JP4568920B2 - Non-aqueous electrolyte secondary battery and non-aqueous electrolyte used therefor - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte secondary battery and a non-aqueous electrolyte used therein , and in particular, a non-aqueous electrolyte secondary battery excellent in low temperature characteristics, long-term stability, cycle characteristics, and the like, and a non-aqueous electrolyte used therein It is about.
[0002]
[Prior art]
In recent years, with the reduction in weight and size of electrical products, development of lithium secondary batteries having high energy density has been promoted, and some of them have already been put into practical use. Currently developed lithium secondary batteries use lithium metal or graphite that absorbs and releases lithium as the negative electrode active material, and lithium transition metals such as LiCoO _2, LiNiO ₂ , and LiMn ₂ O ₄ as the positive electrode active material. A composite oxide is used, and a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent is used as an electrolytic solution. Of these, lithium secondary batteries using graphite as a negative electrode active material are considered promising from the viewpoint of safety and the like.
[0003]
[Problems to be solved by the invention]
As the organic solvent constituting the non-aqueous electrolyte, various solvents such as cyclic carbonates such as ethylene carbonate and propylene carbonate; chain carbonates such as dimethyl carbonate, diethyl carbonate and methyl ethyl carbonate; and lactones such as γ-butyrolactone have been proposed. ing. Among these, propylene carbonate is considered to be one of the most preferable solvents for non-aqueous electrolytes because it has a high dielectric constant, dissolves lithium salts well, has a low melting point, and provides an electrolytic solution having high electrical conductivity even at low temperatures. It has been. However, when a non-aqueous electrolyte obtained by dissolving a lithium salt in propylene carbonate is used in combination with graphite or graphitized carbon having high crystallinity as a negative electrode active material, there is a problem that propylene carbonate decomposes upon charging.
[0004]
Therefore, studies are being made for a non-aqueous electrolyte solution that is stable even when used in combination with graphite and has high electric conductivity. One of them is to use ethylene carbonate instead of propylene carbonate. However, since ethylene carbonate has a high freezing point of 36.4 ° C., it is not used alone, and studies are being made in the direction of using it mixed with a low-viscosity solvent. A low-viscosity solvent generally has a low boiling point, so that when used in a large amount, there is a problem in terms of safety, and when only a small amount is used, there is a problem in terms of electrical conductivity and viscosity at a low temperature. A non-aqueous electrolyte using a mixed solvent of ethylene carbonate and diethyl carbonate has been proposed as one of the solvents for solving such problems. However, this solution is still satisfactory in terms of battery cycle characteristics. Not kimono.
[0005]
In order to solve the above problems, JP-A-5-74486 and JP-A-8-45545 propose to use a nonaqueous electrolytic solution in which a lithium salt is dissolved in an organic solvent containing vinylene carbonate. Has been. Since vinylene carbonate forms a stable protective film on the negative electrode surface, it is said that the storage characteristics of the battery are good.
[0006]
As the most general method for producing vinylene carbonate, a method using dehydrochlorination of chloroethylene carbonate is known, but according to the study by the present inventors, an organic containing vinylene carbonate produced by this method is known. A secondary battery using a non-aqueous electrolyte in which a lithium salt is dissolved in a solvent does not exhibit satisfactory cycle performance. According to the study by the present inventors, the cause is presumed to be due to the oxidative decomposition of the organic halide derived from the raw material contained in the vinylene carbonate. Accordingly, the present invention is that such a well vinylene carbonate with a non-aqueous electrolyte solution as part of the solvent, to provide a non-aqueous electrolyte solution using good nonaqueous electrolyte secondary battery and its cycle-characteristics It is.
[0007]
[Means for Solving the Problems]
A non-aqueous electrolyte secondary battery according to the present invention includes a negative electrode capable of inserting and extracting lithium and an electrode assembly in which a positive electrode is opposed to each other through a separator, and a non-aqueous electrolyte contained in a container. In the non-aqueous electrolyte secondary battery, the non-aqueous electrolyte is produced by dehydrochlorination of chloroethylene carbonate, and contains vinylene carbonate containing an organic halide derived from the chloroethylene carbonate at a ratio of 1 ppm to 50% by weight. LiPF ₆ as a lithium salt in a non-aqueous solvent selected from the group consisting of cyclic carbonates, chain carbonates, lactones, chain carboxylic acid esters, cyclic ethers, chain ethers and sulfur-containing compounds, containing 0.1 to 30 % by weight In contact with the non-aqueous electrolyte of the positive electrode current collector and the part electrically connected to the positive electrode current collector The portion to be made is made of a valve metal or an alloy thereof.
[0008]
DETAILED DESCRIPTION OF THE INVENTION
The non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery according to the present invention is prepared by dissolving a lithium salt in a non-aqueous solvent containing vinylene carbonate. Vinylene carbonate usually contains organic halides such as chloroethylene carbonate, dichloroethylene carbonate, chloromethyl methyl carbonate, and chloroethanol. Since the most common method for producing vinylene carbonate is by dehydrochlorination of chloroethylene carbonate as described above, ordinary vinylene carbonate contains chloroethylene carbonate derived from this raw material. The content of these organic halides in vinylene carbonate is usually 1 ppm to 50 (weight)%.
[0009]
The non-aqueous solvent constituting the non-aqueous electrolyte is composed of vinylene carbonate containing this organic halide and another organic solvent. The content of vinylene carbonate in the non-aqueous solvent is preferably 0.1 to 30% by weight. If the amount is less than 0.1% by weight, the effect is small. Conversely, if the amount exceeds 30% by weight, the low-temperature characteristics of the electrolytic solution may be impaired. A preferable content of vinylene carbonate is 0.1 to 20% by weight.
[0010]
Other organic solvents used in combination with vinylene carbonate include cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate, which are known as solvents for non-aqueous electrolytes, dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, methyl Chain carbonates such as ethyl carbonate, methyl-n-propyl carbonate, ethyl-n-propyl carbonate, lactones such as γ-butyrolactone and γ-valerolactone, chain carboxylic acid esters such as methyl acetate and methyl propionate, tetrahydrofuran, Examples include cyclic ethers such as 2-methyltetrahydrofuran and tetrahydropyran, chain ethers such as dimethoxyethane and diethoxyethane, and sulfur-containing compounds such as sulfolane and diethylsulfone. It is. Among these, it is preferable to use carbonate. Usually, a non-aqueous solvent is used in which the total amount of carbonate and vinylene carbonate selected from the group consisting of cyclic carbonate and chain carbonate accounts for 70% by weight or more. It is preferable to use a non-aqueous solvent in which the total amount is 80% by weight or more, particularly 90% by weight or more.
[0011]
The lithium salt of the electrolyte dissolved in the non-aqueous solvent is an inorganic acid lithium salt such as LiClO ₄ , LiPF ₆ , LiBF ₄ , LiSbF ₆ , or LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C _{2 F 5 SO 2) 2, LiN} (CF 3 SO 2) (C 4 F 9 SO 2), such as LiC (CF ₃ SO _{2) 3} organic lithium salts such as, conventionally used as an electrolyte of the non-aqueous electrolyte solution What is necessary is just to use. These may be used in combination if desired. The concentration of the lithium salt in the non-aqueous electrolyte is usually 0.5 to 2 mol / liter.
[0012]
Usable ones can be used as the positive electrode and the negative electrode of the non-aqueous electrolyte secondary battery according to the present invention. As the positive electrode active material, a lithium transition metal composite oxide capable of inserting and extracting lithium such as LiCoO ₂ , LiNiO ₂ , LiMn ₂ O ₄ is used. As the positive electrode current collector, a valve metal such as aluminum, titanium, or tantalum or an alloy thereof is used. Of these, aluminum or an alloy thereof is preferably used.
Use of a metal other than the valve metal such as stainless steel is not preferable because the organic halide in the non-aqueous electrolyte in contact with the metal is oxidized and decomposed.
[0013]
As the negative electrode active material, graphite is preferably used. As the graphite, both artificial graphite and purified natural graphite can be used. Further, it is also preferable to use those graphites that have been surface-treated with pitch or the like. Among the graphites, the d value (interlayer distance) of the lattice plane (002 plane) determined by X-ray diffraction by the Gakushin method is 0.335 to 0.34 nm, particularly 0.335 to 0.337 nm. It is. If desired, other known negative electrode active materials can be used in place of graphite or in combination with graphite. Examples of such a negative electrode active material include lithium metal and alloys thereof, non-graphite carbon, metal oxides such as tin oxide and silicon oxide. As the current collector for the negative electrode, copper, nickel, stainless steel, or the like is used. Of these, copper is preferably used.
[0014]
As the separator that separates the positive electrode and the negative electrode, a porous film or non-woven fabric of polyolefin such as polyethylene or polypropylene is used.
The structure of the battery may be any conventionally known structure such as a cylinder type in which a sheet-like electrode and a separator are wound in a spiral shape, or a coin type in which a flaky electrode and a separator are laminated.
[0015]
As a container that accommodates the electrode assembly composed of the positive electrode, the negative electrode, and the separator, that is, a battery can, a conventional stainless steel can be used.
However, in the present invention, of the can body and the lead wire and safety valve accommodated in the can body, the portion that is electrically connected to the positive electrode and is in contact with the non-aqueous electrolyte is a valve metal or an alloy thereof. Constitute. Therefore, when using a normal stainless steel can body, the inner surface of the positive electrode side is covered with a valve metal. By doing in this way, the oxidative decomposition | disassembly of the organic halide accompanying the vinylene carbonate in the nonaqueous electrolyte solution which contacts this can be prevented. Moreover, you may comprise a can body with valve metals, such as aluminum and aluminum alloy.
[0016]
Since the non-aqueous electrolyte secondary battery according to the present invention has the above-described configuration, the decomposition of the non-aqueous electrolyte is suppressed and stable performance can be exhibited over a long period of time. This is because vinylene carbonate in the non-aqueous electrolyte forms a stable protective film on the negative electrode to suppress the decomposition of the non-aqueous electrolyte on the negative electrode, and the positive electrode current collector and this are electrically connected The parts that are in contact with the non-aqueous electrolyte are made of valve metal and the surface is covered with an oxide film, so that the oxidative decomposition of organic halides in the non-aqueous electrolyte in contact with these is suppressed. This is thought to be due to
[0017]
【Example】
EXAMPLES The present invention will be described more specifically with reference to the following examples, but the present invention is not limited to these examples.
[0018]
Preparation of vinylene carbonate;
A 200 ml glass four-necked flask equipped with a Dim funnel and a dropping funnel was charged with 52 g (424 mmol) of chloroethylene carbonate and 52 g of ethyl ether. While stirring this mixed solution under a nitrogen atmosphere, 49 g (484 mmol) of triethylamine was dropped from the dropping funnel over about 30 minutes. Next, the liquid temperature was set to 30 ° C., and the reaction liquid was analyzed by gas chromatography, followed by reaction for 35 hours while following the progress of the reaction. The obtained vinylene carbonate contained 33% by weight of chloroethylene carbonate.
[0019]
Preparation of the positive electrode;
6 parts by weight of carbon black and 9 parts by weight of polyvinylidene fluoride were mixed with 85 parts by weight of LiCoO ₂ and mixed well. N-methyl-2-pyrrolidone was added thereto to form a slurry, which was uniformly coated on an aluminum foil having a thickness of 20 μm. After drying, it was punched into a disc shape having a diameter of 12.5 mm to obtain a positive electrode.
[0020]
Preparation of the negative electrode;
6 parts by weight of polyvinylidene fluoride was mixed well with 94 parts by weight of artificial graphite powder KS-44 (manufactured by Timcal) having a d-value of 0.336 nm in the lattice plane (002 plane) in X-ray diffraction. N-methyl-2-pyrrolidone was added thereto to form a slurry, which was uniformly coated on a copper foil having a thickness of 18 μm. After drying, a negative electrode was punched into a disk shape having a diameter of 12.5 mm.
[0021]
Preparation of a non-aqueous electrolyte;
Lithium hexafluorophosphate (LiPF ₆ ) was well dried in a dry argon atmosphere. This lithium hexafluorophosphate was dissolved in a mixed solvent of 9: 1 (weight ratio) of propylene carbonate and vinylene carbonate prepared above so as to have a concentration of 1 mol / liter, and a non-aqueous electrolyte solution was obtained. did.
[0022]
Battery fabrication;
A positive electrode was housed in a stainless steel positive electrode can, and a separator (polypropylene microporous film) impregnated with a nonaqueous electrolyte solution and a negative electrode were sequentially placed thereon. The positive electrode can and the stainless steel sealing plate were caulked and sealed through an insulating gasket to produce a coin-type battery. In addition, in the Example, before accommodating a positive electrode, the inner surface of the positive electrode can made from stainless steel was coat | covered with the aluminum foil, and the nonaqueous electrolyte solution did not contact a positive electrode can.
[0023]
Charge / discharge test;
Using the battery produced as described above, a charge / discharge test was conducted at 25 ° C. and a constant current of 0.5 mA at a charge end voltage of 4.2 V and a discharge end voltage of 2.5 V. The results are shown in FIG. In the battery of the example in which the inner surface of the stainless steel positive electrode can was covered with aluminum foil so that the non-aqueous electrolyte did not contact the positive electrode can, charging and discharging could be stably repeated. On the other hand, in the battery of the comparative example in which the non-aqueous electrolyte is in contact with the stainless steel positive electrode can, charging and discharging were possible only up to the second cycle.
[Brief description of the drawings]
FIG. 1 shows a battery according to the present invention in which aluminum is used as a positive electrode current collector, and the inner surface of a stainless steel positive electrode can is covered with an aluminum foil, and the inner surface of the positive electrode can is not covered with an aluminum foil. 4 is a graph showing the results of a charge / discharge test with a battery manufactured in exactly the same manner.

Claims (8)

Non-aqueous electrolysis in an electrode assembly in which a negative electrode and a positive electrode capable of inserting and extracting lithium are opposed to each other through a separator, and a non-aqueous electrolyte secondary battery in which a non-aqueous electrolyte is contained in a container A cyclic carbonate, a chain , wherein the liquid is produced by dehydrochlorination of chloroethylene carbonate and contains 0.1-30 wt % of vinylene carbonate containing an organic halide derived from the chloroethylene carbonate at a ratio of 1 ppm to 50 wt% LiPF ₆ is dissolved as a lithium salt in a non-aqueous solvent selected from the group consisting of linear carbonates, lactones, linear carboxylic acid esters, cyclic ethers, linear ethers and sulfur-containing compounds , and a positive electrode current collector Of the parts that are electrically connected to the non-aqueous electrolyte, the part that contacts the non-aqueous electrolyte is made of valve metal or an alloy thereof. A secondary battery characterized by being made.   The secondary battery according to claim 1, wherein the vinylene carbonate contains at least chloroethylene carbonate as an organic halide. Non-aqueous solvent are contained little of Kutomo one carbonate selected from the group consisting of cyclic carbonate and chain carbonate, and the total amount of the vinylene carbonate containing the carbonate and the organic halide, a non-aqueous solvent The secondary battery according to claim 1, wherein the secondary battery occupies 70% by weight or more.   The secondary battery according to any one of claims 1 to 3, wherein the valve metal or an alloy thereof is aluminum or an aluminum alloy.   The secondary battery according to any one of claims 1 to 4, wherein the negative electrode uses a carbon material capable of inserting and extracting lithium as an active material.   The secondary battery according to any one of claims 1 to 5, wherein the positive electrode uses a lithium transition metal composite oxide capable of inserting and extracting lithium as an active material.   7. The container according to claim 1, wherein a portion of the portion that is electrically connected to the positive electrode current collector and in contact with the non-aqueous electrolyte is aluminum or an aluminum alloy. Secondary battery. An electrode assembly in which a negative electrode and a positive electrode capable of inserting and extracting lithium are opposed to each other through a separator, and a non-aqueous electrolyte is contained in a container, and is electrically connected to the positive electrode current collector and the positive electrode current collector The non-aqueous electrolyte used in the non-aqueous electrolyte secondary battery in which the portion in contact with the non-aqueous electrolyte is composed of a valve metal or an alloy thereof, and dehydrochlorination of chloroethylene carbonate manufactured by, including vinylene carbonate containing organic halide from the chloroethylene carbonate in a proportion of 50 wt% or less than 1 ppm 0.1 to 30 wt%, a cyclic carbonate, a chain carbonate, a lactone, a chain carboxylic acid ester LiPF ₆ as a lithium salt is dissolved in a nonaqueous solvent selected from the group consisting of cyclic ethers, chain ethers and sulfur-containing compounds. A non-aqueous electrolyte characterized by comprising:

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