JPH0922726A - Solid polymer electrolyte secondary battery with safety for heat - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a safe secondary battery with solid polymer electrolyte, with which the battery function can be stopped at the melting point of polyolefin in the case of overcharge, overdischarge, or misuse by installing a micro-porous film of polyolefin in a solid polymer electrolyte layer. SOLUTION: A solid polymer electrolyte secondary battery concerned includes a positive electrode 2 retaining a non-aqueous electrolytic solution, a negative electrode 4 containing a non-aqueous electrolytic solution and carbonaceous material occluding and releasing lithium ions, and a solid polymer electrolyte layer 5 interposed between the positive and negative electrodes 2, 4 and containing non-aqueous electrolytic solution and polymer retaining it, wherein the electrolyte layer 5 is provided internally with a micro-porous film 6 of polyolefin in such a way as confronting the surface of the layer 5.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a solid polymer electrolyte secondary battery having heat resistance.

[0002]

2. Description of the Related Art In recent years, with the development of electronic equipment, there has been a demand for the development of a secondary battery that is small, lightweight, has a high energy density, and can be repeatedly charged and discharged. As such a secondary battery, a lithium secondary battery including a negative electrode using lithium or a lithium alloy as an active material and a positive electrode using an oxide, sulfide, or selenide such as molybdenum, vanadium, titanium, or niobium as an active material is used. Secondary batteries are known.

However, a secondary battery provided with a negative electrode using lithium or a lithium alloy as an active material has a problem of short charge / discharge cycle life because dendrite of lithium is generated in the negative electrode when the charge / discharge cycle is repeated. .

From the above, a lithium ion secondary battery using a carbonaceous material which absorbs and releases lithium ions such as coke, graphite, carbon fiber, a resin fired body, and pyrolytic vapor phase carbon for the negative electrode is obtained. Proposed. In the lithium ion secondary battery, deterioration of negative electrode characteristics due to dendrite deposition can be improved, so that battery life and safety can be improved.

A polymer electrolyte secondary battery, which is an example of a lithium ion secondary battery, is disclosed in US Pat. No. 5,296,96.
No. 318 discloses a rechargeable lithium intercalation battery having a hybrid polymer electrolyte to which flexibility is imparted by adding a polymer to a positive electrode, a negative electrode and an electrolyte layer. In such a polymer electrolyte secondary battery, a positive electrode having a structure in which a positive electrode layer containing an active material, a non-aqueous electrolytic solution, and a polymer holding the electrolytic solution is supported on a positive electrode current collector, and can absorb and release lithium ions. A negative electrode having a structure in which a negative electrode layer having an active material, a non-aqueous electrolytic solution and a polymer holding the electrolytic solution is supported on a negative electrode current collector, and is interposed between the positive electrode layer and the negative electrode layer, and a non-aqueous It is composed of an electrolytic solution and a solid polymer electrolyte layer having a polymer holding the electrolytic solution.

In the meantime, when the temperature of the polymer electrolyte secondary battery rises up to the melting point of the solid polymer electrolyte layer due to overcharge, overdischarge, misuse, etc., the polymer electrolyte layer melts to form a film. Therefore, the permeation of lithium ions is hindered, the battery function is stopped, and the temperature rise is stopped.

As the polymer in the solid polymer electrolyte layer, a vinylidene fluoride-hexafluoropropylene (VdF-HFP) copolymer is conventionally used. A polymer capable of holding a non-aqueous electrolyte represented by the VdF-HFP copolymer has a high melting point (for example, Vd
The melting point of the F-HFP copolymer is 130 to 160 ° C). Therefore, the secondary battery provided with such a polymer electrolyte layer has a problem that it is excessively heated when it is overcharged or malfunctions.

[0008]

SUMMARY OF THE INVENTION An object of the present invention is to provide a high safety against heat, which is capable of stopping the battery function by the melting point of polyolefin in the case of overcharge, overdischarge or misuse. Another object of the present invention is to provide a solid polymer electrolyte secondary battery.

[0009]

A solid polymer electrolyte secondary battery having heat resistance according to the present invention comprises a positive electrode holding a non-aqueous electrolyte solution, a carbonaceous material capable of inserting and extracting lithium ions, and a non-aqueous material. In a polymer electrolyte secondary battery comprising a negative electrode containing an electrolytic solution, a solid polymer electrolyte layer interposed between the positive electrode and the negative electrode, and containing a non-aqueous electrolytic solution and a polymer holding the electrolytic solution, The solid polymer electrolyte layer is characterized in that a polyolefin microporous film is arranged inside so as to face the surface of the electrolyte layer.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION An example of a solid polymer electrolyte secondary battery having heat resistance according to the present invention will be described below with reference to FIG. The positive electrode is composed of a positive electrode current collector 1 made of, for example, an aluminum foil or an aluminum mesh, and a positive electrode layer 2 carried by the current collector 1. The negative electrode is a negative electrode current collector 3 made of, for example, copper foil or copper mesh.
And the negative electrode layer 4 carried on the current collector 3. The solid polymer electrolyte layer 5 is interposed between the positive electrode layer 2 and the negative electrode layer 4. The electrolyte layer 5 is arranged so that the polyolefin microporous film 6 faces the surface of the electrolyte layer 5 inside.

Next, the solid polymer electrolyte layer 5, the positive electrode layer 2 and the negative electrode layer 4 will be described. 1) Solid Polymer Electrolyte Layer 5 This polymer electrolyte layer 5 is a sheet containing a non-aqueous electrolytic solution and a polymer holding this electrolytic solution, and a polyolefin microporous film 6 faces the surface of the electrolytic layer therein. It is arranged to.

The non-aqueous electrolyte is prepared by dissolving an electrolyte in a non-aqueous solvent. As the non-aqueous solvent,
Ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), dimethyl carbonate (DME), diethylene carbonate (D
EC), methylene ethylene carbonate (MEC), γ
-Butyrolactone (γ-BL), sulfolane, acetonitrile, 1,2-dimethoxymethane, 1,3-dimethoxypropane, dimethyl ether, tetrahydrofuran (THF), 2-methyltetrahydrofuran and the like can be mentioned. The non-aqueous solvent may be used alone,
You may use it in mixture of 2 or more types.

Examples of the electrolyte include lithium perchlorate (LiClO ₄ ) and lithium hexafluorophosphate (L
iPF _6), boric tetrafluoride lithium (LiBF _4), lithium hexafluoroarsenate (LiAsF _6), lithium trifluoromethanesulfonate (LiCF ₃ SO _3), bis (trifluoromethylsulfonyl) imide lithium [LiN
(CF ₃ SO ₂ ) ₂ ] and the like.

It is desirable that the amount of the electrolyte dissolved in the non-aqueous solvent be 0.2 mol / l to 2 mol / l. As the polymer holding the non-aqueous electrolyte, for example, a copolymer of vinylidene fluoride (VdF) and hexafluoropropylene (HFP) can be used. In such a copolymer, VdF contributes to the improvement of the mechanical strength in the skeleton of the copolymer, and HFP is incorporated into the copolymer in an amorphous state to retain the non-aqueous electrolyte solution and lithium. It functions as an ion permeation part. The HFP
The copolymerization ratio of 1 depends on the method of synthesizing the copolymer, but is usually at most about 20% by weight. As the microporous film, for example, a polyethylene microporous film or a polypropylene microporous film can be used. Among them, the polyethylene microporous film is suitable because it has a low melting point of about 120 ° C.

The average pore diameter of the microporous film is a so-called submicron diameter, which is 0.005 μm to
The range is preferably 0.1 μm. This is due to the following reasons. The average pore size is 0.005
If it is less than μm, the permeation of lithium ions may be hindered. On the other hand, when the average pore diameter exceeds 0.1 μm,
Lithium dendrite deposited on the surface of the negative electrode may penetrate the solid polymer electrolyte layer. A more preferable average pore diameter is 0.01 μm to 0.05 μm.

The ratio of the thickness of the electrolyte layer of the microporous film to the total thickness of the porous film is not particularly limited as long as the shutdown characteristics are rapidly exhibited when the battery is heated, but the internal impedance is reduced as much as possible. Therefore, it is preferable to set it in the range of 0.1 to 0.9. This is due to the following reasons. When the thickness ratio of the film is less than 0.1, the film is too thin, so that the film may be fluidized when the film is melted, and lithium ions may permeate the solid polymer electrolyte layer. On the other hand, if the thickness ratio of the film exceeds 0.9, the mobility of lithium ions may decrease, and the capacity of the secondary battery may decrease.
A more preferable ratio of the thickness of the electrolyte layer of the microporous film to the total thickness of the porous film is 0.2.
~ 0.8.

The microporous film is preferably subjected to hydrophilic treatment depending on the type of non-aqueous solvent of the non-aqueous electrolyte. The hydrophilically-treated film can improve the affinity with the non-aqueous solvent, and thus can improve the retention amount of the non-aqueous electrolyte. In particular, a non-aqueous solvent composed of ethylene carbonate and propylene carbonate has a high dielectric constant, but has a poor affinity with polyolefin. When using a non-aqueous solvent of this composition,
By using the polyolefin microporous film that has been subjected to the hydrophilic treatment, a high dielectric constant solid polymer electrolyte layer having the characteristics of the non-aqueous solvent can be obtained.

Examples of the hydrophilic treatment applied to the microporous film include plasma treatment, sulfonation treatment, surfactant treatment, and a method of graft-copolymerizing a vinyl monomer having a hydrophilic group. In particular, the graft copolymerization can be performed without damaging the film, and the vinyl monomer having a hydrophilic group imparted to the film has high hydrophilicity, which is preferable. As the vinyl monomer having a hydrophilic group, for example, acrylic acid, methacrylic acid, esters of acrylic acid or methacrylic acid, vinylpyridine, vinylpyrrolidone, styrenesulfonic acid, styrene and the like are reacted with a direct acid or base to form a salt. And a functional group capable of forming a salt by being hydrolyzed after being graft-copolymerized. Among them, acrylic acid is suitable as the vinyl monomer. 2) Positive Electrode Layer 2 The positive electrode layer 2 is preferably composed of an active material, a conductive material, a non-aqueous electrolytic solution, and a polymer holding the electrolytic solution.

As the active material, various oxides (for example, lithium manganese composite oxide such as LiMn ₂ O ₄ ,
Manganese dioxide, lithium-containing nickel oxide such as LiNiO ₂ , lithium-containing cobalt oxide such as LiCoO ₂ , lithium-containing nickel cobalt oxide, amorphous vanadium pentoxide containing lithium, etc.)
And chalcogen compounds (for example, titanium disulfide, molybdenum disulfide, and the like). Among them, it is preferable to use a lithium manganese composite oxide, a lithium-containing cobalt oxide, and a lithium-containing nickel oxide.

Examples of the conductive material include artificial graphite, carbon black (for example, acetylene black, etc.), nickel powder and the like. As the non-aqueous electrolyte and the polymer, the same ones as those described in the solid polymer electrolyte layer described above are used. 3) Negative Electrode Layer 4 The negative electrode layer 4 is preferably composed of a carbonaceous material that absorbs and releases lithium ions, a nonaqueous electrolytic solution, and a polymer that holds the electrolytic solution.

Examples of the carbonaceous material that absorbs and releases lithium ions include organic polymer compounds (eg, phenol resin, polyacrylonitrile, cellulose).
Examples of the carbonaceous material include a material obtained by firing a coke, a coke, a material obtained by firing a pitch, an artificial graphite, a natural graphite, and the like. Above all, it is preferable to use a carbonaceous material obtained by calcining the organic polymer compound at a temperature of 500 ° C. to 3000 ° C. in an inert gas atmosphere such as an argon gas or a nitrogen gas at normal pressure or reduced pressure. preferable.

As the non-aqueous electrolytic solution and the polymer, the same ones as those described for the solid polymer electrolyte layer are used. The polymer electrolyte secondary battery according to the present invention includes a sheet-like solid polymer electrolyte layer containing a non-aqueous electrolyte solution and a polymer holding the electrolyte solution. Inside the electrolyte layer, a polyolefin microporous film is arranged so as to face the surface of the electrolyte layer. In such a secondary battery, when the temperature rises to the melting point (120 ° C. to 140 ° C.) of the polyolefin that is the material of the film due to overcharging, overdischarging, or misuse, the film melts and the porosity of the film increases. Is blocked. Since lithium ions cannot penetrate such an electrolyte layer, it is possible to prevent the battery function from stopping and the temperature of the secondary battery from rising excessively. Therefore, the safety of the secondary battery can be improved.

In the polymer electrolyte secondary battery, lithium dendrite may be deposited on the surface of the negative electrode due to overcharge or the like. Since the dendrite penetrates the conventional solid polymer electrolyte layer made of a non-aqueous electrolyte and a polymer holding the electrolyte, an internal short circuit may occur when the dendrite occurs in the secondary battery. The internal short circuit causes a decrease in charge / discharge cycle life. By setting the average pore diameter of the polyolefin microporous film of the solid polymer electrolyte layer incorporated in the secondary battery according to the present invention to 0.005 μm to 0.1 μm, this film serves as a barrier and the polymer electrolyte of the lithium dendrite is formed. Layer penetration can be avoided, and the secondary battery can have improved charge / discharge cycle life.

[0024]

Embodiments of the present invention will be described below in detail with reference to the drawings. Example <Preparation of positive electrode layer> Lithium hydroxide monohydrate (LiOH.
H ₂ O) and manganese dioxide (MnO ₂ ) were mixed so that the molar ratio of Li and Mn was 1.5: 1, and the mixture was dehydrated at a temperature of 110 ° C. for 2 hours.
When heated at 20 ° C. for 20 hours, the composition formula becomes Li _x MnO 2.
A lithium manganese composite oxide represented by ₄ was produced.
Vinylidene fluoride-hexafluoropropylene (V
dF-HFP) copolymer (manufactured by Erfatchem Co., trade name KYNAR2750, copolymerization ratio VdF: H
FP is 85:15) to prepare an acetone solution by dissolving 11% by weight in acetone, and then the lithium manganese composite oxide is added to the acetone solution in an amount of 75% by weight based on the solid matter of the copolymer. Acetylene black as a conductive material was mixed in an amount of 10% by weight in terms of solid matter of the copolymer. This suspension was formed into a film by casting, left to stand at room temperature and naturally dried to form a sheet-like positive electrode layer having a thickness of 230 μm. <Production of Negative Electrode Layer> A vinylidene fluoride-hexafluoropropylene copolymer of the same type as that used in the positive electrode layer was dissolved in acetone to prepare 11% by weight of acetone to prepare an acetone solution. Petroleum coke (trade name, manufactured by Petka Co., Ltd .; Melbronn Milled) was mixed in an amount of 70% by weight in terms of solid matter of the copolymer. This suspension was cast to form a film, which was left at room temperature and naturally dried to form a sheet-like negative electrode layer having a thickness of 180 μm. <Production of Solid Polymer Electrolyte Layer> A copolymer of vinylidene fluoride-hexafluoropropylene of the same type as that used for the positive electrode layer was dissolved in acetone at 11% by weight to prepare an acetone solution. The acetone solution was supported by casting on both sides of a PE microporous film having a thickness of 25 μm and an average pore size of 0.02 μm. This was left at room temperature and naturally dried to obtain a sheet-like solid polymer electrolyte layer in which a PE microporous film was arranged so as to face the surface of the electrolyte layer. The ratio of the thickness of the microporous film to the total thickness of the solid polymer electrolyte layer and the porous film was 0.6. <Preparation of Non-Aqueous Electrolyte> In a non-aqueous solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 1 lithium borofluoride as an electrolyte was adjusted to a concentration of 1 mol / l. A non-aqueous electrolytic solution was prepared by dissolving the above.

The sheet-shaped positive electrode layer thus obtained and an aluminum foil as a positive electrode current collector were laminated using a double roll laminator to prepare a sheet-shaped positive electrode. At the same time, the sheet-shaped negative electrode layer and a copper foil as a negative electrode current collector were laminated using a double roll laminator to prepare a sheet-shaped negative electrode. The solid polymer electrolyte layer was interposed between the positive electrode and the negative electrode, and laminated using a double roll laminator. The five-layer laminate is immersed in the non-aqueous electrolyte solution for 10 minutes to impregnate the sheet-shaped positive electrode, the sheet-shaped negative electrode and the solid polymer electrolyte layer with the electrolytic solution, and thus the solid having the structure shown in FIG. 1 described above. A polymer electrolyte secondary battery was manufactured. Comparative Example A solid polymer electrolyte secondary battery having the same configuration as that of the example except that a solid polymer electrolyte layer in which a microporous film was not arranged was used was manufactured.

The obtained secondary batteries of Examples and Comparative Examples were charged with a constant current of 0.2 C and a constant voltage of 4.2 V for 10 hours, and then discharged to a voltage of 2.7 V with a current of 0.2 C. Discharging was repeated and the first cycle of each battery and 500
The discharge capacity at the cycle was measured. As a result, in the secondary batteries of the examples, the discharge capacity in the first cycle was 2.8 mAh / c.
m ² , the discharge capacity at the 500th cycle is 2.4 mAh / c
m ^2, and the secondary battery of Comparative Example discharge capacity at the first cycle is a 3.0mAh / cm ^2, 500 discharge capacity cycle is 2.4 mAh / cm ^2, the intervening battery performance of microporous films It was confirmed that it would not cause any problems.

However, the secondary batteries of Examples and Comparative Examples were subjected to an overcharge test with the charge amount being 250% of the rated capacity, and the battery temperature at that time was measured. The rise could be stopped at a very low temperature of about 120 ° C. On the other hand, the secondary battery of the comparative example,
The temperature rose above 150 ° C, and liquid leakage and violent gas generation were observed.

[0028]

As described above in detail, according to the solid polymer electrolyte secondary battery having the heat resistance according to the present invention,
When the battery is overcharged, overdischarged, or misused, the melting point of the polyolefin can stop the battery function to ensure safety.

[Brief description of drawings]

FIG. 1 is a perspective view showing a solid polymer electrolyte secondary battery having heat resistance according to the present invention.

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 1 ... Positive electrode collector, 2 ... Positive electrode layer, 3 ... Negative electrode collector, 4 ... Negative electrode layer, 5 ... Solid polymer electrolyte layer, 6 ... Polyolefin microporous film.

Claims (1)

[Claims] 1. A positive electrode holding a non-aqueous electrolyte, a negative electrode containing a carbonaceous material that absorbs and releases lithium ions and a non-aqueous electrolyte, and a non-aqueous electrolyte interposed between the positive electrode and the negative electrode. In a polymer electrolyte secondary battery comprising an electrolytic solution and a solid polymer electrolyte layer containing a polymer holding the electrolytic solution, the solid polymer electrolyte layer has a polyolefin microporous film inside facing the surface of the electrolyte layer. A solid polymer electrolyte secondary battery having heat resistance, which is characterized by being arranged as follows.