JP2002008721A - Nonaqueous electrolyte and lithium secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery with a large electric capacity, high cycle characteristics, and high shelf life characteristics, and provide a nonaqueous electrolyte to use in manufacturing the lithium secondary battery. SOLUTION: This nonaqueous electrolyte is prepared by dissolving an electrolyte in a nonaqueous solvent containing a cyclic carbonate, a chain carbonate, and vinylene carbonate, and has a reduction potential of less than 1 volt vs. lithium and/or less mixing content of an organic chlorine compound, and the lithium secondary battery is composed of this nonaqueous electrolyte, a negative electrode of graphite having the spacing of crystal face (d002) of 0.34 nm or less, and a positive electrode.

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 lithium secondary battery using the non-aqueous electrolyte. In particular, the present invention relates to a lithium secondary battery having excellent electric capacity, cycle characteristics and storage characteristics, and a non-aqueous electrolyte and a non-aqueous solvent which can be advantageously used for producing such a lithium secondary battery. .

[0002]

2. Description of the Related Art In recent years, small electronic devices such as personal computers, mobile phones, and camera-integrated video cameras have become remarkably widespread, and small, lightweight, high-capacity secondary batteries have been used as power sources for driving these small electronic devices. Is strongly required. From the viewpoint of small, lightweight and high-capacity secondary batteries, composite oxides such as lithium cobalt oxide, lithium nickel oxide, and lithium manganate are used as the positive electrode active material,
Lithium secondary using a non-aqueous electrolyte in which a lithium material is dissolved in a non-aqueous solvent composed of a cyclic carbonate and a chain carbonate, using a carbon material capable of doping and undoping lithium ions as a negative electrode active material. Batteries are considered suitable, and research and development are currently being actively pursued for further improvements.

[0003] Among carbon materials capable of doping and undoping lithium ions, graphite (graphite) is particularly suitable for lithium secondary batteries because of its favorable characteristics such as large electric capacity and high flatness of potential. It is regarded as one of the most suitable compounds as a negative electrode active material, and is frequently used.

However, in a lithium secondary battery using a graphite-based material as a negative electrode active material, a cyclic carbonate such as ethylene carbonate (EC), propylene carbonate (PC), or butylene carbonate (BC) is used as an electrolyte solvent. In addition, the cyclic carbonate is decomposed by the graphite-based negative electrode active material, and at that time, separation occurs on the surface of the carbon material, and the electric capacity, cycle characteristics,
Battery characteristics such as storage characteristics tend to decrease. In particular, this phenomenon appears remarkably in an electrolytic solution containing propylene carbonate, and there is a problem that propylene carbonate is decomposed on the surface of the graphite negative electrode at the time of initial charging, making charging and discharging difficult.

The addition of various compounds has been proposed as a method for suppressing the decomposition of the cyclic carbonate in the electrolytic solution and the exfoliation of the carbon material by the graphite-based negative electrode active material. For example,
In J. Electrochem. Soc., Vol. 140, No. 6, L101 (1993), a crown ether compound (12-crown-4) was added to an electrolyte containing propylene carbonate and ethylene carbonate as main components. By doing so, it has been proposed that decomposition of the electrolytic solution is suppressed. However, in this case, unless a considerable amount of expensive crown ether is added, the effect of suppressing decomposition is small, and the battery characteristics achieved are not yet sufficient.

Japanese Patent Application Laid-Open No. Hei 8-45545 (US Pat. No. 5,626,981) includes a first solvent having a high dielectric constant and a second solvent having a low viscosity.
A mixture of at least two aprotic organic solvents further contains at least one unsaturated bond and has a crystallinity higher than 0.8 at a potential higher than lithium by 1 volt to form a passivation layer. An electrolyte for a lithium battery in which a lithium salt is dissolved in a solvent system further containing a soluble solvent of the same kind as at least one of the above-mentioned solvents which can be reduced at an anode made of a carbon material is described. Further, it is described that the decomposition of the electrolytic solution is suppressed by the addition of the soluble solvent having a high reduction potential.
According to this method, the additive is reduced at the negative electrode during charging to form a passivation film on the graphite surface, thereby suppressing reduction of other solvents.

However, according to the study of the present inventors,
In the method described above, the initial coulomb (charge / discharge) efficiency is not always high, and the charge / discharge is repeated, so that the electric capacity gradually decreases, and it is difficult to obtain satisfactory cycle characteristics and storage stability. is there.

[0008] Also, 1997 Joint International Meeting
of The Electrochemical Society, Inc. and Internati
onal Society of Electrochemistry, Abstracts, P.15
3 (1997) contains 5% by volume of vinylene carbonate (VC), contains 1 M LiPF ₆ electrolyte, and contains PC / E
In a voltammogram measurement using a cell comprising a graphite electrode (working electrode) / Li (counter electrode) / Li (reference electrode) composed of an electrolyte having a volume ratio of C / DMC (DMC: dimethyl carbonate) of 1/1/3. It has been reported that a reduction peak appears at 1 volt, and this forms a passive film on the negative electrode to suppress the reduction of other solvents.

Further, J. Electrochem. Soc., Vol.
0, No. 9, L161 (1995) states that the addition of chloroethylene carbonate to an electrolytic solution suppresses PC decomposition on the surface of a graphite electrode. It is thought that this is because the decomposition product of chloroethylene carbonate forms a passive film on the graphite surface, but the effect of suppressing the decomposition of the electrolytic solution is not always good.

[0010]

According to the above-mentioned various methods, it has become possible to share a cyclic carbonate, which is an excellent non-aqueous solvent, with a highly crystalline carbon anode such as graphite (graphite). However, even with the use of the non-aqueous solvent system described above, a lithium secondary battery exhibiting sufficiently satisfactory battery characteristics cannot be obtained.

[0011]

Means for Solving the Problems The present inventors have focused on a non-aqueous solvent composition containing a cyclic carbonate, a chain carbonate and vinylene carbonate, which exhibits excellent properties as a non-aqueous solvent for an electrolytic solution. VC)
A study was carried out to determine the effect of the electrolyte on the decomposition of the electrolyte on the graphite electrode surface of the non-aqueous secondary battery. As a result, in a lithium secondary battery, when vinylene carbonate synthesized by a conventional vinylene carbonate synthesis method is used as vinylene carbonate, sufficiently satisfactory battery characteristics cannot be obtained, and the battery characteristics also vary. Was found to be seen. As a result of further study, vinylene carbonate produced by a conventional method contains a considerable amount of an organic chlorine compound by-produced during the production of vinylene carbonate as an impurity, and the vinylene carbonate has a nonaqueous solvent composition. When introduced into the non-aqueous solvent composition, these organic chlorine compounds are mixed into the non-aqueous solvent composition to increase the reduction potential of the non-aqueous electrolyte solution manufactured using the non-aqueous solvent composition, and to lower the battery characteristics, It has been found that the battery characteristics vary.

According to the present invention, an electrolyte is dissolved in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate, and the reduction potential is less than 1 volt with respect to lithium (or a difference of less than 1 volt). In the non-aqueous electrolyte.

The present invention is also directed to a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate, wherein the non-aqueous electrolyte contains an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms. There is also a non-aqueous electrolyte contained in an amount of.

The present invention also resides in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate, and containing an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms.

The present invention further provides a positive electrode, a crystal plane spacing (d)
₀₀₂ ) is a graphite negative electrode having a graphite potential of 0.34 nm or less, and an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate. There is also a lithium secondary battery characterized in that an electrolytic solution is used which is less than 1 volt based on lithium.

Further, according to the present invention, the electrolyte is dissolved in a non-aqueous solvent containing a positive electrode, a graphite negative electrode having a crystal plane spacing (d ₀₀₂ ) of 0.34 nm or less, and a cyclic carbonate, a chain carbonate, and vinylene carbonate. In a lithium secondary battery comprising an electrolytic solution, there is also a lithium secondary battery in which the electrolytic solution contains an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms.

The present invention is particularly characterized by the characteristics or composition of the non-aqueous electrolyte or non-aqueous solvent, and the preferred embodiments are as follows. (1) The reduction potential of the nonaqueous electrolyte is 0.9 volts or less, more preferably 0.8 volts or less, and particularly in the range of 0.7 volts to 0.8 volts, based on lithium. (2) The content of the organic chlorine compound in the non-aqueous solvent is 10 ppm or less, particularly 5 ppm or less, and more preferably 2.5 ppm or less in terms of chlorine atoms. (3) An organic chlorine compound in a non-aqueous solvent is introduced as an impurity of vinylene carbonate. (4) The content of the organic chlorine compound contained as an impurity in vinylene carbonate introduced into the non-aqueous solvent is 100 pp in terms of chlorine atom with respect to vinylene carbonate.
m or less.

Although the reduction potential of the organic chlorine compound in the non-aqueous electrolyte of the present invention and the effect on the improvement of the battery characteristics of the lithium secondary battery due to the reduction of the reduction potential are not clear,
It is estimated as follows.

A vinylene carbonate (VC) product produced by a conventionally used synthetic method contains at least about 3000 ppm of a plurality of organochlorine compounds represented by the following chemical formula. When carbonate is introduced into the non-aqueous solvent of the electrolytic solution, the usual usage amount of about 1 to 10% by mass, the content of the organic chlorine compound in the non-aqueous solvent becomes
It becomes about 30 to 300 ppm.

[0020]

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[0021]

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[0022]

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These organochlorine compounds exhibit a higher reduction potential than vinylene carbonate and other electrolyte compositions, and are reduced on the graphite surface of the negative electrode to form a film prior to reduction of vinylene carbonate and the electrolyte composition. It has the effect of suppressing the decomposition of vinylene carbonate and electrolytic solution.

However, it is presumed that the film formed on the graphite surface of the negative electrode in this way contains chlorine and the film is thick, and therefore does not show a sufficiently satisfactory electrolytic solution decomposition suppressing effect. That is, it is presumed that the organochlorine compound contained as an impurity in vinylene carbonate hinders the intrinsic improvement of the battery performance of vinylene carbonate and does not give a sufficient effect.

Accordingly, the present inventors have intensively studied a method for synthesizing and purifying vinylene carbonate, and as a result, have developed a method for producing high-purity vinylene carbonate having a very low content of organic chlorine compounds. That is, as the conventional vinylene carbonate synthesis method, J. Am. Chem.
As described in c., 75 , 1263 (1953), etc., monochloroethylene carbonate is synthesized by a chlorination reaction of ethylene carbonate (EC) (first step), and this is mixed with an ether-based low boiling solvent. A method for producing vinylene carbonate by performing a dehydrochlorination reaction (second step) with an amine at step (1) is known. The solvent in the second step is replaced with an ester-based high-boiling solvent such as dibutyl carbonate (DBC), and further purified by distillation or crystallization to produce high-purity vinylene carbonate containing almost no organic chlorine compound. Method developed. It was confirmed that the lithium secondary battery using the electrolytic solution containing the high-purity vinylene carbonate as an additive exhibited extremely excellent electric capacity, cycle characteristics and storage characteristics.

[0026]

DESCRIPTION OF THE PREFERRED EMBODIMENTS The content of vinylene carbonate (VC) in a non-aqueous solvent used in the non-aqueous electrolyte of the present invention is as follows.
It is preferably in the range of 0.01% by mass to 10% by mass, and particularly preferably in the range of 0.1% by mass to 5% by mass. If the content of vinylene carbonate is excessively small, the decomposition of the electrolytic solution tends to occur in the graphite negative electrode, and if the content is excessively large, the battery characteristics deteriorate. Further, the content of the organic chlorine compound in the vinylene carbonate product used for preparing the electrolyte solution for a lithium secondary battery of the present invention is preferably 100 ppm or less, particularly preferably 50 ppm or less, in terms of chlorine content.

As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC),
Butylene carbonate (BC) or the like is used, and these are used alone or as a mixture of two or more. Examples of the linear carbonate include dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), and methyl butyl carbonate (MB).
C) and the like, which may be used alone or as a mixture of two or more. The ratio between the cyclic carbonate and the chain carbonate is 2: 8 to 6: 4 in volume ratio.
Is preferably within the range.

The electrolyte used in the non-aqueous electrolyte of the present invention includes LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN
(SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC
And a lithium salt such as (SO ₂ CF ₃ ) ₃ . These electrolytes may be used alone or in combination of two or more. These electrolytes are usually added to the above non-aqueous solvent in an amount of 0.1 to 3M, preferably 0.5 to 1.M.
It is used after being dissolved at a concentration of 5M.

The non-aqueous electrolyte of the present invention can be obtained, for example, by mixing a cyclic carbonate, a chain carbonate and the above-mentioned high-purity vinylene carbonate to prepare a non-aqueous solvent, and dissolving the electrolyte in the non-aqueous solvent.

The electrolytic solution of the present invention is suitably used as a constituent material of a lithium secondary battery. The constituent materials other than the electrolytic solution constituting the secondary battery are not particularly limited, and various constituent materials conventionally used can be used.

For example, as the positive electrode active material, cobalt,
A composite metal oxide of lithium and at least one metal selected from the group consisting of nickel, manganese, chromium, vanadium and iron is used. Examples of such a composite metal oxide include LiCoO ₂ , LiNiO ₂ ,
LiMn ₂ O _{4 and the} like.

For the positive electrode, the above-mentioned positive electrode active material is made of a conductive agent such as acetylene black or carbon black, a binder such as polyvinylidene fluoride (PVDF) or polytetrafluoroethylene (PTFE), and an N-methylpyrrolidone solvent. Then, the positive electrode mixture is kneaded to obtain a positive electrode mixture, the positive electrode mixture is applied to a current collector such as an aluminum foil or a stainless steel sheet, dried at 50 to 250 ° C., and then compression molded.

As the negative electrode active material, it is preferable to use natural or artificial graphite having a crystal plane spacing (d ₀₀₂ ) of 0.34 nm or less. For the negative electrode, the graphite is PVD
F, PTFE, ethylene propylene diene monomer (E
After kneading with a binder such as PDM) and an N-methylpyrrolidone solvent to form a negative electrode mixture, this negative electrode mixture is applied to a current collector such as a copper foil or a stainless steel sheet, and then mixed with a negative electrode.
It is made by drying at 50 ° C. and then compression molding.

The configuration of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a coin battery, a cylindrical battery, a prismatic battery, and a stacked battery having a positive electrode, a negative electrode, a porous membrane separator, and an electrolyte. An example is given.

[0035]

EXAMPLES [Vinylene carbonate] Examples 1 to be described later.
The sources, synthesis methods and properties of the three types of vinylene carbonate used in Comparative Example 3 and Comparative Examples 1 to 6 are shown below. [Procurement of vinylene carbonate, synthesis method]

(1) Vinylene carbonate manufactured by Aldrich Vinylene carbonate commercially available from Aldrich Chemical Company. Inc. as a reagent was used. Less than,
This vinylene carbonate is referred to as "vinylene carbonate manufactured by Aldrich".

(2) Vinylene carbonate synthesis by a known method J. Am. Chem. Soc., 75, 1263 (1953), and J. Am. Ch.
em. Soc., 77, 3789 (1955). That is, ethylene carbonate 6 previously distilled and refined.
While irradiating ultraviolet rays while blowing chlorine gas into
The reaction was conducted at 24 ° C. for 24 hours. After the reaction, 560 g of monochloroethylene carbonate was collected by distillation under reduced pressure. Next, 493 g of monochloroethylene carbonate was dissolved in 500 mL of dry diethyl ether, and 440 g of triethylamine was added dropwise thereto over 6 hours under reflux, and stirring was continued for 14 hours while further refluxing. Thereafter, the solid triethylamine hydrochloride was filtered and washed with a mixed solvent of ether and n-hexane. The filtrate was subjected to simple distillation to distill off the solvent and the excess amine.
Simple distillation was performed under a reduced pressure of Hg, and 290 g of a vinylene carbonate fraction was collected. This vinylene carbonate was further precision distilled under reduced pressure of 30 mmHg to obtain 104 g of vinylene carbonate having a boiling point of 73 ° C. Hereinafter, this vinylene carbonate is referred to as “conventionally synthesized vinylene carbonate”.

(3) Synthesis of high-purity vinylene carbonate First, monochloroethylene carbonate was synthesized by the method of (2). 494 g of the obtained monochloroethylene carbonate was dissolved in 500 mL of dibutyl carbonate, and the solution was charged into a 2 L reactor, and 440 g of triethylamine was added thereto at 50 ° C., and the mixture was allowed to react while being dropped over 6 hours, and stirring was further continued for 14 hours. Was. Thereafter, the reaction solution was cooled to room temperature, and triethylamine hydrochloride was filtered and sufficiently washed with dibutyl carbonate. The obtained filtrate (2100 g) was subjected to simple distillation under a reduced pressure of 30 mmHg to remove excess triethylamine, and then 390 g of a vinylene carbonate fraction was collected. After treating this vinylene carbonate with a silica gel column, 30 mm
By performing precision distillation under a reduced pressure of Hg, vinylene carbonate 19 having a boiling point of 73 ° C. with very few impurities can be obtained.
5 g were obtained. The obtained vinylene carbonate is referred to as “high-purity vinylene carbonate”.

[Gas Chromatography / Mass Spectrometry of Vinylene Carbonate] Aldrich vinylene carbonate,
In the gas chromatographic analysis of vinylene carbonate and the conventional synthesis method, many kinds of impurities were detected in a small amount, and as a result of performing gas chromatography-mass spectrometry, it was estimated that the three kinds of chlorine compounds considered to have been generated during the synthesis of vinylene carbonate. Chlorinated compounds were found to be contained. However, impurities were hardly recognized in the high-purity vinylene carbonate, and the chlorine compound presumed to be the above-mentioned chlorine compound was not detected.

[Amount of Chlorine in Vinylene Carbonate] Vinylene carbonate was subjected to oxy-hydrogen flame treatment to absorb gas into water, and chlorine ions in the absorbing solution were measured by ion chromatography. The results are shown in Table 1. Aldrich vinylene carbonate contained 3200 ppm of chlorine, whereas conventional synthetic vinylene carbonate contained 3550 ppm of chlorine, but the content of high-purity vinylene carbonate was 2 ppm.
It was extremely low at 9 ppm.

[0041]

[Table 1] Table 1 使用 Used vinylene carbonate Chlorine content ─ ─────────────────────────────────── Aldrich vinylene carbonate 3200ppm Conventional synthetic vinylene carbonate 3550ppm High purity vinylene carbonate 29ppm ────────────────────────────────────

Example 1 [Preparation of Electrolyte Solution] High-purity vinylene carbonate was mixed in a mixed solvent of propylene carbonate (PC) and dimethylene carbonate (DMC) at a volume ratio of 1: 2 to 2 mass%. To prepare a non-aqueous solvent, to which LiPF
₆ was dissolved to a concentration of 1 M to prepare an electrolytic solution.

[Production of Lithium Secondary Battery and Measurement of Battery Characteristics] LiCoO ₂ (cathode active material) was 80% by mass, acetylene black (conductive agent) was 10% by mass, and polyvinylidene fluoride (binder) was 10% by mass. %, And the mixture was diluted with N-methylpyrrolidone to prepare a positive electrode mixture. This mixture was applied to an aluminum foil current collector, dried, and compression molded to obtain a positive electrode. On the other hand, natural graphite (d ₀₀₂ = 0.
3354) 90% by mass and 10% by mass of polyvinylidene fluoride (binder) were mixed and diluted with N-methylpyrrolidone to prepare a negative electrode mixture. This mixture was applied to a copper foil current collector, dried, and compression molded to obtain a negative electrode. The ratio between the positive electrode and the negative electrode was set to have substantially the same electric capacity.

A coin-type battery (diameter: 20 mm, thickness: 3.2 m) composed of these positive and negative electrodes, a separator made of a polypropylene microporous film, and an electrolytic solution
m) and charged at room temperature (25 ° C.) with a constant current of 0.8 mA and a constant voltage to a voltage of 4.2 V for 5 hours.
Discharge was performed at a constant current of 8 mA to a voltage of 2.7 V. Figure 1
The graph shows the initial charge / discharge characteristics, the vertical axis shows the battery voltage (V), and the horizontal axis shows the capacity (mAh / g carbon). Furthermore, charge and discharge were repeated, and the cycle change of the discharge capacity was examined.

Comparative Example 1 A secondary battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that vinylene carbonate was not added. However, the propylene carbonate was decomposed at the time of the first charge and did not reach the predetermined voltage, so that discharge was not possible. As a result of disassembling the battery after the charge / discharge, peeling of graphite of the negative electrode was observed.

Comparative Example 2 A secondary battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that vinylene carbonate manufactured by Aldrich was used instead of high-purity vinylene carbonate. FIG. 2 shows the initial charge / discharge characteristics.

Comparative Example 3 A secondary battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that vinylene carbonate of a conventional synthesis method was used instead of high-purity vinylene carbonate.

Table 2 shows the initial coulomb efficiencies of the secondary batteries prepared in Example 1 and Comparative Examples 2 and 3, respectively. As is clear from this, good coulomb efficiency can be obtained by using high-purity vinylene carbonate.

[0049]

[Table 2] (Electrolyte non-aqueous solvent: PC / DMC = 1/2 (volume ratio) + vinylene carbonate) ────────────── Vinylene carbonate Coulomb efficiency ──────────────────────────────── Example 1 78% of high-purity vinylene carbonate (2% by mass) Comparative Example 1 Charging / discharging impossible without addition Comparative Example 2 Vinylene carbonate manufactured by Aldrich (2% by mass) 73% Comparative Example 3 Conventionally synthesized vinylene carbonate (2% by mass) 74% ─────────────────────────────

FIG. 3 shows the cycle characteristics of the secondary batteries of Example 1, Comparative Example 2 and Comparative Example 3, with the discharge capacity (mAh) on the vertical axis and the number of cycles on the horizontal axis.

As can be seen from the graph of FIG.
Compared with the secondary battery of Comparative Example 2 to which vinylene carbonate manufactured was added and the secondary battery of Comparative Example 3 to which vinylene carbonate of the related art was added, the secondary battery of Example 1 to which high-purity vinylene carbonate was added was excellent. Maintains cycle characteristics.

Example 2 Propylene carbonate (P
C) and dimethyl carbonate (DMC) in a volume ratio of 1:
Instead of the mixed solvent of 2, ethylene carbonate (E
C) and dimethyl carbonate (DMC) in a volume ratio of 1: 1
A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that the mixed solvent of was used. FIG. 4 shows an initial charge / discharge curve.

Comparative Example 4 A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 2 except that vinylene carbonate was not used. FIG. 5 shows an initial charge / discharge curve.

Comparative Example 5 A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 2 except that vinylene carbonate manufactured by Aldrich was used instead of high-purity vinylene carbonate.

Comparative Example 6 A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 2, except that vinylene carbonate of a conventional synthesis method was used instead of high-purity vinylene carbonate. FIG. 6 shows an initial charge / discharge curve.

Example 3 Propylene carbonate (P
C) and dimethyl carbonate (DMC) in a volume ratio of 1:
A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that the capacity ratio of propylene carbonate (PC), ethylene carbonate (EC) and dimethyl carbonate (DMC) was changed to 1: 1: 2 instead of 1. Was.

Table 3 shows the initial coulomb efficiencies of the secondary batteries of Examples 2-3 and Comparative Examples 4-6. As is clear from this, good coulomb efficiency can be obtained by using high-purity vinylene carbonate.

[0058]

[Table 3] (Electrolyte non-aqueous solvent: EC / DMC = 1/1 (volume ratio) + vinylene carbonate) ────────────── Vinylene carbonate Coulomb efficiency ──────────────────────────────── Example 2 79% of high-purity vinylene carbonate (2% by mass) Comparative Example 4 No addition 72% Comparative Example 5 Aldrich vinylene carbonate (2% by mass) 75% Comparative Example 6 Conventionally synthesized vinylene carbonate (2% by mass) 74% ─────────────────────────── example 3 high purity vinylene carbonate (2 wt%) ^* 80% ───── ────────────────────────────── Note: The electrolyte non-aqueous solvent in Example 3 was PC / EC / DMC = 1 / 1/2 (volume ratio) + vinylene carbonate.

FIG. 7 shows the cycle characteristics of the secondary batteries of Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 6, respectively.
The vertical axis shows the discharge capacity (mAh), and the horizontal axis shows the cycle number.

As can be seen from the graph of FIG. 7, the secondary battery of Comparative Example 4 in which vinylene carbonate was not added,
Compared with the secondary battery of Comparative Example 5 to which vinylene carbonate manufactured by drich was added and the secondary battery of Comparative Example 6 to which vinylene carbonate of the related art was added, in the secondary battery of Example 2 to which high-purity vinylene carbonate was added, Good cycle characteristics are maintained.

FIG. 8 shows the cycle characteristics of the secondary battery of Example 3 (in which the non-aqueous solvent system was changed).
Ah), and the number of cycles is shown on the horizontal axis, and it can be seen that good cycle characteristics are similarly maintained.

Examples 4 to 6 A battery was prepared and subjected to a charge / discharge test in the same manner as in Example 1 except that the volume ratio of the electrolytic solution solvent was changed as shown in Table 4. Table 4 shows the first Coulomb efficiency
Shown in Further, it was found that it had good cycle characteristics as in Example 1.

[0063]

Table 4 (Non-aqueous solvent for electrolytic solution: basic solvent composition + high-purity vinylene carbonate 2% by mass) ─────────── Basic solvent composition Coulomb efficiency ────────────────────────────────── ４ Example 4 PC / EC / MEC = 5/30/65 81% Example 5 PC / EC / DEC = 5/30/65 80% Example 6 PC / EC / DEC / DMC = 5/30/30 / 35 81% ────────────────────────────────────

[Measurement of Reduction Potential]
Joint International Meeting of The Electrochemi
cal Society, Inc. and International Society of Ele
This was performed according to the method described in ctrochemistry, Abstracts, P.153 (1997).

10 mg of natural graphite powder was weighed,
To this, polyvinylidene fluoride (binder) was mixed at a ratio of 10% by mass, and further N-methylpyrrolidone was added to form a slurry, and a stainless steel current collector (area: 2 cm)
² ) applied on top. Using this as a working electrode, a three-electrode cell using lithium metal for a counter electrode and a reference electrode was assembled.

Separately, propylene carbonate: ethylene carbonate: dimethyl carbonate in a volume ratio of 1:
A non-aqueous solvent mixed at a ratio of 1: 3 was prepared, and LiPF ₆ as an electrolyte was dissolved in the non-aqueous solvent so as to have a concentration of 1 M to prepare a basic electrolytic solution. Using this basic electrolyte as a standard, the following five electrolytes were prepared. (A) The above basic electrolyte itself (b) A high purity vinylene carbonate added to the basic electrolyte by 5% by mass (c) Monochloroethylene carbonate added to the basic electrolyte by 5% by mass (d) ) A solution obtained by adding 5% by mass of high-purity vinylene carbonate and 0.05% by mass of monochloroethylene carbonate to the basic electrolyte solution. (E) 5% by mass of high-purity vinylene carbonate and 0.1% by mass of the basic electrolyte solution. 25% by mass of monochloroethylene carbonate

Next, the above-mentioned three-electrode cell was filled with the above-mentioned electrolytic solution, and the reduction potential was measured at room temperature at a potential scanning rate of 0.1 mV / sec. FIG. 9 shows the result. From this figure, it can be seen that the peak of the reduction potential of the high-purity vinylene carbonate is less than 1 volt, further less than 0.9 volt, and even less than 0.8 volt, especially 0.7 to less than lithium.
It can be seen that it is in the range of 0.8 volts.

[0068]

The electrolytic solution of the lithium secondary battery of the present invention comprises:
It uses a non-aqueous solvent containing high-purity vinylene carbonate, has a reduction potential of less than 1 volt on the basis of lithium, can use graphite as the negative electrode active material, and has a high Coulomb efficiency. Large electric capacity can be obtained. Furthermore, the lithium secondary battery of the present invention has good cycle characteristics.

[Brief description of the drawings]

FIG. 1 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Example 1.

FIG. 2 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Comparative Example 2.

FIG. 3 is a diagram showing cycle characteristics of respective lithium secondary batteries of Example 1, Comparative Example 2 and Comparative Example 3.

FIG. 4 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Example 2.

FIG. 5 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Comparative Example 4.

FIG. 6 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Comparative Example 6.

FIG. 7 shows Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 6.
FIG. 3 is a diagram showing cycle characteristics of the respective lithium secondary batteries.

FIG. 8 is a diagram showing charge / discharge characteristics of a lithium secondary battery of Example 3.

FIG. 9 is a graph showing measurement results of reduction potentials of various vinylene carbonate-containing electrolyte solutions with respect to a lithium electrode.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Inventor Koji Abe 101978 Kogushi, Oji, Ube City, Yamaguchi Prefecture Inside Ube Chemical Plant, Ube Industries, Ltd. F-term in Kobe Industries Ube Chemical Factory (reference) 5H029 AJ03 AJ04 AJ05 AL03 AL07 AM03 AM07 DJ17 EJ11 HJ01 HJ13 5H050 AA07 AA08 AA09 BA17 CA07 CB08 DA03 DA18 HA01 HA13

Claims (20)

[Claims] An electrolyte is dissolved in a non-aqueous solvent containing a cyclic carbonate, a chain carbonate, and vinylene carbonate, and the reduction potential is 1 based on lithium.
Non-aqueous electrolyte characterized by being less than volts. 2. The method according to claim 1, wherein the reduction potential is based on lithium.
2. The non-aqueous electrolyte according to claim 1, which has a voltage of 0.8 volt or less. 3. The method according to claim 1, wherein the reduction potential is based on lithium.
3. The non-aqueous electrolyte according to claim 2, wherein the non-aqueous electrolyte ranges from 0.7 volts to 0.8 volts. 4. An organic chlorine compound is converted to chlorine atoms in an amount of 10 p.
The non-aqueous electrolyte according to claim 1, wherein the non-aqueous electrolyte is contained in an amount of pm or less. 5. The non-aqueous electrolyte according to claim 4, wherein the content of the organic chlorine compound is 5 ppm or less in terms of chlorine atoms. 6. The non-aqueous electrolyte according to claim 5, wherein the content of the organic chlorine compound is not more than 2.5 ppm in terms of chlorine atoms. 7. The non-aqueous electrolyte according to claim 4, wherein the organic chlorine compound is introduced as an impurity of vinylene carbonate. 8. The content of an organic chlorine compound contained as an impurity in vinylene carbonate introduced into an electrolytic solution is 10% in terms of chlorine atoms with respect to vinylene carbonate.
The non-aqueous electrolyte according to claim 7, wherein the concentration is 0 ppm or less. 9. A non-aqueous electrolyte containing an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms in a non-aqueous electrolyte obtained by dissolving an electrolyte in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate. liquid. 10. The non-aqueous electrolyte according to claim 9, wherein the content of the organic chlorine compound is 5 ppm or less in terms of chlorine atoms. 11. A non-aqueous solvent containing a cyclic carbonate, a chain carbonate and vinylene carbonate, and containing an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms. 12. The non-aqueous solvent according to claim 11, wherein the content of the organic chlorine compound is 5 ppm or less in terms of chlorine atoms. 13. The non-aqueous solvent according to claim 12, wherein the content of the organic chlorine compound is 2.5 ppm or less in terms of chlorine atoms. 14. A positive electrode having a crystal plane spacing (d ₀₀₂ ) of 0.3
In a lithium secondary battery comprising a graphite negative electrode of 4 nm or less, and an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate, the reduction potential of the electrolyte is based on lithium. A lithium secondary battery using an electrolyte solution of less than 1 volt. 15. The lithium secondary battery according to claim 14, wherein the reduction potential of the electrolytic solution is 0.8 volt or less based on lithium. 16. The lithium secondary battery according to claim 15, wherein the reduction potential of the electrolyte is in a range of 0.7 volt to 0.8 volt based on lithium. 17. The electrolyte according to claim 14, wherein the electrolyte contains an organic chlorine compound in an amount of 10 ppm or less in terms of chlorine atoms.
6. The lithium secondary battery according to any one of the items 6. 18. The lithium secondary battery according to claim 17, wherein the content of the organic chlorine compound is 5 ppm or less in terms of chlorine atoms. 19. A positive electrode having a crystal plane spacing (d ₀₀₂ ) of 0.3
In a lithium secondary battery comprising a graphite negative electrode having a diameter of 4 nm or less and an electrolyte in which a non-aqueous solvent containing cyclic carbonate, chain carbonate and vinylene carbonate is dissolved, an organic chlorine compound is contained in the electrolyte in a chlorine atom. A lithium secondary battery contained in an amount of 10 ppm or less in conversion. 20. The lithium secondary battery according to claim 19, wherein the content of the organic chlorine compound in the electrolytic solution is 5 ppm or less in terms of chlorine atoms.