JP5259996B2 - Electrolytic solution for lithium secondary battery and lithium secondary battery - Google Patents

Description

  The present invention relates to an electrolyte for a lithium secondary battery in which an initial irreversible capacity is reduced and a discharge capacity loss is suppressed, and a lithium secondary battery using the same.

  In recent years, the application field of lithium secondary batteries has been further improved in performance, such as improvement of output density and energy density, and suppression of capacity loss, along with the expansion of applications from electronic devices such as mobile phones, personal computers and digital cameras. It is being advanced. For in-vehicle use, durability higher than conventional is required on both the high temperature side and low temperature side of the use environment temperature. Particularly in a high temperature environment, since the cell is increased in size, the cell is constantly exposed to a relatively high temperature not only by the use environment but also by self-heating, and improvement of the high temperature durability is very important.

A conventional general lithium secondary battery uses a material capable of reversibly inserting Li ions into a positive electrode active material and a negative electrode active material. For example, a compound such as LiNiO ₂ , LiCoO ₂ , LiMn ₂ O ₄ , or LiFePO ₄ is used for the positive electrode active material. As the negative electrode active material, lithium metal, an alloy thereof, a carbon material, a graphite material, or the like is used. Further, as an electrolytic solution used for a lithium secondary battery, a solution obtained by dissolving an electrolyte such as LiPF ₆ or LiBF ₄ in a mixed solvent such as ethylene carbonate, diethyl carbonate, or propylene carbonate is used.

  Here, in the case of the conventional lithium secondary battery, in the negative electrode, a stable interface (Solid Electrolyte Interface: SEI) that has lithium ion conductivity but no electronic conductivity is formed between the electrolytes. The insertion process of Li ions is excellent in reversibility, but cracks and dissolution occur at the stable interface under high temperature environment, and as a result, the gradual decrease in lithium and the impedance increase over a long charge / discharge cycle. There is. As a result, for example, the capacity output gradually decreases as the number of charge / discharge cycles increases.

In order to deal with such problems, Patent Document 1 discloses that a phosphorous pentoxide (P ₂ O ₅ ) is dispersed and blended as a powder in a positive electrode and exposed to an electrolytic solution, whereby a battery after repeated long-term charge / discharge cycles is obtained. A lithium secondary battery for preventing a decrease in capacity output is disclosed. However, P ₂ O ₅ has low solubility in a typical organic solvent used for the electrolytic solution.

Patent Document 2 also states that a capacity decay rate suppressing additive containing a fluorinated boron compound selected from the group consisting of BF ₃ , BF ₃ complex, HBF ₄ and HBF ₄ complex is added to the non-aqueous electrolyte. There is disclosure. According to Patent Document 2, it is an object to provide a rechargeable non-aqueous lithium battery with an improved capacity decay rate by adding the capacity decay rate suppressing additive. However, in Patent Document 2, no consideration is given to Coulomb efficiency, and there is a problem that the initial irreversible capacity increases although the capacity decay rate can be suppressed by adding the additive.

Japanese Patent Laid-Open No. 8-329985 Japanese Patent Application Laid-Open No. 11-149943

  The present invention has been made in view of the above problems, and its object is to provide an electrolytic solution for a lithium secondary battery that is excellent in thermal stability and can reduce initial irreversible capacity and discharge capacity loss, and It is in providing the used lithium secondary battery.

The inventors of the present application have made various studies on improving the performance of the electrolyte solution for lithium secondary batteries in order to solve the conventional problems. As a result, it has been found that the electrolyte solution for lithium secondary batteries to which the PF ₅ complex is added is excellent in thermal stability and can reduce the initial irreversible capacity and the discharge capacity loss, thereby completing the present invention. It was.

  That is, the electrolytic solution for a lithium secondary battery of the present invention is an electrolytic solution for a lithium secondary battery in which a lithium salt is dissolved in an organic solvent in order to solve the above-described problem, and a PF5 complex is added. It is characterized by being.

In a lithium secondary battery, an irreversible reaction called reductive decomposition of an electrolyte occurs at the negative electrode during initial charging. As a result, a part of lithium from the positive electrode is inactivated in the negative electrode and is not used for charging / discharging, and a capacity difference (irreversible capacity) between the initial charging capacity and the initial discharging capacity occurs. With regard to such a problem of irreversible capacity, the present invention adds a PF ₅ complex to an organic solvent in which a lithium salt is dissolved. Thereby, for example, a lithium secondary excellent in reduction of irreversible capacity as compared with a conventional electrolytic solution to which a fluorinated boron compound selected from the group consisting of BF ₃ , BF ₃ complex, HBF ₄ and HBF ₄ complex is added. A battery electrolyte can be provided. Furthermore, the loss of discharge capacity can be suppressed even after exposure to a high temperature environment of 60 ° C., and the thermal stability can be improved.

  In the above configuration, a BF 3 complex may be further added to the organic solvent. Thereby, the loss of discharge capacity can be further suppressed.

  In the above configuration, the content of the PF5 complex is preferably in the range of 0.1 to 5% by weight with respect to the organic solvent. By making the addition amount 0.1% by weight or more, it is possible to sufficiently reduce the irreversible capacity in the initial charge / discharge, suppress the loss of the discharge capacity, and improve the thermal stability. Moreover, it can prevent that the solubility with respect to the organic solvent of lithium salt falls by making addition amount less than 5 weight%.

  In the above configuration, the content of the PF5 complex and the BF3 complex is preferably in the range of 1 to 5% by weight with respect to the organic solvent. By making the addition amount 1% by weight or more, it is possible to sufficiently reduce the irreversible capacity in the initial charge / discharge, suppress the loss of the discharge capacity, and improve the thermal stability. Moreover, it can prevent that the solubility with respect to the organic solvent of lithium salt falls by making addition amount less than 5 weight%.

Moreover, it is preferable that the said PF5 complex and BF3 complex have linear or cyclic carbonate. Formation of a PF ₅ complex and a BF ₃ complex can be facilitated by being a linear or cyclic carbonate.

  Furthermore, the lithium salt preferably has an anion containing fluorine.

  The lithium salt is preferably at least one selected from the group consisting of LiBF4, LiPF6, and LiN (CF3SO2) 2.

  The organic solvent is preferably a carbonate ester.

  A lithium secondary battery according to the present invention includes the above-described electrolyte for a lithium secondary battery.

  The lithium secondary battery having the above structure can reduce the irreversible capacity in the initial charge / discharge, suppress the loss of the discharge capacity even in a high temperature environment, and improve the thermal stability. .

The present invention has the following effects by the above means. That is, according to the present invention, since the PF ₅ complex is added to the organic solvent in which the lithium salt is dissolved, the irreversible capacity in the initial charge / discharge can be reduced, and the loss of the discharge capacity is suppressed even in a high temperature environment. In addition, thermal stability can be improved.

Embodiments of the present invention will be described below.
The electrolyte solution for a lithium secondary battery of the present invention (hereinafter referred to as “electrolyte solution”) is obtained by adding at least a PF ₅ complex to an organic solvent in which a lithium salt is dissolved. Further, the electrolytic solution of the present invention may be further added BF ₃ complex.

The PF ₅ complex or BF ₃ complex is a complex compound in which a compound having a nucleophilic site in the molecule is combined with the electrophilic force of PF ₅ or BF ₃ . The PF ₅ complex or BF ₃ complex is not particularly limited as long as it is relatively inert to the positive electrode, the negative electrode, and the organic solvent and does not hinder the normal operation of the lithium secondary battery. Furthermore, the compound having a nucleophilic moiety in the molecule is not particularly limited as long as it is stable to PF ₅ or BF ₃ and does not adversely affect the performance of the lithium secondary battery. Specific examples include cyclic carbonates, chain carbonates, phosphate esters, cyclic ethers, chain ethers, lactone compounds, chain esters, nitrile compounds, amide compounds, and sulfone compounds.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and the like. Of these, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferred. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate, methyl propyl carbonate, and methyl butyl carbonate. Of these, dimethyl carbonate and ethyl methyl carbonate are preferred. Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, ethyldimethyl phosphate, and diethylmethyl phosphate. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane. Examples of the lactone compound include γ-butyrolactone. Examples of the chain ester include methyl propionate, methyl acetate, ethyl acetate, and methyl formate. Examples of the nitrile compound include acetonitrile. Examples of the amide compound include dimethylformamide. Examples of the sulfone compound include sulfolane and methyl sulfolane. Moreover, what substituted at least partially the hydrogen of the hydrocarbon group contained in the said organic-solvent molecule | numerator with the fluorine can also be used suitably.

When only the PF ₅ complex is used, the addition amount thereof is preferably in the range of 0.1 to 5% by weight, more preferably in the range of 1 to 3% by weight, based on the weight of the organic solvent. preferable. The lower limit of the amount added varies depending on the expected effect level and usage conditions, but if it is less than 0.1% by weight, the suppression of the discharge capacity loss and the improvement of the thermal stability are insufficient. There is a case. On the other hand, when the addition amount exceeds 5% by weight, there is an effect that the solubility of the lithium salt in the organic solvent is lowered, and the reduction of the irreversible capacity may be insufficient. In addition, the said discharge capacity shows the capability of the lithium secondary battery represented with the quantity of electricity, and is represented by the product of a discharge current (A) and the discharge duration (h) to discharge end voltage.

When a PF ₅ complex and a BF ₃ complex are mixed and used, the total amount added is preferably in the range of 1 to 5% by weight, and in the range of 2 to 5% by weight, based on the weight of the organic solvent. It is more preferable that The lower limit of the amount added varies depending on the expected effect level and use conditions, but if it is less than 1% by weight, the reduction of the irreversible capacity becomes insufficient, the loss of the discharge capacity is suppressed, and the thermal stability. Improvement may be insufficient. On the other hand, when the addition amount exceeds 5% by weight, there is an effect that the solubility of the lithium salt in the organic solvent is lowered, and the reduction of the irreversible capacity may be insufficient. In addition, the mixing ratio of the PF ₅ complex and the BF ₃ complex is preferably 90:10 to 10:90, more preferably 70:30 to 50:50 in terms of weight ratio.

The method for producing the PF ₅ complex or BF ₃ complex is not particularly limited, and for example, it can be obtained by blowing PF ₅ gas or BF ₃ gas into a compound having a nucleophilic site in the molecule. When the PF ₅ gas or BF ₃ gas is absorbed by the compound, it is useful to carry out cooling while suppressing excessive reaction. The complex after absorption can remove unreacted gas by nitrogen flow. Also, if the PF ₅ complex or BF ₃ complex is a product is a solid, obtained by filtration after absorption of PF ₅ gas or BF ₃ gas.

A lithium salt is dissolved in the organic solvent. Thereby, the lithium ion in electrolyte solution can be made into a mobile substance, and a battery characteristic can be improved. Although it does not specifically limit as said lithium salt, What has an anion containing a fluorine is preferable. Examples of the lithium salt having a fluorine-containing anion include LiBF ₄ , LiPF ₆ , LiBF ₃ CF ₃ , LiBF ₃ C ₂ F ₅ , LiCF ₃ SO ₃ , LiC ₂ F ₅ SO ₃ , LiC ₃ F ₇ SO ₃ , LiC ₄ F ₉ SO ₃ , LiN (SO ₂ F) ₂ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) (CF ₃ CO), LiN ( CF ₃ SO ₂ ) (C ₂ F ₅ SO ₂ ), LiC (CF ₃ SO ₂ ) _{3 and the} like. Among these, LiBF ₄ , LiPF ₆ , LiN (CF ₃ SO ₂ ) ₂ from the viewpoint of improving the safety and stability of the electrolytic solution, and improving the electrical conductivity of the lithium salt and the cycle characteristics of the lithium secondary battery. Are preferable, and LiBF ₄ and LiPF ₆ are particularly preferable. Moreover, these lithium salts can be used individually by 1 type or in mixture of 2 or more types.

  The concentration of the lithium salt with respect to the organic solvent is usually 0.1 to 2M, preferably 0.15 to 1.5M, more preferably 0.2 to 1.2M, and particularly preferably 0.3 to 1M. If the concentration is less than 0.1M, the electrical conductivity of the electrolytic solution may be insufficient. On the other hand, if the concentration exceeds 2M, the electrical conductivity may decrease due to an increase in the viscosity of the electrolytic solution, and the battery performance may decrease.

  The organic solvent is not particularly limited, and examples thereof include cyclic carbonate ester, chain carbonate ester, phosphate ester, cyclic ether, chain ether, lactone compound, chain ester, nitrile compound, amide compound, sulfone compound and the like. It is done. Of these organic solvents, carbonates are preferred from the point of being generally used as an organic solvent for a lithium secondary battery.

  Examples of the cyclic carbonate include ethylene carbonate, propylene carbonate, butylene carbonate, and the like. Among these, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferable from the viewpoint of improving the charging efficiency of the lithium secondary battery. Examples of the chain carbonate include dimethyl carbonate, ethyl methyl carbonate, diethyl carbonate and the like. Among these, dimethyl carbonate and ethyl methyl carbonate are preferable from the viewpoint of improving the charging efficiency of the lithium secondary battery. Examples of the phosphate ester include trimethyl phosphate, triethyl phosphate, ethyldimethyl phosphate, diethylmethyl phosphate, and the like. Examples of the cyclic ether include tetrahydrofuran and 2-methyltetrahydrofuran. Examples of the chain ether include dimethoxyethane. Examples of the lactone compound include γ-butyrolactone. Examples of the chain ester include methyl propionate, methyl acetate, ethyl acetate, and methyl formate. Examples of the nitrile compound include acetonitrile. Examples of the amide compound include dimethylformamide. Examples of the sulfone compound include sulfolane and methyl sulfolane. These organic solvents may be used alone or in combination of two or more. Moreover, what substituted at least partially the hydrogen of the hydrocarbon group contained in the said organic-solvent molecule | numerator with the fluorine can be used suitably.

  The electrolytic solution described above can be suitably used for a lithium secondary battery. Next, a lithium secondary battery according to the present embodiment using the electrolytic solution will be described.

As shown in FIG. 1, the lithium secondary battery according to the present embodiment has a positive electrode 1, a separator 3, a negative electrode 2, a spacer in an internal space formed by a positive electrode can 4 and a negative electrode can 5 from the positive electrode can 4 side. 7 has a structure in which laminated bodies laminated in the order of 7 are accommodated. By interposing a spring 8 between the negative electrode can 5 and the spacer 7, the positive electrode 1 and the negative electrode 2 are appropriately pressed and fixed. PF ₅ complex, or electrolyte containing PF ₅ complex and BF ₃ complexes, the positive electrode 1, are impregnated between the separator 3 and the negative electrode 2. In a state where the gasket 6 is interposed between the positive electrode can 4 and the negative electrode can 5, the positive electrode can 4 and the negative electrode can 5 are sandwiched to bond them together, and the laminate is sealed.

The material of the positive electrode active material layer in the positive electrode 1 is not particularly limited, and examples thereof include a transition metal compound having a structure capable of diffusing lithium ions, or a specialized metal compound thereof and an oxide of lithium. _{Specifically, LiCoO 2, LiNiO 2, LiMn} 2 O 4, LiFePO 4 , and the like.

  The positive electrode is formed by press-molding the positive electrode active material listed above together with a known conductive auxiliary agent or binder, or the positive electrode active material together with a known conductive auxiliary agent or binder into an organic solvent such as pyrrolidone. It can be obtained by applying a mixture and pasting it to a current collector such as an aluminum foil, followed by drying.

The material of the negative electrode active material layer in the negative electrode 2 is not particularly limited as long as it is a material capable of inserting and extracting lithium. For example, lithium metal, Sn—Cu, Sn—Co, Sn—Fe or Sn -Alloys such as Ni, metal oxides such as Li ₄ Ti ₅ O ₁₂ or Li ₅ Fe ₂ O ₃ , natural graphite, artificial graphite, boronated graphite, mesocarbon microbeads, pitch-based carbon fiber graphitized materials, carbon nanotubes, etc. Carbon materials and the like.

  The negative electrode can be a foil or powder of the electrode material. In the case of powder, copper paste is formed by pressure molding with a known conductive aid and binder, or mixed with pyrrolidone and other organic solvents together with a known conductive aid and binder. It can be obtained by coating a current collector such as a foil and then drying.

  In the lithium secondary battery according to the present embodiment, a separator 3 is usually interposed between the positive electrode and the negative electrode in order to prevent a short circuit between the positive electrode and the negative electrode. The material and shape of the separator 3 are not particularly limited, but it is preferable that the above-described organic electrolytic solution easily pass therethrough and is an insulator and a chemically stable material. Examples thereof include microporous films and sheets made of various polymer materials. Specific examples of the polymer material include polyolefin polymers such as nylon (registered trademark), nitrocellulose, polyacrylonitrile, polyvinylidene fluoride, polyethylene, and polypropylene. From the viewpoints of electrochemical stability and chemical stability, polyolefin polymers are preferred.

  The optimum working voltage of the lithium secondary battery of the present invention is not particularly limited by the combination of the positive electrode and the negative electrode, but can be used at an average discharge voltage of 2.4 to 4.5V.

  The shape of the lithium secondary battery of the present invention is not particularly limited, and examples thereof include a cylindrical type, a square type, and a laminate type in addition to the coin type cell shown in FIG.

  Hereinafter, preferred embodiments of the present invention will be described in detail by way of example. However, the materials, blending amounts, and the like described in the examples are not intended to limit the scope of the present invention only to them, but are merely illustrative examples, unless otherwise specified.

Example 1
<Creation of electrolyte>
LiPF ₆ has a dew point of −80 ° C. with respect to a mixed solvent composed of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) (volume ratio of EC: EMC = 1: 3, manufactured by Kishida Chemical Co., Ltd., lithium battery grade). It was dissolved in the following argon atmosphere dry box to prepare an electrolyte concentration of 1.0 mol / liter. Next, 3% by weight of PF ₅ -EMC complex was added to the organic solvent to prepare an electrolytic solution according to this example. When the water content of this electrolyte was measured with a Karl Fischer moisture meter (Hiranuma Sangyo Co., Ltd., Hiranuma trace moisture measuring device AQ-7), it was confirmed that it was 30 ppm or less.

<Assembly of lithium secondary battery>
A lithium secondary battery was prepared using a test cell having the structure shown in FIG. 2 (trade name: Tomcell TJ-AC, manufactured by Nippon Tomcell Co., Ltd.). In FIG. 2, the bolt 9, the caulking washer 10, the container 11, the lid 14, the nut 16, the round plate 20, and the spring 21 are all made of stainless steel, and the separator 18 is made of polypropylene and cut into a circular shape. used. The lithium secondary battery was assembled in an argon dry box having a dew point of −80 ° C. or lower.

  First, the bolt 9, the caulking washer 10, the container body 11, the spacer 12, the O-ring 13, the lid 14, the bush 15, the nut 16, the negative electrode 17, the round plate 20, and the spring 21 are vacuum-dried for 24 hours under heating at 120 ° C. After that, it was brought into the dry box. The separator 18 was vacuum-dried for 24 hours under heating at 60 ° C. A natural graphite sheet (product name: Pioxel A100, manufactured by Pionics Co., Ltd.) was used for the negative electrode, cut into a circular shape, vacuum-dried for 12 hours, and then brought into a dry box. In addition, lithium foil foil (the Honjo Metal Co., Ltd. make, battery grade) was used for the positive electrode.

  Next, after impregnating the negative electrode 17 and the separator 18 under reduced pressure, the negative electrode 17, the separator 18, and the spacer 12 are placed on the container body 11 in the order shown in FIG. An appropriate amount was poured. Subsequently, the positive electrode 19, the round plate 20 and the spring 21 were installed in the order shown in FIG. 1, and then the lid 14 was covered from above, and the container body was hermetically sealed with the bolts 9 and nuts 16 to produce a lithium secondary battery. .

(Example 2)
An electrolytic solution and a lithium secondary battery using the same were prepared in the same manner as in Example 1 except that the amount of the PF ₅ -EMC complex added to the organic solvent was changed to 1.5% by weight.

(Example 3)
As in Example 1 except that a mixture of PF ₅ -EMC complex and BF ₃ -EMC complex was used as an additive to be added to the organic solvent, and the addition amount was changed to 3.0% by weight. Then, an electrolytic solution and a lithium secondary battery using the electrolytic solution were produced. The mixing ratio of the PF ₅ -EMC complex and the BF ₃ -EMC complex was 50:50 by weight.

(Comparative Example 1)
As the electrolytic solution, the electrolytic solution and the same were used in the same manner as in Example 1 except that a solution of LiPF ₆ dissolved in a mixed solvent composed of ethylene carbonate (EC) and ethyl methyl carbonate (EMC) was used. A lithium secondary battery was manufactured.

(Comparative Example 2)
An electrolyte solution and a lithium secondary battery using the same were prepared in the same manner as in Example 1 except that BF ₃ -EMC complex was used as an additive to be added to the organic solvent.

(Evaluation of lithium secondary battery characteristics)
A charge / discharge test was performed on each lithium secondary battery produced in each of the examples and comparative examples. The charge / discharge test was carried out in a thermostatic chamber (ESPEC Corporation, trade name: TEMPERATURE CARBINET LU-112) kept in an atmosphere at 25 ° C. That is, after holding the test cell in the thermostat for 2 hours, constant current charging with a current density of 1.0 mAcm ⁻² was performed, and switching to constant voltage charging was performed when the voltage reached 5 mV. After holding at 5 mV for 30 minutes, a constant current discharge of 1.0 mAcm ⁻² was performed, and when the voltage reached 1600 mV, the voltage was switched to constant voltage discharge and held for 30 minutes. This charge / discharge cycle was repeated 10 times.

  Thereafter, the lithium secondary battery is left in a drier maintained at 60 ° C. for 7 days, transferred to a constant temperature oven maintained at 25 ° C. again, and 10 cycles of charge / discharge are repeated as in the charge / discharge conditions. It was.

  Table 1 shows the discharge capacity at the 10th cycle at 25 ° C. before holding at 60 ° C., and the discharge capacity at the first and 10th cycles at 25 ° C. after holding at 60 ° C. In addition, each value shows a relative value when the discharge capacity of Comparative Example 1 in the 10th cycle at 25 ° C. before holding at 60 ° C. is 100.

  As is clear from Table 1, the lithium secondary batteries according to Examples 1 to 3 have a lower discharge capacity loss after holding at 60 ° C. than the lithium secondary battery of Comparative Example 1. It was also confirmed that the initial irreversible capacity before holding at 60 ° C. was smaller than that of the lithium secondary battery of Comparative Example 2.

It is a cross-sectional schematic diagram which shows the outline of the lithium secondary battery which concerns on one Embodiment of this invention. It is a cross-sectional schematic diagram which shows the outline of the lithium secondary battery used in the Example of this invention.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Positive electrode 2 Negative electrode 3 Separator 4 Positive electrode can 5 Negative electrode can 6 Gasket 7 Spacer 8 Spring 9 Bolt 10 Caulking washer 11 Container body 12 Spacer 13 O-ring 14 Lid 15 Bush 16 Nut 17 Negative electrode 18 Separator 19 Positive electrode 20 Round plate 21 Spring

Claims (8)

An electrolyte for a lithium secondary battery in which a lithium salt is dissolved in an organic solvent , wherein a PF ₅ complex is added, and the content of the PF ₅ complex is 0.1 to 5% by weight with respect to the organic solvent. electrolyte for lithium secondary batteries, characterized in der Rukoto range of. The electrolyte solution for a lithium secondary battery according to claim 1, wherein a BF ₃ complex is further added to the organic solvent. _{3. The} electrolyte for a lithium secondary battery according to claim 2 , wherein the content of the PF ₅ complex and the BF ₃ complex is in the range of 1 to 5 wt% with respect to the organic solvent. The electrolyte solution for a lithium secondary battery according to claim 2, wherein the PF ₅ complex and the BF ₃ complex have a linear or cyclic carbonate . The electrolyte solution for a lithium secondary battery according to any one of claims 1 to 4, wherein the lithium salt has an anion containing fluorine . 6. The electrolyte solution for a lithium secondary battery according to claim 5 , wherein the lithium salt is at least one selected from the group consisting of LiBF ₄ , LiPF _6, and LiN (CF ₃ SO ₂ ) ₂ . The electrolyte solution for a lithium secondary battery according to any one of claims 1 to 6 , wherein the organic solvent is a carbonate ester . The lithium secondary battery provided with the electrolyte solution for lithium secondary batteries of any one of Claims 1-7 .

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