JP2014063710A - Electrolytic solution and lithium ion secondary battery - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium ion secondary battery preventing deterioration in capacity due to repetition of charging/discharging, more specifically excellent in cycle life characteristics, in relating to an electrolytic solution or gel electrolyte for a lithium ion secondary battery and a lithium ion secondary battery including the electrolytic solution or gel electrolyte.SOLUTION: An electrolytic solution of the present invention is an electrolytic solution that is obtained by dissolving a first lithium salt in a nonaqueous solvent, and further mixed with an organic acid lithium-boron trifluoride complex, as a second lithium salt.

Description

 The present invention relates to an electrolytic solution using an electrolyte and a lithium ion secondary battery obtained using the electrolytic solution.

 Lithium ion secondary batteries are characterized by high energy density and electromotive force compared to lead-acid batteries and nickel metal hydride batteries, so they are widely used as power sources for mobile phones and laptop computers that require small size and light weight. ing.

 A lithium ion secondary battery is generally composed of a negative electrode, a positive electrode, and a separator that prevents short-circuiting of both electrodes, and holds an electrolytic solution in which a lithium salt is dissolved in a nonaqueous solvent.

In a lithium ion secondary battery, metallic lithium, a lithium alloy, a carbon-based material that can occlude and release lithium, a metal oxide, or the like is usually used as a negative electrode. In addition, transition metal oxides such as lithium cobaltate, lithium nickelate, lithium manganate, and olivine-type lithium iron phosphate are used as the positive electrode.
Many of the current lithium ion secondary batteries are used as an electrolyte solution in various nonaqueous solvents such as ethylene carbonate, propylene carbonate, gamma butyrolactone, methyl carbonate, ethyl methyl carbonate, ethyl carbonate, and lithium hexafluorophosphate (LiPF). ₆ ), lithium boron tetrafluoride (LiBF ₄ ), lithium bis (trifluoromethylsulfonyl) imide (LiN (SO ₂ CF ₃ ) ₂ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate ( LiCF ₃ SO _3), lithium hexafluoro antimonate (LiSbF _6), hexafluoroarsenate arsenate periodate lithium (LiAsF _6), lithium tetraphenylborate _{(LiB (C 6} H ₅₎ by dissolving a lithium salt such as ₄₎ Using non-aqueous electrolyte (example) If, refer to Patent Document 1, Patent Document 2).

Japanese Patent No. 3369583 Japanese Patent No. 4407205

During initial charging of a lithium ion secondary battery, for example, cyclic carbonates such as ethylene carbonate are reduced on the negative electrode surface and are made of a solid electrolyte-like interfacial film made of Li ₂ CO ₃ , Li ₂ O, LiOH, or the like. (Hereinafter referred to as SEI). This SEI film has a role of preventing the breakdown of the negative electrode structure by preventing the solvent molecules solvated with lithium ions from being inserted into the negative electrode during charging and discharging.

 However, when charging and discharging are repeated, the SEI gradually collapses due to factors such as repeated expansion and contraction of the negative electrode and partial overvoltage. Since the carbonate solvent is further decomposed on the negative electrode surface exposed by the collapse of SEI, there is a problem that battery characteristics such as cycle characteristics and storage characteristics are deteriorated.

 Therefore, the present inventor added various organic acid lithium-boron trifluoride complexes to the non-aqueous electrolyte described above as a result of exploring various additives in order to solve the problems of the prior art as described above. As a result, it was found that deterioration of cycle characteristics can be reduced, and the present invention has been completed. That is, the present invention relates to an electrolytic solution for a lithium ion secondary battery and a lithium ion secondary battery including the same, which prevents capacity deterioration due to repeated charge and discharge, that is, a lithium ion secondary battery having excellent cycle life characteristics. It is an issue to provide.

To solve the above problem,
The present invention provides an electrolytic solution obtained by dissolving an organic acid lithium-boron trifluoride complex as the second lithium salt in an electrolytic solution in which the first lithium salt is dissolved in a nonaqueous solvent. I will provide a.

In the electrolytic solution of the present invention, the organic acid lithium-boron trifluoride complex is a lithium formate-boron trifluoride complex, lithium acetate-boron trifluoride complex, lithium oxalate-boron trifluoride complex, succinic acid. It is preferably at least one selected from the group consisting of lithium-boron trifluoride complexes.
In the electrolytic solution of the present invention, the amount of the first lithium salt: the amount of the organic acid lithium-boron trifluoride complex is 99.8: 0.2 to 50:50 (molar ratio). Is preferred.

In the electrolytic solution of the present invention, the first lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroboron (LiBF ₄ ), lithium bis (trifluoromethylsulfonyl) imide lithium (LiN (SO ₂ CF _{3) 2),} lithium perchlorate (LiClO _4), trifluoromethane sulfonic lithium _(LiCF 3 _SO 3), lithium hexafluoro antimonate (LiSbF _6), hexafluoroarsenate arsenate periodate lithium (LiAsF ₆ And at least one selected from the group consisting of lithium tetraphenylborate (LiB (C ₆ H ₅ ) ₄ ).

 In the electrolytic solution of the present invention, the non-aqueous solvent contains one or more selected from the group consisting of cyclic carbonates, chain carbonates, ethers, carboxylic esters, nitriles, amides and sulfones. It is preferable to be made.

 The present invention also provides a lithium ion secondary battery obtained by using the electrolytic solution of the present invention.

 The lithium ion secondary battery to which the electrolytic solution according to the present invention is applied is a lithium ion battery that prevents a capacity deterioration due to repeated charge and discharge by generating a stable SEI film at the time of initial charge, that is, a lithium having excellent cycle life characteristics. An ion secondary battery can be provided.

Embodiments of an electrolytic solution and a lithium ion secondary battery of the present invention will be described.
Note that this embodiment is specifically described in order to better understand the gist of the invention, and does not limit the present invention unless otherwise specified.

<Electrolyte>
The electrolytic solution of the present invention is characterized in that in the electrolytic solution in which the first lithium salt is dissolved in a non-aqueous solvent, an organic acid lithium-boron trifluoride complex is further blended as the second lithium salt. To do.
By adding an organic acid lithium-boron trifluoride complex to an electrolytic solution in which the first lithium salt is dissolved in a non-aqueous solvent, the electrolytic solution of the present invention generates a stable SEI film at the time of initial charge. Therefore, the cycle life characteristics are excellent, and it is suitable for application to a lithium ion secondary battery.

(First lithium salt)
The first lithium salt is not particularly limited as long as it dissolves in a non-aqueous solvent, and preferable ones include lithium hexafluorophosphate (LiPF ₆ ) and lithium tetrafluoroboron (LiBF ₄ ). , lithium perchlorate (LiClO _4), lithium hexafluoro antimonate (LiSbF _6), hexafluoroarsenate arsenate periodate lithium (LiAsF _6), lithium hexafluoro tantalate (LiTaF _6), lithium hexafluoro niobate ( LiNbF ₆ ), lithium tetraphenylborate (LiB (C ₆ H ₅ ) ₄ ), bis (fluorosulfonyl) imide lithium (LiN (FSO ₂ ) ₂ ), bis (trifluoromethylsulfonyl) imide lithium (LiN (SO ₂₎ CF _{3) 2),} bis (pentafluoroethyl sulfonyl) imide (LiN ( _{O 2 C 3 F 5) 2} ), trifluoromethane sulfonic lithium _(LiCF 3 _SO 3), can be mentioned lithium salt and the like.

The said 1st lithium salt may be used individually by 1 type, and may use 2 or more types together. When two or more kinds are used in combination, the combination and ratio may be appropriately selected according to the purpose.
The compounding quantity of said 1st lithium salt is not specifically limited, For example, what is necessary is just to adjust suitably according to the kind of said 1st lithium salt and solvent. Usually, it is preferable that the molar concentration of [the above-mentioned blending amount (number of moles) of the first lithium salt] / [total weight of blended substances] is 0.2 or more, and 0.5 or more. It is more preferable. By setting it as such a range, the ionic conductivity of electrolyte solution improves further. The upper limit of the molar concentration is not particularly limited as long as the effect of the present invention is not hindered, but is preferably 3.0, more preferably 2.0.

 The non-aqueous solvent includes cyclic carbonates such as ethylene carbonate, propylene carbonate and butylene carbonate, lactones such as gamma butyrolactone and bunvalerolactone, chain carbonates such as dimethyl carbonate, methyl ethyl carbonate and diethyl carbonate, tetrahydrofuran 1,2-dimethoxyethane, 2-methyltetrahydrofuran, 1,3-dioxolane, ethers such as 4-methyl-1,3-dioxolane, methyl formate, ethyl formate, propyl formate, methyl acetate, ethyl acetate, propyl acetate , Carboxylic acid esters such as methyl propionate, ethyl propionate and methyl butyrate, nitriles such as acetonitrile, glutaronitrile, adiponitrile, methoxyacetonitrile, 3-methoxypropionitrile S, N, N-dimethylformamide, N, N-amides such as dimethylacetamide and sulfolane, sulfones such as dimethyl sulfoxide can be exemplified.

 The said non-aqueous solvent may be used individually by 1 type, and may use 2 or more types together. When two or more kinds are used in combination, the combination and ratio may be appropriately selected according to the purpose.

(Organic acid lithium-boron trifluoride complex)
In the organic acid lithium-boron trifluoride complex, the portion of the organic acid lithium is preferably a lithium salt of a carboxylic acid. Further, the number of acid groups constituting the lithium salt in the portion of the organic acid lithium is not particularly limited.

 Preferred examples of the organic acid lithium-boron trifluoride complex include lithium formate-boron trifluoride complex, lithium acetate-boron trifluoride complex, lithium propionate-boron trifluoride complex, lithium butyrate-boron trifluoride complex. , Lithium isobutyrate-boron trifluoride complex, lithium oxalate-boron trifluoride complex, lithium lactate-boron trifluoride complex, lithium tartrate-boron trifluoride complex, lithium maleate-boron trifluoride complex, fumarate Lithium acid-boron trifluoride complex, lithium malonate-boron trifluoride complex, lithium succinate-boron trifluoride complex, lithium malate-boron trifluoride complex, lithium citrate-boron trifluoride complex, glutar Lithium acid-boron trifluoride complex, Lithium adipate-boron trifluoride complex, Lithium phthalate Um - boron trifluoride complex, lithium benzoate - boron trifluoride complex can be exemplified. More preferable examples of the organic acid lithium-boron trifluoride complex include lithium formate-boron trifluoride complex, lithium acetate-boron trifluoride complex, lithium oxalate-boron trifluoride complex, lithium succinate-trifluoride. An example is a boron complex.

An organic acid lithium-boron trifluoride complex may be used individually by 1 type, and may use 2 or more types together. When two or more kinds are used in combination, the combination and ratio may be appropriately selected according to the purpose. The compounding quantity of said 1st lithium salt is not specifically limited, For example, what is necessary is just to adjust suitably according to the kind of said 1st lithium salt and solvent. Usually, the blending amount of the first lithium salt: the blending amount of the organic acid lithium-boron trifluoride complex is preferably 99.8: 0.2 to 50:50 (molar ratio). : It is more preferable that it is 1-70: 30 (molar ratio). By setting it as such a range, better SEI is formed and the lithium ion secondary battery excellent in cycle life characteristics is obtained.
In addition, since the organic acid lithium-boron trifluoride complex is directly blended in the electrolytic solution of the present invention, the lithium carboxylate, the boron trifluoride and / or boron trifluoride complex, the organic solvent, Therefore, it does not contain the coordination bond component desorbed from the boron trifluoride complex as a residual impurity, and the coordination bond component is removed. Therefore, the purity is extremely high and the amount of impurities is small.

(Other ingredients)
The electrolyte solution of the present invention may contain other components in addition to the organic acid lithium-boron trifluoride complex as long as the effects of the present invention are not hindered.

(Method for producing electrolyte)
The electrolytic solution of the present invention can be produced by appropriately blending the first lithium salt, the non-aqueous solvent, the organic acid lithium-boron trifluoride complex, and other components as necessary. Each condition such as the order of addition, temperature, time and the like at the time of blending each component can be arbitrarily adjusted according to the type of the blended component.

<Gel electrolyte>
The gel electrolyte of the present invention is characterized in that a matrix polymer is further blended with the electrolytic solution of the present invention. The electrolytic solution is held in a matrix polymer. The gel electrolyte is preferably one that does not exhibit fluidity in an environment of 40 ° C. or lower where a lithium ion secondary battery is normally used.

(Matrix polymer)
The matrix polymer is not particularly limited, and those known in the gel electrolyte field can be used as appropriate.
Specific examples of preferred matrix polymers include polyether polymers such as polyethylene oxide and polypropylene oxide; polyvinyl fluoride, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyvinylidene fluoride-hexafluoride. Fluoropolymers such as acetone copolymers and polytetrafluoroethylene; polyacrylic polymers such as poly (meth) acrylate methyl, poly (meth) ethyl acrylate, polyacrylamide, polyacrylates containing ethylene oxide units; polyacrylonitrile; Polyphosphazene; polysiloxane can be exemplified.

The matrix polymer may be used alone or in combination of two or more. When two or more kinds are used in combination, the combination and ratio may be appropriately selected according to the purpose.
The blending amount of the matrix polymer is not particularly limited. For example, the blending amount of the matrix polymer in the total amount of the blending components may be appropriately adjusted according to the type of the solvent constituting the electrolytic solution. It is preferable that it is 50 mass%. By setting the lower limit value or more, the strength of the gel electrolyte is further improved, and by setting the lower limit value or less, the lithium ion secondary battery shows more excellent battery performance.

(Gel electrolyte production method)
The gel electrolyte of the present invention can be produced by appropriately blending the electrolytic solution of the present invention, the matrix polymer, and other components as necessary. Each condition such as the order of addition, temperature, time and the like at the time of blending each component can be arbitrarily adjusted according to the type of the blended component.

<Lithium ion secondary battery>
The lithium ion secondary battery of the present invention is obtained using the electrolytic solution of the present invention or the gel electrolyte of the present invention.
The lithium ion secondary battery of the present invention can have the same configuration as the conventional lithium ion secondary battery except that the electrolytic solution of the present invention or the gel electrolyte of the present invention is used. For example, a negative electrode, a positive electrode, A separator and the electrolytic solution are provided.

The material of the negative electrode is not particularly limited, but may be exemplified by metal lithium, lithium alloy, carbon-based material capable of inserting and extracting lithium, metal oxide, etc., and may be one or more selected from the group consisting of these materials. preferable.
The material of the positive electrode is not particularly limited, and examples thereof include transition metal oxides such as lithium cobaltate, lithium nickelate, lithium manganate, olivine type lithium iron phosphate, and lithium cobalt / nickel / manganese composite oxide. It is preferable that it is 1 or more types selected from the group which consists of.

 Although the material of the separator is not particularly limited, it can be exemplified by a microporous polymer film, a nonwoven fabric, glass fiber, and the like, and is preferably at least one selected from the group consisting of these materials.

 The shape of the lithium ion secondary battery of the present invention is not particularly limited, and can be adjusted to various shapes such as a cylindrical shape, a square shape, a coin shape, a sheet shape, and a laminate shape.

 What is necessary is just to manufacture the lithium ion secondary battery of this invention using the said electrolyte solution and an electrode according to a well-known method, for example in a glove box or dry air atmosphere.

 Hereinafter, the present invention will be described in more detail with reference to specific examples. However, the present invention is not limited to the following examples.

(1) Chemical substances used The chemical substances used in this example are shown below.
(A) Lithium hexafluorophosphate (LiPF ₆ ) (made by Kishida Chemical Co., Ltd.)
Lithium boron tetrafluoride (LiPF ₄ ) (manufactured by Kishida Chemical Co., Ltd.)
Bis (trifluoromethylsulfonyl) imidolithium (LiN (SO ₂ CF ₃ ) ₂ ) (manufactured by Kishida Chemical Co., Ltd.)
Lithium oxalate (Alfa)
(B) Nonaqueous solvent ethylene carbonate (hereinafter abbreviated as EC) (manufactured by Kishida Chemical Co., Ltd.)
Dimethyl carbonate (hereinafter abbreviated as DMC) (Kishida Chemical Co., Ltd.)
Gamma butyrolactone (hereinafter abbreviated as GBL) (manufactured by Kishida Chemical Co., Ltd.)

(2) Preparation of organic acid lithium-boron trifluoride complex (2-1) Preparation of lithium oxalate-boron trifluoride complex Lithium oxalate (22.3 g, 223 mmol) was weighed into a round bottom flask, Suspended in 200 mL DMC. To this, boron trifluoride diethyl ether complex (63.3 g, 446 mmol) was slowly added dropwise at 23 ° C., followed by stirring at room temperature (23 ° C.) for 24 hours. The reaction solution became transparent and insoluble matter was observed. It was confirmed that a uniform solution was obtained. Subsequently, the solvent and impurities were distilled off from the reaction solution using a rotary evaporator. Thereafter, the precipitated white solid was dried at 50 ° C. to obtain a white powder of lithium oxalate-boron trifluoride complex ((COOLi) _2. (BF ₃ ) ₂ ) (yield 96.5). %).

(3) Manufacture of electrolyte solution and cell All the operations in Examples and Comparative Examples shown below were performed in a dry box.
Table 1 shows the composition of the electrolytic solutions in the examples and comparative examples.

[Example 1]
<Manufacture of electrolyte>
A mixed solvent of EC and DMC (EC: DMC = 30: 70 (volume ratio)) as a nonaqueous solvent was weighed into LiPF ₆ (6.45 g, 42.5 mmol) in a sample bottle, and the concentration of lithium atoms was 1. The electrolyte solution (1) was obtained by mixing so that it might become 0 mol / kg. In addition, the lithium oxalate-boron trifluoride complex (1.12 g, 5.11 mmol) obtained in (2-1) above was mixed with EC and DMC as a nonaqueous solvent (EC: DMC = 30: 70 (volume ratio)) was weighed into a sample bottle and mixed so that the concentration of lithium atoms was 1.0 mol / kg to obtain an electrolytic solution (2).
The electrolytic solution used in this example was obtained by adding 10 parts by weight of the electrolytic solution (2) to 90 parts by weight of the electrolytic solution (1) obtained above. The blending amount of LiPF _{6 and} the blending amount of the lithium oxalate-boron trifluoride complex was 90:10 in molar ratio.

<Manufacture of coin cell>
A negative electrode (manufactured by Hosen Co., Ltd.) and a positive electrode (manufactured by Hosen Co., Ltd.) were punched into a disk shape having a diameter of 16 mm. Further, a glass fiber was punched into a disk shape having a diameter of 17 mm as a separator. The obtained positive electrode, separator and negative electrode were laminated in this order in a battery container made of SUS (CR2032), the separator, the negative electrode and the positive electrode were impregnated with the electrolytic solution obtained above, and a SUS plate was placed on the negative electrode. A coin-type cell was manufactured by placing (covering a thickness of 1.2 mm, a diameter of 16 mm) and capping.

[Reference example]
Except that only the electrolytic solution (1) obtained in Example 1 was used, an electrolytic solution and a coin-type cell were manufactured in the same manner as in the Example.

[Example 2]
<Manufacture of electrolyte>
LiPF ₆ (6.45 g, 42.5 mmol) was weighed in a sample bottle with a mixed solvent of EC, DMC and GBL (EC: DMC: GBL = 34: 33: 33 (volume ratio)) as a non-aqueous solvent. Electrolyte solution (1) was obtained by mixing so that an atom concentration might be 1.0 mol / kg. Further, the lithium oxalate-boron trifluoride complex (1.12 g, 5.11 mmol) obtained in (2-1) above was mixed with EC, DMC and GBL as a nonaqueous solvent (EC: DMC: GBL = 34: 33: 33 (volume ratio)) was weighed into a sample bottle and mixed so that the concentration of lithium atoms was 1.0 mol / kg to obtain an electrolytic solution (2).
The electrolytic solution used in this example was obtained by adding 10 parts by weight of the electrolytic solution (2) to 90 parts by weight of the electrolytic solution (1) obtained above. The blending amount of LiPF _{6 and} the blending amount of the lithium oxalate-boron trifluoride complex was 90:10 in molar ratio.
And the coin-type cell was manufactured by the method similar to Example 1 except having used the electrolyte solution obtained above.

[Example 3]
<Manufacture of electrolyte>
LiBF ₆ (1.86 g, 19.7 mmol) was weighed in a sample bottle with a mixed solvent of EC, DMC and GBL (EC: DMC: GBL = 34: 33: 33 (volume ratio)) as a non-aqueous solvent. Electrolyte solution (1) was obtained by mixing so that an atom concentration might be 1.0 mol / kg.
The electrolyte solution used in this example was obtained by adding 10 parts by weight of the electrolyte solution (2) obtained in Example 2 to 90 parts by weight of the electrolyte solution (1) obtained above. The blending amount of LiPF _{6 and} the blending amount of the lithium oxalate-boron trifluoride complex was 90:10 in molar ratio.
And the coin-type cell was manufactured by the method similar to Example 1 except having used the electrolyte solution obtained above.

[Example 4]
<Manufacture of electrolyte>
Sample of LiN (SO ₂ CF ₃ ) ₂ (7.25 g, 25.3 mmol) and a mixed solvent of EC, DMC and GBL (EC: DMC: GBL = 34: 33: 33 (volume ratio)) as a non-aqueous solvent The electrolyte solution (1) was obtained by measuring in a bottle and mixing so that the density | concentration of a lithium atom might be 1.0 mol / kg.
The electrolyte solution used in this example was obtained by adding 10 parts by weight of the electrolyte solution (2) obtained in Example 2 to 90 parts by weight of the electrolyte solution (1) obtained above. The blending amount of LiPF _{6 and} the blending amount of the lithium oxalate-boron trifluoride complex was 90:10 in molar ratio.
And the coin-type cell was manufactured by the method similar to Example 1 except having used the electrolyte solution obtained above.

[Example 5]
Except that the electrolyte solution was prepared by adding 0.2 parts by weight of the electrolyte solution (2) obtained in Example 2 to 99.8 parts by weight of the electrolyte solution (1) obtained in Example 2. An electrolytic solution was produced in the same manner as in Example 1, and a coin-type cell was further produced.

[Example 6]
Except that the electrolyte solution was prepared by adding 0.5 parts by weight of the electrolyte solution (2) obtained in Example 2 to 99.5 parts by weight of the electrolyte solution (1) obtained in Example 2. An electrolytic solution was produced in the same manner as in Example 1, and a coin-type cell was further produced.

[Example 7]
Except that the electrolyte solution was prepared by adding 1 part by weight of the electrolyte solution (2) obtained in Example 2 to 99 parts by weight of the electrolyte solution (1) obtained in Example 2, Example 1 and An electrolytic solution was manufactured by the same method, and a coin-type cell was manufactured.

[Example 8]
Example 1 is the same as Example 1 except that 5 parts by weight of the electrolytic solution (2) obtained in Example 2 was added to 95 parts by weight of the electrolytic solution (1) obtained in Example 2. An electrolytic solution was manufactured by the same method, and a coin-type cell was manufactured.

[Example 9]
Except that the electrolyte solution was prepared by adding 20 parts by weight of the electrolyte solution (2) obtained in Example 2 to 80 parts by weight of the electrolyte solution (1) obtained in Example 2. An electrolytic solution was manufactured by the same method, and a coin-type cell was manufactured.

[Example 10]
Except that the electrolyte solution was prepared by adding 50 parts by weight of the electrolyte solution (2) obtained in Example 2 to 50 parts by weight of the electrolyte solution (1) obtained in Example 2, and An electrolytic solution was manufactured by the same method, and a coin-type cell was manufactured.

[Comparative Example 1]
Except that only the electrolytic solution (1) obtained in Example 2 was used, an electrolytic solution was produced in the same manner as in Example 1, and further a coin-type cell was produced.

[Comparative Example 2]
Except that only the electrolytic solution (1) obtained in Example 3 was used, an electrolytic solution was produced in the same manner as in Example 1, and further a coin-type cell was produced.

[Comparative Example 3]
Except that only the electrolytic solution (1) obtained in Example 4 was used, an electrolytic solution was produced in the same manner as in Example 1, and further a coin-type cell was produced.

(4) Evaluation of Battery Performance For the coin-type cells of Examples 1 to 10 and Comparative Examples 1 to 3, a constant current and constant voltage charge of 0.2 C at 25 ° C. was set to an upper limit voltage of 4.2 V, and a current value of 0.1 C. Then, 0.2C constant current discharge was performed up to 2.7V. Thereafter, the charge / discharge current was set to 1 C and the charge / discharge cycle was repeated several times to several tens of times in the same manner to stabilize the state of the battery. Thereafter, the charge / discharge cycle was repeated in the same manner at a charge / discharge current of 1C, and the capacity retention rate at 100 cycles (discharge capacity at the 100th cycle (mAh) / discharge capacity at the 1st cycle (mAh)) × 100 Was calculated. The results are shown in Table 2.

As is clear from the results of the examples, by using an electrolytic solution containing a lithium oxalate-boron trifluoride complex as the second lithium salt, a system that does not use the second lithium salt as in the comparative example. In comparison, the capacity retention rate was improved and sufficient charge / discharge characteristics were exhibited.
In Examples 5 to 7, the capacity retention rate is lower than that of the reference example. This is because the safety of gamma-butyrolactone (which is a high-boiling solvent is increased because of the high-boiling solvent), but the battery has a higher safety. This is because the performance is adversely affected. When Comparative Examples 1 to 3 using gamma-butyrolactone as a solvent were compared with Examples 5 to 7, Examples 5 to 7 showed improved capacity retention and sufficient charge / discharge characteristics.

The present invention can be used in the field of lithium ion secondary batteries.

Claims (6)

 An electrolytic solution obtained by dissolving an organic acid lithium-boron trifluoride complex as a second lithium salt in an electrolytic solution in which a first lithium salt is dissolved in a non-aqueous solvent.  The organic acid lithium-boron trifluoride complex is selected from lithium formate-boron trifluoride complex, lithium acetate-boron trifluoride complex, lithium oxalate-boron trifluoride complex, lithium succinate-boron trifluoride complex. The electrolytic solution according to claim 1, wherein the electrolytic solution is one or more selected from the group consisting of:   The blending amount of the first lithium salt: The blending amount of the organic acid lithium-boron trifluoride complex is 99.8: 0.2 to 50:50 (molar ratio). 2. The electrolyte solution according to 2. The first lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium boron tetrafluoride (LiBF ₄ ), lithium bis (trifluoromethylsulfonyl) imide (LiN (SO ₂ CF ₃ ) ₂ ), lithium chlorate (LiClO _4), trifluoromethane sulfonic lithium _(LiCF 3 _SO 3), lithium hexafluoro antimonate (LiSbF _6), hexafluoroarsenate arsenate periodate lithium (LiAsF ₆₎ and lithium tetraphenylborate ( _{LiB (C 6 H 5) 4} ) electrolyte solution according to claim 1, characterized in that at least one member selected from the group consisting of.  The non-aqueous solvent is a mixture of one or more selected from the group consisting of cyclic carbonates, chain carbonates, ethers, carboxylic esters, nitriles, amides and sulfones. The electrolyte solution as described in any one of Claims 1-4.  A lithium ion secondary battery obtained by using the electrolytic solution according to claim 1.