JPH1116579A - Battery - Google Patents

Abstract

(57) [Problem] To provide a battery having flame retardancy and maintaining good battery performance for a long period of time. A polymer containing polyacrylonitrile materials, gamma - made of an electrolyte salt and a non-aqueous solvent and LiPF ₆ containing butyrolactone as a main component, gamma - butyrolactone gel electrolyte content is made less 30 mol% of Is used as an electrolyte material of a battery.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

[0001] The present invention relates to a battery using a gel electrolyte.

[0002]

2. Description of the Related Art A lithium battery comprising a positive electrode and a negative electrode capable of inserting and extracting lithium, and a non-aqueous electrolyte,
Compared with aqueous secondary batteries such as lead batteries and nickel cadmium batteries, they have attracted attention in recent years because of their higher energy density and lower self-discharge rate.

[0003] In order to further improve the battery performance of such a lithium secondary battery, it is of course important to select an electrode material, but the characteristics of an electrolytic solution that plays a role in ionic conduction between the two electrodes greatly affect the battery characteristics. Come. For this reason, the electrolyte is required to have high ionic conductivity as much as possible and electrochemical stability that can withstand a high voltage, and many non-aqueous electrolytes have been proposed from such a point.

For example, as a non-aqueous solvent for an electrolytic solution, cyclic or chain ester organic molecules are often used. Specifically, propylene carbonate (PC), ethylene carbonate (EC), methyl ethyl carbonate (MEC),
Dimethyl carbonate (DMC), gamma-butyrolactone (GBL), 1,2-dimethoxyethane (DM
E), methyl propionate, butyl propionate and the like are typical.

As electrolyte salts, LiPF ₆ , L
iClO ₄ , LiBF ₄ , LiCF ₃ SO ₃ , LiAs
F ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₂ SO ₂ ) ₃
Etc. are known. All of these electrolyte salts show high ionic conductivity of several mS / cm in an appropriate concentration range.

However, the above non-aqueous electrolyte composed of a non-aqueous solvent and an electrolyte salt has a relatively low flash point, so that if the battery is accidentally thrown into a fire, There is a danger that the non-aqueous solvent leaking from the inside will become gaseous and ignite.

Therefore, for example, Japanese Patent Application Laid-Open No. 4-184870 proposes to add a flame retardant such as a phosphoric acid ester to a non-aqueous electrolyte as a method for preventing such ignition. This flame retardant is known as a flame retardant for polymer materials, and its effect is proportional to the amount added.

As other flame retardants, halogen compounds are generally known.

[0009]

However, phosphate-based compounds and halogen-based compounds are not necessarily electrochemically stable substances, and are liable to undergo chemical transformation and decomposition in a high voltage region of 3 V or more. As a result, there is a concern that the battery performance may be degraded. In particular, when the added amount of the flame retardant is increased to obtain a large flame retardant effect, the battery performance is significantly impaired.

The present invention has been proposed in view of such a conventional situation, and has as its object to provide a battery having flame retardancy and maintaining good battery performance for a long period of time. I do.

[0011]

In order to achieve the above object, a battery of the present invention comprises a positive electrode, a negative electrode, a polymer material containing polyacrylonitrile, a non-aqueous solvent containing gamma-butyrolactone, and LiPF _6. And a gel electrolyte composed of an electrolyte salt mainly composed of gamma-butyrolactone in the gel electrolyte.
1% or less. Note that the molar fraction referred to here is 10 times the sum of the number of repeating unit structures of the polymer material and the number of moles of the nonaqueous solvent and the electrolyte salt.
This is the value when 0 mol% is set.

A polymer material containing polyacrylonitrile,
When a gel electrolyte made of a non-aqueous solvent and an electrolyte salt mainly composed of LiPF ₆ is contacted with a flame, a carbonized film is formed on the surface by the carbonizing action of the polymer material and the carbonizing promoting action of LiPF ₆ . This carbonized film prevents ignition and provides excellent flame retardancy.

[0013] The gel electrolyte is mixed with a non-aqueous solvent.
By containing gamma-butyrolactone at a ratio of 0 mol% or less, high ionic conductivity is obtained,
Moreover, it has high chemical stability. For this reason, even if it is in contact with a highly reactive electrode material such as lithium metal, a modification reaction is prevented.

Therefore, a battery using such a gel electrolyte has a low risk of ignition even when the battery is accidentally thrown into a fire. In addition, since the gel electrolyte itself has excellent flame retardancy, it is not necessary to include a general-purpose flame retardant having poor electrochemical stability, and deterioration of battery performance due to the flame retardant can be avoided. Furthermore, since a denaturing reaction hardly occurs in the gel electrolyte, good battery performance is maintained even after long-term storage.

[0015]

DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, specific embodiments of the present invention will be described.

[0016] The battery of the present invention comprises a positive electrode, a negative electrode, and a gel electrolyte.

The gel electrolyte comprises a polymer material, a non-aqueous solvent and an electrolyte salt.

As the polymer material, a polymer containing acrylonitrile in the repeating unit structure is used. This repeating unit structure may be composed of only acrylonitrile, or may be a copolymer with another monomer.
As monomers to be copolymerized, acrylic acid, methacrylic acid, vinyl acetate, itaconic acid, hydrogenated methyl acrylate, hydrogenated ethyl acrylate, acrylamide,
Examples thereof include vinyl chloride, vinylidene fluoride, and vinylidene chloride. Specific examples of the copolymer, acrylonitrile butadiene rubber, acrylonitrile butadiene styrene resin, acrylonitrile chloride polyethylene propylene diene styrene resin, acrylonitrile vinyl chloride resin,
There are acrylonitrile methacrylate resin, acrylonitrile acrylate resin, and the like, and among these, those which can be gelled by combination with a solvent or an electrolyte salt may be selected and used.

The degree of gelation of the gel electrolyte is closely related to the molecular weight of the polymer material, and the number average molecular weight of the polymer material is 5,000 to obtain an appropriate degree of gelation.
It is desirable to be about 500,000.

The solubility parameter of the polymer material is preferably from 8 to 15 (c) from the viewpoint of obtaining a flame retardant effect and compatibility with other components.
al / cm ² ) ^1/2 . The solubility parameter was determined by POLYMER HAN.
D BOOK THIRD EDITION J. Br
andrup. and EH. IMMERGUT W
ILEY INTERSCIENCE. 1989 (P.
IV-340 to 341).

The amount of this polymer material in the gel electrolyte is preferably 5 to 5 in terms of the flame retardancy and compatibility.
The amount is preferably 30 mol%, and further considering the ease of handling after gel molding, 5 to 15 mol%
Preferably, the amount is such that Here, the molar fraction is a value when the total of the number of repeating unit structures of the polymer material and the number of moles of the nonaqueous solvent and the electrolyte salt is 100 mol%.

Next, as the non-aqueous solvent, a solvent containing gamma-butyrolactone is used here. For example, a solvent in a solid state at room temperature such as ethylene carbonate is used in combination with a low melting point solvent having a relatively low melting point, and gamma-butyrolactone is suitable as such a solvent. By using gamma-butyrolactone, the temperature characteristics of the gel electrolyte can be improved and a high ionic conductivity can be obtained. Further, when gamma-butyrolactone is used, a gel electrolyte having excellent chemical stability is obtained as compared with the case of using other low-melting solvents, so that a denaturation reaction of the gel due to contact with the electrode material is prevented. Become. However, if the amount of gamma-butyrolactone is too large, the chemical stability of the gel electrolyte becomes low, and the charge transfer resistance of the gel electrolyte may increase due to denaturation. It is desirable.

This gamma-butyrolactone is desirably used as a mixture with a high dielectric constant solvent having a high dielectric constant. Examples of such a high dielectric constant solvent include ethylene carbonate and propylene carbonate. It is particularly preferable to use ethylene carbonate because it has high stability and is advantageous in suppressing denaturation of the gel electrolyte. Specific combinations include a mixed system of gamma-butyrolactone and ethylene carbonate, and a mixed system of gamma-butyrolactone, ethylene carbonate and polycarbonate.

As the electrolyte salt, LiPF ₆ is used here because it promotes carbonization of the polymer material when the gel comes into contact with the flame and contributes to the improvement of flame retardancy. Note that Li
PF ₆ may be used alone or as a mixture with another electrolyte salt. The electrolyte salt to be mixed is LiN (CF ₃ S
O ₂ ) ₂ , LiC (CF ₂ SO ₂ ) ₃ , LiBF ₄ , LiAs
F ₆ , LiCF ₃ SO ₃ , LiClO ₄ , NaClO _4, etc. can be used. Among them, LiN (CF ₃ SO ₂ ) ₂ is LiPF ₆
Next, the effect of imparting flame retardancy is high, which is preferable.

The amount of the electrolyte salt in the gel electrolyte is desirably selected so that the concentration of lithium ions is 3 to 9 mol%. If the concentration of lithium ions is less than 3 mol%, sufficient ion conductivity cannot be obtained, which may cause an increase in polarization during electrode reaction. When the concentration of lithium ions exceeds 9 mol%, it becomes difficult to dissolve the polymer material. Further, in consideration of the salt concentration dependence of the ionic conductivity, the lithium ion concentration is preferably in the range of 4 to 9 mol% in order to secure sufficient ionic conductivity.

When the gel electrolyte made of the above materials is brought into contact with a flame, a carbonized film is formed on the surface by the carbonizing action of the polymer material and the carbonizing promoting action of LiPF ₆ . This carbonized film prevents ignition and provides excellent flame retardancy.

The gel electrolyte has high ionic conductivity and high chemical stability. For this reason, even if it is in contact with a highly reactive electrode material such as lithium metal, a modification reaction is prevented.

Such a gel electrolyte is produced, for example, as follows.

First, a non-aqueous electrolyte is prepared by dissolving an electrolyte salt in a non-aqueous solvent. Subsequently, this non-aqueous electrolyte is heated,
A polymer material containing polyacrylonitrile is added. Then, when the polymer material is completely dissolved, the solution is quickly spread on the substrate and allowed to cool to obtain a gel electrolyte.

As described above, the gel electrolyte does not require a special crosslinking operation, and can be easily prepared only by cooling after dissolution.

The battery of the present invention comprises such a gel electrolyte, a positive electrode and a negative electrode, and each form is appropriately selected according to the shape of the battery. For example, in a flat battery, a first gel electrolyte, a separator, and a second gel electrolyte are sandwiched between a plate-shaped positive electrode and a negative electrode, and this electrode element is housed in a predetermined battery exterior material. A battery is configured.

A battery using such a gel electrolyte is as follows:
Even if batteries are accidentally thrown into a fire, the risk of ignition is low. In addition, since the gel electrolyte itself has excellent flame retardancy, it is not necessary to include a general-purpose flame retardant having poor electrochemical stability, and deterioration of battery performance due to the flame retardant can be avoided. Furthermore, since a denaturing reaction hardly occurs in the gel electrolyte, good battery performance is maintained even after long-term storage.

In addition, since the non-aqueous electrolyte is fixed in the gel electrolyte, no liquid leakage occurs even when some load is applied to the battery.

Further, when looking at the structure of the electrode, the gel electrolyte is interposed between the positive electrode and the negative electrode, and the gel electrolyte adheres to both electrode surfaces, so that the distance between the electrodes is always constant. . For this reason, for example, in a conventional flat battery using a liquid non-aqueous electrolyte as it is, a pressurizing means was necessary to maintain a stable positional relationship between the two electrodes, but such a pressurizing means became unnecessary. Structure is simplified.

This battery may be of a primary battery type or a secondary battery type. For example, in the case of a secondary battery specification, the following positive electrode material and negative electrode material are used.

First, as a positive electrode material, a general formula Li _x M
O ₂ (where M is one or more transition metals, preferably M
At least one of n, Co, Ni, and Fe. Also,
0.05 ≦ x ≦ 1.10) is used.

As the negative electrode material, lithium metal,
A lithium alloy, and a material capable of inserting and extracting lithium, such as a carbonaceous material, a silicon compound, and a tin compound, are used. Of these, as carbonaceous materials,
Pyrolytic carbons, cokes (pitch coke, needle coke, petroleum coke, etc.), graphites, glassy carbons, carbons using organic polymer as precursor (furan resin, etc. fired at appropriate temperature, etc.) ), Carbon fiber, activated carbon and the like.

[0038]

EXAMPLES Specific examples of the present invention will be described based on experimental results.

Example 1 A gel electrolyte was prepared as follows.

First, ethylene carbonate (EC) and gamma-butyrolactone (GBL) were added at 60 mol%: 20.
mol%, and LiPF ₆ was further mixed with 10 m
ol% was added to prepare a non-aqueous electrolyte.

Then, the non-aqueous electrolyte was poured into a beaker and heated to 120 ° C. while stirring in a dry atmosphere having a dew point of −50 ° C. or less. Next, polyacrylonitrile (PAN, number average molecular weight:
A powder having a solubility of 150,000 and a solubility parameter of 12) was added so that the mole fraction of acrylonitrile was 10 mol%, and the mixture was further heated and stirred for 20 minutes to obtain a transparent and highly viscous solution. Thereafter, the heating was stopped, and this highly viscous solution was quickly spread on a flat chalet, and a glass test tube (inner diameter 12 mm × length 15 mm) was prepared.
0 mm), and the mixture was allowed to cool all day and night to produce a gel electrolyte.

Example 2 A gel electrolyte was prepared in the same manner as in Example 1 except that the contents of ethylene carbonate (EC) and gamma-butyrolactone (GBL) were changed as shown in Table 1.

Example 3 Propylene carbonate (PC) was added to a non-aqueous solvent except that the content of ethylene carbonate (EC), gamma-butyrolactone (GBL) and propylene carbonate (PC) was as shown in Table 1. In the same manner as in Example 1, a gel electrolyte was produced.

Comparative Example 1 A gel electrolyte was prepared in the same manner as in Example 1 except that the contents of ethylene carbonate (EC) and gamma-butyrolactone (GBL) were changed as shown in Table 1.

Comparative Example 2 Propylene carbonate (PC) was added to a nonaqueous solvent without adding gamma-butyrolactone, and the contents of ethylene carbonate (EC) and propylene carbonate (PC) were as shown in Table 1. And LiPF ₆
A gel electrolyte was prepared in the same manner as in Example 1 except that the content of was changed to 8 mol%.

The gel electrolyte prepared as described above was evaluated for ionic conductivity, flame retardancy and charge transfer resistance. The measuring method of these characteristics is shown below.

<Measurement of Ionic Conductivity> The gel electrolyte in the flat chalet was cut into a cylindrical shape having a diameter of 1.0 cm, sandwiched between two platinum disk electrodes (1.0 cm in diameter), and an impedance analyzer (trade name) HP4192
The ion conductivity was measured by the complex impedance method using A). The measurement conditions are as follows. The measurement was performed at a temperature of 25 ° C.

Applied voltage: 0.5 V Sweep frequency range: 5 to 13 MHz <Test for Flame Retardancy> The test for flame retardancy was carried out using the apparatus shown in FIG.

This apparatus has a structure in which two columns 2 are fixed on a base 1, and a wire mesh (made of stainless steel, mesh shape: 5 mm × 5 mm square) 3 is stretched between the two columns 2. Have been. A butane gas burner 4 is provided below the wire mesh 3. The butane gas burner 4 is oriented such that the gas outlet 5 has an angle θ of 45 ° with respect to the horizontal plane, and the blue flame 7 ignited by the gas.
Is adjusted to such a height position as to blow upward beyond the wire net 3. On the other hand, the measurement sample 6 is obtained by cutting the gel electrolyte in a test tube to have a length of 130 mm, and is placed on the wire mesh 3.

The flame retardancy was evaluated by applying a flame 7 from the butane gas burner 4 to a portion 6 mm from the end of the measurement sample 6 and continuing to indirectly flame for 30 seconds in this state. This was performed by observing the degree of combustion in this portion. Here, the case where the combustion part of the gel electrolyte did not reach the mark line S of 25 mm from the end was determined as “non-flammable”, and the case where the combustion part exceeded the mark line S of 25 mm was determined as “flammability”. .

<Measurement of Charge Transfer Resistance> The gel electrolyte in the flat chalet was cut into a column having a diameter of 1.0 cm, sandwiched between two pieces of lithium metal (1.0 cm in diameter), and then subjected to an impedance analyzer (product). First name HP41
92A), the change with time of the charge transfer resistance was observed by the complex impedance method. The measurement conditions are as follows.

Applied voltage: 0.01 V Sweep frequency range: 10 mHz to 1 kHz The increase in the charge transfer resistance with time is caused by the contact of the gel electrolyte with lithium metal as the electrode material,
It is considered that the resulting passive film is deposited on the metal surface. Therefore, the charge transfer resistance can be regarded as an index reflecting the chemical stability of the gel electrolyte against lithium metal.

Table 1 shows the evaluation results of the ionic conductivity and the flame retardancy measured as described above. FIG. 2 shows the change over time in the charge transfer resistance.

[0054]

[Table 1]

However, the molar fraction of polyacrylonitrile (PAN) is calculated based on the molecular weight of the repeating unit structure.

First, as shown in Table 1, the gel electrolytes of Examples 1 to 3 and Comparative Example 1 containing gamma-butyrolactone were the gel electrolytes of Comparative Example 2 not containing gamma-butyrolactone. Higher ion conductivity is obtained as compared with.

This indicates that the ionic conductivity of the gel electrolyte can be increased by adding gamma-butyrolactone to the solvent.

Next, among the time-dependent changes in the charge transfer resistance of FIG. 2, the gel electrolytes of Examples 1, 2 and Comparative Example 1 using a mixed system of gamma-butyrolactone and ethylene carbonate were compared. The gel electrolyte of Comparative Example 1 in which the content of butyrolactone was 60 mol% was 10
The charge transfer resistance sharply rises relatively early before 0 hour, whereas the content of gamma-butyrolactone is 30
The increase in the charge transfer resistance is suppressed in the gel electrolytes of Examples 1 and 2 in which the amount is not more than mol%.

This indicates that the content of gamma-butyrolactone must be suppressed to 30 mol% or less in order to suppress the charge transfer resistance of the gel electrolyte.

The gel electrolytes of Example 3 and Comparative Example 2 containing propylene carbonate tend to have a larger increase in charge transfer resistance than the gel electrolytes not containing propylene carbonate. Therefore, in order to suppress an increase in charge transfer resistance, it is desirable to minimize the use of propylene carbonate.

Next, a primary battery and a secondary battery were actually manufactured by incorporating the above-mentioned gel electrolyte, and their characteristics were evaluated.

Manufacturing of Primary Battery The manufacturing process of the battery is shown in FIGS.

First, the positive electrode plate 8 shown in FIG. 3 was manufactured as follows.

Mixing 85% by weight of manganese dioxide, 10% by weight of graphite and 5% by weight of polyvinylidene fluoride,
Furthermore, dimethylformamide was added as a powder in substantially the same amount, and then kneaded to prepare a positive electrode mixture.

The positive electrode mixture was applied to a 2 cm × 2 cm area on a stainless steel mesh as a current collector, dried in a dryer at a temperature of 120 ° C., and roll-pressed. Then, as shown in FIG. 3, the electrode plate was formed into a shape such that the band-shaped current collector 8b protruded from the application region 8a of the positive electrode mixture, and the positive electrode plate 8 was produced.

Next, a lithium metal plate having a thickness of 30 μm was cut into a size of 2 cm × 2 cm, and pressed on a current collector having substantially the same planar shape as the above-mentioned positive electrode plate to produce a negative electrode plate 9.

Then, a gel electrolyte was applied to the positive electrode plate 8 and the negative electrode plate 9.

Subsequently, a nonwoven fabric made of polypropylene (thickness: 150 μm) to be a diaphragm 10 as shown in FIG. 4 was prepared, and a gel electrolyte 11 was applied to the diaphragm 10 as shown in FIG.

The gel electrolyte applied to the positive electrode, the negative electrode, and the diaphragm had the same composition as that of Example 2 described above.

Then, as shown in FIG. 6, the positive electrode plate 8, the diaphragm 10, and the negative electrode plate 9 were overlapped in this order and pressed to form an electrode element.

Then, the obtained electrode element is covered with a heat-fusible laminate film (manufactured by Dai Nippon Printing Co., Ltd.) as a bag-shaped battery exterior material 12, and as shown in FIG. A flat primary battery was manufactured by vacuum packing 12a. In addition, this heat-fusible laminate film is
It consists of a polyethylene terephthalate film, an aluminum foil, and a polypropylene film, with the polyethylene terephthalate film side being the outside of the battery.

The discharge characteristics of the thus prepared primary battery were examined. The discharge current density at the time of discharge is 500 μA
/ Cm ² , and the discharge was terminated when the closed circuit voltage reached 1.0 V. FIG. 8 shows the discharge characteristics.

As shown in FIG. 8, this primary battery has an average discharge voltage of 2.6 to 2.8 V, and has good flatness of the discharge curve. From this, it was found that this gel electrolyte exhibited sufficient performance as an electrolyte material for a primary battery.

Preparation of Secondary Battery 90% by weight of lithium nickelate, 8% by weight of graphite, and 2% by weight of polyvinylidene fluoride were mixed, and dimethylformamide was added in a substantially equal amount of powder, followed by kneading to form a positive electrode. Materials were produced.

The positive electrode mixture was applied to a 2 cm × 2 cm area on a stainless steel mesh as a current collector, dried in a dryer at a temperature of 120 ° C., and roll-pressed. Then, this electrode plate was molded into a shape such that the band-shaped current collector protruded from the application region of the positive electrode mixture, to produce a positive electrode plate.

Next, 90% by weight of mesophase small sphere type carbon and 10% by weight of polyvinylidene fluoride were mixed, and dimethylformamide was added in a substantially equal amount of powder, followed by kneading to prepare a negative electrode mixture.

This negative electrode mixture was applied to a 2 cm × 2 cm area on a stainless steel mesh as a current collector, dried in a dryer at a temperature of 120 ° C., and roll-pressed. Then, this electrode plate was molded into a shape such that the band-shaped current collector protruded from the application region of the negative electrode mixture, and a negative electrode plate was produced.

A flat secondary battery was manufactured in the same steps as shown in FIGS. 4 to 7 except that the positive electrode plate and the negative electrode plate manufactured as described above were used.

The charge / discharge characteristics of this secondary battery were examined.

The charging is performed by the constant current / constant voltage method which switches to the constant voltage method at 4.2 V, and the charging current is 512 μA / c.
m ² , and the charging time was set to 10 hours. Also, the discharge
The measurement was performed by a constant current method, and a continuous discharge was performed at 512 μA / cm ² until the closed circuit voltage became 2.5 V.

FIG. 9 shows the charge / discharge characteristics at the third charge / discharge cycle. From FIG. 9, it was found that the average voltage of the battery was 3.61 V and the charge / discharge efficiency was 99%, indicating that the battery exhibited excellent reversibility. From this, it was found that this gel electrolyte exhibited sufficient performance as an electrolyte material for a secondary battery having a high energy density.

[0082]

As is clear from the above description, the battery of the present invention is characterized in that a polymer material containing polyacrylonitrile, a non-aqueous solvent containing gamma-butyrolactone and L
iPF ₆ consists electrolyte salt consisting mainly of, the gamma -
Since a gel electrolyte having a butyrolactone content of 30 mol% or less is used, it has excellent flame retardancy and
Good battery performance is obtained, and the battery performance can be maintained even after long-term storage.

[Brief description of the drawings]

FIG. 1 is a schematic diagram showing a configuration of an apparatus used for measurement of a flame retardancy test.

FIG. 2 is a characteristic diagram showing a change with time of a charge transfer resistance.

FIG. 3 is a schematic view illustrating a positive electrode plate and a negative electrode plate, illustrating a method of manufacturing a battery in the order of steps.

FIG. 4 is a schematic view showing a diaphragm.

FIG. 5 is a schematic view showing a step of applying a gel electrolyte to a diaphragm.

FIG. 6 is a schematic view showing a process of storing a positive electrode plate, a diaphragm, and a negative electrode plate in a battery exterior material.

FIG. 7 is a schematic view showing a step of heat-sealing a battery exterior material.

FIG. 8 is a characteristic diagram showing discharge characteristics of a primary battery to which a gel electrolyte is applied.

FIG. 9 is a characteristic diagram showing charge / discharge characteristics of a secondary battery to which a gel electrolyte is applied.

Claims (3)

[Claims] 1. A positive electrode and a negative electrode, a polymer material containing polyacrylonitrile, gamma-
A non-aqueous solvent containing butyrolactone and a gel electrolyte comprising an electrolyte salt mainly composed of LiPF ₆ , wherein the content of gamma-butyrolactone in the gel electrolyte is 30 mol% or less. Battery. 2. The battery according to claim 1, wherein the non-aqueous solvent is a mixed solvent of gamma-butyrolactone and ethylene carbonate. 3. The battery according to claim 1, wherein the non-aqueous solvent is a mixed solvent of gamma-butyrolactone, ethylene carbonate and propylene carbonate.