JPH1116561A - Battery separator, its manufacture and nonaqueous secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a separator having excellent mechanical strength and ion conductivity by using a polyvinylidene fluoride resin porous body, and dispersing an inorganic filler in the porous body. SOLUTION: This polyvinylidene fluoride resin preferably consists of a vinylidene fluoride homopolymer, and the polyvinylidene fluoride resin consists of a copolymer of at least one monomer selected from ethylene tetrafluoride, propylene hexafluoride, ethylene trifluoride, and ethylene trifluoride chloride with vinylidene fluoride. The ratio of the vinylidene fluoride component in the copolymer is preferably 50 wt.% or more. To 100 pts.wt. of the polyvinylidene fluoride resin, 2-200 pts.wt. of an inorganic filler is contained. The inorganic filler is selected from inorganic oxides and silicates, and preferably selected from alumina and silica.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a battery separator, a method of manufacturing the same, and a non-aqueous secondary battery using the separator. It is an object of the present invention to provide a method for manufacturing a non-aqueous secondary battery such as a lithium secondary battery having a small thickness and excellent flexibility and mechanical strength.

[0002]

2. Description of the Related Art A separator used for a non-aqueous battery such as a lithium battery ensures conductivity by preventing a short circuit between a positive electrode and a negative electrode and by holding an electrolyte in a myriad of holes formed in the separator. Has a role. Conventionally, as typical examples, a porous film made of polyethylene (PE) or polypropylene (PP), a two-layer film in which PE and PP are bonded, a three-layer film in which PE is sandwiched between PP, and the like have been used. Was. However, PE and PP are flammable materials, and particularly for lithium batteries, materials with higher safety are demanded.

Recently, there has been proposed a lithium battery in which a solution prepared by dissolving a Li salt such as LiPF6 in a carbonate-based solvent is used as a separator (polymer gel electrolyte) by swelling a vinylidene fluoride-based copolymer film (published patent). Gazette, Hei 8-507407 and Hei 8-50910
0). In the production of such a battery, a film containing a vinylidene fluoride copolymer and a plasticizer is prepared, and then the plasticizer is removed from the film using a low-boiling solvent, or the electrolyte salt is removed. A complicated process of replacing with a solution is required. Further, a vinylidene fluoride copolymer film swollen with a solvent usually has insufficient temperature resistance at a high temperature (50 ° C. or more) or has poor battery characteristics such as capacity at a low temperature (0 ° C. or less). Easy to fall.

Further, it has been proposed to use a porous film made of polyvinylidene fluoride resin as a separator (Japanese Patent Laid-Open Publication No. Hei 8-250127; Toriyama et al., Proceedings of the 37th Battery Symposium, page 239). , 1
996). According to these documents, polyvinylidene fluoride resin swells (plasticizes) with an organic electrolytic solution, and its matrix portion also contributes to ion conduction. Therefore, if this porous film is used for a separator, It is said that higher ionic conductivity can be obtained than when a porous film made of polyethylene or polypropylene is used.
However, in this case, in order to obtain high ion conductivity, it is necessary to use a porous film having large porosity, and the mechanical strength of the separator becomes a problem.

[0005]

DISCLOSURE OF THE INVENTION The present inventors have solved the drawbacks of conventional porous bodies made of polyethylene or polypropylene and polymer gel electrolytes, and have obtained polyvinylidene fluoride resins having excellent mechanical strength and ion conductivity. As a result of investigations to obtain a battery separator made of a porous body, the present invention has been reached.

[0006]

SUMMARY OF THE INVENTION The present invention relates to a battery separator comprising a porous polyvinylidene fluoride resin, wherein an inorganic filler is dispersed in the porous body.

The "polyvinylidene fluoride resin" used in the present invention is a thermoplastic fluorine resin containing at least 50% by weight, preferably 75% by weight or more of vinylidene fluoride as a constituent unit of the resin. Therefore, the copolymer is not limited to a homopolymer composed of vinylidene fluoride alone, and may be a copolymer of vinylidene fluoride and one or more monomers selected from the following monomers. Examples of the copolymerizable monomer include ethylene tetrafluoride, ethylene trifluoride, ethylene trifluoride chloride, vinyl fluoride, propylene hexafluoride, ethylene, and perfluoroalkyl vinyl ether. As the polyvinylidene fluoride-based copolymer, a copolymer of vinylidene fluoride and propylene hexafluoride (propylene hexafluoride content of 15% by weight or less) is preferably used. Such a thermoplastic polyvinylidene fluoride resin is obtained by a commonly used polymerization method such as emulsion polymerization and suspension polymerization, and has a melt flow rate (MFR) value of 0.005 to 0.005.
100 g / 10 min (measured at 230 ° C. under a load of 2.16 kg) is preferable, and more preferably 0.01 to 10 min.
20 g / 10 min.

In the present invention, the inorganic filler is used in an amount of 2 to 200 parts by weight based on 100 parts by weight of the polyvinylidene fluoride resin.
Parts by weight, more preferably 5 to 50 parts by weight.

The inorganic filler used in the present invention includes:
Numerous types of materials generally used for polymer materials can be used (Reference: “Filler Encyclopedia” edited by Filler Research Group, Taiseisha, 1994). Especially,
Inorganic oxides and silicates are preferably used. Examples of the inorganic oxide, silica, alumina, diatomaceous earth, titanium oxide, calcium oxide, zinc oxide, magnesium oxide,
There are tin oxide and ferrite. Of these, alumina and pyrogenic silica are particularly preferably used. Examples of the silicates include calcium silicate, talc, mica, montmorillonite, bentonite, sepiolite, imogolite, sericite, glass fiber, and glass beads. The shape of such an inorganic filler is not particularly limited, and fibrous, needle-like, plate-like, and granular shapes can be used.

In the above-mentioned inorganic filler, the surface is hydrophobized by reacting a functional group having a hydroxyl group such as a silanol group present on the surface with a silane compound such as halosilane, alkoxysilane, silazane or siloxane. You may.

The method for producing the battery separator of the present invention is not particularly limited, and a known method for producing a porous body from a polymer resin can be applied. As a typical method, there is a method called a sol-gel method using a solvent and a non-solvent of a polymer resin to be used. That is, it is possible to produce a porous body by dispersing the above-mentioned inorganic filler in a solution obtained by dissolving a polyvinylidene fluoride resin in a solvent, and bringing this solution into contact with a non-solvent of the polyvinylidene fluoride resin. it can. In this case, as a solvent for dissolving the polyvinylidene fluoride resin, N-methylpyrrolidone,
Formamide, dimethylformamide, dimethylacetamide, dimethylsulfoxide, γ-butyrolactone,
Tetramethyl urea, trimethyl phosphate, acetone, methyl ethyl ketone, tetrahydrofuran and the like are used. Moreover, you may use as a mixed solvent of these. The concentration of the polyvinylidene fluoride resin contained in the solution is usually 5 to 50% by weight, preferably 10 to 50% by weight.
30% by weight. As the poor solvent, water and alcohols are preferably used, and water is particularly preferable.

When the polyvinylidene fluoride resin solution in which the inorganic filler prepared as described above is dispersed is brought into contact with a poor solvent, the sol-gel transition starts immediately, and a porous body is formed. In this case, it is desirable to use a poor solvent having a low content of a solvent that dissolves the polyvinylidene fluoride resin, and to contact the poor solvent for a sufficiently long time to extract the solvent in the porous body. The contact time varies depending on the size of the porous body, but is usually preferably 10 seconds or more, and more preferably 30 seconds or more.

The preferred shape of the porous body as the battery separator is a film. 5-2 as thickness
00 μm, more preferably 10 to 100
μm. The pore size is preferably 20 μm or less, and more preferably 0.05 to 10 μm. The porosity is preferably 5 to 85%, more preferably 20 to 85%.
80%.

The porous body of the present invention may further comprise, if necessary,
By performing stretching (hot stretching or cold stretching) and / or treatment, the crystallinity can be increased or the crystal structure can be fixed. The stretching ratio is 1.1 to 6 times and less than the breaking stretching ratio, and preferably 1.2 to 4 times, depending on the type of resin (molecular weight and comonomer ratio) and the production conditions of the porous body. Stretching may be uniaxial or biaxial (simultaneous, sequential). Recommended heat treatment temperature is 70 ~
The temperature is 155 ° C, preferably 100 to 150 ° C. The heat treatment may be in a free state or a limited shrinkage, a fixed length, or a strained state. In the case of a strained state, 10% or less, preferably 5%.
The following tension rates are good. The point is that the spherulite generated when melted and solidified is not destroyed, and it is preferable to grow the spherulite in a state before the spherulite is broken and transferred to microcrystals.

The porous material of the present invention has a high energy (usually 2
To 40 megarads) or by irradiation with an electron beam or γ-ray or a chemical deHF reaction. Thereby, the heat resistance and mechanical strength of the porous body can be improved.

Hereinafter, the application of the porous film obtained as described above to a lithium battery will be described. That is, an anode having an anode active material layer formed on at least one surface of a current collector, a cathode having a cathode active material layer formed on at least one surface of a current collector, and an anode and a cathode using the porous film described above. A porous film is inserted between them so that they do not come into direct contact with each other, and they are laminated or wound in a roll (spiral) and stored in a suitable container, and LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li
Dissolve at least one lithium salt selected from N (CF ₃ SO ₂ ) ₂ , LiCF ₃ SO ₃ , LiSbF _{6 and the} like in a suitable solvent (mainly a carbonate system such as ethylene carbonate, propylene carbonate and dimethyl carbonate) The resulting electrolyte solution is added, the container is sealed, and a battery is finally obtained. That is, the porous film of the present invention is used in a state where the pores are filled with the above electrolyte solution, and at the same time, the matrix portion of the porous film made of polyvinylidene fluoride resin is swollen with the electrolyte solution. Becomes

Here, as the current collector of the electrode, a metal foil,
Although there are a metal mesh, a three-dimensional porous body, and the like, as a metal used for the current collector, a metal that is difficult to alloy with lithium is desirable, and particularly, iron, nickel, cobalt, copper, aluminum, titanium, vanadium, chromium, Manganese is used alone or in these alloys.

The negative electrode active material of the electrode active material may be any material that can dope and dedope lithium ions. Such materials include coke materials such as petroleum coke and carbon coke, carbon blacks such as acetylene black, graphite, glassy carbon, activated carbon, carbon fiber, and organic polymers fired in a non-oxidizing atmosphere. There is a carbonaceous material such as a fired organic polymer obtained. In some cases, copper oxide is added.

Examples of the positive electrode active substance include transition metal oxides such as manganese oxide and vanadium pentoxide, transition metal oxides such as iron sulfide and titanium sulfide, and composite compounds of these with lithium (for example, Lithium cobalt composite oxide, lithium cobalt nickel composite oxide, lithium manganese oxide) and the like can be used.
When these materials are incorporated into the positive electrode, a paste is prepared by mixing a solvent with a mixture of a powdered positive electrode active material and an appropriate amount of a conductor (often using carbon) and a binder. After application to the body and drying, pressing is performed as necessary to obtain an electrode.

[0020]

EXAMPLES The present invention will be described below in detail with reference to examples, but the present invention is not limited to these examples.

[0021]

Example 1 15 parts by weight of Kynar 741 (polyvinylidene fluoride resin manufactured by Elf Atchem Co., Ltd., having an MFR value of 0.3 g / 10 minutes (measured at 230 ° C. under a load of 2.16 kg)) were replaced by 85 parts by weight of N -Dissolved in methylpyrrolidone. 4 parts by weight of amorphous silica (primary particle diameter, 20 nm) obtained by a combustion method from silicon tetrachloride and hydrogen was dispersed in this solution, cast on a polyethylene terephthalate support film, and immersed in water for 2 hours. , A porous film having a thickness of 50 μm was obtained. The silica content in this film was 21% by weight. The film had a porosity of about 40%, and many pores of about 1 μm were found inside.

This film was treated with ethylene carbonate (E
C) and propylene carbonate (PC) were immersed for 1 hour in an electrolyte solution in which LiPF ₆ was dissolved at a concentration of 1 M in a 1: 1 mixture, and the conductivity was measured using a normal bridge. About three times the value was obtained. When the film swollen with this electrolyte solution was heated to 60 ° C., it was found that the shape of the film was stable without breaking.

[0023]

Example 2 100 parts by weight of a coal pitch coke pulverized by a ball mill as a negative electrode active material carrier was used as a binder, polyvinylidene fluoride (manufactured by Elf Atochem Co., Ltd .;
10 parts by weight of Kynar 301F (230 ° C., MFR under a load of 2.16 kg of 0.03 g / 10 min) was added to a solution of N-methylpyrrolidone to form a slurry (paste). This slurry was applied to both sides of a copper foil having a thickness of 20 μm, left at 120 ° C. for 1 hour, dried under reduced pressure, and pressed to obtain a negative electrode having a thickness of 140 μm and a width of 20 mm.

Next, a positive electrode was obtained as follows. 100 parts by weight of LiCoO ₂ as a positive electrode active material and 6 parts by weight of graphite as a conductive agent, and 10 parts by weight of polyvinylidene fluoride (Kyner 301F) as a binder are dispersed in N-methylpyrrolidone to form a slurry ( Paste). This slurry was applied to both surfaces of a 20-μm-thick aluminum foil, left at 120 ° C. for 1 hour, dried under reduced pressure, and pressed to obtain a positive electrode having a thickness of 160 μm and a width of 20 mm.

The porous film obtained in Example 1 was used as the obtained negative electrode, positive electrode, and separator,
After laminating the separator, the negative electrode, the separator, the positive electrode, and the separator in this order, the spirally wound electrode body was produced by spirally winding the laminate. Next, after a lead wire was attached to each electrode of the electrode body, the electrode body was housed in a stainless steel can, and propylene carbonate was added thereto as an electrolytic solution.
LiPF in an equal volume mixed solvent with 2-dimethoxyethane
A solution of ₆ dissolved in 1M was injected.

In the charge / discharge test, the battery was charged to 4.1 V at a current density of 30 mA per g of carbon, and then discharged to 2.5 V at the same current. After the second time, charging and discharging were repeated under the same conditions, and the battery was evaluated by discharge capacity. As a result, the discharge capacity at the 100th cycle was as good as 60% or more of that at the 10th cycle.

[0027]

Example 3 In Example 1, Kyner 2801 (propylene hexafluoride content, about 10% by weight, a copolymer of vinylidene fluoride and propylene hexafluoride, manufactured by Elf Atochem Co., Ltd.) was used in place of Kynar 741. MFR value 0.2g / 1
At 0 minutes (measured at 230 ° C. under a load of 2.16 kg), a porous film was produced in the same manner as in Example 1 except that fine particles of α-alumina were used instead of silica. The alumina content in this film was 21% by weight. The film had a porosity of about 30%, and many pores of about 1 μm were found inside. Further, when the conductivity was measured in the same manner as in Example 1, a value approximately three times as large as that in Comparative Example 2 was obtained.

[0028]

Comparative Example 1 A solution prepared by dissolving 15 parts by weight of Kynar 741 in 85 parts by weight of N-methylpyrrolidone was cast on a support film made of polyethylene terephthalate and immersed in water for 2 hours to form a porous film having a thickness of 50 μm. A functional film was obtained. This film had a porosity of about 40%, and many pores of about 1 μm were found inside. This film was immersed in an electrolyte solution of LiPF ₆ at a concentration of 1 M in a 1: 1 mixture of ethylene carbonate (EC) and propylene carbonate (PC) for 1 hour, and then the conductivity was measured using a normal bridge. However, it was 0.3 mS / cm.

[0029]

Comparative Example 2 A porous film was produced in the same manner as in Comparative Example 1 except that the Kynar 2801 was used instead of the Kynar 741. The film had a porosity of about 30%, and many pores of about 1 μm were found inside. Also, when the conductivity was measured in the same manner as in Comparative Example 1,
0.4 mS / cm.

[0030]

As described above, the battery separator of the present invention has high ionic conductivity, is easy to manufacture, and has excellent mechanical properties. If this is applied to a lithium battery separator, the polyvinylidene fluoride-based resin is flame-retardant, so that the battery has higher safety than when a polyethylene separator is used.

Claims (11)

[Claims] 1. A battery separator comprising a polyvinylidene fluoride resin porous body, wherein an inorganic filler is dispersed in the porous body. 2. The battery separator according to claim 1, wherein the polyvinylidene fluoride resin is a vinylidene fluoride homopolymer. 3. The copolymer of vinylidene fluoride and at least one monomer selected from the group consisting of ethylene tetrafluoride, propylene hexafluoride, ethylene trifluoride, and ethylene trifluoride chloride. 2. The battery separator according to claim 1, wherein the ratio of the vinylidene fluoride component in the copolymer is 50% by weight or more. 4. The battery separator according to claim 1, wherein the inorganic filler is contained in an amount of 2 to 200 parts by weight based on 100 parts by weight of the polyvinylidene fluoride resin. 5. The battery separator according to claim 1, wherein the inorganic filler is selected from inorganic oxides and silicates. 6. The battery separator according to claim 5, comprising at least one inorganic filler selected from the group consisting of alumina and silica. 7. The method according to claim 1, wherein the inorganic filler is dispersed in a solution obtained by dissolving the polyvinylidene fluoride resin in a solvent, and the solution is brought into contact with a non-solvent of the polyvinylidene fluoride resin. The battery separator as described. 8. A polyvinylidene fluoride resin obtained by dispersing an inorganic filler in a solution obtained by dissolving a polyvinylidene fluoride resin in a solvent, and bringing this solution into contact with a non-solvent of the polyvinylidene fluoride resin. A method for producing a battery separator made of a porous material. 9. A non-aqueous secondary battery comprising a positive electrode, a separator, and a negative electrode, wherein the separator is made of a porous polyvinylidene fluoride resin in which an inorganic filler is dispersed. battery. 10. The non-aqueous secondary battery according to claim 7, wherein the polyvinylidene fluoride resin porous body is made of a vinylidene fluoride homopolymer. 11. A polyvinylidene fluoride resin porous material,
Ethylene tetrafluoride, propylene hexafluoride, ethylene trifluoride,
And a copolymer of at least one monomer selected from ethylene trifluoride chloride and vinylidene fluoride, wherein the copolymer has a vinylidene fluoride component ratio of 50% by weight or more. Item 7. A non-aqueous secondary battery according to Item 7.