JP2000164248A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To prevent drop in conductivity, prevent decomposition of an electrolyte with an inexpensive additive by including a specified amount of cyclic carbonate in a nonaqueous solvent of an electrolyte in which a lithium salt is dissolved. SOLUTION: A compound of 0.05% or more but less than 5% by volume represented by the formula is contained in a nonaqueous solvent. In the formula, R1, R2 show each 1-3C alkyl group. For example, 4,5-dimethyl-1,3-oxole-2-one, 4-5-diethyl-1,3-oxole-2-one and the like are listed. The nonaqueous solvent contains 20% or more of cyclic alkylene carbonate having 2-4C alkylene group (for example, ethylene carbonate) and 20% or more of chain carbonate of dialkyl carbonate having 1-4C alkyl group (for example, dimethyl carbonate), and a mixture solvent containing 70% or more these carbonates of the total solvent is used. As a lithium salt, LiClO4, LiPF6 and the like are listed.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte secondary battery having excellent cycle characteristics.

[0002]

2. Description of the Related Art In recent years, with the reduction in weight and size of electric appliances, development of lithium secondary batteries having a high energy density has been promoted. Currently, lithium secondary batteries, which are being developed mainly, use lithium metal or graphite that absorbs and releases lithium as a negative electrode active material, and LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O as positive electrode active materials.
A non-aqueous electrolyte in which a lithium salt is dissolved in an organic solvent is used as an electrolyte using a lithium-containing composite oxide such as ₄ . Among them, lithium secondary batteries using graphite as a negative electrode active material are considered promising from the viewpoint of safety.

[0003]

However, a lithium secondary battery using graphite as a negative electrode active material has a problem that the battery capacity gradually decreases when charging and discharging are repeated.
It is considered that this is because the decomposition of the electrolytic solution occurs on the surface of graphite as the negative electrode active material, and the charge / discharge efficiency does not become sufficiently high. As a countermeasure, JP-A-7-122296 discloses that It has been proposed to use a mixed solvent to which the compound represented by (1) is added.

[0004]

Embedded image

(In the formula, R ¹ and R ² each independently represent an alkyl group having 1 to 3 carbon atoms.) The same publication discloses that dimethyl carbonate contains a compound of formula (1), A diagram showing the discharge capacity when charging and discharging are repeated for a lithium secondary battery using LiPF ₆ dissolved in a mixed solvent added to be 0 to 100% as an electrolytic solution is described by the following equation. When the volume occupied by the compound of (1) is out of the range of 10 to 50% by volume,
It is shown that the discharge capacity decreases rapidly. However, when the compound of the formula (1) is added in a high concentration, the conductivity of the electrolytic solution is greatly reduced, and the compound is more expensive than other commonly used solvents. There is a need to develop a method to prevent the decomposition of the electrolyte.

[0006]

The present inventors have studied in detail the effect of the addition amount of the compound of the formula (1) on the charge / discharge characteristics of the battery.
It has been found that a small amount of less than volume% is more preferable,
The present invention has been reached. That is, according to the present invention, a nonaqueous electrolyte secondary battery comprising at least a negative electrode containing graphite as a part of the active material, a positive electrode, and an electrolyte obtained by dissolving a lithium salt in a nonaqueous solvent In the above, by adding a compound represented by the formula (1) in a nonaqueous solvent in an amount of 0.05% by volume or more and less than 5% by volume, the charge / discharge characteristics can be further improved.

[0007]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, a non-aqueous electrolyte obtained by dissolving a lithium salt in a small amount of a compound represented by the formula (1) dissolved in a non-aqueous solvent is used as an electrolyte. As the solvent, ethylene carbonate, propylene carbonate, which is conventionally known as a solvent of the non-aqueous electrolyte,
Cyclic carbonates such as butylene carbonate; dimethyl carbonate, diethyl carbonate, di-n-propyl carbonate, methyl ethyl carbonate, methyl-
chain carbonates such as n-propyl carbonate, ethyl-n-propyl carbonate, γ-butyrolactone,
lactone such as γ-valerolactone, methyl acetate, chain carboxylate such as methyl propionate, tetrahydrofuran, 2-methyltetrahydrofuran, cyclic ether such as tetrahydropyran, dimethoxyethane, chain ether such as diethoxyethane, sulfolane, A sulfur-containing compound such as diethyl sulfone is used. Preferably, a mixed solvent mainly composed of a cyclic carbonate and a chain carbonate, each of which accounts for 20% by volume or more is used. If desired, other solvents may be contained in the mixed solvent, but the amount thereof should be less than 30% by volume unless otherwise specified so as not to impair the properties of the carbonate solvent. It is. Other solvents in the mixed solvent are the total 2
It is preferably 0% by volume or less, particularly preferably 10% by volume or less.

[0008] In this specification, all the volume percentages are values measured at room temperature, that is, 25 ° C. However, those solid at 25 ° C. are heated to the melting point and measured in a molten state. In the solvent, 4,5-dimethyl-1,3-dioxol-2-one, 4,5-diethyl-1,3-dioxol-2-one, 4,5-di-n-propyl-1,
3-dioxol-2-one, 4-ethyl-5-methyl-1,3-dioxol-2-one, 4-methyl-5
n-propyl-1,3-dioxol-2-one, 4-
Ethyl-5-n-propyl-1,3-dioxole-2
A compound represented by the formula (1), such as -one, is added in an amount of 0.05% by volume or more and less than 5% by volume. As the compound represented by the formula (1), a compound in which R ¹ and R ² are each a methyl group or an ethyl group is preferably used.

According to the study of the present inventors, if the amount of the compound of the formula (1) is too large, the charge / discharge efficiency of a battery manufactured using an electrolytic solution containing the compound as a solvent is rather deteriorated. If the amount of the compound of the formula (1) is less than 0.05% by volume, the effect of adding the compound of the formula (1) is hardly exhibited. The preferable addition amount of the compound of the formula (1) is 0.1% by volume or more and 4% by volume or less, particularly 0.3% by volume or more.
5% by volume or less.

As a lithium salt to be dissolved in a solvent, L
iCLO_Four, LiPF₆, LiBF _FourInorganic acid such as lithium
Salt or LiCF_ThreeSO_Three, LiN (CF_ThreeSO_Two)
_Two, LiN (C_TwoF_FiveSO_Two)_Two, LiN (CF_ThreeSO
_Two) (C_FourF₉SO_Two), LiC (CF_ThreeSO_Two)_Threeetc
Non-aqueous electrolytes such as lithium organic acid salts
Can be used. these
May be used in combination if desired. The electrolyte in the electrolyte
The concentration of the salt is usually 0.5 to 2 mol / l.
You.

The non-aqueous electrolyte secondary battery according to the present invention can be manufactured in the same manner as a normal non-aqueous electrolyte secondary battery except that the above-mentioned non-aqueous electrolyte is used. As the positive electrode active material, a lithium transition metal composite oxide capable of inserting and extracting lithium, such as LiCoO ₂ , LiNiO ₂ , and LiMn ₂ O ₄ , is used. As the positive electrode current collector, a metal such as aluminum, titanium, and tantalum or an alloy thereof is used. Among them, it is preferable to use aluminum or its alloy.

It is preferable to use graphite as the negative electrode active material. As the graphite, either artificial graphite or purified natural graphite can be used. It is also preferable to use those obtained by subjecting these graphites to surface treatment with a pitch or the like. Among the graphites, the d value (interlayer distance) of the lattice plane (002 plane) obtained by X-ray diffraction by the Gakushin method is 0.335 to 0.335.
It is 0.34 nm, especially 0.335 to 0.337 nm. If desired, another material known as a negative electrode active material can be used in combination with the above graphite. Examples of such a material include lithium metal and alloys thereof, non-graphite carbon, and metal oxides such as tin oxide and silicon oxide. Copper, nickel, stainless steel, or the like is used as the negative electrode current collector. Among them, it is preferable to use copper.

As a separator for separating the positive electrode and the negative electrode, a porous film of a polyolefin such as polyethylene or polypropylene or a nonwoven fabric 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 spirally spread, and a coin type in which a flaky electrode and a separator are laminated.

[0014]

The present invention will be described more specifically with reference to the following examples. In the following examples, the positive electrode and the negative electrode used were prepared as follows. Positive electrode: 85 parts by weight of LiCoO _2, 6 parts by weight of carbon black and polyvinylidene fluoride (product of Kureha Chemical Company, KF
(-1000) 9 weight was added and mixed well. N-
Add methylpyrrolidone to make slurry, thickness 20μ
m of aluminum foil. After this was dried, it was punched into a disk having a diameter of 12.5 mm to obtain a positive electrode. Negative electrode: Polyvinylidene fluoride 6 was added to 94 parts by weight of natural graphite powder (NG-7, manufactured by Kansai Thermochemical Co., Ltd.) having a lattice plane (002 plane) d value of 0.336 nm in X-ray diffraction.
The parts by weight were added and mixed well. N-methyl-2-
Pyrrolidone was added to form a slurry, which was applied on a copper foil having a thickness of 18 μm so as to have a uniform thickness. After this was dried, it was punched into a disk having a diameter of 12.5 mm to obtain a negative electrode.

Examples 1 and 2 and Comparative Examples 1 to 4 Under a dry argon atmosphere, a mixed solvent shown in Table 1 was added to a mixed solvent in which both R ¹ and R ² in the formula (1) were methyl groups.
5-Dimethyl-1,3-dioxol-2-one was dissolved. Lithium hexafluorophosphate (LiPF ₆ ), which was sufficiently dried in a dry argon atmosphere, was dissolved in the solution to a concentration of 1 mol / liter to prepare an electrolyte solution. The positive electrode was accommodated in a stainless steel can that also served as the positive electrode conductor, and the negative electrode was placed thereon via a separator impregnated with an electrolytic solution. This can body and the sealing plate also serving as the negative electrode conductor,
It was caulked and sealed via an insulating gasket to produce a coin-type battery. For this coin-type battery, at 25 ° C., a charging end voltage of 4.2 V at a constant current of 0.5 mA,
A test was conducted in which charge and discharge at a discharge end voltage of 2.5 V were repeated. Table 1 shows the charge / discharge efficiency at the fifth cycle. Charge / discharge efficiency (%) = (discharge capacity / charge capacity) × 100

[0016]

[Table 1]

Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising at least a negative electrode containing graphite as an active material, a positive electrode, and an electrolyte obtained by dissolving a lithium salt in a non-aqueous solvent.
A secondary battery comprising a nonaqueous solvent containing at least 0.05% by volume and less than 5% by volume of a compound represented by the formula (1). Embedded image (Wherein, R ¹ and R ² each independently represent a carbon atom of 1
Represents an alkyl group of 3 to 3) 2. The method according to claim 1, wherein the non-aqueous solvent contains the compound represented by the formula (1) at a concentration of 0.3% by volume or more and 3.5% by volume or less. Non-aqueous electrolyte secondary battery. 3. The non-aqueous solvent comprises an alkylene group having 2 carbon atoms.
A cyclic carbonate selected from the group consisting of alkylene carbonates of 1 to 4 and a chain carbonate selected from the group consisting of dialkyl carbonates having 1 to 4 carbon atoms in the alkyl group, each containing at least 20% by volume, and 3. The non-aqueous electrolyte secondary battery according to claim 1, wherein the carbonate is a mixed solvent occupying 70% by volume or more of the whole.