JP2000058118A - Nonaqueous electrolyte secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a battery superior in low-temperature characteristic and a cycle characteristic by constituting the battery with a negative electrode using a carbon material as an active material, a positive electrode using a lithium composite oxide as an active material and a nonaqueous electrolyte dissolved with the support salt containing lithium borate tetrafluoride and lithium phosphate hexafluoride in an organic solvent containing sulfolane and diethyl carbonate. SOLUTION: Although the mixing ratio of sulfolane(SL) and diethyl carbonate(DEC) used as the organic solvent of a nonaqueous electrolyte secondary battery can be set to various values according to the performance of the secondary battery to be obtained, the mixing ratio is preferably set to SL:DEC=1:2-2:1 in a volume ratio in consideration of the function of SL as a high-dielectric constant solvent and the function of DEC as a low-viscosity solvent. The mole ratio between LiBF4 and LiPF6 in the nonaqueous electrolyte is preferably set in the range of LiBF4:LiPF6=1:1-3:1.

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 secondary battery utilizing insertion and extraction of lithium ions, and more particularly to a non-aqueous electrolyte secondary battery having excellent cycle characteristics and low-temperature characteristics.

[0002]

2. Description of the Related Art A non-aqueous electrolyte secondary battery including a negative electrode using a carbon material as an active material, a positive electrode using a lithium composite oxide as an active material, and a non-aqueous electrolyte, that is, a so-called lithium secondary battery, has been developed. Because of its high voltage and high energy density, it is expected to be used as a power source for electric vehicles. However,
When a lithium secondary battery is used as a power supply for an electric vehicle, in addition to good cycle characteristics in which the discharge capacity does not deteriorate even with repeated charge and discharge, stable charge and discharge can be performed even at a low temperature of about -30 ° C. It is also required to have excellent low-temperature characteristics such as being able to perform.

[0003] The organic solvent used for the non-aqueous electrolyte of the lithium secondary battery is generally a mixed solvent obtained by mixing a high dielectric constant solvent and a low-viscosity solvent, and is used for a lithium secondary battery using a carbon material for the negative electrode. At present, a mixed solvent using ethylene carbonate (EC), which is a kind of cyclic carbonate, as a high dielectric constant solvent is mainly used. However, a lithium secondary battery using this EC as a high dielectric constant solvent cannot satisfy required low-temperature characteristics. This is E
C has a high freezing point of about 40 ° C. and a low viscosity solvent having a low freezing point, for example, diethyl carbonate (DEC: freezing point-4)
This is because, even if the solvent is excessively mixed with (5 ° C.), when the temperature reaches around −30 ° C., EC is deposited, and charging and discharging cannot be performed satisfactorily.

[0004] Sulfolane (SL) is well known as a solvent having a high dielectric constant instead of the cyclic carbonate ester which is currently the mainstream. This SL has a freezing point of 28 ° C., which is lower than EC, and has better electrochemical stability than EC.
Promising in terms of low temperature characteristics and cycle characteristics. However,
Since it is a solid at room temperature like EC, it is premised that it is mixed with a low-viscosity solvent having a low freezing point.

A non-aqueous electrolyte secondary battery using a mixed solvent obtained by combining SL and a low-viscosity solvent having a low freezing point is not intended to improve low-temperature characteristics. Some are as shown. This secondary battery uses metallic lithium as a negative electrode,
Dimethyl carbonate (D
MC) or diethyl carbonate (DEC) as a low-viscosity solvent. However, the non-aqueous electrolyte secondary battery using the mixed solvent disclosed in the above publication has the following disadvantages.

A secondary battery using a mixed solvent of SL and DMC has excellent charge / discharge efficiency and cycle characteristics in a normal temperature range, but the freezing point of DMC is 2 ° C. The electrolyte solidifies and falls into a state where charging and discharging are impossible. On the other hand, in the secondary battery using the mixed solvent of SL and DEC, although the freezing point of DEC is relatively low at −40 ° C., there is a disadvantage that DEC is easily decomposed and cycle characteristics are poor.

On the other hand, various studies have been made on supporting salts to be dissolved in an organic solvent in order to improve the cycle characteristics. For example, Japanese Patent Application Laid-Open No. H8-17468 discloses lithium tetrafluoroborate ( LiBF ₄ ) and lithium hexafluorophosphate (LiPF ₆ ) in a molar ratio of 1: 2
It is shown that a non-aqueous electrolyte dissolved in a ratio of 1:11 can constitute a secondary battery having good cycle characteristics. However, this technique relates to a case where a mixed solvent of propylene carbonate (PC), which is a cyclic ester, and DEC is used as an organic solvent, and a case of a non-aqueous electrolyte using a mixed solvent of SL and DEC described above. However, if this technology is applied as it is, improvement in cycle characteristics cannot be expected.

[0008]

As described above, at present, at present, it is chemically and electrochemically stable, and has good cycle characteristics and low-temperature characteristics, especially in an environment of -30 ° C to -40 ° C. There was no non-aqueous electrolyte secondary battery with which discharge performance was compatible. The present invention has been made in view of the above circumstances, and by making the organic solvent used for the non-aqueous electrolyte and the supporting salt to be dissolved therein suitable, the low-temperature characteristics are excellent, and the cycle characteristics are also excellent. Another object of the present invention is to provide a non-aqueous electrolyte secondary battery.

The present inventor has searched for a low-viscosity solvent to be mixed with the sulfolane and a supporting salt to be dissolved while taking advantage of the advantage of sulfolane as a high dielectric constant solvent.
The following invention has been reached.

[0010]

According to the present invention, there is provided a non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material as an active material, a positive electrode using a lithium composite oxide as an active material, and a supporting salt dissolved in an organic solvent. A non-aqueous electrolyte secondary battery having a non-aqueous electrolyte solution, wherein the supporting salt includes lithium tetrafluoroborate and lithium hexafluorophosphate, and the organic solvent is sulfolane and diethyl carbonate. And characterized in that:

In other words, the non-aqueous electrolyte secondary battery of the present invention is a so-called lithium secondary battery, in which the organic solvent of the non-aqueous electrolyte is a low dielectric constant solvent such as sulfolane (SL) and a low-viscosity solvent. The freezing point of diethyl carbonate (D
EC) to form a secondary battery that can be charged and discharged even under low-temperature conditions. Further, in this mixed solvent, lithium tetrafluoroborate (LiBF ₄ ) and lithium hexafluorophosphate ( By dissolving two kinds of supporting salts of LiPF ₆ ), DE which has been conventionally regarded as a problem is solved.
By suppressing the decomposition of C, it is possible to obtain a secondary battery in which the discharge capacity does not deteriorate even by repeated charging and discharging.

The molecular structural formulas of SL and DEC used as the electrolyte solvent of the non-aqueous electrolyte secondary battery of the present invention are shown below.

[0013]

Embedded image

SL plays a role of dissociating the lithium salt as a solvent having a high dielectric constant, and DEC is a kind of chain carbonate and ensures quick movement of dissociated ions as a low-viscosity solvent. The freezing point of SL is 28 ° C., which is lower than that of ethylene carbonate (EC), which is a cyclic carbonate. The freezing point of DEC is also -45 ° C., and dimethyl carbonate, which is another chain carbonate (DMC: freezing point of 2 ° C.) ) Etc. are lower. Therefore, the electrolyte using this mixed solvent is -30 ° C to -40 ° C.
It does not freeze even at low temperatures of ℃.

In general, a lithium salt that is ionized by being dissolved in an organic solvent to generate lithium ions is used as a supporting salt for an electrolyte solution of a lithium secondary battery. In the non-aqueous electrolyte secondary battery of the present invention, two kinds of lithium salts, LiBF ₄ and LiPF ₆ , are used. Both LiBF ₄ and LiPF ₆ have a high degree of dissociation and have good electrical conductivity of the electrolytic solution, so that they are suitable supporting salts for lithium secondary batteries. In the secondary battery of the present invention, the use of these two supporting salts, LiBF ₄ and LiPF ₆ , suppresses the decomposition of DEC, which is the low-viscosity solvent, and improves the cycle characteristics.

The reason why the cycle characteristics are improved is not clear, but is presumed as follows. In a lithium secondary battery using a carbon material for the negative electrode, the negative electrode and the electrolyte react during the first charging. The decomposition product of the electrolytic solution forms a film on the surface of the negative electrode, and suppresses the subsequent reaction between the electrolytic solution and the negative electrode. The performance of the film formed by the decomposition of the electrolytic solution governs the charge / discharge efficiency of the battery.

Turning now to the solvent, J. Power.
Sources., 68, 91-98 (1997) states that the decomposition product of DEC does not form a film on the negative electrode surface because it is dissolved in the solvent (in a commonly used mixed solvent of EC and DEC, Decomposition products are the main component of the coating, which exhibits good properties). In the mixed solvent of SL and DEC of the present invention, the decomposition product of SL is
It is expected that the effect of suppressing the decomposition of the solvent is low.

Next, considering the supporting salt, LiBF
It is described in the above-mentioned literature that supporting salts such as ₄ and LiPF ₆ also decompose during charging and remain on the negative electrode surface. In particular, when LiBF ₄ is used alone as a supporting salt,
The impedance of the film formed by the decomposition of LiBF ₄ is high, which adversely affects the charge / discharge efficiency and cycle characteristics of the battery.

When LiBF ₄ is used alone as a mixed solvent of SL and DEC, the main component of the film formed on the negative electrode surface is expected to be a decomposition product of LiBF _4. Lithium secondary batteries using are poor in charge / discharge efficiency and cycle characteristics. When LiPF ₆ is used as a supporting salt in the mixed solvent of SL and DEC, L
The coating formed by the decomposition of iPF ₆ has a low effect of suppressing the decomposition of the solvent, and also has poor charge / discharge efficiency and cycle characteristics of the battery. However, LiBF ₄ and LiPF ₆
It is considered that, when is mixed, a film having a low balance in a certain composition and a well-balanced film for suppressing the reaction between the negative electrode and the electrolytic solution is obtained.

[0020]

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS The non-aqueous electrolyte secondary battery of the present invention will be described in detail below based on embodiments. The description will first be made on the organic solvent and the supporting salt of the non-aqueous electrolyte solution, which are the characteristic parts of the present invention. Next, the other components such as the negative electrode and the positive electrode are performed. In the secondary battery of the present invention, the mixing ratio of SL and DEC used as the organic solvent can be various values according to the performance of the secondary battery to be obtained. However, the function of SL as a high dielectric constant solvent and D
In consideration of the function of EC as a low-viscosity solvent, the range of the mixing ratio is limited, and in the volume ratio, SL: DEC =
It is desirable to mix in the range of 1: 2 to 2: 1.

The electrolyte of the secondary battery according to the present invention may be used in a small amount as needed to improve the characteristics of the secondary battery unless the target low-temperature characteristics and cycle characteristics are impaired. A third solvent may be added to the above-mentioned mixed solvent. Organic solvents that can be used as the third solvent include propylene carbonate, ethylene carbonate, dimethyl carbonate, ethyl methyl carbonate, γ-butyrolactone, 1,2-dimethoxyethane, 1,2-diethoxyethane, tetrahydrofuran, 2-methylfuran. Examples include tetrahydrofuran and 3-methylsulfolane.

The molar ratio between LiBF ₄ and LiPF ₆ in the non-aqueous electrolyte is preferably in the range of LiBF ₄ : LiPF ₆ = 1: 1 to 3: 1. As a result of various studies on the mixing ratio of the supporting salt, it was found that when LiBF ₄ and LiPF ₆ were used in a mixture, a secondary battery having good charge / discharge efficiency and cycle characteristics in this range was obtained. . When the ratio deviates from the above range, the charge / discharge efficiency and the cycle characteristics deteriorate.

In order to improve the electric conductivity of the electrolytic solution, the concentration of the supporting salt in the non-aqueous electrolytic solution is preferably set to a high concentration. It is considered that the electric conductivity is rather deteriorated due to a decrease in the number of free ions due to an increase in the interaction between them and an increase in the viscosity of the electrolytic solution. From this, in the secondary battery of the present invention, the total concentration of the supporting salt, that is, the total concentration of LiBF ₄ and LiPF ₆ is 0.8 to 1.2 mol / l.
It is desirable to be within the range.

As in the case of the above-mentioned organic solvent, the electrolyte for the secondary battery of the present invention is required to improve the characteristics of the secondary battery unless the intended low-temperature characteristics and cycle characteristics are impaired. If the amount is slightly in accordance with
_{4. A} third supporting salt may be added in addition to LiPF ₆ . Lithium salts that can be used as the third supporting salt include LiClO ₄ , LiCF ₃ SO ₃ , L
iAsF ₆ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ S
O ₂ ) _{2 and the} like.

The lithium secondary battery includes a negative electrode, a positive electrode, a separator and the like in addition to the nonaqueous electrolyte. Hereinafter, aspects of these components that can be used in the nonaqueous electrolyte secondary battery of the present invention will be described. For the negative electrode, a paste is prepared by mixing a binder with a negative electrode active material capable of inserting and extracting lithium ions, adding an appropriate solvent, and forming a paste.
It is formed by coating and drying on the surface of the metal foil current collector. In the non-aqueous electrolyte secondary battery of the present invention, a carbon material, for example, a fired organic compound such as graphite or a phenol resin, or a powdered material such as coke is used as the negative electrode active material. The binder plays a role of binding the active material particles, and a fluorine-containing resin such as polyvinylidene fluoride or the like, and a solvent for dispersing the active material and the binder such as N-methyl-2-pyrrolidone or the like. Organic solvents can be used. Further, instead of these materials, synthesis of one or more cellulose ether-based substances selected from the group of methylcellulose, carboxymethylcellulose, and the like as a negative electrode binder and styrene-butadiene rubber latex, carboxy-modified styrene-butadiene rubber latex, and the like. A composite binder with a rubber-based latex-type adhesive may be used, and water may be used as a solvent. As the negative electrode current collector, a copper foil or the like can be used.

As the positive electrode, a conductive material and a binder are mixed with a positive electrode active material capable of inserting and extracting lithium ions, and an appropriate solvent is added to form a paste like a negative electrode. It is formed by coating and drying on the surface. In the nonaqueous electrolyte secondary battery of the present invention, LiCoO ₂ ,
One or more kinds of lithium composite oxide powders such as LiNiO ₂ and LiMn ₂ O ₄ can be used. Above all, spinel-structured LiMn ₂ O ₄ is suitable as a positive electrode active material of a secondary battery for an electric vehicle that requires a large amount of an active material because it is inexpensive because it does not contain resources such as Co and Ni, which are small resources.

The conductive material is for ensuring the electrical conductivity of the positive electrode active material layer, and may be one or a mixture of two or more kinds of powdered carbon materials such as carbon black, acetylene black, and graphite. Can be used. Examples of the positive electrode binder include polytetrafluoroethylene, polyvinylidene fluoride, a fluorine-containing resin such as fluororubber, polypropylene,
A thermoplastic resin such as polyethylene can be used.
An organic solvent such as N-methyl-2-pyrrolidone can be used as a solvent in which the active material, the conductive material, and the binder are dispersed. An aluminum foil or the like can be used for the positive electrode current collector.

The separator sandwiched between the positive electrode and the negative electrode separates the positive electrode from the negative electrode and holds the electrolyte, and a thin microporous film such as polyethylene or polypropylene can be used. The non-aqueous electrolyte secondary battery of the present invention composed of the above, the shape is a coin type,
Various types such as a laminated type and a cylindrical type can be used. In any case, a separator is sandwiched between the positive electrode and the negative electrode to form an electrode body, and the electrode body is made to conduct from the positive electrode and the negative electrode to the positive electrode terminal and the negative electrode terminal communicating with the outside, respectively. Is sealed in a battery case together with the above non-aqueous electrolyte to complete the battery.

[0029]

EXAMPLES Based on the above embodiment, as an example, a non-aqueous electrolyte in which LiBF ₄ and LiPF ₆ are dissolved in a mixed solvent of sulfolane (SL) and diethyl carbonate (DEC) at various molar ratios is produced. did. A non-aqueous electrolyte in which LiBF ₄ or LiPF ₆ is dissolved alone in a mixed solvent of SL and DEC; SL and dimethyl carbonate (DMC)
And a non-aqueous electrolyte using a mixed solvent of ethylene carbonate (EC) and DEC were prepared as comparative examples. Then, the state of the non-aqueous electrolytes of the examples and comparative examples at a low temperature was observed, and the low-temperature characteristics of the electrolytes were evaluated. Furthermore, a coin-type secondary battery was actually constructed using an electrolytic solution having good low-temperature characteristics, and the cycle characteristics of the battery were evaluated. Evaluations of low-temperature characteristics and cycle characteristics of the respective examples, comparative examples, and secondary batteries using these electrolytes are shown below.

<Example> First, a mixed solvent in which SL and DEC were mixed at a volume ratio of 1: 1 was prepared. In this mixed solvent, LiBF ₄ and LiPF ₆ as supporting salts were dissolved in the following molar ratio, and prepared so that the total concentration of the supporting salt was 1 mol / l to obtain a non-aqueous electrolyte. LiBF ₄ and LiP
A non-aqueous electrolyte having a molar ratio with F _{6 of} 3: 1 was used as the non-aqueous electrolyte of Example 1, and a 1: 1 mixture with the non-aqueous electrolyte of Example 3 was used. did. In addition, the organic solvent and the supporting salt used for the preparation of the non-aqueous electrolyte in the examples were all manufactured by Toyama Pharmaceutical Co., Ltd. The same applies to the non-aqueous electrolyte of the following comparative example.

<Comparative Example> SL and DEC were mixed in a volume ratio of 1:
A non-aqueous electrolyte in which only LiBF ₄ was dissolved as a supporting salt in the mixed solvent mixed in 1 was prepared, and this was used as a non-aqueous electrolyte in Comparative Example 1. A non-aqueous electrolyte in which only LiPF ₆ was dissolved in the same mixed solvent was prepared, and this was used as a non-aqueous electrolyte of Comparative Example 2.

Next, the organic solvent was changed, and a non-aqueous electrolyte solution in which LiBF ₄ was dissolved in a mixed solvent in which SL and DMC were mixed at a volume ratio of 1: 1 was prepared. A water electrolyte was used. Further, the mixed solvent was changed, and a non-aqueous electrolyte solution in which LiBF ₄ was dissolved in a mixed solvent in which EC and DEC were mixed at a volume ratio of 1: 1 was prepared. This was used as the non-aqueous electrolyte solution of Comparative Example 4. . Next, the mixing ratio of the solvent of Comparative Example 4 was changed, and a non-aqueous electrolyte solution in which LiBF ₄ was dissolved in a mixed solvent in which EC and DEC were mixed at a volume ratio of 3: 7 was prepared. The non-aqueous electrolyte of Example 5 was used. In addition, the concentration of the supporting salt was 1 mol / l in each of the non-aqueous electrolytes of the comparative example.

<Evaluation of Low Temperature Characteristics of Nonaqueous Electrolyte> The nonaqueous electrolytes of the above Examples and Comparative Examples were poured into 10 ml sample cans and kept at a constant temperature of −20 ° C., −30 ° C., and −40 ° C. Each of the tanks was allowed to stand for one day or more, and the state of the electrolyte was visually observed. The results of this observation are shown in Table 1 below.

[0034]

[Table 1]

As shown by these results, SL, DMC and E
The non-aqueous electrolyte solutions of Comparative Examples 3 and 4 using a mixed solvent in which C and DEC were mixed at a volume ratio of 1: 1 were at −20 ° C.
And a secondary battery using these mixed solvents is-
It turns out that charging and discharging cannot be performed at 20 ° C. In addition, even in the non-aqueous electrolyte of Comparative Example 5 in which DEC, which is a solvent having a low freezing point, was excessively mixed, crystals were precipitated at −20 ° C., indicating that satisfactory low-temperature characteristics could not be obtained.

On the other hand, the non-aqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 and 2 using a mixed solvent in which SL and DEC were mixed at a volume ratio of 1: 1 had a low temperature of -40 ° C. Even when left below, it exists as a liquid. Therefore, a non-aqueous electrolyte using a mixed solvent of SL and DEC can be charged and discharged even in a low temperature atmosphere of -40 ° C.
That is, it can be evaluated as a non-aqueous electrolyte that can constitute a secondary battery having good low-temperature characteristics.

<Preparation of Coin-Type Secondary Battery> The non-aqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 and 2 using a mixed solvent of SL and DEC having good low-temperature characteristics were used. hand,
A coin-type secondary battery was actually manufactured. Hereinafter, this secondary battery will be described. The negative electrode was manufactured as follows. Using artificial graphite (MCMB25-28: manufactured by Osaka Gas) as a carbon material as an active material, 95 parts by weight of this artificial graphite was mixed with 5 parts by weight of polyvinylidene fluoride as a binder, and N-methyl-2 as a solvent was mixed. -A paste-like negative electrode mixture was prepared by dispersing the mixture in pyrrolidone. This negative electrode mixture is uniformly applied to one surface of a 15 μm-thick strip-shaped copper foil serving as a negative electrode current collector,
After drying, compression molding was performed with a roll press.

The positive electrode was manufactured as follows. manga
Lithium phosphate (Li_1.03Mn_1.97O _Four： Honjo Chemical
90% by weight of this lithium manganate
And 7 parts by weight of graphite as a conductive material and a binder.
And mix 7 parts by weight of polyvinylidene fluoride,
Dispersed in N-methyl-2-pyrrolidone
Was prepared. This positive electrode mixture serves as a positive electrode current collector.
Evenly coated on one side of a 20μm thick aluminum strip
After being clothed and dried, it was compression molded by a roll press.

Next, the positive electrode was cut into a circular shape having a diameter of 15 mmφ, and the negative electrode was cut into a circular shape having a diameter of 18 mmφ. Type battery case. And Examples 1-3, Comparative Example 2,
After injecting each of the electrolyte solutions 3 into the battery case, the battery case was sealed and a coin-type secondary battery was obtained.

<Evaluation of Cycle Characteristics of Secondary Battery> A charge / discharge cycle test was performed on the coin-type secondary batteries manufactured as described above under a temperature condition of 25 ° C., and these coin-type secondary batteries were evaluated. The discharge capacity was measured. The charge-discharge cycle test was conducted at a constant current of 1.0 mA / cm ² and a charge end voltage of 4.2.
After the charging until the voltage reached V, the discharge was performed at a constant current of 1.0 mA / cm ² until the discharge end voltage reached 3.0 V as one cycle, and the cycle was performed up to 50 cycles. FIG. 1 shows the discharge capacity at the first cycle (initial capacity) and the discharge capacity at the 50th cycle of each secondary battery measured by the charge / discharge cycle test.

As a result of this charge / discharge cycle test, LiBF
_The secondary batteries using the non-aqueous electrolytes of Examples 1 to 3 in which both ₄ and LiPF ₆ were dissolved as supporting salts are the same as those of Comparative Examples 1 and 2 in which LiBF ₄ or LiPF ₆ was dissolved alone. It was confirmed that the discharge amount at the 50th cycle was larger than that of the secondary battery using the water electrolyte. Next, LiBF ₄ in the supporting salt, as revealed by the charge-discharge cycle test
FIG. 2 shows the relationship between the ratio of the discharge capacity and the discharge capacity. As can be seen from this figure, the ratio of LiBF ₄ in the supporting salt was 50%.
It was found that the secondary battery using the non-aqueous electrolyte in the range of ７５75 mol% was a secondary battery having a large discharge capacity at the 50th cycle. That is, among the non-aqueous electrolyte solutions in which both LiBF ₄ and LiPF ₆ are dissolved as supporting salts,
It was confirmed that a nonaqueous electrolyte having a molar ratio of BF ₄ to LiPF _{6 in} the range of 1: 1 to 3: 1 can constitute a secondary battery having better cycle characteristics.

From the evaluation of the low-temperature characteristics and the cycle characteristics described above, the non-aqueous electrolyte in which both LiBF ₄ and LiPF ₆ are dissolved as a supporting salt in a secondary battery of the present invention, that is, a mixed solvent in which SL and DEC are mixed is shown. The secondary battery using the liquid was proved to be a battery excellent in both low-temperature characteristics and cycle characteristics.

[0043]

The present invention provides a non-aqueous electrolyte comprising a negative electrode using a carbon material as an active material, a positive electrode using a lithium composite oxide as an active material, and a non-aqueous electrolyte obtained by dissolving a supporting salt in an organic solvent. In the secondary battery, the supporting salt includes lithium tetrafluoroborate and lithium hexafluorophosphate, and the organic solvent includes sulfolane and diethyl carbonate. With such a configuration, the non-aqueous solvent secondary battery of the present invention can be charged and discharged even at a low temperature of −40 ° C., and can suppress the deterioration of the discharge capacity even by repeated charging and discharging. It has become possible. Therefore, according to the present invention, a non-aqueous electrolyte secondary battery excellent in both low-temperature characteristics and cycle characteristics can be provided.

[Brief description of the drawings]

FIG. 1 is a diagram showing the results of charge / discharge cycle tests of secondary batteries using the nonaqueous electrolytes of Examples 1 to 3 and Comparative Examples 1 and 2.

FIG. 2 Li in a supporting salt in a charge / discharge cycle test
Shows the relationship between the ratio and the discharge capacity of BF ₄

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Claims (3)

[Claims] 1. A non-aqueous electrolyte secondary battery comprising a negative electrode using a carbon material as an active material, a positive electrode using a lithium composite oxide as an active material, and a non-aqueous electrolyte in which a supporting salt is dissolved in an organic solvent. A non-aqueous electrolyte secondary battery, wherein the supporting salt includes lithium tetrafluoroborate and lithium hexafluorophosphate, and the organic solvent includes sulfolane and diethyl carbonate. 2. The non-aqueous electrolyte secondary battery according to claim 1, wherein the molar ratio between the lithium tetrafluoroborate and the lithium hexafluorophosphate is in the range of 1: 1 to 3: 1. 3. The volume ratio of the sulfolane to the diethyl carbonate is in the range of 1: 2 to 2: 1.
3. The non-aqueous electrolyte secondary battery according to 1.