JP2001057235A - Non-aqueous electrolyte and non-aqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To improve the life of a battery by using an electrolyte composed of a non-aqueous solvent including an additive, and lithium salt, and having below specific % of leakage current control rate. SOLUTION: A non-aqueous electrolyte composed of a non-aqueous solvent including an addtive, and lithium salt, and having a leakage current control rate of below 95% is used. The non-aqueous electrolyte of 3g, having 1 mol/l of concentration of LiPF6 to natural graphite of 1 g, is allowed to exist in an electrochemical element consisting of an electrode including natural graphite as an active material, and another electrode including metallic lithium as an active material, voltage of 0.01 V is applied thereto at 60 deg.C, and a value of current flowing after 25 hours from the start of the application of voltage is measured and converted into a value per graphite of 1 g to be regarded as a leakage current value. A ratio of leakage current value α when a mixture solvent composed of 39.8 wt.% of ethylene carbonate, 59.7 wt.% of dimethylcarbonate and 0.5 wt.% of additive, to a leakage current value β when a mixture solvent composed of 40 wt.% of ethylene carbonate and 60 wt.% of dimethylcarbonate, is a leakage current control rate.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery, and more particularly to a non-aqueous electrolyte having an improved battery life and a secondary battery using the same.

[0002]

BACKGROUND OF THE INVENTION A battery using a non-aqueous electrolyte has a high voltage, a high energy density, and a high reliability such as a storage property, so that it is widely used as a power source for consumer electronic devices. Have been. Above all, a typical secondary battery using such a non-aqueous electrolyte is a lithium ion secondary battery.

As an electrolytic solution used in the lithium ion secondary battery, a carbonate compound having a high dielectric constant such as propylene carbonate or ethylene carbonate is used as a solvent, or a mixed solvent with a low-viscosity carbonate compound such as diethyl carbonate is used. And LiB
_{F 4, LiPF 6, LiClO 4} , LiAsF 6, LiCF 3 S
A solution in which an electrolyte such as O ₃ or Li ₂ SiF ₆ is dissolved is frequently used.

[0004] As the negative electrode of the lithium ion secondary battery, a carbon material capable of inserting and extracting lithium is mainly used. Specifically, it is classified into a graphitic carbon material and an amorphous carbon material. Have been. The graphitic carbon material has its (0
02) A highly crystalline material having a plane spacing of 0.34 nm or less, and the amorphous carbon material is a material having a (002) plane spacing of more than 0.34 nm. A lithium ion battery using a graphitic carbon material has advantages such as a small change in charge / discharge voltage, a large specific capacity and a large discharge capacity per volume, and a lithium ion battery using an amorphous carbon material. The ion battery has advantages such as a large discharge capacity per weight and excellent charge / discharge cycle characteristics.

[0005] In a battery using such an electrolytic solution and a negative electrode, a reductive decomposition reaction of a solvent is likely to occur at the time of initial charging,
The charge / discharge efficiency at that time tends to decrease. In particular, when a highly crystalline carbon material such as graphite is used for the negative electrode, and propylene carbonate or butylene carbonate is used as the non-aqueous solvent, the reductive decomposition reaction of the solvent occurs vigorously during charging, and the insertion reaction of lithium ions into the graphite hardly occurs. May not progress.

For this reason, ethylene carbonate which is solid at room temperature as a non-aqueous solvent, but in which reductive decomposition reaction does not easily occur continuously, is mixed with propylene carbonate, or the content of propylene carbonate in the electrolyte is restricted. The prescription is taken. Thereby, the reductive decomposition reaction of the non-aqueous solvent is suppressed, and the charge / discharge efficiency at the time of the first charge / discharge has been improved. However, there is a need for a countermeasure against a reduction in battery life which is likely to be caused by a minute reductive decomposition reaction, which is likely to occur when a high-temperature storage or charge / discharge cycle is repeated.

As a method for improving the high-temperature storage characteristics and charge / discharge cycle characteristics of a battery, a method of adding various additives to an electrolyte has been proposed. In general, the amount of electrolytic reaction of an electrolytic solution on an electrode is very small, while the amount of charge / discharge reaction to an electrode that occurs in parallel with the electrolytic reaction is very large. The decomposition current is buried in the charge / discharge reaction current, and a normal method such as cyclic voltammetry for evaluating electrochemical stability cannot accurately measure a minute electrolysis reaction on an electrode. Therefore, it is unknown at present what kind of additives can suppress the slight electrolysis reaction on the electrode and improve the high-temperature storage characteristics and the charge-discharge cycle characteristics of the battery by what effect. It is.

[0008]

SUMMARY OF THE INVENTION Accordingly, the present invention provides a non-aqueous electrolyte having improved high-temperature storage characteristics and the like of a battery, and a secondary battery having an improved battery life using the non-aqueous electrolyte. The purpose is to provide.

[0009]

That is, the present invention relates to a non-aqueous electrolyte comprising a non-aqueous solvent containing an additive and a lithium salt, wherein the additive has a leakage current suppression ratio determined by the following measuring method. Is 95% or less.

[Method of Measuring Leakage Current Suppression Rate] In an electrochemical device composed of an electrode using natural graphite as an active material and an electrode using lithium metal as an active material, the LiPF ₆ concentration per 1 g of natural graphite was determined. With 3 g of 1 (mol / l) non-aqueous electrolyte interposed, 0.01 V was applied to this device at 60 ° C., and the current value flowing 25 hours after the start of voltage application was measured to obtain a value per 1 mg of natural graphite. The converted value was defined as a leakage current value (μA / mg). At this time, 40% by weight of ethylene carbonate and dimethyl carbonate 6 were used as the non-aqueous solvent.
Leakage when using a mixed solvent composed of 39.8% by weight of ethylene carbonate, 59.7% by weight of dimethyl carbonate and 0.5% by weight of an additive with respect to the leakage current value β when using a mixed solvent of 0% by weight The ratio of the current value α is defined as a leakage current suppression rate (%). Leakage current suppression rate = (α / β) x 100 (%)

Examples of such additives include divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), tricarboxyethyl isocyanurate, and tri (acryloyloxyethyl) isocyanurate. A compound which is dissolved in such a non-aqueous solvent and becomes insoluble in an electrolytic solution when electrochemically decomposed is preferably used.

The non-aqueous solvent containing the additive includes a cyclic carbonate (A) represented by the general formula (1), a chain ester (B) represented by the general formula (2), and an additive ( C), and the mixing ratio (% by weight) of each component is A: B: C = (10-99.99) :( 0-89.9)
9): (0.01 to 5) is preferable.

[0013]

Embedded image (Wherein, R ₁ and R ₂ represent a hydrogen atom or a carbon number of 1 to 3)
Are the same or different from each other)

[0014]

Embedded image (Wherein R ₃ is —H, —CH ₃ , —CH ₂ CH ₃ , —OC
H ₃ represents —OCH ₂ CH ₃ , and R ₄ represents —CH ₃ , —CH _{2
CH 3, -CH 2 CH 2 CH} 3, -CH represents a (CH _{3) 2)}

The present invention also relates to a non-aqueous electrolyte secondary battery including a negative electrode made of a carbon material capable of doping / dedoping lithium ions, a positive electrode, and the above-mentioned non-aqueous electrolyte, and which is stored at a high temperature. A battery with excellent characteristics and improved battery life.

[0016]

DETAILED DESCRIPTION OF THE INVENTION Next, the nonaqueous electrolytic solution according to the present invention and each structure of a secondary battery using the same will be specifically described.

Additive The non-aqueous electrolyte according to the present invention is an electrolyte containing a non-aqueous solvent containing an additive and a lithium salt as an electrolyte. The agent has a leakage current suppression rate of 95 when measured by the method described below.
% Or less, preferably 85% or less, more preferably 70%
It is as follows.

The current flowing when a constant voltage is applied to the electrochemical element composed of the positive electrode, the negative electrode and the non-aqueous electrolyte depends on the current consumed in the charge / discharge reaction of the electrochemical element and the non-aqueous solvent on the electrode. The current consumed in the electrolysis is roughly classified, but if the time during which a constant voltage is continuously applied to the electrochemical element is extended, the current consumed in the charge / discharge reaction gradually decreases,
Approximately approaches zero. Therefore, it is considered that the current value measured after the constant voltage is continuously applied for a certain period of time substantially corresponds to the current value consumed for electrolysis of the non-aqueous solvent on the electrode. In this specification, this current value is hereinafter referred to as “leakage current value”.

The leakage current value is measured using the following device. That is, an electrode using natural graphite as an active material is prepared as one electrode constituting an electrochemical element. As natural graphite, for example, trade name LF- manufactured by Chuetsu Graphite Works, Ltd.
18A can be used, the amount of natural graphite is 15 mg or more, and the basis weight of the electrode is about 10 to 15 mg / cm ² . The natural graphite used was not appropriately treated after surface treatment, and was subjected to long-term storage for 2 weeks or more as an electrochemical cell, repeated charge / discharge operations for 20 cycles or more, and a high-temperature storage test. It is not appropriate to reuse from the thing.

An electrode using metallic lithium as an active material is prepared as the other electrode. When an electrically insulating porous film impregnated with a non-aqueous electrolyte is interposed between the two electrodes,
An electrochemical device for measurement is completed. The non-aqueous electrolyte, LiPF ₆ is 1 in a non-aqueous solvent containing an additive is an electrolyte (m
ol / l). Here, the weight ratio between the electrode and the non-aqueous electrolyte is 1 to 2 for the natural graphite.

After aging the electrochemical device thus manufactured in advance, a voltage of 0.01 V is applied at 60 ° C., and the application is continued for 25 hours. The value of the current flowing through this electrochemical element upon application, that is, the value of leakage current (μA)
Decreases gradually as shown in FIG.
The value becomes almost constant over time. Therefore, the measurement of the leakage current was performed by applying 0.01 V at 60 ° C. and applying 2 V from the start of application.
After 5 hours, the measured value (μA) is converted to a value per 1 mg of natural graphite to obtain a leakage current value (μA / mg).

First, LiPF ₆ was added to a mixed solvent containing 39.8% by weight of ethylene carbonate, 59.7% by weight of dimethyl carbonate, and 0.5% by weight of an additive to obtain 1 (mo).
1 / l), the leakage current value was measured using a non-aqueous electrolyte dissolved at a concentration of 1 / l, and the value was defined as α. Next, 40% by weight of ethylene carbonate and 60% by weight of dimethyl carbonate
A non-aqueous electrolyte solution is prepared by dissolving LiPF ₆ at a concentration of 1 (mol / l) in a mixed solvent with the above, and the leakage current value is measured in the same manner, and the value is defined as β. The leakage current suppression rate (%)
It is expressed by the following equation. Leakage current suppression rate = (α / β) x 100 (%)

The leakage current value β is a value when the charge / discharge current of the battery when the mixed solvent of ethylene carbonate and dimethyl carbonate is used becomes approximately 0, and the leakage current value α is ethylene carbonate, Since the value of the charge / discharge current of the battery when the mixed solvent of dimethyl carbonate and the additive was used was approximately 0, the leakage current suppression rate was determined when the additive was a non-aqueous solvent on the electrode. Can be treated as one index indicating the action of suppressing the electrolysis of.

The smaller the value of the suppression ratio, the more the additive used has a greater effect of suppressing the electrolysis of the non-aqueous solvent in the battery, and the less likely is the occurrence of extra side reactions which adversely affect the battery characteristics. Suggests that Accordingly, when the leakage current suppression rate of the additive is 95% or less, the high-temperature storage characteristics and the charge / discharge cycle characteristics of the battery are improved, the battery life is improved, and the non-aqueous electrolyte for primary batteries and secondary batteries is improved. It is shown that it is optimal.

The additive having a leakage current suppression rate of 95% or less is a compound soluble in a solvent and hardly soluble in an electrolytic solution when electrochemically decomposed. For example, an unsaturated compound having a multiple bond such as a carbon-carbon double bond, a compound having a plurality of reaction points in a molecule such as isocyanuric esters, and the like can be suitably used.

The following compounds can be mentioned as specific examples. (A) an unsaturated carbonate compound having a double bond such as divinylethylene carbonate, vinylethylene carbonate, acryloyloxymethylethylene carbonate, methacryloyloxymethylethylene carbonate, or phenylene (methyl carbonate); (b) maleic acid, anhydride Unsaturated compounds such as maleic acid, N-ethylmaleimide, phthalic acid, phthalic anhydride, sulfolene, divinyl sulfone

(C) Tricarboxyethyl isocyanurate, tri (methoxycarbonylethyl) isocyanurate, tri (ethoxycarbonylethyl) isocyanurate, trialkyl isocyanurate (alkyl group: methyl, ethyl, propyl), tri (isocyanurate) Methoxysilylpropyl), tri (hydroxyethyl) isocyanurate, tri (glycidyl) isocyanurate, tri (acryloyloxyethyl) isocyanurate, tri (methacryloyloxyethyl) isocyanurate, trivinyl isocyanurate, triallyl isocyanurate, isocyanuric acid Isocyanuric esters such as triphenyl

Among them, divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), tricarboxyethyl isocyanurate, and tri (acryloyloxyethyl) isocyanurate are preferred.

Component Composition of Nonaqueous Electrolyte Solution The nonaqueous electrolyte solution of the present invention is an electrolyte solution in which a lithium salt is dissolved in a nonaqueous solvent containing an additive, which acts as an electrolyte as a whole. Leakage current suppression rate of 9
It is less than 5%. Such an additive is desirably mixed in a non-aqueous solvent containing it in the range of 0.01 to 5% by weight.

The non-aqueous solvent used in the non-aqueous electrolyte is
One or more solvents selected from carbonates, carboxylate esters, ethers, carbamates, amides, ureas, sulfones, phosphates, and the like, which are commonly used in electrolytes, can be used. Among them, from the viewpoint of electrochemical stability, carbonate esters and carboxylate esters are preferable. Particularly preferred non-aqueous solvents are the cyclic carbonates (A) and chain esters (B) described in detail below.

The cyclic carbonate (A) is represented by the following general formula (1).

Embedded image Here, R ₁ and R ₂ are a hydrogen atom or a carbon atom having 1 to 3 carbon atoms.
And may be the same or different from each other.

Specific examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, 1,2-
Butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, 1,2-hexylene carbonate,
2,3-hexylene carbonate, 3,4-hexylene carbonate, and 4,5-heptylen carbonate are exemplified. Among these, ethylene carbonate and propylene carbonate are preferred, and ethylene carbonate is most desirable from the viewpoint of electrochemical stability, particularly for an electrode using graphite as an active material.

The chain ester (B) is represented by the following general formula (2).

Embedded image Here, R ₃ is —H, —CH ₃ , —CH ₂ CH ₃ , —OCH
₃ represents —OCH ₂ CH ₃ , and R ₄ represents —CH ₃ , —CH ₂ C
H ₃ , —CH ₂ CH ₂ CH ₃ , and —CH (CH ₃ ) ₂ .

Specific examples of the chain ester include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, ethyl propyl carbonate,
Examples thereof include carbonates such as ethyl isopropyl carbonate, dipropyl carbonate, and diisopropyl carbonate, and carboxylic esters such as methyl formate, ethyl formate, methyl acetate, ethyl acetate, methyl propionate, ethyl propionate, and propyl propionate. Among these, dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, and methyl isopropyl carbonate are particularly preferable from the viewpoint of stability to the negative electrode.

When the cyclic carbonate (A) and the chain ester (B) are used in combination, the following examples can be given as preferred combinations. (1) ethylene carbonate and dimethyl carbonate,
Ethylene carbonate, methyl ethyl carbonate, ethylene carbonate and diethyl carbonate (2) Ethylene carbonate, propylene carbonate, dimethyl carbonate, ethylene carbonate, propylene carbonate, methyl ethyl carbonate, ethylene carbonate, propylene carbonate, diethyl carbonate, ethylene carbonate, dimethyl carbonate, methyl Ethyl carbonate, ethylene carbonate, dimethyl carbonate and diethyl carbonate (3) ethylene carbonate, propylene carbonate, dimethyl carbonate and methyl ethyl carbonate,
Ethylene carbonate, propylene carbonate, dimethyl carbonate, diethyl carbonate, etc.

The mixing ratio (% by weight) of each component of the non-aqueous solvent containing the additive is such that the cyclic carbonate (A), the chain ester (B), and the additive (C) are: A: B: C = (10-99.99): (0-89.9
9): (0.01-5) Preferably, A: B: C = (15-55) :( 42-84.95):
It is desirable to be in the range of (0.02-3).

When the mixing ratio of each component constituting the non-aqueous solvent is within the above range, the increase in the viscosity of the non-aqueous electrolyte can be suppressed, and the degree of dissociation of the lithium salt as the electrolyte can be increased.
Further, the stability of the non-aqueous electrolyte can be improved. Therefore, the conductivity of the electrolytic solution and the life characteristics of the battery related to the charge and discharge characteristics of the battery can be simultaneously improved.

Other solvents can be added to this non-aqueous solvent within a range not departing from the object of the present invention. Examples of such solvents include phosphate esters such as trimethyl phosphate, chain ethers such as dimethoxyethane,
Cyclic ethers such as tetrahydrofuran; amides such as dimethylformamide; chain carbamates such as methyl-N, N-dimethylcarbamate; cyclic esters such as γ-butyrolactone; cyclic sulfones such as sulfolane; cyclic carbamates such as N-methyloxazolidinone; Cyclic amides such as -methylpyrrolidone,
Examples include cyclic ureas such as N, N-dimethylimidazolidone.

Lithium salts usable as electrolytes include LiF, LiCl, LiBr, LiI, Li ₂ S
O ₄ , LiOH, LiSO ₃ CH ₃ , LiSO ₃ C ₆ H ₄ CH
₃ , LiPF ₆ , LiBF ₄ , LiClO ₄ , LiAsF ₆ , Li ₂
Compounds such as SiF ₆ , LiC ₄ F ₉ SO ₃ , and LiC ₈ F ₁₇ SO ₃ are exemplified.

Further, a lithium salt represented by the following general formula can also be used. LiOSO ₂ R ₈ , LiN (SO ₂
R ₉ ) (SO ₂ R ₁₀ ), LiC (SO ₂ R ₁₁ ) (SO
₂ R ₁₂ ) (SO ₂ R ₁₃ ), LiN (SO ₂ OR ₁₄ ) (SO ₂
OR ₁₅ ) (where R _{8 to} R ₁₅ may be the same or different and are perfluoroalkyl groups having 1 to 6 carbon atoms). These lithium salts may be used alone, or two or more thereof may be used in combination.

Of these, LiPF₆, LiBF_Four, LiO
SO_TwoR₈, Lin (SO_TwoR₉) (SO _TwoR_Ten), LiC
(SO_TwoR₁₁) (SO_TwoR₁₂) (SO_TwoR₁₃), LiN (S
O_TwoOR_{1 Four}) (SO_TwoOR_Fifteen) Is preferable, and furthermore, L
iPF₆, LiBF_FourIs most preferably used.

The lithium salt as an electrolyte is dissolved in a non-aqueous solvent containing the above-mentioned additive in a concentration range of 0.1 to 3 (mol / l), preferably 0.5 to 2 (mol / l). Is used. If necessary, additives such as a stabilizer can be appropriately added to the non-aqueous electrolyte.

The above-described non-aqueous electrolyte is desirably used in the form of a solution in which a lithium salt is dissolved in a non-aqueous solvent in order to increase lithium ion conductivity, and the solution is converted into a porous insoluble polymer. Even when used in the state of being impregnated, or in the state of a gel polymer electrolyte in which a polymer substance is swollen with the solution, and further used in the state of being impregnated in an inorganic carrier such as alumina or silica. Is also good.

The secondary battery according to the secondary batteries present invention, doping of lithium ions as a negative electrode active material, a negative electrode containing a carbon material capable of dedoping, a positive electrode, is composed of a non-aqueous electrolyte described above ing.

As the negative electrode active material, a carbon material capable of doping and undoping lithium ions is used. Among them, desirable carbon materials are roughly classified into a graphitic carbon material and an amorphous carbon material. You. The graphitic carbon material is a material made of highly crystalline carbon having a (002) plane spacing of 0.34 nm or less, and the amorphous carbon material is (0)
02) A material having a plane spacing of more than 0.34 nm.
In particular, when a graphitic carbon material is used, the effect of reducing leakage current is high. Specifically, natural graphite, mesophase carbon fiber, mesophase carbon microbeads, and other substrates containing various carbons are heated at 2000 ° C. or more. Examples include fired products and pyrolytic graphite.

As the positive electrode active material constituting the positive electrode,
A composite oxide composed of lithium and a transition metal, for example, Li
CoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ ,
LiNi _X Co _(1-X) O ₂ , or a transition metal oxide or sulfide such as MoS ₂ , V ₂ O ₅ , TiO ₂ , MnO ₂ , or a conductive polymer such as a polyaniline-disulfide compound may be used. it can. In particular, a composite oxide composed of lithium and a transition metal is preferable.

A non-aqueous electrolyte secondary battery using such a material can be used after being molded into a cylindrical, coin, sheet, or square shape.

A cylindrical non-aqueous electrolyte secondary battery will be described as a representative example. As shown in FIG.
A negative electrode 1 formed by applying a negative electrode active material to a negative electrode current collector 9 and a positive electrode 2 formed by applying a positive electrode active material to a positive electrode current collector 10 are:
Wound through the separator 3 into which the non-aqueous electrolyte is injected,
With the insulating plate 4 placed above and below the wound body, the battery can 5
It is stored in. A battery lid 7 is attached to the battery can 5 by caulking via a sealing gasket 6, and is electrically connected to the negative electrode 1 or the positive electrode 2 via a negative electrode lead 11 and a positive electrode lead 12, respectively. Alternatively, it functions as a positive electrode.

In FIG. 1, a porous membrane is used as the separator 3. However, as the electrolyte, a gel polymer electrolyte obtained by swelling a polymer substance with a non-aqueous electrolyte, or a non-aqueous electrolyte In the case of impregnating an inorganic carrier such as silica or the like, a separator is not necessarily required.

In this battery, the positive electrode lead 12 is electrically connected to the battery lid 7 via the current interrupting thin plate 8. When the pressure inside the battery rises, the current interrupting thin plate 8
Is pushed up and deformed, and the positive electrode lead 12
It is cut off leaving a welded part, and the current is cut off.

FIG. 2 shows an example of a coin-type non-aqueous electrolyte secondary battery. In this battery, the disc-shaped negative electrode 13 and the disc-shaped positive electrode 1
4. The separator 15, the aluminum or stainless steel plate 17, and the spring 20 are the negative electrode 13, the separator 1
5, positive electrode 14, aluminum or stainless steel plate 1
7. The springs 20 are stacked in this order. Each component is accommodated in the battery can 16 in this stacked state, and the battery can lid 19 is attached by caulking via a gasket 18. As the negative electrode 13, the separator 15, and the positive electrode 14,
The same as described above is used. The battery can 16, the battery can lid 19, and the spring 20 are made of a material such as stainless steel which is hardly corroded by the electrolytic solution.

[0052]

Next, the present invention will be described in detail with reference to examples, but the present invention is not limited to these examples.

First, a natural graphite electrode and a LiCoO ₂ electrode were prepared, and a Li-natural graphite battery and L
An iCoO ₂ -natural graphite battery was manufactured, and a leakage current value, a leakage current suppression rate, and a high-temperature storage characteristic of each battery were measured.

<Preparation of natural graphite electrode> Natural graphite (manufactured by Chuetsu Graphite Co., Ltd., LF-18A) was prepared as a graphitic carbon material, and 87 parts by weight of the natural graphite powder and polyvinylidene fluoride (PVDF) as a binder were prepared. 13 parts by weight and dispersed in N-methylpyrrolidinone as a solvent to prepare a natural graphite mixture slurry. This negative electrode mixture slurry has a thickness of 18 μm.
m, and applied to a current collector made of a strip-shaped copper foil, dried, compression-molded, and punched into a disk having a diameter of 14 mm to obtain a coin-shaped natural graphite electrode. This natural graphite electrode mixture had a thickness of 110 μm and a weight of 20 mg / φ14 mm.

<Preparation of LiCoO ₂ electrode>
₂ (Honjo FMC Energy Systems, HLC-2
1) 90 parts by weight, 6 parts by weight of graphite as a conductive material, 1 part by weight of acetylene black, and 3 parts by weight of polyvinylidene fluoride (PVDF) as a binder are mixed and dispersed in N-methylpyrrolidinone as a solvent. , LiCoO ₂ mixture slurry was prepared. This LiCoO ₂ mixture slurry is applied to an aluminum foil having a thickness of 20 μm, dried, compression-molded, and punched into a disk having a diameter of 13 mm.
An iCoO ₂ electrode was produced. The thickness of the LiCoO ₂ mixture is 9
0 μm, and the weight was 35 mg / φ13 mm.

<Production of Li-Natural Graphite Battery> A coin-type battery shown in FIG. 2 was produced. That is, a natural graphite electrode 14 having a diameter of 14 mm, a metal lithium foil 13 having a diameter of 16 mm and a thickness of 0.3 mm, and a thickness of 25 μm and a diameter of 19 mm
15 made of microporous polypropylene film
Into a 2032 stainless steel battery can 16, a metal lithium foil 13, a separator 15, and a natural graphite electrode 14.
In this order.

Thereafter, the non-aqueous electrolyte solution 0.1.
05 ml, further 1.5 mm thick and 15.
The 5 mm stainless steel plate 17 and the spring 20 were housed in the battery can 16. Finally, by caulking the battery can lid 19 via the gasket 18 made of polypropylene,
Maintains airtightness inside the battery, diameter 20mm, height 3.2m
m coin-shaped Li-natural graphite batteries were produced.

[0058] <LiCoO ₂ - natural graphite Fabrication of Battery> diameter 13 mm LiCoO ₂ electrode 14, natural graphite electrodes 13, and the thickness 25μm of diameter 14 mm, microporous polypropylene film separator made of 15 diameter 16mm
The natural graphite electrode 13, the separator 15, the LiCoO ₂ electrode 1
No. 4 was laminated.

After that, the non-aqueous electrolytic solution 0.1.
Inject 03ml, further thickness 1.2mm, diameter 16m
The aluminum plate 17 and the spring 20 were accommodated in the battery can 16. Finally, the battery can lid 19 is caulked via a polypropylene gasket 18 to maintain the airtightness of the battery, and has a diameter of 20 mm and a height of 3.2 mm.
Coin type LiCoO ₂ -natural graphite battery was manufactured.

<Measurement of Leakage Current Value> Prior to the measurement of the leakage current value of a Li-natural graphite battery, aging was performed under the following conditions. Aging conditions are as follows: discharge to 0 V at a current density of about 0.6 mA / cm ² , hold at 0 V for a total discharge time of 10 hours, and then charge to 1.2 V at a current density of about 0.6 mA / cm ² After that, the voltage is maintained at 1.2 V and the total charging time is set to 10 hours. Next, about 1.2 mA / cm
After discharging to 0 V at a current density of ² and holding at 0 V for a total discharge time of 5 hours, then charging to 1.2 V at a current density of about 1.2 mA / cm ² and holding at 1.2 V A charge / discharge operation with a total charge time of 5 hours was defined as one cycle, and only this operation was performed for two cycles.

Next, the temperature of the battery was raised to 60 ° C.
Discharging was continued for a total of 25 hours under the conditions of a constant current of mA and a constant voltage of 0.01 V. At this time, the value of the current flowing through the battery was measured, and the change in the current value with respect to the discharge time was tracked. The current value after 25 hours was measured for calculating the leakage current value.

<High Temperature Storage Test> In the case of a Li-natural graphite battery, aging was first performed under the same conditions as described above.
Then, as the third cycle of the latter half, 2 mA constant current,
The discharge was performed for a total of 10 hours under the condition of a constant voltage of 0.01 V. Next, the battery was stored in a thermostat at 60 ° C. for 14 days, and then charged for 5 hours under the conditions of a constant current of 2 mA and a constant voltage of 1.2 V. At this time, the ratio of the charge capacity after storage to the discharge capacity in the third cycle before high-temperature storage was determined as the remaining capacity ratio. The high-temperature storage characteristics were evaluated based on the residual capacity ratio.

In the case of a LiCoO ₂ -natural graphite battery,
First, aging was performed under the following conditions. That is, 0.5
Under a condition of a constant current of mA and a constant voltage of 4.2 V, the battery was charged until the current value at 4.2 V became 0.05 mA, and then charged for 1 m.
A constant current of A and a constant voltage of 3.0 V were discharged until the current value at 3.0 V became 0.05 mA. Then 1
Under the condition of a constant current of mA and a constant voltage of 3.85 V, 3.85
The battery was charged until the current value at V became 0.05 mA.

Then, it was stored in a thermostat at 60 ° C. for one day,
Then, under the conditions of a constant current of 1 mA and a constant voltage of 3.0 V,
The discharge was performed until the current value at 3.0 V became 0.05 mA. At this time, the ratio of the discharge capacity after storage to the charge capacity up to 3.85 V before high-temperature storage is defined as the remaining capacity ratio (%).
Asked.

Example 1 15.2 g (10%) of LiPF ₆
0 mmol) with ethylene carbonate (EC, 39.
8% by weight), dimethyl carbonate (DMC, 59.7)
% By weight) and divinylethylene carbonate (DVE
C, 0.5% by weight) to make 100 ml at 25 ° C. to prepare a non-aqueous electrolyte having a LiPF ₆ concentration of 1 (mol / l).

The leakage current value of this non-aqueous electrolyte was measured. As can be seen from FIG. 3, the current flowing in the Li-natural graphite battery, that is, the leakage current (μA) becomes almost constant in 15 to 25 hours, and the leakage current value after 25 hours is 0.22 μA per 1 mg of natural graphite. Met. Further, the high-temperature storage characteristics of the Li-natural graphite battery were examined, and the remaining capacity ratio (%) is shown in Table 1.

(Example 2) In Example 1, ethylene carbonate (EC), dimethyl carbonate (DM
C) and divinyl ethylene carbonate (DVEC)
The leakage current value was measured in the same manner as in Example 1 except that the mixing ratio was changed. In addition, this non-aqueous electrolyte is
Applied to iCoO ₂ -natural graphite battery, residual capacity ratio (%)
Was measured and the results are shown in Table 1.

(Examples 3 to 7) In Example 1, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), and tri (acryloyloxyethyl) isocyanurate were used in place of divinylethylene carbonate. Was performed in the same manner as in Example 1 except that each was used. Table 1 shows the results of measuring the leakage current value.

Comparative Example 1 The same procedure as in Example 1 was carried out except that a mixed solvent of ethylene carbonate (EC, 40% by weight) and dimethyl carbonate (DMC, 60% by weight) was used as the non-aqueous electrolyte. The current value was measured and is shown in Table 1. The leakage current suppression rate (%) of the above example was calculated using this leakage current value, and is also shown in Table 1. Furthermore, the remaining capacity ratio (%) of the Li-natural graphite battery and the LiCoO ₂ -natural graphite battery using the non-aqueous electrolyte was also measured, and the results are shown in Table 1.

[0070]

[Table 1]

From the results of Examples 1 to 7 and Comparative Example 1,
It was found that the leakage current suppression rate measured in each example was 93% or less, the value was small, the capacity remaining rate in a high-temperature storage test was high, and the battery life was improved.

[0072]

According to the non-aqueous electrolyte of the present invention, the additive used in the non-aqueous electrolyte has a leakage current suppression rate of 95% or less, so that the electrolysis reaction of the non-aqueous solvent on the electrode can be minimized. It is a stable and non-aqueous electrolyte. A secondary battery using such a non-aqueous electrolyte has improved high-temperature storage characteristics, and is thought to contribute to improvement in charge / discharge cycle characteristics and extension of battery life.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view of a cylindrical secondary battery to which a non-aqueous electrolyte according to the present invention is applied.

FIG. 2 is a cross-sectional view of a coin-type secondary battery to which the non-aqueous electrolyte according to the present invention is applied.

FIG. 3 is a chart showing a change in a leakage current value (μA) flowing through a battery with respect to a discharge time (hr).

[Explanation of symbols]

 1, 13 ... Negative electrode 2, 14 ... Positive electrode 3, 15 ... Separator 5, 16 ... Battery can 7, 19 ... Battery cover 9 ... ...... Negative electrode current collector 10 ...... Positive electrode current collector

Claims (6)

[Claims] 1. A non-aqueous electrolyte comprising a non-aqueous solvent containing an additive and a lithium salt, wherein the additive has a leakage current suppression rate of 95% or less, which is determined by the following measuring method. Non-aqueous electrolyte. [Method of Measuring Leakage Current Suppression Rate] In an electrochemical element composed of an electrode using natural graphite as an active material and an electrode using metal lithium as an active material, the LiPF ₆ concentration was 1 (mol / g) per 1 g of natural graphite. / L) of non-aqueous electrolyte of 3 g, a current of 0.01 mg was applied to the device at 60 ° C., and a current flowing 25 hours after the start of voltage application was measured.
And the converted value is used as the leakage current value (μA /
In this case, 39.8% by weight of ethylene carbonate and 59.9% of dimethyl carbonate with respect to the leakage current value β when a mixed solvent of 40% by weight of ethylene carbonate and 60% by weight of dimethyl carbonate were used as the non-aqueous solvent.
The ratio of the leakage current value α when using a mixed solvent consisting of 7% by weight and 0.5% by weight of the additive is defined as a leakage current suppression rate (%). Leakage current suppression rate = (α / β) x 100 (%) 2. The method according to claim 1, wherein the non-aqueous solvent is at least one compound selected from the group consisting of cyclic carbonates, chain carbonates, and chain carboxylic esters. Non-aqueous electrolyte. 3. The method according to claim 1, wherein the additive is a compound which is dissolved in a non-aqueous solvent and becomes hardly soluble in an electrolytic solution when decomposed electrochemically. Non-aqueous electrolyte. 4. The method according to claim 1, wherein said additives are divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene di (methyl carbonate), tricarboxyethyl isocyanurate, and tri (acryloyloxyethyl) isocyanurate. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte is at least one compound selected from the group consisting of: 5. The non-aqueous solvent containing the above-mentioned additive comprises a cyclic carbonate (A) represented by the general formula (1), a chain ester (B) represented by the general formula (2), and Agent (C) and the mixing ratio of each component (% by weight)
A: B: C = (10-99.99) :( 0-89.9
9): (0.01 to 5), The non-aqueous electrolyte according to any one of claims 1 to 3. Embedded image (Wherein, R ₁ and R ₂ represent a hydrogen atom or a carbon number of 1 to 3)
Which may be the same or different from each other) (Wherein R ₃ is —H, —CH ₃ , —CH ₂ CH ₃ , —OC
H ₃ represents —OCH ₂ CH ₃ , and R ₄ represents —CH ₃ , —CH _{2
CH 3, -CH 2 CH 2 CH} 3, -CH represents a (CH _{3) 2)} 6. A non-aqueous electrolyte comprising a negative electrode made of a carbon material capable of doping / dedoping lithium ions, a positive electrode, and the non-aqueous electrolyte according to any one of claims 1 to 4. Liquid secondary battery.