JP2001057234A - 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 and lithium salt, and having a leakage current value below a specific value per a specific amount of natural graphite. SOLUTION: A non-aqueous electrolyte is composed of a non-aqueous solvent and lithium salt and has a leakage current value of below 0.25 μA per natural graphite of 1 mg. The non-aqueous electrolyte of 3 g to natural graphite of 1 g is allowed to exist in an electrochemical element composed 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 current flowing after 25 hours from the start of the application of voltage is measured as a leakage current value preferably. The non-aqueous solvent is preferably composed of 95-99.99 wt.% of cyclic carbonic ester and/or chained carbonic ester, and 0.01-5 wt.% of a compound capable of being hardly soluble in the electrolyte when it is electrochemically decomposed. A non- aqueous secondary battery consists of a negative electrode composed of a carbon material capable of doping and dedoping a lithium ion, a positive electrode and the non-aqueous electrolyte.

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
The (02) plane is 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 batteries.

[0009]

That is, the present invention relates to a non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, wherein the non-aqueous electrolyte has a natural graphite having a leakage current value determined by the following measuring method. It relates to a non-aqueous electrolyte having a concentration of 0.25 μA or less per 1 mg.

[Measurement Method of Leakage Current Value] A non-aqueous electrolytic solution was added to 1 g of natural graphite in an electrochemical element comprising an electrode using natural graphite as an active material and the other electrode using metallic lithium as an active material. With 3 g interposed therebetween, 0.01 V is applied to the device at 60 ° C., and the value of the current flowing 25 hours after the start of voltage application is measured, and the value is defined as the leakage current value.

The above-mentioned non-aqueous solvents include cyclic carbonates and / or chain carbonates, divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), divinyl sulfone, isocyanurate and the like. It is preferable to use a compound such as tricarboxyethyl acid or tri (acryloyloxyethyl) isocyanurate, which is hardly soluble in the electrolytic solution when electrochemically decomposed. Lithium salts dissolved in the electrolyte include Li
PF ₆ , LiBF ₄ or the like is desirable, and its concentration is 0.1 to 3
(Mol / l) is preferable.

The present invention also relates to a non-aqueous electrolyte secondary battery including a negative electrode made of a carbon material capable of doping and undoping lithium ions, a positive electrode, and the above-mentioned non-aqueous electrolyte.

[0013]

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.

[0014] Non-aqueous electrolytic solution present invention related to non-aqueous electrolyte solution is an electrolyte containing a nonaqueous solvent and a lithium salt as an electrolyte, when measured by a method described later to the non-aqueous electrolyte solution It is desirable that the observed leakage current value is 0.25 μA or less, preferably 0.20 μA or less, more preferably 0.15 μA or less per mg of natural graphite.

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 current consumed by the non-aqueous solvent on the electrode. The current is roughly divided into the current consumed in the electrolysis. However, if the time during which a constant voltage is continuously applied to the electrochemical element is extended, the value of the current consumed in the charging / discharging gradually decreases and approaches zero. Therefore, 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.

Therefore, the above-mentioned current value is a value when the charging / discharging current in the battery has approximately approached 0, and if the current value is 0.25 μA / mg or less, the Decomposition of the non-aqueous solvent is suppressed, and extra side reactions that adversely affect battery characteristics are less likely to occur. As a result, the high-temperature storage characteristics and charge / discharge cycle characteristics of the battery are improved, and the battery life is improved. . Therefore, such a non-aqueous electrolyte is suitable as an electrolyte for primary batteries and secondary batteries. In this specification, this current value is hereinafter referred to as “leakage current value”.
Call.

The value of the leakage current 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 is not suitable if it has been subjected to a surface treatment. It has been subjected to long-term storage of 2 weeks or more as an electrochemical cell, repeated charge / discharge operations for 20 cycles or more, and subjected to a high-temperature storage test. It is not appropriate to re-use it.

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. Non-aqueous electrolytes include
LiPF _{6 as an} electrolyte is dissolved in a nonaqueous solvent at a concentration of 0.5 to 2, preferably 0.7 to 1.5 (mol / 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, 0.01 V is applied at 60 ° C., and the application is continued for 25 hours. The current value flowing through the electrochemical element upon application, that is, the leakage current value, gradually decreases as shown in an example in FIG. 3, and becomes substantially constant after 15 hours. Therefore, the measurement of leakage current is 6
A voltage of 0.01 V was applied at 0 ° C., a measured value 25 hours after the start of application was obtained, the measured value (μA) was converted to a value per 1 mg of natural graphite, and the converted value was used as a leakage current value (μA / m
g).

Structure of Nonaqueous Electrolyte The nonaqueous electrolyte according to the present invention is an electrolyte in which a lithium salt is dissolved in a nonaqueous solvent and which acts as an electrolyte. The above leakage current value is measured to be 0.25 μA or less per 1 mg of natural graphite.

The lithium salt usable as the electrolyte includes 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
Lithium salts 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.

As the non-aqueous solvent, besides carbonate ester,
Methyl formate, ethyl formate, propyl formate, methyl acetate,
Chain carboxylic acid esters such as ethyl acetate, propyl acetate, methyl propionate, and ethyl propionate; 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-dimethyl carbamate; cyclic esters such as γ-butyrolactone;
Solvents such as cyclic sulfones such as sulfolane, cyclic carbamates such as N-methyloxazolidinone, cyclic amides such as N-methylpyrrolidone, cyclic ureas such as N, N-dimethylimidazolidone, and mixtures thereof can be used.

Among these non-aqueous solvents, from the viewpoint of electrochemical stability, a carbonic acid ester or a chain carboxylic acid ester is preferable, and a cyclic carbonic acid ester or a chain carbonic acid ester is more preferable. 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 and the like. Examples of the chain carbonate include dimethyl carbonate, methyl ethyl carbonate, diethyl carbonate, methyl propyl carbonate, methyl isopropyl carbonate, and ethyl propyl carbonate.

The cyclic carbonate and the chain carbonate are desirably used as a mixture of the two, and the mixing ratio at that time is expressed by weight%, and the ratio of cyclic carbonate: chain carbonate is 5:95 to 95: 5, preferably 20:
80 to 60:40.

It is preferable that the non-aqueous electrolyte according to the present invention contains, in addition to the non-aqueous solvent, a small amount of an additive soluble in the solvent. Such additives include:
A compound that becomes poorly soluble in an electrolytic solution when electrochemically decomposed, for example, an unsaturated compound having a multiple bond such as a carbon-carbon double bond, or an intramolecular compound such as isocyanurate. A compound having a plurality of reaction points 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 groups: 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

Of these, divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), divinyl sulfone, tricarboxyethyl isocyanurate, and tri (acryloyloxyethyl) isocyanurate are preferred. .

It is desirable that these additives are added and mixed in the non-aqueous electrolyte so as to have a ratio of 95 to 99.99% by weight of the non-aqueous solvent and 0.01 to 5% by weight of the additives. Within this range, the effect of reducing the leakage current value is exhibited, and the high-temperature storage characteristics and charge / discharge cycle of the battery can be improved.

The lithium salt as an electrolyte is dissolved in a non-aqueous solvent containing the above-mentioned additive, and has a concentration of 0.1 to 3 (mol / l), preferably 0.5 to 2 (mol / l). Prepared and 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. It can be used in the state of being impregnated, or in the form of a gel polymer electrolyte in which a polymer substance is swollen with the electrolytic solution, or in the state of being impregnated in an inorganic carrier such as alumina or silica. You may.

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 being doped with and dedoped with 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 formed 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. 1, the battery includes 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. Is wound through a separator 3 into which a non-aqueous electrolyte is injected.
Is stored in the battery can 5 with the. 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 in which a polymer substance is swollen 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 containing the non-aqueous electrolyte, the aluminum or stainless steel plate 17 and the spring 20
3, a separator 15, a positive electrode 14, an aluminum or stainless steel plate 17, and a spring 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 those described above are 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.

[0041]

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 prepared, and the leakage current value and the high-temperature storage characteristics 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> LiCoO
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 electrolytic 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.

<Preparation of LiCoO ₂ -Natural Graphite Battery> A LiCoO ₂ electrode 13 having a diameter of 13 mm, a natural graphite electrode 13 having a diameter of 14 mm, and a separator 15 made of a microporous polypropylene film having a thickness of 25 μm and a diameter of 16 mm
The natural graphite electrode 13, the separator 15, the LiCoO ₂ electrode 1
No. 4 was laminated.

Then, 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 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 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 of LiPF ₆ (10
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 became almost constant in 15 to 25 hours, and the leakage current value after 25 hours was 1 mg of natural graphite.
0.22 μA per unit. 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. Also, this non-aqueous electrolytic solution is
The present invention was applied to an O ₂ -natural graphite battery, and the residual capacity ratio (%) was measured.

(Examples 3 to 10) In Example 1, instead of divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), tricarboxyethyl isocyanurate, and triisocyanurate ( (Acryloyloxyethyl) and divinyl sulfone were used in the same manner as in Example 1. 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. Furthermore, Li-natural graphite batteries and Li-
The remaining capacity ratio (%) of the CoO ₂ -natural graphite battery was also measured and is shown in Table 1.

[0059]

[Table 1]

From the results of Examples 1 to 10 and Comparative Example 1, it was found that the smaller the leakage current value, the higher the residual capacity ratio in the high-temperature storage test, and the longer the battery life.

[0061]

The nonaqueous electrolyte of the present invention has a leakage current value of 0.25 μA or less per 1 mg of natural graphite, so that the electrolysis reaction of the nonaqueous solvent on the electrode can be minimized. , Stable non-aqueous electrolyte. Further, a secondary battery using such a non-aqueous electrolyte has improved high-temperature storage characteristics, and is considered 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 (5)

[Claims] 1. A non-aqueous electrolyte containing a non-aqueous solvent and a lithium salt, wherein the non-aqueous electrolyte has a leakage current value of 0.25 μA or less per 1 mg of natural graphite determined by the following measuring method. Non-aqueous electrolyte solution characterized by the above-mentioned. [Measurement method of leakage current value] Electrode using natural graphite as active material,
A non-aqueous electrolyte solution containing 1 g of natural graphite
To the device, 0.01 g at 60 ° C.
V is applied, and the value of the current flowing 25 hours after the start of the voltage application is measured and defined as the leakage current value. 2. The non-aqueous solvent comprises a cyclic carbonate and / or a chain carbonate and a compound which becomes insoluble in an electrolytic solution when electrochemically decomposed. 2. The non-aqueous electrolyte according to 1. 3. The non-aqueous solvent contains 95 to 99.99% by weight of a cyclic carbonate and / or a chain carbonate.
The non-aqueous electrolytic solution according to claim 1 or 2, comprising: 0.01 to 5% by weight of a compound which becomes insoluble in the electrolytic solution when electrochemically decomposed. 4. The compound which becomes insoluble in an electrolytic solution when electrochemically decomposed is divinylethylene carbonate, vinylethylene carbonate, maleic anhydride, phthalic anhydride, phenylene (methyl carbonate), divinyl sulfone. 4. The non-aqueous electrolyte according to claim 2, wherein the non-aqueous electrolyte is at least one compound selected from the group consisting of tricarboxyethyl isocyanurate and tri (acryloyloxyethyl) isocyanurate. 5. A non-aqueous electrolyte comprising a negative electrode made of a carbon material capable of doping and undoping lithium ions, a positive electrode, and the non-aqueous electrolyte according to any one of claims 1 to 4. Liquid secondary battery.