JP4470902B2 - Non-aqueous electrolyte and lithium secondary battery - Google Patents

Description

  The present invention relates to a non-aqueous electrolyte and a lithium secondary battery using the non-aqueous electrolyte. The present invention particularly relates to a lithium secondary battery excellent in electric capacity, cycle characteristics, and storage characteristics, and a non-aqueous electrolyte and a non-aqueous solvent that can be advantageously used in the production of such a lithium secondary battery. .

  In recent years, small electronic devices such as personal computers, mobile phones, and camera-integrated video cameras have been widely used, and a small, lightweight and high-capacity secondary battery is strongly demanded as a power source for driving these small electronic devices. Yes. From the viewpoint of a compact, lightweight and high-capacity secondary battery, a carbon material that can be doped / undoped with lithium ions using a composite oxide such as lithium cobaltate, lithium nickelate, and lithium manganate as the positive electrode active material Lithium secondary batteries using a non-aqueous electrolyte in which a lithium salt is dissolved in a non-aqueous solvent composed of a cyclic carbonate and a chain carbonate are suitable for use as a negative electrode active material. Research and development is actively underway.

  Among carbon materials that can be doped / undoped with lithium ions, graphite, in particular, has preferable characteristics such as high electric capacity and high potential flatness, and therefore, a negative electrode active material for lithium secondary batteries. It is regarded as one of the most suitable compounds and is frequently used.

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

  Addition of various compounds has been proposed as a method for suppressing the decomposition of the cyclic carbonate in the electrolytic solution by the graphite-based negative electrode active material and the peeling of the carbon material. For example, Non-Patent Document 1 discloses that the decomposition of the electrolytic solution is suppressed by adding a crown ether compound (12-crown-4) to the electrolytic solution containing propylene carbonate and ethylene carbonate as main components. Has been proposed. However, in this case, if a considerable amount of expensive crown ether is not added, the effect of suppressing decomposition is small, and the battery characteristics to be achieved are still not sufficient.

  Patent Document 1 and Patent Document 2 further include at least one mixture of at least two aprotic organic solvents including a first solvent having a high dielectric constant and a second solvent having a low viscosity. At least one of the above-mentioned solvents, which can be reduced at an anode made of a carbon material having a crystallinity higher than 0.8 at a potential higher than lithium by 1 volt to form a passivating layer. An electrolyte solution for a lithium battery is described in which a lithium salt is dissolved in a solvent system further containing the same type of soluble solvent. And it is described that decomposition | disassembly of electrolyte solution can be suppressed by addition of the soluble solvent which shows said high reduction potential. In this method, the additive is reduced at the negative electrode during charging to form a passive film on the graphite surface, thereby suppressing the reduction of other solvents.

  However, according to the research of the present inventors, in the above-described method, the initial coulomb (charge / discharge) efficiency is not necessarily high, and the electric capacity gradually decreases by repeating charge / discharge, and satisfactory cycle characteristics are obtained. It is difficult to obtain storage stability.

Non-Patent Document 2 includes 5% by volume of vinylene carbonate (VC), 1M LiPF ₆ electrolyte, and a volume ratio of PC / EC / DMC (DMC: dimethyl carbonate) is 1/3. In a voltammogram measurement using a cell made of an electrolyte and made of graphite electrode (working electrode) / Li (counter electrode) / Li (reference electrode), a reduction peak appears at 1 volt, which forms a passive film on the negative electrode. It has been reported to suppress the reduction of other solvents.

Furthermore, Non-Patent Document 3 states that the PC decomposition on the surface of the graphite electrode is suppressed by adding chloroethylene carbonate to the electrolytic solution. This is thought to be due to the fact that the decomposition product of chloroethylene carbonate forms a passive film on the graphite surface, but the inhibitory effect on the decomposition of the electrolytic solution is not necessarily good.
J. Electrochem. Soc., Vol.140, No.6, L101 (1993) 1997 Joint International Meeting of The Electrochemical Society, Inc. and International Society of Electrochemistry, Abstracts, P.153 (1997) J. Electrochem. Soc., Vol. 140, No. 9, L161 (1995) JP-A-8-45545 US Pat. No. 5,626,981

  By the above various methods, it has become possible to share a cyclic carbonate, which is an excellent nonaqueous solvent, with a highly crystalline carbon negative electrode such as graphite (graphite), but also by using these nonaqueous solvent systems. However, it is not possible to obtain a lithium secondary battery exhibiting sufficiently satisfactory battery characteristics.

  The inventor of the present invention pays attention to a non-aqueous solvent composition containing a cyclic carbonate, a chain carbonate, and vinylene carbonate, which exhibits excellent characteristics as a non-aqueous solvent for an electrolytic solution, and particularly a non-aqueous secondary battery using vinylene carbonate (VC). Intensive research was conducted on the effect of inhibiting the decomposition of the electrolyte on the graphite electrode surface. As a result, when using vinylene carbonate synthesized by a conventional vinylene carbonate synthesis method as a vinylene carbonate in a lithium secondary battery, sufficiently satisfactory battery characteristics cannot be obtained, and the battery characteristics also vary. It was found that can be seen. As a result of further investigation, the vinylene carbonate produced by the conventional method contains a considerable amount of an organic chlorine compound as a by-product during the production of the vinylene carbonate, and the vinylene carbonate has a non-aqueous solvent composition. When introduced into a product, these organochlorine compounds are mixed into the non-aqueous solvent composition to increase the reduction potential of the non-aqueous electrolyte prepared using the non-aqueous solvent composition, It was found that the battery characteristics varied.

The present invention relates to a nonaqueous electrolytic solution in which an electrolyte is dissolved in a nonaqueous solvent containing cyclic carbonate, chain carbonate, and vinylene carbonate in an amount in the range of 0.1 to 5 % by mass, and an organic chlorine compound is chlorine. A crystal plane as a negative electrode, characterized in that it does not contain more than 2.5 ppm in terms of atoms, and the reduction potential is in the range of 0.7 to 0.8 volts, based on lithium It exists in the nonaqueous electrolyte for lithium secondary batteries containing the graphite negative electrode whose space | interval ( _d002 ) is 0.34 nm or less .

The present invention further includes a non-aqueous solvent comprising a positive electrode, a graphite negative electrode having a crystal plane spacing (d ₀₀₂ ) of 0.34 nm or less, and cyclic carbonate, chain carbonate, and vinylene carbonate in an amount in the range of 0.1 to 5 % by mass. In the lithium secondary battery comprising the electrolyte solution in which the electrolyte is dissolved, the electrolyte solution does not contain an organic chlorine compound in an amount exceeding 2.5 ppm in terms of chlorine atoms, and the reduction potential is lithium. There is also a lithium secondary battery characterized by using an electrolyte solution in a range of 0.7 volts to 0.8 volts with reference to.

The present invention is particularly characterized by the characteristics or composition of the non-aqueous electrolyte or non-aqueous solvent, and preferred embodiments thereof are as follows.
( 1 ) An organic chlorine compound in a non-aqueous electrolyte is introduced as an impurity of vinylene carbonate.
( 2 ) The content of the organochlorine compound contained as an impurity in the vinylene carbonate introduced into the non-aqueous electrolyte is 100 ppm or less in terms of chlorine atoms with respect to vinylene carbonate.

  Although the reduction potential due to the reduction 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 are not clear, it is estimated as follows.

  The vinylene carbonate (VC) product produced by a conventionally used synthesis method contains a plurality of organochlorine compounds as represented by the following chemical formula of at least about 3000 ppm. When about 1 to 10% by mass, which is a normal use amount, is introduced into the nonaqueous solvent of the electrolytic solution, the content of the organic chlorine compound in the nonaqueous solvent is about 30 to 300 ppm in terms of chlorine atoms. .

  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 prior to the reduction of vinylene carbonate or the electrolyte composition to form a film. It has the effect of suppressing the decomposition of vinylene carbonate and electrolyte.

  However, the film formed on the graphite surface of the negative electrode in this way contains chlorine, and since the film becomes thick, it is presumed that it does not exhibit a sufficiently satisfactory electrolyte decomposition inhibiting effect. That is, it is presumed that the organic chlorine compound contained as an impurity in vinylene carbonate inhibits the original battery performance improvement of vinylene carbonate and does not give a sufficient effect.

Thus, as a result of intensive studies on the synthesis and purification methods of vinylene carbonate, the present inventor has developed a method for producing high-purity vinylene carbonate with a very low content of organochlorine compounds. That is, as a conventional vinylene carbonate synthesis method, as described in J. Am. Chem. Soc., 75 , 1263 (1953), monochloroethylene carbonate is obtained by chlorination reaction of ethylene carbonate (EC). A method of producing vinylene carbonate by synthesizing (first step) and performing a dehydrochlorination reaction (second step) with an amine in an ether-based low boiling point solvent 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. Developed a method. It was confirmed that the lithium secondary battery using the electrolytic solution containing this high-purity vinylene carbonate as an additive exhibits extremely excellent electric capacity, cycle characteristics and storage characteristics.

The electrolyte solution of the lithium secondary battery of the present invention uses a non-aqueous solvent containing high-purity vinylene carbonate with a small amount of mixed organic chlorine compounds, and the reduction potential is 0.7 volts to 0.8 as a lithium standard. In the range of bolts, graphite can be used as the negative electrode active material, and a large electric capacity can be obtained as can be seen from high Coulomb efficiency. Furthermore, the lithium secondary battery of the present invention has good cycle characteristics.

The content of vinylene carbonate (VC) in the non-aqueous solvent used in the non-aqueous electrolyte of the present invention is 0 . It is preferable that it exists in the range of 1 mass%-5 mass%. When the content of vinylene carbonate is excessively small, the electrolytic solution is easily decomposed at the graphite negative electrode, and when it is excessively large, battery characteristics are deteriorated. In addition, the content of the organic chlorine compound in the vinylene carbonate product used for the preparation of the electrolyte solution for a lithium secondary battery of the present invention is preferably 100 ppm or less, particularly 50 ppm or less as a chlorine content converted value.

  As the cyclic carbonate, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and the like are used, and these may be used alone or as a mixture of two or more. As the chain carbonate, dimethyl carbonate (DMC), methyl ethyl carbonate (MEC), diethyl carbonate (DEC), methyl propyl carbonate (MPC), methyl butyl carbonate (MBC) and the like are used. It is used also as a mixture of the above. The ratio between the cyclic carbonate and the chain carbonate is preferably in the range of 2: 8 to 6: 4 by volume ratio.

The electrolytes used in the non-aqueous electrolyte of the present invention include LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂ CF ₃ ) Lithium salt such as ₃ . These electrolytes may be used alone or in combination of two or more. These electrolytes are usually used after being dissolved in the non-aqueous solvent at a concentration of 0.1 to 3M, preferably 0.5 to 1.5M.

  The nonaqueous electrolytic solution of the present invention can be obtained, for example, by mixing a cyclic carbonate, a chain carbonate and the high-purity vinylene carbonate to prepare a nonaqueous solvent, and dissolving the electrolyte therein.

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

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

  For the positive electrode, the positive electrode active material is kneaded with 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 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 was kneaded with a binder such as PVDF, PTFE, ethylene propylene diene monomer (EPDM), and an N-methylpyrrolidone solvent to form a negative electrode mixture. It is applied to a current collector such as a sheet made, dried at 50 to 250 ° C., and then compression molded.

  The configuration form of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a positive electrode, a negative electrode, a porous membrane separator, and a coin battery, a cylindrical battery, a square battery, and a stacked battery having an electrolyte solution. It is done.

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

(1) Aldrich vinylene carbonate
Vinylene carbonate commercially available from Aldrich Chemical Company. Inc. was used. Hereinafter, this vinylene carbonate is referred to as “Aldrich vinylene carbonate”.

(2) Synthesis of vinylene carbonate by known methods
Synthesized according to the methods of J. Am. Chem. Soc., 75, 1263 (1953) and J. Am. Chem. Soc., 77, 3789 (1955). That is, the reaction was carried out at 65 ° C. for 24 hours under ultraviolet irradiation while blowing chlorine gas into 600 g of ethylene carbonate that had been purified by distillation in advance. 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 over 6 hours under reflux, and stirring was continued for 14 hours while further refluxing. Thereafter, 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 excess amine, and then further subjected to simple distillation under reduced pressure of 30 mmHg to fractionate 290 g of vinylene carbonate fraction. This vinylene carbonate was further subjected to precision distillation 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 “conventional synthetic vinylene carbonate”.

(3) Synthesis of high-purity vinylene carbonate First, monochloroethylene carbonate was synthesized by the method (2). 494 g of the obtained monochloroethylene carbonate was dissolved in 500 mL of dibutyl carbonate and charged into a 2 L reactor, and 440 g of triethylamine was added dropwise at 50 ° C. over 6 hours, followed by further stirring for 14 hours. It was. Thereafter, the reaction solution was cooled to room temperature, and triethylamine hydrochloride was filtered and thoroughly washed with dibutyl carbonate. A simple distillation was performed on 2100 g of the obtained filtrate under a reduced pressure of 30 mmHg to distill off excess triethylamine, and 390 g of vinylene carbonate fraction was fractionated. After treating this vinylene carbonate with a silica gel column, precision distillation was performed under a reduced pressure of 30 mmHg to obtain 195 g of vinylene carbonate having a boiling point of 73 ° C. with very few impurities. The obtained vinylene carbonate is referred to as “high purity vinylene carbonate”.

[Gas chromatography mass spectrometry of vinylene carbonate]
Gas chromatographic analysis of Aldrich vinylene carbonate and conventional synthetic vinylene carbonate detected a small amount of many types of impurities. It was confirmed that the chlorinated compounds estimated to be chlorinated compounds were included. However, almost no impurities were observed in the high-purity vinylene carbonate, and no chlorine compound presumed to be the chlorine compound was detected.

[Chlorine content in vinylene carbonate]
The vinylene carbonate is subjected to oxyhydrogen flame combustion treatment, gas is absorbed in water, and chlorine ions in the absorption liquid are measured by an ion chromatograph. Table 1 shows the results. Aldrich vinylene carbonate contained 3200 ppm as chlorine and 3550 ppm in conventional synthetic vinylene carbonate, but the content in high-purity vinylene carbonate was 29 ppm, which was very low.

Table 1
────────────────────────────────────
Vinylene carbonate used Chlorine content ─────────────────────────────────────
Aldrich vinylene carbonate 3200ppm
Conventional synthetic vinylene carbonate 3550ppm
High purity vinylene carbonate 29ppm
────────────────────────────────────

[Example 1]
(Preparation of electrolyte)
A non-aqueous solvent is prepared by adding a high-purity vinylene carbonate to a mixed solvent of propylene carbonate (PC) and dimethyl carbonate (DMC) in a volume ratio of 1: 2 so as to be 2% by mass, and LiPF ₆ Was dissolved to a concentration of 1M to prepare an electrolytic solution.

[Production of lithium secondary battery and measurement of battery characteristics]
80% by mass of LiCoO ₂ (positive electrode active material), 10% by mass of acetylene black (conductive agent), and 10% by mass of polyvinylidene fluoride (binder) are mixed, and this is diluted with N-methylpyrrolidone. Thus, a positive electrode mixture was prepared. This mixture was applied to an aluminum foil current collector, dried and compression molded to obtain a positive electrode. On the other hand, 90% by mass of natural graphite (d ₀₀₂ = 0.3354) 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 be approximately the same electric capacity.

  A coin-type battery (diameter 20 mm, thickness 3.2 mm) composed of these positive and negative electrodes, a separator made of a polypropylene microporous film, and an electrolyte solution was prepared, and a constant current of 0.8 mA was obtained at room temperature (25 ° C.). After charging for 5 hours at a current and constant voltage to a voltage of 4.2 V, the battery was discharged to a voltage of 2.7 V at a constant current of 0.8 mA. FIG. 1 shows the initial charge / discharge characteristics in a graph in which the vertical axis represents the battery voltage (V) and the horizontal axis represents the capacity (mAh / g carbon). Furthermore, charge / discharge was repeated and the cycle change of the discharge capacity was also examined.

[Comparative Example 1]
A secondary battery was prepared and charged / discharged in the same manner as in Example 1 except that vinylene carbonate was not added. However, at the first charge, the propylene carbonate decomposed and did not reach a predetermined voltage, and thus could not be discharged. As a result of disassembling the battery after charging and discharging, exfoliation of graphite on the negative electrode was observed.

[Comparative Example 2]
A secondary battery was prepared in the same manner as in Example 1 except that Aldrich vinylene carbonate was used instead of high-purity vinylene carbonate, and a charge / discharge test was performed. The initial charge / discharge characteristics are shown in FIG.

[Comparative Example 3]
A secondary battery was produced in the same manner as in Example 1 except that conventional synthetic vinylene carbonate was used instead of high-purity vinylene carbonate, and a charge / discharge test was performed.

  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.

Table 2
(Electrolytic solution non-aqueous solvent: PC / DMC = 1/2 (volume ratio) + vinylene carbonate)
────────────────────────────────────
Vinylene carbonate Coulomb efficiency ─────────────────────────────────────
Example 1 High purity vinylene carbonate (2% by mass) 78%
────────────────────────────────────
Comparative example 1 No addition Comparative example 2 incapable of charging / discharging Aldrich vinylene carbonate (2% by mass) 73%
Comparative Example 3 Conventional synthetic 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 vertical axis representing discharge capacity (mAh) and the horizontal axis representing the number of cycles.

  As can be seen from the graph of FIG. 3, when compared to the secondary battery of Comparative Example 2 to which Aldrich vinylene carbonate was added and the secondary battery of Comparative Example 3 to which conventional synthetic vinylene carbonate was added, high-purity vinylene carbonate was added. In the secondary battery of Example 1, good cycle characteristics are maintained.

[Example 2]
A mixed solvent of ethylene carbonate (EC) and dimethyl carbonate (DMC) having a volume ratio of 1: 1 was used instead of a mixed solvent of propylene carbonate (PC) and dimethyl carbonate (DMC) having a volume ratio of 1: 2. A battery was produced in the same manner as in Example 1, and a charge / discharge test was performed. FIG. 4 shows an initial charge / discharge curve.

[Comparative Example 4]
A battery was prepared and a charge / discharge test was conducted 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 charged and discharged in the same manner as in Example 2 except that Aldrich vinylene carbonate was used instead of high-purity vinylene carbonate.

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

[Example 3]
The volume ratio of propylene carbonate (PC), ethylene carbonate (EC) and dimethyl carbonate (DMC) was changed to 1: 1: 2 instead of the volume ratio of 1: 2 of propylene carbonate (PC) and dimethyl carbonate (DMC). A battery was prepared in the same manner as in Example 1 and a charge / discharge test was conducted.

  Table 3 shows 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.

Table 3
(Electrolytic solution non-aqueous solvent: EC / DMC = 1/1 (volume ratio) + vinylene carbonate)
────────────────────────────────────
Vinylene carbonate Coulomb efficiency ─────────────────────────────────────
Example 2 High purity vinylene carbonate (2% by mass) 79%
────────────────────────────────────
Comparative Example 4 72% not added
Comparative Example 5 Aldrich vinylene carbonate (2% by mass) 75%
Comparative Example 6 Conventional synthetic vinylene carbonate (2% by mass) 74%
────────────────────────────────────
Example 3 High purity vinylene carbonate (2% by mass) _* 80%
────────────────────────────────────
Note: The electrolyte nonaqueous solvent of Example 3 is 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, 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. 7, the secondary battery of Comparative Example 4 to which no vinylene carbonate was added, the secondary battery of Comparative Example 5 to which vinylidene carbonate made by Aldrich was added, and the comparative battery of Comparative Example 6 to which conventional synthetic vinylene carbonate was added. Compared to the secondary battery, the secondary battery of Example 2 to which high-purity vinylene carbonate was added maintained good cycle characteristics.

  FIG. 8 shows the cycle characteristics of the secondary battery of Example 3 (in which the non-aqueous solvent system was changed), with the vertical axis representing the discharge capacity (mAh) and the horizontal axis representing the number of cycles. It can be seen that good cycle characteristics are maintained.

[Examples 4 to 6]
A battery was prepared and a charge / discharge test was conducted in the same manner as in Example 1 except that the volume ratio of the electrolyte solvent was changed as shown in Table 4. The initial coulomb efficiency is shown in Table 4. Moreover, it turned out that it has favorable cycling characteristics similarly to Example 1.

Table 4
(Electrolytic solution non-aqueous solvent: basic solvent composition + high-purity vinylene carbonate 2% by mass)
────────────────────────────────────
Basic solvent composition Coulomb efficiency ────────────────────────────────────
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]
The reduction potential was measured according to the method described in 1997 Joint International Meeting of The Electrochemical Society, Inc. and International Society of Electrochemistry, Abstracts, P.153 (1997).

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

Separately, a nonaqueous solvent in which propylene carbonate: ethylene carbonate: dimethyl carbonate is mixed at a volume ratio of 1: 1: 3 is prepared, and LiPF ₆ is dissolved as an electrolyte to a concentration of 1M to obtain a basic electrolyte solution. Prepared. Using this basic electrolytic solution as a standard, the following five types of electrolytic solutions were prepared.

(A) The basic electrolyte itself (b) 5% by mass of high-purity vinylene carbonate added to the basic electrolyte (c) 5% by mass of monochloroethylene carbonate added to the basic electrolyte (d ) 5% by mass of high purity vinylene carbonate and 0.05% by mass of monochloroethylene carbonate with respect to the basic electrolyte (e) 5% by mass of high purity vinylene carbonate and 0. 25% by weight monochloroethylene carbonate added

Subsequently, the above-mentioned electrolyte solution was filled in the triode cell, and the reduction potential was measured at room temperature at a potential scanning rate of 0.1 mV / sec. The result is shown in FIG. From this figure, the peak of reduction potential of High purity vinylene carbonate is to lithium 0. It can be seen that it is in the range of 7 to 0.8 volts.

FIG. 3 is a diagram showing charge / discharge characteristics of the lithium secondary battery of Example 1. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery of the comparative example 2. 6 is a diagram showing cycle characteristics of lithium secondary batteries of Example 1, Comparative Example 2, and Comparative Example 3. FIG. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery of Example 2. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery of the comparative example 4. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery of the comparative example 6. 6 is a diagram showing cycle characteristics of lithium secondary batteries of Example 2, Comparative Example 4, Comparative Example 5, and Comparative Example 6. FIG. It is a figure which shows the charging / discharging characteristic of the lithium secondary battery of Example 3. It is a figure which shows the measurement result of the reduction potential with respect to the lithium electrode of various vinylene carbonate containing electrolyte solution.

Claims (2)

Cyclic carbonate, a non-aqueous electrolyte solution of an electrolyte is dissolved in a nonaqueous solvent containing an amount of vinylene carbonate a chain carbonate and 0.1 to 5 wt% range, 2 organochlorine compound with chlorine atoms in terms of without containing in an amount of more than .5 ppm, and reduction potential, relative to the lithium, which lies in the range of 0.7 volts to 0.8 volts, lattice spacing as the negative electrode (d ₀₀₂ ) Is a non-aqueous electrolyte for a lithium secondary battery including a graphite negative electrode having a size of 0.34 nm or less . The electrolyte is dissolved in a positive electrode, a graphite negative electrode having a crystal plane spacing (d ₀₀₂ ) of 0.34 nm or less, and a non-aqueous solvent containing cyclic carbonate, chain carbonate, and vinylene carbonate in an amount ranging from 0.1 to 5 % by mass. In the lithium secondary battery including the electrolyte solution, the electrolyte solution contains no organic chlorine compound in an amount exceeding 2.5 ppm in terms of chlorine atoms, and the reduction potential is 0 based on lithium. A lithium secondary battery using an electrolyte solution in a range of .7 volts to 0.8 volts.

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