JP2000200623A - Lithium battery having fluorine-containing polymer electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the cycle durability of a battery by setting copolymer containing monomer unit composed of vinylidene fluoride and α-fluoroacrylate to a matrix of electrolyte and providing polymer electrolyte containing solution composed of solute of lithium salt and solvent solving the lithium salt. SOLUTION: α-fluoroacrylate of monomer unit is expressed by the formula. In the formula, R is CH2(CF2)nH, CH2(CF2)nF, (CH2)nH, (CF2)nF; n=1-6. The weight ratio of vinylidene fluoride to α-fluoroacrylate is preferably 1/99-99/1 and the molecular weight is desirably to be 10,000-200,000. Carbonate is optimal for the solvent of the lithium salt solution and propylene carbonate and ethylene carbonate are adaptable. This lithium battery using fluorine-containing electrolyte is superior in stability of polymer electrolyte, has superior electric conductivity, and superior in the cycle durability because of its superior bond between the electrolyte and electrode material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a lithium battery using a fluorine-containing polymer electrolyte. In particular, it relates to a lithium secondary battery having excellent cycle durability.

[0002]

2. Description of the Related Art Lithium secondary batteries, which are typical devices, have a high energy density, and have recently been rapidly used in mobile phones,
It has been used for personal computers, etc., showing a great extension. For example, LiMnO ₂ , LiCoO ₂ , Li
A metal oxide such as NiO ₂ is used for the anode, and a carbon material capable of inserting and extracting lithium and lithium ions is used for the cathode.
Many studies have been made on lithium secondary batteries using a non-aqueous electrolyte composed of an organic solvent and a lithium salt.

[0003] JP-A-4-506726, JP-A-8-1996
Japanese Patent Application Laid-Open No. 507407 proposes a polymer electrolyte. However, polyethylene oxide-based polymer electrolytes have poor retention stability of organic electrolytes, and network-structured polyacrylate-based polymer electrolytes are electrochemically unstable. Yes, not suitable for 4V class batteries.

[0004] JP-A-10-294131 and JP-A-10-284128 disclose vinylidene fluoride-hexafluoropropylene copolymer, vinylidene fluoride
It has been reported that a perfluoroalkyl vinyl ether copolymer has been proposed, which solves the conventional problems and improves the charge / discharge cycle durability when used in a lithium secondary battery. Lithium secondary batteries have a wide range of uses and are being studied in search of batteries having better cycle durability.

[0005]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a lithium secondary battery which uses a fluorinated electrolyte to improve the retention stability of the electrolyte and has excellent cycle durability.

[0006]

In a lithium battery (comprising an anode, a cathode, and an electrolyte), the electrolyte is composed of a monomer unit composed of vinylidene fluoride and a monomer unit composed of α-fluoroacrylate represented by the formula (1) (provided that the monomer unit is composed of α-fluoroacrylate). And a polymer electrolyte containing a solution comprising a solute of a lithium salt and a solvent capable of dissolving the lithium salt, based on a copolymer containing a fluorine atom (a part of which may be replaced by a chlorine atom or a bromine atom). There is provided a lithium battery characterized by a certain feature.

Embedded image [Wherein R is CH ₂ (CF ₂ ) nH, CH ₂ (CF ₂ )
nF, _(CH 2) nH or _(CF 2) nF _(n is 1 to
Integer of 6)] When the lithium battery of the present invention is used particularly as a secondary battery, a lithium secondary battery using a lithium intercalation compound for the cathode is preferable from the viewpoint of safety.

[0007] The matrix of the polymer electrolyte in the present invention is a copolymer containing a monomer unit composed of vinylidene fluoride (provided that a part of fluorine atoms may be substituted with chlorine atoms or bromine atoms). This copolymer can be copolymerized with other monomer units which can be copolymerized with these in an amount of 20% by weight or less. As other monomer units, for example, tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, vinyl fluoride,
Hexafluoropropylene, perfluoroalkyl vinyl ether, ethylene, ethyl vinyl ether, cyclohexyl vinyl ether, acrylic acid, acrylic acid ester, methacrylic acid, methacrylic acid ester and the like can be mentioned.

The weight ratio of vinylidene fluoride to α-fluoroacrylate in the copolymer of the present invention is from 1/99 to 99 /
1 is preferred. When the weight ratio exceeds 99/1, the crystallinity of the copolymer increases, the flexibility decreases, and the molding processability deteriorates, so that it becomes difficult for the lithium salt solution to enter the parent polymer electrolyte, It is not good because the electric conductivity of the electrolyte becomes low. When the weight ratio is less than 1/99, the flexibility of the electrolyte increases and the strength decreases, so that 99/5 to 5 /
95 is preferred. In order to increase the strength of the electrolyte, a copolymer in which the weight ratio of vinylidene fluoride to α-fluoroacrylate is 40/60 to 99/5 is preferable. The molecular weight of the copolymer used in the present invention is preferably 10,000 to 200,000. If it exceeds 200,000, the viscosity increases, the amount of lithium salt retained decreases, and the electric conductivity of the polymer electrolyte decreases, which is not good. When the molecular weight is less than 10,000, the mechanical strength of the polymer electrolyte is extremely reduced. Therefore, the molecular weight is more preferably 30,000 to 100,000.

[0009] As a solvent for the lithium salt solution of the present invention, a carbonate ester is preferable. For example, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and the like can be mentioned. In the present invention, the above carbonates can be used alone or in combination of two or more,
It may be used by mixing with other solvents.

[0010] It is preferred to use at least one of the salts. The lithium salt solution is prepared by adding a lithium salt to the solvent in an amount of 0.2 to 2.0.
It is preferably dissolved in a concentration of mol / l, preferably 0.5 to 1.
5 mol / l is more preferred. Outside this range, the ionic conductivity decreases and the electrical conductivity of the polymer electrolyte decreases. In the present invention, the polymer electrolyte in which the lithium salt solution is uniformly distributed in the matrix polymer electrolyte is used, but the content of the lithium salt solution is 30 to 80% by weight.
, And more preferably 40 to 65% by weight.

The polymer electrolyte of the present invention can be prepared by various methods. For example, a copolymer serving as a base is dissolved in an organic solvent, and mixed with a solution in which a lithium salt is dissolved in the solvent. This mixed solution is applied on a glass substrate by a bar coater or a doctor blade, and dried to remove an organic solvent, thereby producing a polymer electrolyte film. As the organic solvent, tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, acetone, acetonitrile, dimethyl carbonate, ethyl acetate, butyl acetate and the like can be used.
Organic solvents at a temperature of less than or equal to C are preferred.

In the case of a secondary battery, the cathode active material is a material capable of occluding and releasing lithium ions, and examples thereof include a lithium metal, a lithium alloy, and a carbon material.
As the carbon material, a material obtained by thermally decomposing an organic substance under various thermal decomposition conditions, graphite, or the like can be used.

In the case of a secondary battery, the anode active material is a material capable of inserting and extracting lithium ions.
Ti, Zr, Nb, Cr, Mn, Fe, Co, Ni, Z
Oxides and composite oxides containing a metal such as n, In, Sn, and Sb as a main component, or a composite oxide of the metal and lithium can be used. The anode and the cathode of the present invention are desirably prepared by kneading the active material with an organic solvent so as to make the mud, apply it to a metal foil current collector, and dry it. In the present invention, the battery element can also be formed by forming the copolymer into a porous film without dissolving it in an organic solvent, placing the copolymer between an anode and a cathode, and absorbing a lithium salt solution.

[0014]

EXAMPLES The present invention will be described specifically with reference to Examples.
The present invention is not limited by these examples.

Example 1 In an autoclave (with a stirrer) having an internal volume of 11, 500 g of distilled water, 60 g of t-butanol, 0.6 g of sec-butanol, 6 g of C ₈ F ₁₇ COONH ₄ and 6 g of Na
₂ HPO ₄ · 12H ₂ O and 12 g, 6 g of ammonium persulfate, FeSO ₄ · 7H ₂ O and 0.009 g, EDT
A · 2H ₂ O, 11 g, CH ₂ ＣＦCF—COOCH ₂ C
After adding 65 g of F ₂ CF ₂ CF ₃ and purging with nitrogen gas, 100 g of vinylidene fluoride was charged, and a 1% by weight aqueous solution of formaldehyde sodium sulfoxylate dihydrate was added at 25 ° C. at a rate of 20 ml / hr. While keeping the pressure at 25 atm, vinylidene fluoride was charged and polymerization was carried out. After 5 hours, the reaction was stopped to obtain a dispersion having a concentration of 30%. After coagulation, washing and drying, the copolymer was recovered. The composition of this copolymer is such that the weight ratio of vinylidene fluoride / α-fluoroacrylate is 80/20, and the intrinsic viscosity using tetrahydrofuran as a solvent is 1.3 dl / g.
Met.

The copolymer 1 was placed in an argon gas atmosphere.
0 parts by weight was heated and dissolved in 32 parts by weight of tetrahydrofuran with stirring. (Solution 1) Next, LiPF ₆ was dissolved in a mixed solvent of ethylene carbonate / propylene carbonate (1/1 volume ratio) at a concentration of 1 mol / l in an argon gas atmosphere. (Solution 2) 5 per 21 parts by weight of solution 1
A part by weight of solution 2 was added, and the mixture was heated to 60 ° C. and stirred. This mixed solution was applied on a glass substrate using a doctor blade, and dried at 40 ° C. for 1 hour to remove tetrahydrofuran, thereby producing a 100 μm-thick transparent polymer electrolyte film. The composition of this film is a copolymer, a mixed solvent of ethylene carbonate / propylene carbonate,
LiPF ₆ was 50/44/6 by weight. The film was peeled from the glass substrate, and the electrical conductivity was measured by an AC impedance method in an argon gas atmosphere at 25 ° C. The electric conductivity was 7 × 10 ⁻⁴ s / cm.

LiCoO ₂ powder as an anode active material is 11
Parts by weight, 1.5 parts by weight of acetylene black as a conductive material, 6 parts by weight of the above copolymer, 11 parts by weight of solution 2 and 70 parts by weight of acetone were mixed in an argon gas atmosphere, and heated while stirring. To prepare the slurry. The slurry was applied to an aluminum foil using a doctor blade and dried to produce an anode. Meso-fez carbon fiber (average diameter 8 μm, average length 5
0 μm) 12 parts by weight, 6 parts by weight of the above copolymer, solution 2
Was mixed with 70 parts by weight of acetone in an argon gas atmosphere, and heated while stirring to prepare a slurry. The slurry was applied to a copper foil using a doctor blade and dried to produce a cathode having a thickness of 20 μm.

The above-mentioned polymer electrolyte film is formed into a 1.5 cm square, and the anode and the cathode are opposed to each other with the film sandwiched therebetween, and placed between two polytetrafluoroethylene plates each having a thickness of 1.5 mm and a length of 3 cm square. By tightening, the outside was covered with an exterior film to assemble a lithium ion secondary battery element. All of these operations were performed in an argon gas atmosphere. The charge and discharge conditions were a charge and discharge cycle test at a constant current of 0.5 coulomb with a charge voltage up to 4.2 V and a discharge voltage up to 2.5 V. As a result, the capacity retention after 1000 cycles was 95%.

[0019] Instead of Example _{2 CH 2 = CF-COOCH 2} CF 2 CF 2 CF 3 CH 2 = CF-COOCH 2 CF 2 CF 2 CF 2 C
Copolymers of Example 1 and similarly vinylidene fluoride and α- fluoroacrylate except that charged the F ₃ (88 in weight ratio
/ 12) were synthesized. The intrinsic viscosity was 1.5 dl / g. A polymer film having a thickness of 100 μm was prepared in the same manner as in Example 1 except that this copolymer was used. The electrical conductivity of this film was 5 × 10 ⁻⁴ s / cm. A battery element was manufactured using this polymer electrolyte, and a charge / discharge cycle test was performed. 90% capacity retention after 1000 cycles
Met.

Example 3 A lithium secondary battery element was prepared in the same manner as in Example 1 except that a lithium-aluminum alloy foil having a thickness of 100 μm was used as a cathode, and a charge / discharge cycle test was performed. Was 88%.

[0021]

The lithium battery of the present invention has excellent stability of the polymer electrolyte, maintains good electric conductivity, and has good adhesion between the polymer electrolyte and the electrode active material. Are better.

Claims (4)

[Claims] In a lithium battery (comprising an anode, a cathode and an electrolyte), the electrolyte is composed of a monomer unit composed of vinylidene fluoride and a monomer unit composed of α-fluoroacrylate represented by the formula (1) (provided that a fluorine atom (A part of which may be substituted with a chlorine atom or a bromine atom) is a fluorine-containing polymer electrolyte containing a solution comprising a solute of a lithium salt and a solvent capable of dissolving the lithium salt as a base. A lithium battery, characterized in that: Embedded image [Wherein, R is CH ₂ (CF ₂ ) _n H, CH ₂ (CF ₂ )
_{n F, (CH 2) n} H or _{(CF 2)} n F _(n is 1 to
6) 2. The copolymer according to claim 1, wherein the weight ratio of the monomer unit composed of vinylidene fluoride and the monomer unit composed of α-fluoroacrylate is 95/5 to 5/95.
The lithium battery as described. 3. The lithium battery according to claim 1, wherein the solvent contained in the polymer electrolyte is a carbonate ester. 4. The lithium battery according to claim 1, wherein the polymer electrolyte contains 30 to 80% by weight of a solution in which a lithium salt is dissolved.