JPH0652896A - Nonaqueous secondary battery - Google Patents

Abstract

PURPOSE:To provide a nonaqueous secondary battery of great current efficiency and a large capacity. CONSTITUTION:In a nonaqueous secondary battery comprising a positive electrode 7, an organic solvent electrolyte and a negative electrode 8, the negative electrode 8 uses as active material a carbonaceous material containing graphite whose carbon network face has a spacing d002 of less than 0.337nm. The organic solvent electrolyte 17 is composed chiefly of propylene carbonate, ethylene carbonate and/or cyclohexyl carbonate and gamma-butyrolactone.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a high capacity non-aqueous secondary battery using an organic solvent as an electrolytic solution.

[0002]

2. Description of the Related Art It is known that high capacity is obtained by using metallic lithium as a negative electrode of a non-aqueous secondary battery using an organic solvent as an electrolytic solution. However, in the metallic lithium negative electrode,
Internal short circuit due to dendritic lithium (lithium dendrite) generated by repeated charging / discharging and reduction in current efficiency are major obstacles to commercialization of high-capacity and long-life batteries. Also, in the case of a battery using metallic lithium, when the battery becomes a high temperature state due to heat generation at the time of a short circuit, because of the high reactivity of metallic lithium, there is a risk of ignition and rupture of the battery can, which is also a great safety point. I have a problem.

For the purpose of improving such drawbacks, as a negative electrode used in a non-aqueous secondary battery using an organic solvent as an electrolytic solution,
Attention is paid to carbonaceous materials. The carbonaceous materials used for the negative electrode are roughly classified into the following three types according to their electrochemical properties. The first group is one in which the distance between carbon mesh planes represented by graphite is narrow (d ₀₀₂ <0.337 nm) and the carbon mesh planes grow in the stacking direction of the carbon mesh planes. It is known that such a carbon material intercalates both cations and anions between the carbon network planes to form an intercalation compound, and is also considered to be used as a conductive material, an organic synthesis reaction catalyst, or a battery. Has been.

The second group has a very large surface area (S _A > 100 m ² / g) typified by activated carbon and a large spacing between carbon mesh planes (d ₀₀₂ > 0.337 nm), and has not progressed crystallization. is there. Since this type has a large surface adsorption amount, the lithium storage amount per carbon is large, but the current efficiency is low and the cycleability is low. In the third group, the carbon net plane has grown to some extent, but the carbon net plane spacing is wider than that in the first group (d ₀₀₂ > 0.337 nm). This group shows various electrochemical characteristics depending on its structure, but unlike the first group, it can occlude lithium with almost no reaction with the electrolytic solution, and is an electrolytic solution conventionally used in metal lithium negative electrodes and lithium alloy negative electrodes. Can be applied. However, the utilization rate (lithium storage amount per carbon) is smaller than that of the first group.

The use of graphite belonging to the first group as the negative electrode of a battery is disclosed in JP-A-57-208079.
JP-A-58-192266, JP-A-59-143
280, JP-A-60-54181, JP-A-60-182670, and JP-A-60-221973.
Japanese Patent Laid-Open No. 61-7567, Japanese Patent Laid-Open No. 1-31
It is proposed in Japanese Patent No. 1565, Japanese Patent Laid-Open No. 4-171677, and the like. In these publications, as organic solvents that can be used, propylene carbonate (hereinafter abbreviated as PC), tetrahydrofuran (hereinafter abbreviated as THF),
γ-butyrolactone (hereinafter abbreviated as γ-BL), 1,
2-dimethoxyethane (hereinafter abbreviated as DME), sulfolane, ethylene carbonate (hereinafter abbreviated as EC)
Etc., an electrolyte system that can be used in conventional lithium batteries is described. As an example, LiClO ₄ or LiBF ₄ is used as an electrolyte, and PC is used as a typical solvent.
Alternatively, THF is used. Examples using a mixed solvent are JP-A-57-208079 and JP-A-4-1-1.
Japanese Patent No. 71677 discloses PC / DME and PC / EC, respectively.

However, when LiClO ₄ or LiBF ₄ was used as the electrolyte and PC was used as the solvent, an attempt was made to charge with graphite (electrochemical lithium intercalation), and the graphite itself expanded due to gas generation due to PC decomposition, I couldn't charge it at all. The PC-based electrolytic solution is an electrolytic solution generally used in a metallic lithium negative electrode, but cannot be used when graphite is used as a negative electrode, which is completely different from the case of a metallic lithium negative electrode.

Further, when LiBF ₄ was used as an electrolyte and a charge / discharge was attempted using a mixed solvent of PC / DME and PC / EC with graphite as an electrode, charge / discharge was possible, but the current efficiency at the beginning of the charge / discharge cycle was increased. Was found to be extremely low and not practical. When intercalating lithium ions as cations, graphite has a high utilization rate (lithium storage amount per carbon) of 16.7%, but when it is intended to be used as the negative electrode of a battery, it is electrochemically charged as described above. Cannot effectively absorb and release lithium.
This is because J. Electrochem. Soc. , 1
17 P. 222 (1970) and JP-A-63-2555.
As described in Comparative Example 1 of the publication, the intercalation compound in which lithium ions are occluded in graphite has high reactivity with an organic solvent, and the reaction with the electrolytic solution has priority over the function as an electrode. The utility value is low.

An example in which graphite is used as a negative electrode is disclosed in US Pat. No. 4,423,125 and J. Ele
ctrochem. Soc. , 137 P.I. 2009
(1990). US Patent No. 44231
In the specification of No. 25, dioxolane is used as the electrolytic solution.
Dioxolane is chemically unstable, and also electrochemically at 3.5 V or more, the electrolytic solution is polymerized and an active material having a high voltage cannot be used for the positive electrode, which is disadvantageous. J. E
retrochem. Soc. , 137 P.I. 200
9 (1990) describes electrochemical lithium intercalation using graphite or petroleum coke as an electrode and a mixed solvent of PC and EC as an electrolytic solution. In petroleum coke, the side reaction that occurs during the initial charge depends on the surface area, whereas in graphite, a side reaction that does not depend on the surface area occurs in addition to the side reaction that depends on the surface area during the initial charge, so that the initial current efficiency is low. Have been described. When a battery is assembled in such a system, a large number of positive electrodes are required because the initial current efficiency is low, the utilization rate of the positive electrode as a battery per active material cannot be increased, and it is difficult to increase the capacity. For this reason, in order to increase the capacity, a method of assembling both the positive electrode and the negative electrode in a charged state (a state in which negative electrode carbon occludes lithium and the positive electrode has an empty site for receiving lithium) may be used. However, since the charged electrode has extremely high reactivity, it poses a safety problem and requires complicated steps such as assembling the battery under an inert gas, which is not practical. In addition, in this system, side reactions continue after 2 cycles and the current efficiency is 1
It states that it will not be 00%. High current efficiency is especially important for battery cycleability. In order to discharge a certain capacity when the current efficiency of the negative electrode is low, the positive electrode always needs to have a charge amount equal to or greater than the discharge capacity, and the positive electrode is gradually overloaded. Bring If the positive electrode is charged with a constant capacity so that the positive electrode does not become overcharged, the current efficiency is low, and the capacity is lowered by repeating the cycle.

In any case, in order to obtain a secondary battery having a high capacity and good cycle characteristics, what is required for the negative electrode is that the electrode is stable during assembly, the current efficiency is high, and the utilization rate is high. In a conventional electrolytic solution system used in a lithium battery having metallic lithium or a lithium alloy as a negative electrode, the carbonaceous materials of the first group react with the electrolytic solution, and thus have not yet been put into practical use. The second group has low current efficiency and low utility value as a negative electrode, and the third group has some good current efficiency, but this is also the utilization rate (per carbon atom). The lithium storage capacity) is about 10%, and a negative electrode material having a higher utilization factor and a higher current efficiency has been desired in order to increase the capacity of the battery.

[0010]

SUMMARY OF THE INVENTION It is an object of the present invention to provide a non-aqueous secondary battery which has a high utilization factor for high capacity and high current efficiency.

[0011]

In order to solve the above-mentioned problems, the inventors of the present invention have diligently studied a combination of a carbonaceous material used for a negative electrode and an organic solvent electrolyte solution, and found that a large amount of lithium ion However, when it is used as a negative electrode of a battery, graphite, which is said to be unable to effectively charge and discharge due to its reaction with an electrolytic solution, is mainly composed of PC, EC and / or cyclohexyl carbonate (hereinafter referred to as CHC). It has been found that the use of an electrolytic solution composed of γ-BL) and γ-BL allows unexpected charging / discharging, has a large capacity for charging / discharging, and has high current efficiency, and has completed the present invention.

That is, according to the present invention, in a non-aqueous secondary battery comprising a chargeable / dischargeable positive electrode, a carbonaceous negative electrode, and an organic solvent-based electrolytic solution, the carbonaceous negative electrode active material is mainly a carbon mesh plane spacing d _002. Is less than 0.337 nm, and the organic solvent-based electrolyte is mainly propylene carbonate, ethylene carbonate and / or cyclohexyl carbonate, and γ-butyrolactone. It is a thing.

The present invention will be described in detail below. In the present invention
The spacing d of the carbon mesh surface₀₀₂Is less than 0.337 nm
Lead is an ordered stack of carbon nets, such as graphite.
It refers to a carbonaceous material that is properly laminated. Carbonaceous material
Depending on the starting material and its treatment (manufacturing) method, various structures may be used.
However, the carbon mesh of all materials is processed by high temperature.
Face spacing d₀₀₂Becomes smaller, and the layer thickness L of the carbon mesh surface is
c tends to be large, and graphite has the smallest interplanar spacing d.
₀₀₂= 0.3354 nm. This d₀₀₂Decrease and
The increase in Lc varies greatly depending on the carbonaceous material, and high temperature treatment
Easy graphitization at (~ 3000 ℃)
Carbon and graphitization are difficult to proceed (d₀₀₂Is small
It is classified as non-graphitizable carbon. This carbonaceous material
When graphitizing the material, d₀₀₂, Lc and dense
Degree, surface area, electric resistance, etc. vary greatly, but
The surface spacing is particularly important for the formation of an object.

The carbonaceous material of the present invention has a d ₀₀₂ of 0.337.
It must be less than nm. d ₀₀₂ is 0.337
If the thickness is more than nm, the current efficiency becomes low and the amount of lithium stored per carbon (utilization rate) becomes low, which is not preferable. In addition, although the current efficiency may be somewhat lowered, the negative electrode of the present invention can also be prepared by using the graphite and another carbonaceous material in combination. Examples thereof include coke, acetylene black, activated carbon, carbide of mesophase microbeads (mesophase microcarbon), needle coke and the like.

D ₀₀₂ used in the present invention is 0.337n
The graphite of less than m may be either artificial graphite or naturally occurring graphite (natural graphite), or may be a mixture of both. Although the artificial graphite is not particularly limited, it is generally 2500 ° C. or higher, using petroleum pitch, coal tar pitch, pyrolytic carbon, mesophase microbeads or mesophase microcarbon, needle coke, condensed polycyclic hydrocarbon, etc. as a starting material.
More preferably, it is obtained by heat treatment at 3000 ° C. or higher.

The lamination thickness Lc of the graphitic carbon mesh surface used in the present invention is not particularly limited, but Lc is also an important parameter for graphitization, and preferably 30
nm or more, more preferably 50 nm or more. Thirty
If it is less than nm, the utilization rate tends to be low. The surface area is also not particularly limited, but when the surface area is large, many side reactions are likely to occur, so it is preferably 50 m ² /
It is preferably g or less, more preferably 20 m ² / g or less.

The shape of the graphite used in the present invention may be powdery or fibrous, and is not particularly limited, but powdery one is preferably used because the packing density is easily increased. The particle size is 0.1 to 100 μm, preferably 1 to 50
A powder having a particle size of μm is preferably used. The electrolytic solution of the present invention includes PC, EC and / or CHC, and γ-
It is essential to contain BL. In a non-aqueous secondary battery in which the graphite used in the present invention is used as the negative electrode active material and a PC single solvent electrolyte containing an electrolyte is used, the PC is decomposed and the graphite expands and falls off from the electrode to charge. I can't. Further, in the γ-BL single solvent type electrolyte solution, although the current efficiency is good, a slight reaction of γ-BL itself occurs when charging and discharging are repeated, and the output characteristics after the long-term charge / discharge cycle deteriorates due to electrolyte solution deterioration. . Further, the EC-only or CHC-only solvent-based electrolytic solution has a high solidifying temperature, becomes solid at around room temperature, and cannot be used at low temperatures.

However, when an electrolytic solution mainly containing PC, EC and / or CHC, and γ-BL is used, decomposition of PC, expansion of graphite, and deterioration of electrolytic solution are suppressed, and the graphite negative electrode is charged. Discharge becomes possible. PC,
The mixing ratio of EC and / or CHC and γ-BL is preferably used within a range satisfying the following conditions (A), (B) and (C).

(A) 0% by volume <PC <80% by volume (B) 0% by volume <EC and / or CHC <90% by volume (C) 10% by volume ≦ γ-BL <95% by volume PC is 80% by volume In the above case, the current efficiency decreases due to the decomposition reaction of PC. EC or CHC 90% by volume
As described above, the battery characteristics at low temperatures deteriorate as described above. Further, when the content of γ-BL is 95% by volume or more, the output characteristics after a long-term charge / discharge cycle deteriorate. When the content of γ-BL is less than 10% by volume, it is difficult to suppress the decomposition reaction of PC, and the current efficiency becomes low. PC, EC and / or CHC,
In addition, the mixing ratio of γ-BL is (A ′), (B ′),
It is more preferably used within the range of satisfying the condition (C ').

(A ') 0% by volume <PC <60% by volume (B') 0% by volume <EC and / or CHC <80
% (C ′) 20% by volume ≦ γ-BL <90% by volume In a non-aqueous secondary battery in which a graphite-based material used in the present invention is used as a negative electrode active material and a PC single solvent electrolyte containing an electrolyte is used, As the graphite decomposes and the graphite expands and falls off from the electrode, it cannot be charged. When EC and / or CHC and γ-BL are added to this PC system, decomposition of PC and expansion deterioration of graphite are suppressed, charge and discharge become possible, and secondary current with high current efficiency and excellent cycleability is provided. It becomes a battery.

Organic solvents other than PC, EC, CHC, γ-BL, such as ethers, ketones, carbonates, nitriles, amides, sulfone compounds, esters, aromatic hydrocarbons, etc. are added in small amounts. You may. Further, these may be used in combination. Among these, ethers, ketones, nitriles, esters and the like are preferable.

As a specific example, dimethoxyethane (DM
E), tetrahydrofuran (THF), 2-methyl-tetrahydrofuran, anisole, 1,4-dioxane,
4-methyl-2-pentanone, cyclohexane, acetonitrile, propionitrile, butyronitrile, butylene carbonate, diethyl carbonate (hereinafter abbreviated as DEC) dimethylformamide, dimethylacetamide, dimethylsulfoxide, sulfolane, methyl formate,
Ethyl formate, methyl acetate, ethyl acetate, propyl acetate,
Examples thereof include ethyl propionate.

The positive electrode to be combined with the negative electrode of the present invention is not particularly limited, but MnO ₂ , MoO ₃ ,
V ₂ O ₅ , V ₆ O ₁₃ , Fe ₂ O ₃ , Fe ₃ O ₄ , lithium-containing transition metal chalcogen compound (Li _(1-X) Co
O ₂ , Li _(1-X) NiO ₂ ), TiS ₂ , MoS ₃ , F
eS ₂ , CuF ₂ , NiF _{2 and} other inorganic compounds, fluorinated carbon, graphite, vapor grown carbon fiber and / or pulverized product thereof, carbon material such as pitch-based carbon fiber and / or pulverized product thereof, polyacetylene, poly- Examples thereof include conductive polymers such as p-phenylene. For a positive electrode not containing lithium, the negative electrode of the present invention can be used by occluding lithium, or the negative electrode of the present invention can be used by bonding a required amount of metallic lithium to form a battery.

However, such a battery is inactive during assembly.
Assembling process such as assembly under gas is required
It becomes complicated. Transition metal chalcogenides containing lithium
When the compound is used, both the positive electrode and the negative electrode have stable discharge in air.
It is possible to assemble the battery in the state, restrictions on processing and assembly
There is little danger of heat generation and explosion due to battery short circuit.
It is not rugged and is preferable for safety. Richiu like this
Examples of the transition metal chalcogen compound containing rum include L
i_(1-X)CoO₂, Li_(1-x)NiO₂, Li _(1-x)C
o_(1-y)Ni_yO₂, LiMn₂O_Four, Li_(1-X)Co
_(1-Y)M_YO₂(M is a transition metal other than Co and Ni, A
l, In, Sn, etc.), Li_(1-X)A _ZCo_(1-Y)
M_YO₂(A is an alkali metal other than Li)
It

The electrolyte used in the present invention is not particularly limited, but LiBF ₄ , LiAsF ₆ , LiPF ₄ are used.
₆ , LiClO ₄ , CF ₃ SO ₃ Li, LiI, LiA
_{lCl 4, NaClO 4, NaBF 4} , NaI, (n-
Bu) ₄ NClO ₄ , (n-Bu) ₄ NBF ₄ , KPF
₆ grade is used. Moreover, you may mix and use these electrolytes. LiBF ₄ is preferable from the viewpoint of battery performance, safety in handling, toxicity and the like.

Further, when an electrode is formed by using the graphite of the present invention, a current collector, a mixture or the like may be used. Cu, Ni or the like is used as the current collector, and Teflon or the like is used as the mixture. Polymers such as polyethylene, nitrile rubber, polybutadiene, butyl rubber, polystyrene, styrene / butadiene rubber, polysulfide rubber, nitrocellulose, cyanoethyl cellulose and acrylonitrile, vinyl fluoride, vinylidene fluoride and chloroprene are used.

As a method for forming this electrode, a method of mixing an electrode active material and an organic polymer and compression molding, a method of dispersing the electrode active material in a solvent solution of the organic polymer, followed by coating and drying, an organic polymer After the electrode active material is dispersed in the combined aqueous or oily dispersion, a method of coating and drying is known, but it is not particularly limited, but it is not preferable if the distribution of the binder becomes nonuniform, and thus it is preferable. Is a method of dispersing an electrode active material in an aqueous or oily dispersion of an organic polymer and then coating and drying, more preferably using a non-fluorine-containing organic polymer containing particles of 0.5 μm or less in the organic polymer. Good.

If necessary, components such as a separator, a terminal and an insulating plate are used as the constituent elements of the battery.

[0029]

The present invention will be described in more detail with reference to the following examples and comparative examples, but the invention is not limited thereto. Further, in Examples 1 to 4 and Comparative Examples 1 to 2, metallic lithium was used as the counter electrode in order to see the performance of the negative electrode alone. In this case, the carbonaceous negative electrode is conventionally the positive electrode, but since it receives lithium ions during the discharge and is reduced, it is called the negative electrode here, and the reduction direction is called charging.

In Tables 1 and 2, the current efficiency is the discharge electricity quantity /
The amount of electricity charged is the amount of electricity discharged / the amount of electricity per weight of the negative electrode active material (12 g is 96485 coulomb), the overvoltage is the value of the voltage drop immediately after the start of discharge, and the number is the number of cycles.

[0031]

Example 1 Graphite [SP10, d manufactured by Nippon Graphite Co., Ltd.]
₀₀₂ = 0.3355 nm, Lc> 100 nm, BET surface area due to N ₂ adsorption = 13 m ² / g] 100 parts by weight, styrene / butadiene latex [L1571 manufactured by Asahi Kasei Corporation] (solid content 48% by weight) 4.17 Parts by weight,
Carboxymethylcellulose (BSH12 manufactured by Dai-ichi Kogyo Seiyaku Co., Ltd.) aqueous solution (solid content 1% by weight) as a thickener 1
30 parts by weight and 30 parts by weight of water were added and mixed to obtain a coating liquid. This coating liquid was applied and dried using a 10 μm Cu foil as a base material to obtain a negative electrode having a thickness of 100 μm and 95 g / m ² .
The negative electrode was peeled off leaving a portion of 1 cm × 1 cm to obtain a working electrode shown in FIG.

On the other hand, as the counter electrode, a SUS net pressure-bonded with metallic lithium was used, and as the reference electrode, metallic lithium was used. The above electrodes are immersed in an electrolyte solution in an atmosphere of Ar gas at 1M
LiBF ₄ was added to PC + EC + γ-BL (volume ratio 20: 2
0:60) The battery of FIG. 1 was assembled using the electrolytic solution dissolved in the mixed solvent. This battery was repeatedly charged with a constant voltage of 1 mA to 10 mV and discharged with a constant current of 1 mA to 1 V. Table 1 shows the current efficiency and utilization rate in the charge / discharge cycle of this battery.

[0033]

Example 2 PC + EC + γ-BL as the solvent of the electrolytic solution
(Volume ratio 20:20:60) PC instead of mixed solvent
The same procedure as in Example 1 was performed except that + EC + γ-BL (volume ratio 10:80:10) was used. The results are shown in Table 1.

[0034]

Example 3 PC + EC + γ-BL as the solvent of the electrolytic solution
(Volume ratio 20:20:60) PC + instead of mixed solvent
The same procedure as in Example 1 was carried out except that CHC + γ-BL (volume ratio 13:13:74) was used. The results are shown in Table 1.

[0035]

[Comparative Example 1] PC + EC + γ-BL as a solvent for the electrolytic solution
(Volume ratio 20:20:60) The same procedure as in Example 1 was repeated except that PC was used instead of the mixed solvent. When energization was started, graphite was expanded and dropped from the Cu foil with the generation of bubbles due to decomposition of PC, and charging could not be performed.

[0036]

[Comparative Example 2] PC + EC + γ-BL as a solvent for the electrolytic solution
(Volume ratio 20:20:60) PC + instead of mixed solvent
The same procedure as in Example 1 was carried out except that EC (volume ratio 50:50) was used. The results are shown in Table 1.

[0037]

Example 4 PC + EC + γ-BL as the solvent of the electrolytic solution
(Volume ratio 20:20:60) PC + instead of mixed solvent
The same procedure as in Example 1 was carried out except that EC + γ-BL (volume ratio 85: 5: 10) was used. The results are shown in Table 1. Example 5 shows an example of a battery in which a lithium-containing chalcogen compound is combined as the positive electrode.

[0038]

Example 5 100 parts by weight of LiCoSn _0.02 O ₂ having a particle size of 3 μ was used as a conductive filler for graphite (Lon.
20 parts by weight of TZ6 (trade name: KS6) and 100 parts by weight of polyvinylidene fluoride 5% by volume dimethylformamide solution as a binder were added and mixed to obtain a coating liquid. 15μ
Using mAl foil as a base material, applying and drying this coating solution to a thickness of 1
A positive electrode of 20 μm and 290 g / m ² was obtained. 1 cm × 1 c of the positive electrode and the negative electrode (95 g / m ² ) obtained in Example 1
It cut out to m and assembled the battery shown in FIG. As the electrolytic solution, 1 M LiBF ₄ was used as PC + EC + γ-B as in Example 1.
An electrolytic solution dissolved in a mixed solvent of L (volume ratio 20:20:60) was used. This battery was repeatedly charged with a constant voltage of 1 mA to 4.2 V and discharged with a constant current of 1 mA to 2.7 V. The results are shown in Table 2.

[0039]

[Comparative Example 3] PC + EC + γ-BL as a solvent for the electrolytic solution
(Volume ratio 20:20:60) γ-B instead of mixed solvent
The same procedure as in Example 5 was carried out except that L single solvent was used. The results are shown in Table 2.

[0040]

[Table 1]

[0041]

[Table 2]

[0042]

INDUSTRIAL APPLICABILITY The non-aqueous secondary battery of the present invention has a large current efficiency and a large utilization rate, and is useful because it can have a high capacity.

[Brief description of drawings]

FIG. 1 is a cross-sectional view showing a configuration example of a battery of the present invention.

FIG. 2 is a cross-sectional view showing a configuration example of a battery of the present invention.

[Explanation of symbols]

 1. Working electrode (carbonaceous negative electrode) 2. Counter electrode (lithium metal) 3. Reference electrode (lithium metal) 4. Electrolyte solution 5. Glass container 6. Ar gas 7. Positive electrode 8. Negative electrode 9. Current collector 10. Current collector 11. SUS Net 12. SUS net 13 External electrode terminal 14 External electrode terminal 15 Battery case 16 Separator 17 Electrolyte

Claims (1)

[Claims] 1. In a non-aqueous secondary battery comprising a chargeable / dischargeable positive electrode, a carbonaceous negative electrode, and an organic solvent-based electrolytic solution, the active material of the carbonaceous negative electrode is mainly a carbon mesh plane spacing d _002.
Is less than 0.337 nm, and the organic solvent electrolyte is mainly propylene carbonate, ethylene carbonate and / or cyclohexyl carbonate, and γ-butyrolactone.