JPH10284124A - Lithium battery containing polymer electrolyte - Google Patents

Abstract

PROBLEM TO BE SOLVED: To stabilize electrolyte retaining property and enhance durability for charging and discharging cycle by using a copolymer containing a polymerization unit base on tetrafluoroethylene and a polymerization unit based on hexafluoropropylene in a specific ratio as a matrix for the electrolyte and by using a solvent which can dissolve lithium salt and salt. SOLUTION: Electrolyte of a lithium battery has as a matrix a copolymer containing a polymerization unit based on tetrafluoroethylene for 30-70 wt.% and a polymerization unit based on hexafluoropropylene for 8-50 wt.%. Solute of lithium salt and a solution which can dissolve lithium salt are included. A melting point of the copolymer is preferably not less than 50 deg.C. Solvent for the lithium salt is preferably carbonic estel such as propylene carbonate, dimethyl carbonate and the like. It is preferably that the solution in which the lithium salt is dissolved in the solvent is contained in the polymer electrolyte for 30-80 wt.%. Also, a molecular weight of the copolymer used is preferably 10,000-1,000,000.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a lithium battery using a polymer electrolyte, and more particularly to a lithium secondary battery having excellent cycle life.

[0002]

2. Description of the Related Art A battery using a material capable of occluding and releasing an alkali metal or an alkali metal ion as an electrode active material has attracted attention as having a high energy density. Among them, a lithium secondary battery has a particularly high energy density. Therefore, it is being widely used as a power source for electronic devices.

In recent years, measures have been taken to prevent liquid leakage caused by using liquid electrolytes for primary batteries and secondary batteries, to reduce the ignitability of flammable electrolytes, and to facilitate the incorporation into electronic devices by forming batteries into films. Polymer electrolytes have been proposed from the standpoints of improvement in space efficiency and effective use of space (Japanese Patent Application Laid-Open Nos. Hei 8-507407 and Hei 4-506726).

Among them, polyethylene oxide-based polymer electrolytes are electrochemically stable, but have a drawback in that the solvent retention of the organic electrolyte is low. A polyacrylate-based polymer electrolyte having a three-dimensional structure has good solvent retention, but is electrochemically unstable and is not suitable for a 4V-class battery. The polymer electrolyte made of polyvinylidene fluoride is electrochemically stable and has a feature that the polymer is hardly burned because it contains fluorine atoms. However, when the temperature of the polymer electrolyte is increased, the electrolyte oozes out of the polymer. On the other hand, there is an attempt to solve this problem by using a vinylidene fluoride / hexafluoropropylene copolymer.

Further, the conventional lithium secondary battery using a polymer electrolyte has a drawback that the charge / discharge cycle durability is inferior to that of a battery using a liquid electrolyte.

[0006]

DISCLOSURE OF THE INVENTION The present invention, by examining a novel composition of a polymer electrolyte, has a good electrolyte holding property, is stable, and has an excellent charge / discharge cycle durability especially when used as a secondary battery. It is intended to provide a lithium battery.

[0007]

The present invention relates to a lithium battery having a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte comprises 30 to 70% by weight of polymerized units based on tetrafluoroethylene (hereinafter referred to as TFE) and hexafluoropropylene ( Polymerized units 13 to 32 based on HFP)
Polymer comprising a matrix containing a copolymer containing 8 to 50% by weight of polymerized units based on vinylidene fluoride (hereinafter referred to as VDF) in a matrix and containing a solute of a lithium salt and a solvent capable of dissolving the lithium salt Provided is a lithium battery, which is an electrolyte.

[0008] The lithium battery of the present invention can be used as either a primary battery or a secondary battery. In particular, when used as a secondary battery, a so-called lithium ion secondary battery using a lithium intercalation compound for the negative electrode is preferable, considering that lithium is not deposited on the negative electrode and safe.

The polymer electrolyte in the present invention is a solid electrolyte in which a lithium salt solution is uniformly distributed in a matrix by impregnating the matrix with a solution in which a lithium salt is dissolved in a solvent.

In a matrix of a polymer electrolyte,
If the polymerized unit based on TFE is more than 70% by weight, the crystallinity and oleophobicity of the polymer will be high, and the lithium salt solution will not easily penetrate into the polymer, and the electric conductivity of the polymer electrolyte will be low. If the amount of polymerized units based on TFE is less than 30% by weight, the flexibility of the polymer electrolyte is increased, and the strength is reduced. If the polymerization unit based on HFP is more than 32% by weight, the strength when a lithium salt solution is contained to form a polymer electrolyte decreases, so that it is necessary to carry out appropriate crosslinking for improving the strength. On the other hand, if the content is less than 13% by weight, the melting point becomes high, which is not preferable in molding. If the polymerization unit based on VDF is more than 50% by weight, the melting point will be high, and if it is less than 8% by weight, the affinity of the copolymer with the lithium salt solution will be reduced.

Particularly preferably, 40 to 65% by weight of polymerized units based on TFE and 15 to 25 polymerized units based on HFP
Use is made of copolymers containing, by weight, 15 to 45% by weight of polymerized units based on VDF.

The copolymer containing a polymerized unit based on TFE, a polymerized unit based on HFP, and a polymerized unit based on VDF contains 10% by weight of a polymerized unit based on another monomer capable of forming a copolymer therewith. It may be a copolymer appropriately contained within a range not exceeding.

Other monomers include, for example, chlorotrifluoroethylene, trifluoroethylene, vinyl fluoride, ethylene, propylene, isobutylene, ethyl vinyl ether, chloroethyl vinyl ether, cyclohexyl vinyl ether, methyl isopropyl ether, vinyl pivalate, acetic acid Vinyl, vinyl benzoate, Veo
va-9 and Veova-10 (trade name, manufactured by Shell), ethyl allyl ether, cyclohexyl allyl ether, norbornadiene, crotonic acid and its ester, acrylic acid and its alkyl ester, methacrylic acid and its alkyl ester, and the like.

In the polymer electrolyte matrix of the present invention, the weight ratio of other components added as required, the molecular weight of the copolymer, and the like depend on the solubility of the matrix in an organic solvent for forming a polymer electrolyte film. Or dispersibility, miscibility of the matrix with the lithium salt solution and retention of the lithium salt solution, adhesion of the polymer electrolyte to the current collector metal, strength, moldability, handling, ease of matrix availability, etc. Can be selected.

The molecular weight of the copolymer used in the present invention is preferably 10,000 to 1,000,000. If the molecular weight is more than 1,000,000, the dissolution viscosity is extremely high, so that it is difficult to uniformly mix with the lithium salt solution, or the holding amount of the lithium salt solution is reduced, and the electric conductivity of the polymer electrolyte is undesirably reduced. On the other hand, if it is less than 10,000, the mechanical strength of the polymer electrolyte is significantly reduced, which is not preferable. Particularly preferably, it is set to 30,000 to 500,000.

In the present invention, carbonate is preferable as the solvent in which the lithium salt is dissolved and contained in the polymer electrolyte. Carbonate can be used either cyclic or chain. Examples of the cyclic carbonate include propylene carbonate and ethylene carbonate. Examples of the chain carbonate include dimethyl carbonate, diethyl carbonate, methyl ethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate and the like.

In the present invention, the above-mentioned carbonates can be used alone or in combination of two or more. It may be used by mixing with other solvents. Further, depending on the material of the negative electrode active material, the combined use of a chain carbonate and a cyclic carbonate may improve the discharge characteristics, cycle durability, and charge / discharge efficiency.

The lithium salt used in the present invention includes:
ClO ₄ ⁻ , CF ₃ SO ₃ ⁻ , BF ₄ ⁻ , PF ₆ ⁻ , AsF ₆ ⁻ ,
It is preferable to use at least one of lithium salts having an anion of SbF ₆ ⁻ , CF ₃ CO ₂ ⁻ , (CF ₃ SO ₂ ) ₂ N ⁻ or the like.

In the present invention, the lithium salt is preferably dissolved in the solvent at a concentration of 0.2 to 2.0 mol / l. Outside this range, the ionic conductivity decreases and the electrical conductivity of the polymer electrolyte decreases. More preferably, 0.5 to 1.5 mol / l is selected.

In the polymer electrolyte of the present invention, the above-mentioned lithium salt solution is uniformly distributed in a matrix. The content of the lithium salt solution in the polymer electrolyte is 30.
~ 80% by weight is preferred. If the amount is less than 30% by weight, the electric conductivity is undesirably low. If it exceeds 80% by weight, the polymer electrolyte may not be able to maintain a solid state, which is not preferable. Particularly preferably, 40 to 65% by weight is employed.

The polymer electrolyte of the present invention can be prepared by various methods. For example, a copolymer serving as a matrix component is dissolved or uniformly dispersed in an organic solvent, and mixed with a solution in which a lithium salt is dissolved in a solvent (hereinafter, this mixture is referred to as a mixture for forming a polymer electrolyte). The mixed solution is applied, cast or spin-coated on a glass plate with a bar coater or a doctor blade, and then dried to remove mainly the organic solvent in which the copolymer is dissolved or dispersed, thereby obtaining a polymer electrolyte film. When the solvent in which the lithium salt is dissolved partially evaporates during drying, the film is newly impregnated with the solvent or the film is exposed to the solvent vapor to obtain a desired composition.

The organic solvent for dissolving or dispersing the copolymer to form a film includes tetrahydrofuran, methyl ethyl ketone, methyl isobutyl ketone, toluene, xylene, N-methylpyrrolidone, acetone,
Acetonitrile, dimethyl carbonate, ethyl acetate,
A volatile organic solvent having a boiling point of 100 ° C. or lower such as tetrahydrofuran or acetone is preferable in order to selectively remove the organic solvent by drying.

The negative electrode active material in the present invention is a material capable of releasing lithium ions in the case of a primary battery, and a material capable of absorbing and releasing lithium ions in the case of a secondary battery. Although the material forming these negative electrode active materials is not particularly limited, for example, lithium metal, lithium alloy, carbon material, oxides mainly composed of metals of Groups 14 and 15 of the periodic table, carbon compounds, silicon carbide compounds, silicon oxide compounds , Titanium sulfide, boron carbide compounds and the like.

As the carbon material, those obtained by thermally decomposing organic substances under various thermal decomposition conditions, artificial graphite, natural graphite, soil graphite, expanded graphite, flaky graphite and the like can be used. As the oxide, a compound mainly composed of tin oxide can be used.

The positive electrode active material in the present invention is a material capable of occluding and releasing lithium ions in the case of a primary battery, and a material capable of occluding and releasing lithium ions in the case of a secondary battery. For example, Ti, Zr, Hf of Group 4 of the periodic table, V, Nb, Ta of Group 5, Cr, Mo, W of Group 6, Mn of Group 7,
Group 8 Fe, Ru, Group 9 Co, Group 10 Ni, Group 11 Cu, Group 12 Zn, Cd, Group 13 Al, Ga, I
n, Group 14 Sn, Pb, Group 15 Sb, Bi and 16
Oxides and composite oxides containing a metal such as group Te as a main component, chalcogenides such as sulfides, oxyhalides,
A composite oxide of the metal and lithium can be used. Further, a conductive polymer material such as a polyaniline derivative, a polypyrrole derivative, a polythiophene derivative, a polyacene derivative, a polyparaphenylene derivative, or a copolymer thereof can also be used.

In the present invention, when a secondary battery using a material capable of inserting and extracting lithium as a negative electrode active material is used, lithium is contained in the negative electrode and / or the positive electrode. Generally, a lithium-containing compound is used at the time of synthesis of the positive electrode active material, and lithium is contained in the solid matrix of the positive electrode active material.
Further, lithium can be contained in the negative electrode by a chemical or electrochemical method before assembling the battery, or lithium can be incorporated by bringing lithium metal into contact with the negative electrode and / or the positive electrode when assembling the battery.

As the lithium-containing compound used for the positive electrode active material, a composite oxide of lithium and manganese, a composite oxide of lithium and cobalt, and a composite oxide of lithium and nickel are particularly preferable.

The positive and negative electrodes of the present invention are preferably obtained by kneading an active material with an organic solvent to form a slurry, applying the slurry to a metal foil current collector, and drying. More preferably, the mixed solution for forming a polymer electrolyte is impregnated or applied to the positive electrode and the negative electrode, and the polymer electrolyte is permeated to the inside of the electrode layer. Alternatively, the slurry may be preliminarily mixed with a mixed solution for forming a polymer electrolyte and then applied to the metal foil current collector.

In the present invention, the copolymer is formed into a porous film without being dissolved or dispersed in an organic solvent,
A battery element can also be formed by applying a slurry containing an active material to a metal foil current collector, sandwiching it between a positive electrode and a negative electrode obtained by drying, and then absorbing a solution in which a lithium salt is dissolved.

The shape of the lithium battery of the present invention is not particularly limited. A sheet shape (a so-called film shape), a folded shape, a wound-type cylindrical shape with a bottom, a button shape, and the like are selected according to the application.

[0031]

EXAMPLES The present invention will be specifically described below with reference to examples, but the present invention is not limited to these examples. The volume flow rate, which is a measure of the molecular weight of the polymer, was measured by the following method. Using a Koka type flow tester, 265 ℃, 5kg /
The volume of the molten sample extruded per unit time from a nozzle having a diameter of 1 mm and a length of 2 mm under a pressure of cm ² was measured. The unit is mm ³ / sec.

The start and end points of the melting range were determined by differential thermal analysis. As a measuring method, using a differential thermal analysis measuring apparatus, the heating rate was 10 ° C./min in a nitrogen gas atmosphere,
Indium was used as a reference substance.

Example 1 1 l of stainless steel autoclave with a stirrer was used as a container, and ion-exchanged water 615 was used.
g, 2.56 g of ammonium perfluorooctanoate,
0.41 g of oxalic acid and ammonium hydrogen oxalate
24 g were charged. Subsequently, nitrogen was enclosed, and purging and degassing were repeatedly performed. Next, 55.2 g of HFP (= 0.40)
Mol) was supplied to the vessel. Furthermore, 21.5 g of TFE (=
0.21 mol) and 8.4 g of VDF (= 0.13 mol)
, Whereby a pressure of 15 kg / cm ² was generated. The temperature in the vessel was maintained at 43 ° C., the rotation speed of the stirring blade was set to 200 rpm, and the polymerization was started by adding a 15 wt% KMnO ₄ aqueous solution.

As the polymerization proceeds, gaseous TFE, VD
F and liquid HFP were supplied and the pressure was maintained. After 4 hours, the vessel was cooled and the remaining gas monomers were purged to stop the polymerization. The monomer supplied up to this point was 54 g of TFE (= 0.
54 mol), 45.4 g (= 0.71 mol) of VDF and 125.5 g (= 0.91 mol) of HFP.

The milky white dispersion 73 obtained by opening the container was obtained.
0 g was acidified with concentrated hydrochloric acid, stirred with a stirrer equipped with a propeller, and the obtained precipitate was washed several times with ion-exchanged water. The copolymer was dried at 110 ° C. for 10 hours, and the weight of the obtained copolymer was 108 g.

The composition of the copolymer calculated from the ¹³ C-NMR spectrum is such that a polymer unit based on TFE, a polymer unit based on VDF, and a polymer unit based on HFP have a weight ratio of 4
5.0 / 37.6 / 17.3. The melting range of this copolymer was confirmed to be 114 to 126 ° C. by differential thermal analysis. The volume flow rate value is 20 (mm ³ / s
ec).

In an argon atmosphere, 10 parts by weight of this copolymer was dissolved in 32 parts by weight of acetone while heating with stirring. This is designated as solution 1. Next, LiPF ₆ was dissolved at a concentration of 1 mol / l in an argon atmosphere in a solvent in which ethylene carbonate and propylene carbonate were mixed at a volume ratio of 1/1. This is designated as solution 2.

[0038] 21 parts by weight of solution 1 and 5 parts by weight of solution 2 were added, and the mixture was heated to 60 ° C and stirred. This solution was applied on a glass plate using a bar coater, and dried at 40 ° C. for 1 hour to remove acetone to obtain 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 / 44.3 / 5.7 by weight. The film was peeled from the glass substrate, and the electrical conductivity was measured by an AC impedance method at 25 ° C. in an argon atmosphere. The electric conductivity was 5.0 × 10 ⁻⁴ S / cm.

LiCoO ₂ powder was used as a positive electrode active material in 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 atmosphere, and heated while stirring. Got a slurry. This slurry was applied to a 20 μm-thick aluminum foil having a roughened surface using a bar coater and dried to obtain a positive electrode.

Mesophase carbon fiber powder (average diameter 8 μm, average length 50 μm, (0
02) A spacing of 0.336 nm) 12 parts by weight of the copolymer, 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 atmosphere, and heated with stirring to obtain a slurry. Was. Thickness of this slurry obtained by roughening the surface 2
A negative electrode was obtained by coating and drying on a 0 μm copper foil with a bar coater.

The polymer electrolyte film is 1.5 cm
Formed into corners, through which the effective electrode area 1 cm x 1 cm
The positive electrode and the negative electrode were opposed to each other, sandwiched between two 1.5 cm-thick 3 cm square poly TFE back plates, and the outside thereof was covered with an exterior film to assemble a lithium ion secondary battery element. This operation was all performed in an argon atmosphere.

A charge / discharge cycle test was performed under the conditions of a constant current of 0.5 C, a charge voltage of up to 4.2 V, and a discharge voltage of up to 2.5 V. As a result, the capacity retention after 500 cycles was 93%.

Example 2 In Example 1, the initial TFE was changed to 2
3.5 g (= 0.23 mol) and HFP 51.2 g (=
0.37 mol), 5.9 g of VDF (= 0.09 mol)
And for the monomers supplied as the polymerization proceeds, T
62.5 g (= 0.62 mol) of FE and 14
1.3 g (= 1.02 mol), VDF 37.4 g (=
The copolymerization was carried out in the same manner as in Example 1 except that the amount was changed to 0.58 mol, to obtain 113 g of a TFE / HFP / VDF copolymer (59.3 / 16.2 / 24.5 in weight ratio). The melting range of this copolymer was confirmed to be 125 to 136 ° C. by differential thermal analysis. The volume flow rate is 10 (mm ³
/ Sec).

A polymer electrolyte film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that this copolymer was used.
When the electric conductivity of this film was measured in the same manner as in Example 1, it was 4.5 × 10 ⁻⁴ S / cm.

A battery element was assembled in the same manner as in Example 1 except that this polymer electrolyte was used, and a charge / discharge cycle test was performed in the same manner as in Example 1. The capacity retention after 500 cycles was 91%.

[Example 3] In the example 1, the initial TFE was 2
5.2 g (= 0.25 mol) and 48 g of VDF (= 0.25 mol)
07 mol), and 53.2 g (= 0.38 mol) of HFP.
E was 66.4 g (= 0.66 mol) and VDF was 30.8 g
g (= 0.48 mol) and 152.4 g of HFP (= 1.
Copolymerization was carried out in the same manner as in Example 1 except that the amount was changed to 10 mol) to obtain 102 g of TFE / HFP / VDF copolymer (weight ratio: 62.1 / 16.1 / 21.8). The melting range of this copolymer was confirmed by differential thermal analysis to be 153 to 168 ° C. The volume flow rate value is 6 (mm ³ /
sec).

A polymer electrolyte film having a thickness of 100 μm was obtained in the same manner as in Example 1 except that this copolymer was used.
When the electric conductivity of this film was measured in the same manner as in Example 1, it was 4.0 × 10 ⁻⁴ S / cm.

A battery element was assembled in the same manner as in Example 1 except that this polymer electrolyte was used, and a charge / discharge cycle test was performed in the same manner as in Example 1. The capacity retention after 500 cycles was 90%.

Example 4 A lithium secondary battery element was assembled 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 negative electrode, and a charge / discharge cycle test was performed as in Example 1. The capacity retention after 500 cycles was 87%.

[0050]

As described above, the lithium battery of the present invention has good retention of the lithium salt solution of the polymer electrolyte, good electric conductivity, and good adhesion between the polymer electrolyte and the electrode active material. Excellent durability. Further, the lithium battery of the present invention can be applied to both a primary battery and a secondary battery by selecting a positive electrode active material and a negative electrode active material.

 ──────────────────────────────────────────────────続 き Continued on the front page (72) Inventor Masayuki Tamura 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Prefecture Inside the Asahi Glass Co., Ltd. (72) Inventor Atsushi Funaki 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa Asahi Glass Central Research Laboratory (72) Inventor Tohru Ishida 1150 Hazawa-cho, Kanagawa-ku, Yokohama-shi, Kanagawa-ken Central Research Laboratory of Asahi Glass Co., Ltd.

Claims (4)

[Claims] 1. A lithium battery comprising a positive electrode, a negative electrode and an electrolyte, wherein the electrolyte comprises 30 to 70% by weight of polymerized units based on tetrafluoroethylene, 13 to 32% by weight of polymerized units based on hexafluoropropylene, and vinylidene fluoride. A lithium battery comprising: a polymer electrolyte comprising, as a matrix, a copolymer containing 8 to 50% by weight of a polymer unit based on the polymer electrolyte, the solution comprising a solute of a lithium salt and a solvent capable of dissolving the lithium salt. 2. The lithium battery according to claim 1, wherein the melting point of the copolymer is 50 ° C. or higher. 3. The lithium battery according to claim 1, wherein the solvent 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 obtained by dissolving a lithium salt in a solvent.