JP2001185215A - Nonaqueous electrolyte and nonaqueous electrolyte secondary battery using the same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a nonaqueous electrolyte having extremely small decomposition amount on the negative electrode and a nonaquaous electrolyte secondary battery which is superior in low-temperature property, long-term stability and cycle property. SOLUTION: This nonaqueous electrolyte, that contains at least one kind of sulphur-containing compound selected from among 1,3-dithiol-2-thiones, 1, 3-ditholane-2-thiones and compounds as shown in equation (1). In the equation, R1 represents an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group or an aryl group, and R2 represents an alkyl group, an alkenyl group or an alkynyl group containing at least one sulphur atom. However, R1 and R2 may be mutually bonded to form a ring. Furthermore, this is the nonaqueous electrolyte secondary battery, which has as an embodyment the negative electrode which at least passed through the process wherein sulphur-containing compound that is contained in the nonaqueous electrolyte is made to be reduced on the negative electrode.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a non-aqueous electrolyte and a non-aqueous electrolyte secondary battery using the same. More specifically, the present invention relates to a non-aqueous electrolyte containing a specific sulfur-containing compound and a non-aqueous electrolyte secondary battery using the same. The electrolytic solution of the present invention has very little decomposition on the negative electrode, and a non-aqueous electrolytic solution secondary battery using the same has excellent low-temperature characteristics, long-term stability, and cycle characteristics. The size and performance of the secondary battery can be reduced.

[0002]

2. Description of the Related Art With the recent reduction in the weight and size of electric products, lithium secondary batteries having a high energy density have attracted attention and have been rapidly spreading in recent years. However, there is still a problem that a good cycle life cannot be obtained, and therefore, development of a battery that has good cycle characteristics such as charge / discharge efficiency and cycle life and also has good use characteristics at low temperatures. Is desired. Non-aqueous organic solvents such as carbonates or esters such as ethylene carbonate, propylene carbonate, diethyl carbonate, and γ-butyrolactone have been used as a solvent for the electrolyte solution of such a lithium secondary battery. Among them, propylene carbonate is a solvent having a high dielectric constant, dissolves a lithium salt-based solute (electrolyte) well, and has a high electric conductivity even at a low temperature, so that it has excellent performance as a main solvent of an electrolytic solution. However, in a lithium secondary battery (lithium ion battery) using graphite or graphitized carbon as a negative electrode material having a flat charge / discharge curve and a high battery capacity, it is clear that propylene carbonate is reductively decomposed on the surface of the negative electrode material. Therefore, there is a problem that propylene carbonate cannot be used as an electrolytic solution for batteries of the same type.

[0003] In order to avoid such a reductive decomposition reaction on the surface of the carbon negative electrode material, ethylene carbonate is used as a main solvent in the same type of lithium secondary batteries currently in practical use. Thermodynamically, almost all solvents should undergo a reductive decomposition reaction at a potential at which lithium is inserted into a layered compound such as graphite. However, lithium can be inserted in some non-aqueous electrolytes such as ethylene carbonate because a stable film derived from the electrolyte is formed on the carbon surface and the carbon surface is inactivated. It is considered. It is believed that this film exhibits lithium ion conductivity but does not have electron conductivity, which inhibits further decomposition of the electrolyte and allows lithium to be inserted into carbon. Such a film is formed during the first charging.
However, ethylene carbonate cannot be used alone because it has a higher freezing point of 36.4 ° C. than propylene carbonate, and is used as a mixture with a low-viscosity solvent. Various solvents have already been studied as a low-viscosity solvent to be mixed with ethylene carbonate. Generally, the low-viscosity solvent often has a low boiling point, so that a large amount of the solvent causes a problem in terms of safety. If only a small amount is added, problems arise in terms of electric conductivity and viscosity at low temperatures. From such a background, a mixed solvent of ethylene carbonate and diethyl carbonate or the like is used as an electrolyte for a lithium secondary battery.

[0004]

However, the above-mentioned mixed solvents also have problems in low-temperature characteristics and the like. The present invention provides a nonaqueous electrolyte secondary battery in which the decomposition of the solvent on a graphite-containing negative electrode is extremely low and a low-temperature characteristic, long-term stability, and cycle characteristics using the same are improved. The purpose is to provide.

[0005]

Means for Solving the Problems The inventors of the present invention have conducted intensive studies in view of such circumstances, and as a result, as for a non-aqueous electrolytic solution containing a specific sulfur-containing compound, the decomposition of the above solvent on a graphite-containing negative electrode has been considered. The present inventors have found that low-temperature characteristics and cycle life of a non-aqueous electrolyte secondary battery using the same are improved, and completed the present invention.

That is, the gist of the present invention is as follows. In a non-aqueous electrolyte obtained by dissolving a lithium salt as a solute in a non-aqueous solvent, the non-aqueous solvent contains 1,3-dithiol-2-thiones, 1,3-dithiolan-2-thiones and the following general formula (I) Non-aqueous electrolyte solution characterized by containing at least one sulfur-containing compound selected from the compounds represented by the following formula:

[0007]

Embedded image

(Wherein, R ¹ represents an alkyl group, alkenyl group, alkynyl group, aralkyl group or aryl group, and R ² represents an alkyl group, alkenyl group or alkynyl group containing at least one sulfur atom. , R ¹
And R ² may combine with each other to form a ring)

[0009] 2. 2. A non-aqueous electrolyte secondary battery including a negative electrode and a positive electrode capable of inserting and extracting lithium, the non-aqueous electrolyte described in 1 above, a separator and an outer can. 2. A non-aqueous electrolyte secondary battery comprising a negative electrode as a component, which has undergone at least a step of reducing the sulfur-containing compound contained in the non-aqueous electrolytic solution on the negative electrode.

[0010]

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail. (1) Non-aqueous Electrolyte The non-aqueous electrolyte of the present invention is a non-aqueous solvent, that is, the organic solvent is 1,3-dithiol-2-thiones, 1,3
-Dithiolan-2-thiones and at least one sulfur-containing compound selected from the compounds of the formula (I). It is presumed that such a sulfur-containing compound (hereinafter sometimes referred to as a film-forming compound) forms a film that protects the surface of the active material of the negative electrode by reduction.

The carbon number of 1,3-dithiol-2-thiones and 1,3-dithiolan-2-thiones is usually
It is 3-9, preferably 3-5. And as a specific example of 1,3-dithiol-2-thiones, for example,
3-dithiol-2-thione, 4-methyl-1,3-dithiol-2-thione, 4-ethyl-1,3-dithiol-2-thione, 4-propyl-1,3-dithiol-
2-thione, 4,5-dimethyl-1,3-dithiol-
2-thione, and the like.

Specific examples of 1,3-dithiolan-2-thiones include 1,3-dithiolan-2-thione, 4-methyl-1,3-dithiolan-2-thione,
4-ethyl-1,3-dithiolan-2-thione, 4-propyl-1,3-dithiolan-2-thione, 4,5-dimethyl-1,3-dithiolan-2-thione, and the like. In addition, among these 1,3-dithiol-2-thiones and 1,3-dithiolan-2-thiones, 1,3
-Dithiol-2-thione, 1,3-dithiolan-2-
Thione and 4-ethyl-1,3-dithiol-2-thione are preferred. Next, the compound of the formula (I) will be described.

[0013]

Embedded image

(Wherein, R ¹ represents an alkyl group, alkenyl group, alkynyl group, aralkyl group or aryl group, and R ² represents an alkyl group, alkenyl group or alkynyl group containing at least one sulfur atom. , R ¹
And R ² may combine with each other to form a ring)

In the formula (I), when R ¹ is an alkyl group, the alkyl group may be linear, branched or cyclic, and generally has 1 to 18, preferably 1 to 18, carbon atoms.
13, more preferably 1 to 8, further preferably 1 to 5, and specific examples thereof include, for example, a methyl group, an ethyl group, a propyl group, and a butyl group. Of these, a methyl group and an ethyl group are preferred.

When R ¹ is an alkenyl group or an alkynyl group, these groups may be linear, branched or cyclic, and usually have 2 to 18, preferably 2 to 13, carbon atoms. More preferably 2 to 8, even more preferably 2
To 5, and specific examples thereof include, for example, an ethenyl group,
And a propenyl group, a butenyl group, an ethynyl group, a propynyl group, a butynyl group, and the like.

When R ¹ is an aralkyl group, the aralkyl group may have a substituent, and usually has 7 to 18, preferably 7 to 13, carbon atoms. Is, for example, a benzyl group, a phenylethyl group, a phenylpropyl group and the like. Furthermore, R ¹
Is an aryl group, the aryl group may have a substituent, and the number of carbon atoms is usually 6 to 18, preferably 6 to 13. Specific examples thereof include, for example, a phenyl group and a tolyl group. Group, xylyl group, naphthyl group and the like.

When R ² is an alkyl group containing at least one sulfur atom, the group may be linear, branched or cyclic, and usually has 1 to 1 carbon atom.
7, preferably 1 to 12, more preferably 1 to 7, and still more preferably 1 to 4, and specific examples thereof include a methylthio group, an ethylthio group, a propylthio group, and a butylthio group.

When R ² is an alkenyl group or an alkynyl group containing at least one sulfur atom, the group may be linear, branched or cyclic, and usually has 2 to 2 carbon atoms. 17, preferably 2 to 12, more preferably 2 to 7, and still more preferably 2 to 4, and specific examples thereof include an ethenylthio group, a propenylthio group, an ethynylthio group, and a propynylthio group.

Specific examples of such a compound of the formula (I) include, for example, 3-methylthio-
2-butanone and the like, and cyclic compounds include thiophenone derivatives such as tetrahydrothiophen-3-one, 2 (5H) -thiophenone and tetrahydrothiopyran-4-one.

The content of the sulfur-containing compound in the organic solvent (including the sulfur-containing compound) is usually 0.1 to 0.1%.
20% by weight, preferably 0.1 to 15% by weight, more preferably 0.1 to 10% by weight, further preferably 1 to 7% by weight, particularly preferably 1 to 3% by weight. Can be When the content is less than 0.1% by weight,
If a sufficient protective film is not formed, and if the content exceeds 20% by weight, the viscosity of the electrolytic solution increases, the electric conductivity decreases, and the performance of the battery deteriorates, which is not preferable.

Examples of the organic solvent used in addition to the sulfur-containing compound include cyclic carbonates such as ethylene carbonate and propylene carbonate, chain carbonates such as dimethyl carbonate, diethyl carbonate and ethyl methyl carbonate, γ-butyrolactone, and the like. cyclic esters such as γ-valerolactone, methyl acetate, chain esters such as methyl propionate, tetrahydrofuran, 2-methyltetrahydrofuran, cyclic ethers such as tetrahydropyran, dimethoxyethane, chain ethers such as dimethoxymethane, Examples thereof include sulfur-containing organic solvents such as sulfolane and diethyl sulfone. These solvents may be used as a mixture of two or more kinds.

As the solute, a lithium salt is usually used.
Used. Lithium salts are particularly limited
However, specific examples thereof include, for example, LiCl
O_Four, LiPF₆, LiBF_Four, LiAsF₆, LiS
bF₆, An inorganic lithium salt or LiCF selected from_Three
SO_Three, LiN (CF_ThreeSO_Two)_Two, LiN (CF_ThreeC
F _TwoSO_Two)_Two, LiN (CF_ThreeSO_Two) (C_FourF₉S
O_Two), LiC (CF_ThreeSO_Two)_ThreeFluorine-containing organic materials such as
Titanium salts can be mentioned. There are two types of these electrolytes
Or more of them may be used in combination. Lithium in electrolyte in electrolyte
The molar concentration of the sodium salt is usually 0.5 to 2.0 mol / liter.
Torr, preferably 0.5-1.5 mol / l
You. Less than 0.5 mol / l or 2.0 mol / l
Above 10%, the electrical conductivity of the electrolyte is low and the battery performance is low.
It is not preferable because it lowers.

(2) Non-Aqueous Electrolyte Secondary Battery The non-aqueous electrolyte secondary battery of the present invention comprises a negative electrode and a positive electrode capable of inserting and extracting lithium, the non-aqueous electrolyte, a separator and an external battery. And cans.

(I) Negative Electrode The negative electrode used in the present invention is composed of an active material whose surface is at least partially coated with the above-mentioned film-forming substance. The battery of the present invention has a large capacity, a small irreversible capacity observed in the initial cycle, a high storage stability and reliability of the battery when left at a high temperature, and an extremely excellent high efficiency discharge characteristic and a low temperature discharge characteristic. It can be.

(Active Material) As an active material constituting an electrode, a pyrolytic substance of an organic substance, a carbonaceous material capable of absorbing and releasing lithium such as artificial graphite and natural graphite, and a lithium oxide such as tin oxide and silicon oxide. A releasable metal oxide material, as well as metal sulfide materials such as zinc sulfide, silicon sulfide, and silver sulfide can be used. Among these materials, carbonaceous materials are preferable, and graphite materials are more preferable. Further, in the case of a carbonaceous material, a halogen gas treatment such as an acid treatment, an alkali treatment, or a fluorine treatment, a wet oxidation treatment using hydrochloric acid, nitric acid, sulfuric acid, or the like, and a mixing with an organic compound serving as a carbon precursor are performed. For example, a carbonaceous or graphitic material that has been subjected to a surface treatment in advance, such as a carbonization, can be used. These active materials may be used alone or in combination of two or more.

The carbonaceous or graphitic material in the present invention is obtained by processing natural or artificial graphite particles or carbonaceous particles as a graphite precursor, or carbon fibers, graphitized carbon fibers or the like into powder. These carbonaceous and graphitic powders before treatment are:
The interlayer distance (d ₀₀₂ ) is 0.340 nm or less, the crystallite size (Lc) is 30 nm or more, and the true density is 2.25 g / cc.
It is preferable that it is above. Further, the interlayer distance (d ₀₀₂ ) is more preferably 0.337 nm or less, and 0.336 n
m or less is most preferable. The crystallite size (Lc) is 50
nm or more is more preferable, and 100 nm or more is most preferable.

In Raman spectrum analysis using argon ion laser light,
Range of ^-1 to the peak P _A (peak intensity I _A) and 1350
~1370cm intensity ratio R = I _B / I _A in the range of ^-1 to a peak P _B (peak intensity I _B) is 0.0 to 1.0, preferably 0.0 to 0.5, 1580-16
The half width of the peak in the range of 20 cm ^-1 is preferably 26 cm ^-1 or less. Further, the intensity ratio R of the Raman spectrum is more preferably 0.4 or less, most preferably 0.3 or less. Half width is more preferably 25 cm ^-1 or less range of peak of the 1580~1620cm ^-1, and most preferably 24cm ^-1 or less.

The crystallinity of carbonaceous or graphitic particles can also be determined by electrochemical capacity using lithium ions. The charge or discharge rate of the carbonaceous or graphitic material used in the present invention was 0.2 mA / cm ² . The electric capacity of the half battery is preferably 330 mAh / g or more, more preferably 350 mAh / g or more.
That is, it is a highly crystalline carbon material having a carbon hexagonal network structure developed to some extent, and when metal ions are intercalated, a composition expressed as C ₆ Li, that is, 1 atom of lithium is added to 6 atoms of carbon. It is particularly preferred that the material be capable of forming the contained stage 1 structure.

The median diameter of the carbonaceous or graphitic material of the present invention is preferably in the range of 1 to 50 μm, preferably 5 to 40 μm, more preferably 10 to 30 μm, and particularly preferably 10 to 20 μm. The amount of fine powder of 10 μm or less is 25% or less, preferably 17% by volume-based particle size distribution.
%, More preferably 14% or less, even more preferably 12% or less. BET of carbonaceous or graphitic particles
Law specific surface area, 0.5 m ² / g or more 25.0m and ² / g or less, preferably 0.5 m ² / g or more 10.0 m ²
/ G or less, more preferably 0.5 m ² / g or more and 7.0 m
² / g or less, more preferably 0.5 m ² / g or more and 5.0.
m ² / g or less. As a method of achieving both the particle diameter and the BET specific surface area, there is a control of the specific surface area by a classification operation. By performing the fine powder removal by the classification operation, the specific surface area can be effectively reduced.

(Binder) The negative electrode for a non-aqueous electrolyte secondary battery of the present invention is obtained by adding a binder, a solvent and the like to an active material such as graphite.
A slurry is formed, and the slurry is applied to a metal current collector substrate and dried to form a sheet electrode. Further, the active material can be directly formed into a pellet electrode by a method such as roll forming or compression forming. Furthermore, the active material may be formed using an organic binder commonly used in carbon engineering such as tar, and sintered to form an electrode. In this case, the active material itself can be a precursor thereof, and its own binding component can be used as a binder.

Examples of the binder that can be used for the above purpose include:
Stable to electrolyte solvents, polyethylene, polypropylene, polyethylene terephthalate, aromatic polyamide, cellulose-based resin-based polymers, styrene-butadiene rubber, isoprene rubber, butadiene rubber, ethylene
Rubbery polymers such as propylene rubber, styrene / butadiene / styrene block copolymers, hydrogenated products thereof, styrene / ethylene / butadiene / styrene copolymers, styrene / isoprene / styrene block copolymers, hydrogenated products thereof Thermoplastic elastomeric polymer, syndiotactic 12-polybutadiene, ethylene / vinyl acetate copolymer, propylene / α-olefin (2 to 2 carbon atoms)
12) soft resinous polymers such as copolymers, fluorine-based polymers such as polyvinylidene fluoride, polytetrafluoroethylene, polytetrafluoroethylene / ethylene copolymers,
A polymer composition having an ion conductivity of an alkali metal ion, particularly, a lithium ion may be used. Examples of the polymer having ion conductivity include polyethylene oxide,
Polyether polymer compounds such as polypropylene oxide, crosslinked polymers of polyether compounds, polyepichlorohydrin, polyphosphazene, polysiloxane, polyvinylpyrrolidone, polyvinylidene carbonate, polyacrylonitrile and other high molecular compounds, lithium salts,
Or a system in which an alkali metal salt mainly composed of lithium is compounded, or an organic compound having a high dielectric constant such as propylene carbonate, ethylene carbonate, and γ-butyrolactone and an organic compound having a low viscosity such as a linear carbonate are blended therein. A system can be used. The ionic conductivity of such an ionic conductive polymer composition at room temperature is as follows:
It is preferably at least 10 ^-5 s / cm, more preferably 10 ^-3 s / cm.
s / cm or more.

The electrode material used in the present invention and the above-mentioned binder can be mixed in various forms. That is, a form in which both particles are mixed, a form in which a fibrous binder is entangled with the particles of the electrode material, a form in which a layer of the binder is attached to the particle surfaces of the electrode material, and the like are given. The mixing ratio of the electrode material and the binder is preferably 0.1 to 30% by weight, more preferably 0.5 to 10% by weight, based on the active material such as graphite. If the binder is added in an amount larger than this, the internal resistance of the electrode increases, which is not preferable. If the amount is less than this, the binding property between the current collector and the electrode material powder is poor.

(Current Collector) The electrode current collector used in the present invention is made of a metal such as copper, nickel or stainless steel, and may be in the form of a foil or a mesh. . Among these, copper foil is preferred from the viewpoint of easy processing into a thin film and the point of cost.

(Anode Density) The negative electrode for a non-aqueous secondary battery used in the present invention is formed into an arbitrary shape by a method such as roll molding or compression molding. At this time, the density (unit: g / cm) of the active material layer on the electrode formed by a method such as roll forming or compression forming.
³ , hereinafter referred to as electrode density) is usually 10 or less, preferably more than 0.8 and less than 2.0, more preferably more than 1.1 and less than 1.9, more preferably 1.2 or more. By setting the ratio to 1.8 or less, the capacity per unit volume of the battery can be maximized without impairing high-efficiency discharge and low-temperature characteristics.

(Surface Protective Film) The surface protective film of the negative electrode is obtained by chemically or electrochemically reducing a film-forming compound on the negative electrode surface. As a chemical reduction method, nickel, platinum, metals such as copper, copper chromite, copper chromate, oxides such as molybdenum oxide, molybdenum sulfide, catalytic reduction method using a sulfide such as tungsten sulfide as a catalyst, A method using a metal such as tin, iron, zinc or sodium, or a metal hydride such as lithium aluminum hydride or lithium borohydride as a reducing agent may be used. As a method of forming a protective film by these chemical reduction methods, a surface protective film forming compound is applied to the active material by dipping or the like after the electrode is formed, and an electrode material protective film is formed by using the above-described reduction method. Alternatively, a compound such as a surface protective film can be attached to the active material before the electrode is formed by using a method such as mixing, and the electrode can be formed after the protective film is formed by the above-described reduction method. In any of the methods, the film-forming compound may be used alone or as a dilute solution, or a catalyst or a reducing agent may be attached to the surface of the active material before preparing the electrode.

The electrochemical reduction method includes immersing the electrode in a non-aqueous electrolyte containing the compound for forming a surface protective film, and setting the electrode potential near the reduction potential of the selected compound. can get. The film formation operation can be performed in a state where the electrode is finally incorporated in the non-aqueous secondary battery in which the electrode is used, or as a step before being incorporated in the non-aqueous secondary battery, in a different cell. It is also possible to carry out. Since a certain amount of electricity is consumed to form the surface protective film, when incorporated into a non-aqueous secondary battery, there is a problem that the coulomb efficiency of the battery is reduced accordingly, but operation is simple. It is practically preferable.

When the film forming operation is performed in the former non-aqueous secondary battery, it is equivalent to the operation in the bipolar cell. When the film forming operation is performed in the latter cell, in addition to the working electrode and the counter electrode, If necessary, the reference electrode can be immersed to perform a surface protection film forming operation, that is, an electrolytic reduction operation, as a three-electrode cell. When a different cell is used, it is possible to use an electrode having the same dimensions as when the battery is finally incorporated into a non-aqueous secondary battery, or to use an electrode having a different size from that when incorporating the battery into a non-aqueous secondary battery. Perform film formation processing,
It is also possible to reshape the electrode to any dimensions after the treatment. Further, when a different cell is used, a large-scale electrolytic cell for synthesis (refer to page 10 of “Organic Electrosynthesis” published by Kodansha Co., Ltd. in 1981) or a roll-winding method for electrodes may be used. Continuous electrolytic cell (US Pat. No. 5,
595, 837) can be used.

The formation of the surface protective film in the present invention can be carried out by setting the electrode potential at which the protective film is to be formed, ie, the working electrode potential, near the reduction potential of the surface protective film forming compound. The working electrode potential can be obtained by setting the potential near the reduction potential by the constant potential method, or by leaving the working electrode potential to the self-regulating potential specific to the electrolytic system by the constant current method. The reduction potential is 0 to 3.3 with respect to the unipolar potential of Li / Li +.
V, preferably 0 to 2.5 V, more preferably 0.5 to
The voltage is set in the range of 2.2 V, more preferably 1.0 to 1.8 V, and even more preferably 1.0 to 1.5 V. In the case of the constant current method, the self-regulating potential inherent to the electrolytic system in the range of 3.3 V to 0 V with respect to the monopolar potential of Li / Li + (reduction potential of the surface protective film forming compound determined by the electrolytic system). Is satisfied, the current density can be set as the range in which the potential self-regulation works. Further, the lower limit of the potential in the constant current method does not have to reach the potential at which Li is occluded as a layered compound in the negative electrode active material, as long as the potential passes the above-described reduction potential. In the present invention, the constant current method and the constant potential method can be used alone or in combination, as long as the current passes through the above-mentioned reduction potential and a current necessary and sufficient to form a surface protective film is applied. . (II) Positive Electrode As the positive electrode material constituting the battery, a material capable of inserting and extracting lithium, such as a lithium transition metal composite oxide material such as lithium cobalt oxide and lithium nickel oxide, can be used. As the shape of the positive electrode, a sheet electrode applied to a current collector after mixing with a binder and a conductive agent, if necessary, and a pellet electrode subjected to press molding can be used. As the material of the positive electrode current collector, a valve metal such as aluminum, titanium, and tantalum or an alloy thereof can be used.

(III) Non-Aqueous Electrolyte As the non-aqueous electrolyte, the above-mentioned non-aqueous electrolyte containing a film-forming compound is used. When a negative electrode having a film formed in advance in a different cell is used, the film forming compound does not have to be charged in the electrolytic solution. It is preferable to add about 3% by weight of a film-forming compound.

(IV) Separator As the separator constituting the battery, a porous sheet or nonwoven fabric made of a polyolefin such as polyethylene or polypropylene can be used.

(V) Battery Shape The shape of the battery is, for example, a cylinder type having a sheet electrode and a separator in a spiral shape, a cylinder type having an inside-out structure combining a pellet electrode and a separator, and a coin type having a laminate of a pellet electrode and a separator. Can be used.

(VI) Material of Battery Outer Can As the material of the battery outer can, stainless steel is preferably used, but valve metals such as aluminum can also be used. As a method of protecting with valve metal, there is a method of protecting with plating or foil. Further, aluminum or an aluminum alloy may be used as the outer can material. In addition, the outer can mentioned here includes a lead wire housed inside the battery, a safety valve that operates when the internal pressure inside the battery rises, and the like.

[0044]

The present invention will be described in more detail with reference to the following examples and comparative examples. However, the present invention is not limited to these examples unless it exceeds the gist of the present invention.

Examples 1-4 and Comparative Examples 1-3 (Surface protective film forming treatment) 95 parts by weight of artificial graphite powder (trade name "KS44") manufactured by Timcal Co., Ltd. were mixed with 5 parts by weight of polyvinylidene fluoride, and N -Methyl-2-pyrrolidone was added and dispersed to form a slurry. This was applied on a copper foil by a doctor blade method and preliminarily dried. Further, after being pressed by a press so that the electrode plate density becomes about 1.3 g / cc, it was punched into a disk shape and dried under reduced pressure to obtain a negative electrode.

Regarding the electrolyte solution, propylene carbonate (hereinafter abbreviated as “PC”) and 1,3-dithiolan-2-thione (Example 1) under a dry argon atmosphere.
And propylene carbonate (PC), 1,3-dithiol-2-thione (Example 2), tetrahydrothiophen-3-one (Example 3) and 2 (5H) -thiophenone (Example 4) in Comparative Examples As examples, tetrahydrothiophene (Comparative Example 2) and thiophene (Comparative Example 3) each containing no surface film-forming compound in the electrolyte solution (Comparative Example 1) and containing a sulfur element in the ring but having no carbonyl bond were used. % By weight (PC is 93% by weight) mixed with a sufficiently dried lithium hexafluorophosphate (LiP
F ₆ ) as an electrolyte, 1 mol / liter (1M)
To prepare an electrolytic solution. Thereafter, a coin cell in which the electrode and the lithium metal electrode were opposed to each other centering on the separator impregnated with the electrolytic solution was prepared in a dry argon atmosphere, and the surface protective film was formed. In forming the surface protective film, the current value was kept constant at 0.2 mA, and current was supplied until the potential difference between both electrodes became 0 V. In Examples 1 to 4, energization was possible up to 0 V, but in Comparative Examples 1 to 3, PC decomposition occurred violently with the generation of gas.
V could not be supplied.

(Evaluation of Battery) Next, the coin cells of Examples 1 to 4 were dedoped with lithium until the potential difference became 1.5 V, and then dismantled in a dry argon atmosphere to form a surface protective film. After taking out the latter electrode,
A coin cell using the electrode as the negative electrode material and LiCoO ₂ as the positive electrode material was prepared again. The positive electrode active material is LiCoO
₂ (85 parts by weight), carbon black (6 parts by weight) and polyvinylidene fluoride (9 parts by weight) were added and mixed, and N-methyl-2-pyrrolidone was added to be dispersed to form a slurry. Apply on aluminum foil by blade method,
Pre-drying was performed. Furthermore, after being press-bonded with a press machine, it was punched into a disk shape and dried under reduced pressure to obtain a positive electrode. The electrode after the formation of the surface protective film was taken out and used as a negative electrode. In the electrolytic solution, the same electrolytic solution as that used in Comparative Example 1, that is, a single solvent of PC as the solvent, and lithium hexafluorophosphate (LiPF ₆ ) dissolved in the solute at a ratio of 1 mol / liter (1M) was used. The used electrolyte was used. After preparing these, a coin cell was prepared in which the negative electrode and the positive electrode were opposed to each other centering on the separator impregnated with the electrolytic solution. Thereafter, the current value is set to 0.5
The charge / discharge test was performed at a constant end-of-charge voltage of 4.2 V and an end-of-discharge voltage of 2.5 V. This charge / discharge reaction was repeated a total of 5 times (5 cycles). The obtained charge / discharge test results were arranged by the following two indices. First, as an index of the formation of the protective film, attention was paid to the presence or absence of the electrochemical reduction decomposition reaction of propylene carbonate in the first cycle. If the protective film is not sufficiently formed, a reductive decomposition reaction of propylene carbonate occurs on the negative electrode interface, and a large amount of gas is generated, the battery deteriorates, and the charge / discharge reaction should not proceed further. Further, using the charge capacity of the fifth cycle as a denominator, the charge / discharge efficiency value having the discharge capacity as a numerator was compared as an index of stability of the obtained protective film. Table 1 summarizes the results. In a battery that has been subjected to a surface protective film forming treatment, a charge / discharge reaction can be performed even in the case of a single solvent of PC.

[0048]

[Table 1]

[0049]

According to the present invention, it is possible to obtain a non-aqueous electrolyte secondary battery which has very little decomposition on the negative electrode, and has excellent low-temperature characteristics, long-term stability and cycle characteristics. Can be made.

 ────────────────────────────────────────────────── ─── Continuing on the front page (72) Inventor Jin Suzuki 8-3-1 Chuo, Ami-cho, Inashiki-gun, Ibaraki Prefecture Mitsubishi Chemical Corporation Tsukuba Research Laboratories F-term (reference) CJ14 CJ28 HJ01 HJ02 5H050 AA06 AA07 AA09 BA17 CA08 CB07 DA03 DA09 EA10 EA23 EA24 FA04 GA15 GA27 HA01

Claims (6)

[Claims] 1. A non-aqueous electrolyte comprising a lithium salt dissolved as a solute in a non-aqueous solvent.
3-dithiol-2-thiones, 1,3-dithiolane-
A non-aqueous electrolyte solution comprising at least one sulfur-containing compound selected from 2-thiones and a compound represented by the following general formula (I). Embedded image (In the formula, R ¹ represents an alkyl group, an alkenyl group, an alkynyl group, an aralkyl group or an aryl group, R ² represents an alkyl group, an alkenyl or alkynyl group containing a sulfur atom of at least Ichike. However, R ¹ And R ² may combine with each other to form a ring) 2. The non-aqueous electrolyte according to claim 1, wherein the compound of the formula (I) is a cyclic keto compound containing a sulfur atom in the ring. 3. The non-aqueous electrolyte according to claim 1, wherein the content of the sulfur-containing compound in the non-aqueous solvent is 0.1 to 20% by weight. 4. The method according to claim 1, wherein the lithium salt is LiPF ₆ , LiBF
₄ , LiN (CF ₃ (SO ₂ ) ₂ ) or Li (CF ₃ C
4. The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte is F ₂ SO ₂ ) ₂ . 5. A non-aqueous electrolyte comprising a negative electrode and a positive electrode capable of inserting and extracting lithium, the non-aqueous electrolyte according to claim 1, and a separator and an outer can. Next battery. 6. A non-aqueous electrolyte secondary battery comprising, as a component, a negative electrode that has undergone at least a step of reducing the sulfur-containing compound contained in the non-aqueous electrolyte solution according to claim 1 or 2 on the negative electrode.