fluoride resin tends to swell or dissolve significantly at a high temperature with a lithium salt-containing organic solvent. By having a crosslinked structure, high high-temperature stability is obtained. This crosslinked structure can be introduced at any stage during polymerization, before or after formation of the porous thin film. As a crosslinking method, a method of using a polyfunctional monomer during polymerization, a method of irradiating radiant energy such as an electron beam, γ-ray, X-ray, or ultraviolet light after polymerization, or a method of containing a radical initiator after polymerization to generate heat or radiation A method of causing a reaction by energy irradiation or the like can be used. When a crosslinked structure is introduced after the polymerization, a monofunctional or / and polyfunctional monomer component may be newly allowed to coexist. Among these methods, a method in which radiation energy such as an electron beam, γ-ray, X-ray, or ultraviolet ray is irradiated after polymerization is preferable since impurities and unreacted functional groups hardly remain. In particular, when the thickness of the porous film is 100 μm or less, crosslinking by electron beam irradiation is economical, and is particularly preferable. When crosslinking is performed by electron beam irradiation, the irradiation amount is preferably in the range of 5 to 100 Mrad, and more preferably 8 to 50 Mr.
range of ad. If it is less than 5 Mrad, the effect of crosslinking is not sufficient, and if it exceeds 100 Mrad, the collapse of the polymer becomes remarkable.

The formation of the crosslinked structure can be confirmed by the solubility of the uncrosslinked polymer in a soluble solvent. That is, since the polymer having a crosslinked structure has a component that is insoluble in a soluble solvent and does not dissolve uniformly, formation of a crosslinked structure can be determined. This soluble solvent is not particularly limited because it varies depending on the composition of the polymer. Carbonate and the like can be used. The dissolution can be promoted by heating.

The membrane of the present invention can be obtained by impregnating the polymer membrane of the component (A) with the organic solvent containing the lithium salt of the component (B), and has excellent lithium ion conductivity. It is preferable that the polymer membrane before the impregnation is a porous membrane having communication holes in order to quickly perform the impregnation. In producing the porous membrane, the continuity of the membrane can be controlled by appropriately adjusting the film forming conditions. The continuity of the pores can be evaluated by measuring the amount of water permeation described later, and the permeation amount is from several tens to several tens of thousands (liter / m ² / h) by a known film forming method.
r / 0.1 MPa, 25 ° C.).
Further, the pore size can be controlled by appropriately selecting the film forming conditions, and a membrane having an average pore size of an arbitrary value of 0.01 to 10 μm can be obtained. Permeability is 1
A film of 00 liter / m ² /hr/0.1 MPa and 25 ° C. or higher is particularly preferable because the rate of impregnation with the lithium salt-containing organic solvent is high. Further, the average pore size is 0.1 to 5 μm
Is particularly preferred. If it is less than 0.1 μm, the impregnation rate tends to be low, and if it exceeds 5 μm, an internal short circuit tends to occur.

The lithium salt-containing organic solvent as the component (B) constituting the diaphragm in the present invention is obtained by dissolving a lithium salt in a non-aqueous organic solvent. As the lithium salt, an electrochemically stable lithium salt is preferable. Examples of the lithium salt include lithium fluoroalkylsulfonic acid salts such as CF ₃ SO ₃ Li and C ₄ F ₉ SO ₃ Li, and (CF ₃ SO
_{2) 2-sulfonyl} imide lithium salts such NLi, LiB
F ₄ , LiPF ₆ , LiClO ₄ , LiAsF _{6 and the} like can be mentioned. These can be used alone or as a mixture of two or more.

The non-aqueous organic solvent for dissolving these lithium salts may be any solvent which is chemically stable and dissolves lithium salts. Examples thereof include ethylene carbonate, propylene carbonate, dimethyl carbonate, and the like.
Diethyl carbonate, carbonate compounds such as methyl ethyl carbonate, tetrahydrofuran, dimethoxyethane, diglyme, triglyme, tetraglyme,
Examples include ether compounds such as oligoethylene oxide, lactone compounds such as butyrolactone and propiolactone, and nitrile compounds such as acetonitrile and propionitrile. These can be used alone or as a mixture of two or more. Among these solvents, those capable of dissolving the above-mentioned Li salt at a concentration of 0.2 mol / liter or more are preferable, and solvents having low viscosity are preferable because they have high impregnating properties into the polymer film.

In the present invention, the thickness of the diaphragm is 10 to 30.
0 μm, with a range of 20 to 100 μm being particularly preferred. If the thickness exceeds 300 μm, the effective electric resistance becomes too high, and the energy density per volume of the battery becomes low. On the other hand, when the thickness is less than 10 μm, the film strength is insufficient, and the battery assembly yield tends to decrease. The structure of the diaphragm in the present invention is not particularly limited, for example, a structure having a hole communicating from the positive electrode side to the negative electrode side,
A structure having independent holes inside the film or a structure having substantially no holes can be adopted. Among these structures, the structure is preferable because the battery has good low-temperature characteristics, and the structure is most preferable. The above structure
The structure reflects the structure of the polymer film before the lithium salt-containing organic solvent is contained, but the structure can be changed by means such as heating after the inclusion. That is, after containing a lithium salt-containing organic solvent, 40 to
By heating to about 100 ° C. for a short time, the polymer can be swollen to reduce the surface pore diameter, or it can be made substantially non-porous.

[0019]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, the present invention will be described in more detail by way of examples. In addition, each measurement was performed by the following method. (1) Measurement of VdF Content in Constitution Vinylidene Fluoride Resin The porous membrane sample of the production example was dissolved in d-dimethylsulfoxide to form a 10 wt% solution, and ¹⁹ F-NMR measurement (NMR: JNM-LAMBDA400 manufactured by JEOL Ltd.) Using a mold).

The signal intensity around -78 ppm derived from the CF ₃ group of hexafluoropropylene and the signal intensity of a plurality of signals around -95 ppm and -110 to -125 ppm derived from the CF ₂ group of vinylidene fluoride are usually determined. The molar percentage of CF ₂ groups was determined by the method and converted to weight%. (2) Observation of the cross-sectional structure The porous membrane sample was immersed in liquid nitrogen in a state of being impregnated with ethanol, frozen, then cut, and the cross section was SEM.
(SEM S-800, manufactured by Hitachi, Ltd.). (3) Measurement of Porosity The porous membrane sample was immersed in ethanol (special grade reagent) to perform a hydrophilic treatment, and then immersed in pure water at room temperature for 2 hours or more to completely replace the inside of the void with pure water. Next, after wiping off the water on the membrane surface, the weight (A) of the porous membrane containing pure water in the voids was measured. Subsequently, the porous membrane sample was placed in a vacuum at 60 ° C.
For 4 hours or more to remove water in the voids and measure the weight (B) of only the polymer part. From these weights and the true specific gravity (dp, dw) of the constituent polymer of the membrane and water, it was calculated by the following equation.

Porosity (%) = 100 × ((A−B) / d)
w) / (B / dp + (AB) / dw) The true specific gravity of the constituent polymer and water is 1.77, respectively.
1.0. (4) Measurement of Water Permeability (Communication) After punching out a porous membrane sample to a diameter of 25 mm, it was immersed in ethanol (special grade reagent) to make it hydrophilic. Subsequently, the porous membrane was immersed in ultrapure water and replaced with pure water, and the porous membrane was incorporated into a membrane filter holder having an effective area of 3.5 cm ² and filled with ultrapure water. A hydrostatic pressure of 0.1 MPa was applied for 5 minutes, and the weight of the permeated water was measured. Measure the temperature of the ultrapure water at this time, and from the true density and viscosity of the pure water at that temperature,
The amount of water permeated at 25 ° C. per hour and per m ² (liter / m ² /hr/0.1 MPa, 25 ° C.) was calculated. (5) Tensile strength properties A test piece was prepared by cutting a porous membrane sample into a JIS No. 5 dumbbell shape, and tensile strength at break and tensile elongation at break were measured using an Instron universal testing machine (manufactured by Shimadzu Corporation). . The number of repetitions was set to 5, and the average value was taken. In addition,
The chuck distance is 80 mm and the head speed is 50 mm / m
It was measured under the conditions of "in". (6) Solvent swelling resistance The porous membrane sample cut into 50 mm x 50 mm was
It was immersed in propylene carbonate (special grade reagent) adjusted to 3 ° C. and left for one day. Thereafter, the film was taken out and the lengths of two sides (L1, L2) of the film were measured immediately. The area change rate was determined from the following equation.

Area change rate (%) = 100 × (L1 × L2)
-2500) / 2500

[0023]

[Production Example 1] 13 parts by weight of vinylidene fluoride homopolymer (Kynar 761 manufactured by Elf Atochem), 4 parts by weight of vinylidene fluoride-hexafluoropropylene copolymer (Kynar 2801 manufactured by Elphatochem: a product containing 88% by weight of vinylidene fluoride), polyvinylpyrrolidone A solution consisting of 15 parts by weight (K-30 manufactured by BASF) and 68 parts by weight of N-methyl-2-pyrrolidone (special grade reagent manufactured by Tokyo Chemical Industry Co., Ltd.) was prepared and cast on a glass plate at 50 ° C. Immediately, it was immersed in a 75% by weight aqueous solution of N-methyl-2-pyrrolidone at 30 ° C. to solidify it, washed with water and ethanol, and dried by heating.

When this porous membrane was subjected to FT-IR measurement, the presence of about 2 wt% polyvinylpyrrolidone was confirmed. Next, the porous film was irradiated with an electron beam (irradiation amount 1
0 Mrad) to obtain a crosslinked porous membrane. Observation of the cross-sectional structure of this porous film revealed that pores of 5 μm or less were continuous three-dimensionally, and had a sponge-like structure. Table 2 shows the physical properties of this porous membrane. The VdF content in the constituent vinylidene fluoride resin was measured for the film before electron beam irradiation. Further, about 1 g of the crosslinked membrane was sampled, immersed in 50 g of N-methyl-2-pyrrolidone (special grade reagent manufactured by Tokyo Chemical Industry Co., Ltd.), and stirred for 24 hours. I was able to confirm that.

[0025]

Production Examples 2 to 7 A porous membrane was obtained in the same manner as in Production Example 1, except that the kind and amount of polymer in the stock solution, the composition of the coagulating solution and the amount of electron beam irradiation were as shown in Table 1. It was confirmed that the crosslinked film was crosslinked in the same manner as in Production Example 1. Table 2 shows the physical properties of this porous membrane.

[0026]

Example 1 Using the porous membrane of Production Example 1, a secondary battery was assembled as follows. First, L having an average particle size of 10 μm
The iCoO ₂ powder and carbon black were mixed and dispersed in an N-methyl-2-pyrrolidone solution (5% by weight) of polyvinylidene fluoride (KF # 1100, manufactured by Kureha Chemical Industry) to prepare a slurry. The weight composition of the solid content in the slurry was LiCoO ₂ (89%), carbon black (8%).
%) And a polymer (3%). This slurry was applied on an aluminum foil by a doctor blade method, dried, and then pressed to produce a positive electrode sheet having a thickness of 110 μm.

Next, needle coke powder having an average particle size of 10 μm was dispersed in a carboxymethyl cellulose solution and a styrene-butadiene latex (L1571 manufactured by Asahi Kasei Kogyo Co., Ltd.) dispersion mixture to prepare a slurry. The solid content weight composition in the slurry was as follows: needle coke / carboxymethyl cellulose / styrene butadiene = 100 /
0.8 / 2. The slurry was applied to a metal copper sheet by a doctor blade method, dried, and then pressed to form a film having a thickness of 12 mm.
A 0 μm negative electrode sheet was produced.

The positive electrode sheet, the porous film, and the negative electrode sheet prepared as described above are stacked and wound, pressed and flattened. Then, the SU having a width of 34 mm, a height of 47 mm, and a thickness of 8.6 mm is obtained.
It was inserted into an S container. Then, a lithium salt-containing organic solvent (ethylene carbonate / propylene carbonate / γ
-A solution in which LiBF ₄ was dissolved in a 1: 1: 2 mixed solvent of butyrolactone at a concentration of 1.5 mol / liter) was injected to assemble a prismatic battery.

The battery was charged and discharged at a current density of 1 mA / cm ² . Charging was performed at 4.2V constant potential charging after constant current charging, and discharging was performed at 2.7V constant current discharging with a cutoff voltage of 2.7V. The low-temperature characteristics and high-temperature storage characteristics of the battery were measured by the following methods. Table 3 shows the results. Also,
The following operation was performed for 20 of the batteries, and the short-circuit occurrence rate was determined to be 0%. (A) Low temperature characteristics First, charging and discharging were repeated 20 times at 20 ° C. Subsequently, charge / discharge was repeated 5 times at 0 ° C. At this time, the percentage of the fifth discharge capacity at 0 ° C. to the 20th discharge capacity at 20 ° C. was determined.

Low temperature characteristics (%) = (discharge capacity at 0 ° C.) /
(Discharge capacity at 20 ° C.) × 100 (b) High-temperature storage characteristics First, charge and discharge were repeated 20 times at 20 ° C. Then, after storing at 60 ° C. for 24 hours, the temperature was returned to 20 ° C.
The charge and discharge at ℃ was repeated 5 times. 20 before storage at 60 ° C
The percentage of the fifth discharge amount at 0 ° C. after the storage at 60 ° C. with respect to the fifth discharge amount was determined.

High-temperature storage characteristics (%) = (discharge capacity at 0 ° C. after storage) / (discharge capacity at 20 ° C. before storage) × 100 (c) Short-circuit occurrence rate First, charge and discharge 20 times at 20 ° C. After doing, 60
It was left for 24 hours under an environment of ° C. Next, the temperature was returned to 20 ° C., and the battery was charged and discharged 100 times. Those having a voltage drop of 0.2 V or more before and after standing for 48 hours were regarded as internal short-circuited products, and the percentage thereof was calculated.

Short-circuit occurrence rate (%) = (number of internal short-circuit products)
/ 20 × 100

[0033]

Examples 2 to 5 A battery was assembled in the same manner as in Example 1 except that the used porous membrane and the lithium salt-containing organic solvent were changed as shown in Table 3, and the battery performance (low-temperature characteristics and high-temperature storage characteristics) Was examined. Table 3 shows the results. Next, the occurrence rate of short-circuit was examined in the same manner as in Example 1.
%Met.

[0034]

Comparative Examples 1 and 2 An attempt was made to assemble a battery in the same manner as in Example 1 except that the porous membranes of Production Examples 5 and 6 were used. However, the strength properties of the film are low,
Assembly was difficult.

[0035]

Comparative Example 3 A battery was assembled in the same manner as in Example 1 except that the porous membrane of Production Example 7 was used, and the battery performance (low-temperature characteristics and high-temperature storage characteristics) was examined. Table 3 shows the results.

[0036]

[Table 1]

[0037]

[Table 2]

[0038]

[Table 3]

[0039]

As described above, the non-aqueous secondary battery of the present invention is characterized by using a diaphragm having a specific composition. Thereby, it is possible to realize a battery with less occurrence of an internal short circuit at the time of battery assembly, excellent low-temperature characteristics and high-temperature storage characteristics, and it is possible to provide a polymer battery that is more safe than conventional non-aqueous secondary batteries. .

Claims (1)

[Claims] 1. A secondary battery having a positive electrode, a negative electrode, a diaphragm, and a non-aqueous electrolyte, wherein the diaphragm comprises (A) a vinylidene fluoride-based homopolymer and a copolymer, and
A non-aqueous secondary battery characterized in that 90 to 98 wt% of the whole thereof is composed of a vinylidene fluoride resin composed of vinylidene fluoride monomer units and (B) a lithium salt-containing organic solvent.