JPH09293533A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enhance the safety of a battery by containing the specified amount of specific compound in an electrolyte. SOLUTION: An electrolyte contains a lithium salt as an electrolyte, fluorine- containing alkane represented by the formula, and an organic solvent for dissolving the salt. In the formula, X and Y show a fluorine atom or a hydrogen atom, and (n) is an integer of 5-8. The fluorine-containing alkane which is actually electrochemically inactive is contained 0.5-30wt.% in the electrolyte. Firing of a battery is caused by ignition of the inflammable vapor of the organic electrolyte exploded out from a battery container in some cause. The vapor is diluted with the gas of the fluorine compound, and ignition is prevented, and the safety of a battery is ensured. The fluorine-containing alkane is preferable to be self-nonflammable.

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery. In particular, the present invention relates to a non-aqueous electrolyte secondary battery used in a field having a large capacity, a high energy density, and a high demand for maintenance-free, such as an electric vehicle and a load leveling of electric power.

[0002]

2. Description of the Related Art In recent years, in response to downsizing and weight saving of electronic devices, lithium-ion secondary batteries for electronics have been put into practical use as power sources thereof and are used in handy video cameras, portable personal computers and the like. . Furthermore, electric vehicles have been attracting attention due to environmental issues and the like, and attention has been paid to sealed, maintenance-free lithium ion secondary batteries with high energy density (Japanese Patent Laid-Open No. 59-8).
1869, JP-A-3-74061, JP-A-7-21
1349 publication).

[0003]

Non-aqueous electrolyte secondary batteries generally use a flammable non-aqueous organic compound as an electrolyte, and it is known that ignition problems may occur. When lithium, a lithium alloy, or the like is used as the negative electrode active material, these serve as ignition sources and cause ignition. Even a lithium-ion secondary battery using a compound capable of inserting and extracting lithium as a negative electrode active material causes a problem when it is not used correctly. In particular, a lithium ion secondary battery may cause a problem of ignition when overcharged to a level far exceeding a recommended charging voltage.

Various lithium ion secondary batteries using a solid electrolyte have been proposed (Solid State Ionics, p. 895 (1988)) and Electrochim. Act from the viewpoint of the above-mentioned battery safety.
a, Vol. 11, pp. 1669 (1988)), but sufficient performance has not been obtained yet.

[0005]

[Means for Solving the Problems] As a result of intensive studies in view of the above problems, the presence of a nonflammable compound in a nonaqueous electrolyte solution reduces the performance of a conventional nonaqueous electrolyte secondary battery. It was found that it is possible to improve the safety of the battery without any problems, and the present invention has been completed. That is, the present invention is at least a positive electrode active material, a negative electrode active material, a separator,
A non-aqueous electrolyte secondary battery comprising a non-aqueous electrolyte, characterized in that the non-aqueous electrolyte contains 0.5 to 30% by weight of a fluorine-containing alkane represented by the following formula (I). An aqueous electrolyte secondary battery is provided.

[0006]

Embedded image

(In the formula, X and Y each independently represent a fluorine atom or a hydrogen atom, and n is a number of 5 to 8.)

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION The non-aqueous electrolyte secondary battery of the present invention
At least a negative electrode, a positive electrode, a separator, and a nonaqueous electrolytic solution. FIG. 1 shows a non-aqueous electrolyte secondary battery as an example thereof.
In the figure, 1 is a negative electrode current collector plate, 2 is a positive electrode current collector plate, 3 is a negative electrode, 4
Is a positive electrode, 5 is a separator impregnated with an electrolytic solution, 6 is a connecting plate, 7 is a polypropylene container, and 8 is a rubber stopper.

Negative electrode: The negative electrode active material may be lithium and a lithium alloy, but a carbon material having a higher safety and capable of inserting and extracting lithium is preferable. This carbon material is not particularly limited, but is not limited to graphite, coal-based coke, petroleum-based coke, petroleum-based pitch carbide, petroleum-based pitch carbide, needle coke, pitch coke, phenolic resin, crystalline cellulose, etc. Partially graphitized carbon materials, furnace black, acetylene black, pitch-based carbon fibers and the like can be mentioned.

As the negative electrode, it is possible to use a negative electrode active material and a binder (binder) which are slurried with a solvent and applied to form a sheet, which is then dried. Examples of the binder (binder) include polyvinylidene fluoride, polytetrafluoroethylene, EPDM (ethylene-propylene-diene terpolymer), SBR (styrene-butadiene rubber), NBR (acrylonitrile-butadiene rubber), Examples include, but are not limited to, fluororubber.

As the solvent for forming a slurry, an organic solvent that dissolves the binder is usually used. For example, N-methylpyrrolidone, dimethylformamide, dimethylacetamide, methyl ethyl ketone, cyclohexanone, methyl acetate, methyl acrylate, diethyltriamine, N-
N-dimethylaminopropylamine, ethylene oxide, tetrahydrofuran, etc. may be mentioned, but not limited thereto. In some cases, the active material is slurried with latex such as SBR by adding a dispersant and a thickener to water. When using a current collector for the negative electrode, copper, nickel,
Stainless steel, nickel plated steel, etc. are used.

Positive electrode: The positive electrode active material is preferably a metal oxide compound, a chalcogenite compound or the like which can occlude or intercalate lithium, and LixCo
O ₂ , LixMnO ₂ , LixMn ₂ O ₄ , LixV ₂
O ₅ , LixTiS _{2 and the} like can be mentioned. As the positive electrode, a sheet-like product can be used in which a positive electrode active material, a binder (binder), and a conductive agent are slurried with a solvent and applied, and dried.

As the binder and the solvent used for forming the slurry, those exemplified in the production of the negative electrode can be used.
As the conductive agent of the positive electrode, fine particles of graphite, carbon black such as acetylene black, and fine particles of amorphous carbon such as needle coke are used. When a current collector is used for the positive electrode, aluminum, stainless steel, nickel-plated steel or the like is used.

Separator: As the separator, a microporous polymer film is used. It is made of nylon, cellulose acetate, nitrocellulose, polysulfone, polyacrylonitrile, polyvinylidene fluoride, or a polyolefin-based polymer such as polypropylene, polyethylene or polybutene. Is used. The chemical and electrochemical stability of the separator is an important factor. From this point of view, a polyolefin-based polymer is preferable, and from the viewpoint of self-closing temperature which is one of the purposes of the battery separator, polyethylene is preferable.

In the case of a polyethylene separator, ultrahigh molecular weight polyethylene is preferable from the viewpoint of shape retention at high temperature, and the lower limit of its viscosity average molecular weight is preferably 50.
Million, more preferably 1 million, most preferably 1.5 million. On the other hand, the upper limit of the molecular weight is preferably 5,000,000, more preferably 4,000,000, and most preferably 3,000,000. This is because if the molecular weight is too large, the fluidity is so low that the pores of the separator may not close when heated.

Electrolytic solution: In the present invention, the electrolytic solution contains a lithium salt as an electrolyte and an organic solvent which dissolves the fluorine-containing alkane represented by the above formula (I) and the lithium salt as an organic solvent. The substantially electrochemically inactive fluorine-containing alkane is contained in the electrolytic solution in an amount of 0.5 to 30% by weight. The ignition of the battery is caused by the ignition of the flammable gas which is ejected from the battery container for some reason. The vapor of the organic electrolyte is a source of combustible gas. By diluting the ejected vapor with the fluorine-containing organic compound gas of the present invention, it is possible to substantially prevent ignition and ensure the safety of the battery. Therefore, the fluorinated alkane is preferably self-inflammable.

The lower limit of the vapor pressure of the fluorinated alkane at 25 ° C. is preferably 25 mmHg, more preferably 50 mmHg, and most preferably 70 mmHg. This is because if the vapor pressure of the fluorinated alkane is low, the effect of making the electrolyte nonflammable is poor. Also, the upper limit of the vapor pressure of the fluorine-containing alkane at 25 ° C. is preferably 760 mmH.
g, more preferably 600 mmHg, most preferably 500 mmHg. Fluorine-containing alkanes are liquid at room temperature. Furthermore, if the vapor pressure is too high, the internal pressure of the battery rises and there is a risk of explosion, which is not preferable. For the above reasons, the carbon number of the fluorine-containing alkane is in the range of 5-8. Table 1 shows the physical properties of typical fluorinated alkanes that are liquid at room temperature.

[0018]

[Table 1]

The larger the amount of fluorine atoms is, the more the effect of incombustibility is increased, and the intermolecular interaction is reduced, so that the volatility becomes high. For the above reasons, the fluorine-containing alkane is preferably a compound having a large amount of fluorine atoms. A fluorine-containing alkane in which some of the F atoms of the fluorine-containing alkane are replaced with H may be used. H / F of the fluorine-containing alkane (H and F
Represents the number of hydrogen atoms and fluorine atoms respectively in the fluorine-containing alkane molecule), preferably 0.2 or less, more preferably 0.1 or less, and most preferably 0.

The proportion of the fluorinated alkane in the electrolytic solution is in the range of 0.5 to 30% by weight. This is because if the proportion of the fluorine-containing alkane is low, the incombustibility effect will be poor.
If the proportion of the fluorine-containing alkane is too large, for example, solution resistance increases (lithium ion conductivity decreases), and desired battery characteristics are not expressed, which is not preferable.

As the main organic solvent other than the fluorine-containing alkane, for example, carbonates, ethers, ketones, sulfolane compounds, lactones, nitriles, chlorinated hydrocarbons, amines, esters, amides, phosphoric acid Ester compounds and the like can be used. When these representative ones are listed, propylene carbonate, ethylene carbonate, vinylene carbonate, tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-
Dioxane, 4-methyl-2-pentanone, 1,2-dimethoxyethane, 1,2-diethoxyethane, γ-butyrolactone, 1,3-dioxolane, 4-methyl-1,
3-dioxolane, diethyl ether, sulfolane, methylsulfolane, acetonitrile, propionitrile,
Benzonitrile, butyronitrile, valeronitonil,
1,2-dichloroethane, dimethylformamide, dimethylsulfoxide, trimethyl phosphate, triethyl phosphate and the like may be used alone or in combination of two or more kinds.

As the above-mentioned organic solvent, it is preferable to use a high dielectric constant solvent in order to dissociate the electrolyte. The high dielectric constant solvent means a compound having a relative dielectric constant of 20 or more at 25 ° C. In particular, as the high dielectric constant solvent, it is preferable that the electrolytic solution contains ethylene carbonate, propylene carbonate, and a compound in which hydrogen atoms thereof are replaced with another element such as halogen or an alkyl group.

The proportion of the high dielectric constant compound in the electrolytic solution is
Preferably 20 to 95% by weight, more preferably 30 weight
The amount is at least%, more preferably at least 40% by weight. Conversion
If the compound content is low, the desired battery characteristics cannot be obtained.
This is because there are cases. Conventionally known electrolytes
Can be used with a gap, LiClO_Four, LiAsF ₆, Li
PF₆, LiBF_Four, LiB (C₆H₆)_Four, LiC
l, LiBr, CH_ThreeSO_ThreeLi, (C-F_ThreeSO_Two)
NLi or the like is used. The electrolyte concentration in the electrolyte is
Usually, it is 0.5 to 1.5 mol / liter and 3 to 3 in the electrolytic solution.
An amount of 20% by weight.

[0024]

The present invention will be described in more detail below with reference to examples, but the present invention is not limited to the following examples without departing from the gist thereof. The evaluation methods used in the examples are as follows. In Examples and Comparative Examples, “parts” means “parts by weight”. (Example 1)

(Negative electrode) 90 parts of coal-based needle coke having an average particle size of 10 μm (weight ratio; the same applies hereinafter unless otherwise specified) was mixed with a solution of 10 parts of polyvinylidene fluoride in N-methylpyrrolidone (2% by weight). Then, a negative electrode mixture slurry was prepared. It was applied on both sides of a copper foil having a thickness of 20 μm, dried to evaporate the solvent, and roll-treated to prepare a negative electrode sheet. The size of the negative electrode mixture application part is 12 cm x 15 c
m and the thickness was 250 μm on each side. The left and right sides of the copper foil were designed to be coated with the negative electrode mixture leaving ears of 25 mm on the left and 15 mm on the right. In addition, the electrodes forming the ends of the unit cells are prepared by applying the negative electrode mixture only on one surface.

(Positive electrode) 1 mol of lithium carbonate and 2 mol of cobalt carbonate were mixed and pulverized in a ball mill and heat-treated in air at 850 ° C. for 5 hours, and then pulverized and mixed again in a ball mill, and further 850 ° C. for 5 hours. Heat-treated in 9
To 0 part, 5 parts of acetylene black as a conductive agent was added and mixed, and mixed with 5 parts of polyvinylidene fluoride N-methylpyrrolidone solution (2% by weight) to prepare a positive electrode mixture slurry. It was applied on both sides of an aluminum foil having a thickness of 25 μm, dried to evaporate the solvent, and roll-treated to produce a positive electrode. The size of the part where the positive electrode mixture is applied is 12 cm x 15
cm, and the thickness was 250 μm on each side. The left and right sides of the aluminum foil were designed to be coated with the positive electrode mixture leaving ears of 25 mm on the right and 15 mm on the left. The electrodes forming the ends of the unit cells were prepared by applying the positive electrode mixture only on one surface.

(Separator) 20 parts of ultrahigh molecular weight polyethylene powder having a melting point of 135 ° C. and a molecular weight (viscosity average) of 2 × 10 ⁶ and 80 parts of ceryl alcohol are fed to an extruder and continuously kneaded at 230 ° C. After extruding into a film form from a T-die, melt deformation was applied in the machine (MD) direction to obtain a film having a film thickness of 50 μm. The obtained film is 80
Dip in isopropyl alcohol at ℃, extract and remove the ceryl alcohol from the film, then the surface temperature 1
Heat with pinch rolls heated at 25 ° C for 30 seconds to 25
A porous film having a thickness of μm was obtained.

(Assembly of Unit Cell) The unit cell was assembled by alternately laminating the negative electrode and the positive electrode with a separator interposed therebetween. At this time, the electrodes at both ends used an electrode mixture coated on one side only. A metal rod was separately welded to the negative electrode and the positive electrode to form a current collector in which the negative electrode and the positive electrode were electrically connected separately. The cells are fastened with a non-conductive frame in the stacking direction. By stacking 26 sets and half of the electrodes of the above size (the electrodes at both ends are half because the electrode mixture is applied only on one surface), a single cell having a charge / discharge capacity of about 350 Wh was produced.

(Assembly of assembled battery) As an electrolytic solution, 49.5 parts of ethylene carbonate and 3,2-dimethoxyethane 3 were used.
2.4 parts and linear perfluoroalkane (C ₆ F ₁₄ ).
A solution prepared by dissolving 9.1 parts of lithium hexafluorophosphate (LiPF ₆ ) as an electrolyte in an organic solvent consisting of 10 parts was used.

The above unit cell was vacuum degassed at 1 × 10 ^-2 Torr or less and then placed in a dry box which had been replaced with Ar gas. The above-mentioned 10 cells were stored in a polypropylene container equipped with a partition wall, the above-mentioned electrolytic solution was injected,
I closed the top lid. At this time, the negative electrode terminal and the positive electrode terminal of each cell protruded to the upper part of the container through the upper lid. This terminal was sealed with a suitable sealant at the penetrating portion of the upper lid. At this time, the diameter of 10
mm holes were made. 1 mA / c at 25 ° C
After charging the battery at a constant current density of m ² until the battery voltage reached 4.0 V, the hole at the top of the container was covered with a rubber stopper coated with an adhesive.

(Evaluation of flammability of battery)
After charging in an atmosphere of ℃ at a constant current density of 1 mA / cm ² until the battery voltage reached 4.3 V, the rubber stopper on the upper part of the container was removed, and the test flame was brought closer according to JIS K2810. There wasn't. In the following, the results of evaluation in the same manner while changing the temperature at which the battery pack is placed are summarized in Table 2
Shown in

(Examples 2 to 3) Lithium secondary batteries were obtained and evaluated in the same manner except that the type of the linear perfluoroalkane of the electrolyte was as shown in Table 2. (Examples 4 to 6 and Comparative Example 1) A lithium secondary battery was obtained and evaluated in the same manner as in Example 1 except that the composition in the electrolytic solution was changed as shown in Table 2. Only the battery of Comparative Example 1 ignited.

[0033]

[Table 2]

[0034]

INDUSTRIAL APPLICABILITY The non-aqueous electrolyte secondary battery containing the fluorine-containing alkane in an amount of 0.5 to 30% by weight in the non-aqueous electrolyte of the present invention can remarkably improve its safety without impairing the battery characteristics. . This non-aqueous electrolyte secondary battery is suitable for a large battery in which a large amount of organic electrolyte exists, especially a large battery represented by an automobile.

[Brief description of drawings]

FIG. 1 is a cross-sectional view of a non-aqueous electrolyte secondary battery.

Claims (2)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising at least a positive electrode active material, a negative electrode active material, a separator, and a non-aqueous electrolytic solution, wherein the non-aqueous electrolytic solution contains a fluorine-containing alkane represented by the following formula (I): 0.5 to 30% by weight of a non-aqueous electrolyte secondary battery. Embedded image (In the formula, X and Y each independently represent a fluorine atom or a hydrogen atom, and n is a number of 5 to 8.) 2. The electrolytic solution contains lithium salt as an electrolyte.
˜20 wt%, 0.5 to 30 wt% of the fluorine-containing alkane represented by the above formula (I) as a solvent, and 20 wt% or more of a high dielectric constant solvent selected from ethylene carbonate and propylene carbonate. , Claim 1
3. The non-aqueous electrolyte secondary battery according to 1.