JP2003272701A - Nonaqueous electrolyte and lithium secondary battery using the same - Google Patents

Abstract

<P>PROBLEM TO BE SOLVED: To provide a lithium secondary battery with high battery characteristics such as cycle characteristics, electric capacity, and a shelf life. <P>SOLUTION: This nonaqueous electrolyte prepared by dissolving electrolyte salts in nonaqueous solvents contains at least one kind of pentafluorophenyl compounds selected from pentafluorophenyl ether compounds, pentafluorobiphenyl compounds, and pentafluorophenyl alkane compounds in the nonaqueous electrolyte. <P>COPYRIGHT: (C)2003,JPO

Description

Detailed Description of the Invention

[0001]

TECHNICAL FIELD The present invention relates to a non-aqueous electrolyte capable of providing a lithium secondary battery having excellent battery characteristics such as cycle characteristics, electric capacity, and storage characteristics, and a lithium using the same. Regarding secondary batteries.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries have been widely used as driving power sources for small electronic devices and the like. A lithium secondary battery is mainly composed of a positive electrode, a non-aqueous electrolytic solution and a negative electrode. In particular, a lithium secondary battery using a lithium composite oxide such as LiCoO ₂ as a positive electrode and a carbon material or lithium metal as a negative electrode is It is preferably used. And
As the non-aqueous electrolyte solution for the lithium secondary battery, ethylene carbonate (EC), propylene carbonate (P
Carbonates such as C) are preferably used.

[0003]

However, regarding the battery cycle characteristics and battery characteristics such as electric capacity,
There is a demand for secondary batteries having even more excellent characteristics.
As the positive electrode, for example, LiCoO ₂ , LiMn ₂ O ₄ , Li
In a lithium secondary battery using NiO ₂ or the like, a solvent in a non-aqueous electrolyte is partially oxidatively decomposed during charging, and the decomposed product inhibits a desired electrochemical reaction of the battery. This results in poor performance. This is probably due to the electrochemical oxidation of the solvent at the interface between the positive electrode material and the non-aqueous electrolyte. Further, for example, a lithium secondary battery using a highly crystallized carbon material such as natural graphite or artificial graphite as a negative electrode, the solvent in the non-aqueous electrolytic solution is reductively decomposed on the surface of the negative electrode during charging, and as a non-aqueous electrolytic solution solvent. Even in the generally widely used EC, a part of reductive decomposition occurs during repeated charging and discharging, resulting in deterioration of battery performance. Therefore, at present, the battery cycle characteristics and battery characteristics such as electric capacity are not always satisfactory.

The present invention solves the problems relating to the non-aqueous electrolyte for a lithium secondary battery as described above, is excellent in battery cycle characteristics, and is also excellent in battery characteristics such as electric capacity and storage characteristics in a charged state. Another object of the present invention is to provide a non-aqueous electrolyte solution that can be used in a lithium secondary battery that can form a lithium secondary battery, and a lithium secondary battery using the same.

[0005]

The present invention provides a non-aqueous electrolytic solution in which an electrolyte salt is dissolved in a non-aqueous solvent, wherein the non-aqueous electrolytic solution contains a pentafluorophenyl ether compound, a pentafluorobiphenyl compound and a pentafluoro The present invention relates to a non-aqueous electrolytic solution containing at least one pentafluorophenyl compound selected from phenylalkane compounds. The present invention also provides a positive electrode,
A lithium secondary battery comprising a non-aqueous electrolytic solution in which an electrolyte salt is dissolved in a negative electrode and a non-aqueous solvent, wherein the non-aqueous electrolytic solution contains a pentafluorophenyl ether compound, a pentafluorobiphenyl compound and a pentafluorophenyl alkane compound. The present invention relates to a lithium secondary battery containing at least one pentafluorophenyl compound.

[0006]

BEST MODE FOR CARRYING OUT THE INVENTION In the present invention, the pentafluorophenyl ether compound has the following general formula (I):

[Chemical 7] (In the formula, R ₁ represents an alkyl group having 1 to 12 carbon atoms or a haloalkyl group having 1 to 12 carbon atoms). Further, the pentafluorobiphenyl compound has the following general formula (II),

[Chemical 8] _(Wherein, shows the _{R 2, R 3, R 4} , R 5, R 6 are each independently a hydrogen atom, a halogen atom, an alkoxy group having 1 to 12 carbon atoms, an acyloxy group having 2 to 12 carbon atoms.) Compounds represented by are preferred. Further, the pentafluorophenylalkane is represented by the following general formula (III),

[Chemical 9] (In the formula, R ₇ represents an alkyl group having 1 to 12 carbon atoms.)
Compounds represented by are preferred.

In the present invention, specific examples of the compounds represented by the above general formulas (I) to (III) contained in the non-aqueous electrolyte will be described in detail below.

In the above general formula (I), R ₁ is an alkyl group having 1 to 12 carbon atoms or a haloalkyl group having 1 to 12 carbon atoms such as methyl group, ethyl group, propyl group, butyl group, pentyl group and hexyl group. Substituents such as a group, a heptyl group, an octyl group, a nonyl group, a decyl group and a dodecyl group can be mentioned. Further, a branched alkyl group such as an isopropyl group, a tert-butyl group and a 2-ethylhexyl group can be mentioned. Further, a haloalkyl group in which at least one of the hydrogen atoms contained in the substituent is substituted with a halogen atom can be mentioned, and specific examples thereof include a trifluoromethyl group, a 1,2-dichloroethyl group, a pentafluoroethyl group and a hepta. A fluoropropyl group may be mentioned. Further, at least one alkyl group substituted with an aromatic group of the hydrogen atom of the above substituent group such as a benzyl group, or an allyl group _{(CH 2 = CH-CH 2} -) of such a methylene group (CH ₂ Examples of the alkyl group include a substituent having an unsaturated bond such as =). Specific examples of the pentafluorophenyl ether compound having an alkyl group or a haloalkyl group include 2,3,4,5,6
Pentafluoroanisole, 2 ', 2', 2 '-(trifluoroethoxy) pentafluorobenzene, pentafluoro- (2,2,3,3-tetrafluoropropoxy) benzene and the like are preferable.

In the general formula (II), R ₂ ,
R ₃ , R ₄ , R ₅ , and R ₆ are each independently a hydrogen atom,
A halogen atom, an alkoxy group having 1 to 12 carbon atoms, and an acyloxy group having 2 to 12 carbon atoms are preferable. Examples of the halogen atom include a fluorine atom, a chlorine atom and a bromine atom. Further, as the alkoxy group having 1 to 12 carbon atoms, methoxy group, ethoxy group, propoxy group, butoxy group, pentyloxy group, hexyloxy group, heptyloxy group, octyloxy group, nonyloxy group, decyloxy group, dodecyloxy group. Substituents such as Further, a branched alkoxy group such as an isopropoxy group, a tert-butoxy group and a 2-ethylhexyloxy group can be mentioned. Further, a substituent in which at least one of the hydrogen atoms contained in the substituent is substituted with a halogen atom or an aryl group having 6 to 18 carbon atoms, and specific examples thereof include a 1-chloroethoxy group and a 2-chloroethoxy group. Base,
Examples thereof include a 2,2,2-trifluoroethoxy group, a 2,2,2-trichloroethoxy group, and an alkoxy group such as a benzyloxy group. Furthermore, carbon number 2 to 1
Examples of the acyloxy group of 2 include methylcarbonyloxy group, ethylcarbonyloxy group, propylcarbonyloxy group, butylcarbonyloxy group, pentylcarbonyloxy group, hexylcarbonyloxy group, heptylcarbonyloxy group, octylcarbonyloxy group, nonylcarbonyloxy group. And substituents such as a group, a decylcarbonyloxy group and a dodecylcarbonyloxy group. Also included are branched acyloxy groups such as the isopropoxycarbonyloxy group. Specific pentafluorobiphenyl compounds include decafluorobiphenyl, 2,3,4,5,6-pentafluoro-1,1'-
Biphenyl, 2,3,5,6-tetrafluoro-4-pentafluorophenylanisole, 2-bromo-2 ',
3,3 ', 4,4'5,5', 6,6'-nonafluorobiphenyl, 4-acetoxy-2,2 ', 3,3',
Suitable examples include 4 ', 5,5', 6,6'-nonafluorobiphenyl and the like.

Further, in the general formula (III), R ₇
Examples of the alkyl group having 1 to 12 carbon atoms include substituents such as methyl group, ethyl group, propyl group, butyl group, pentyl group, hexyl group, heptyl group, octyl group, nonyl group, decyl group and dodecyl group. Can be mentioned. Further, a branched alkyl group such as an isopropyl group, a tert-butyl group and a 2-ethylhexyl group can be mentioned. Further, at least one alkyl group substituted with an aromatic group of the hydrogen atom of the above substituent group such as a benzyl group, or an allyl group _{(CH 2 = CH-CH 2} -) of such a methylene group (CH ₂ Examples of the alkyl group include a substituent having an unsaturated bond such as =). Specific pentafluorophenylalkane compounds include 2,3,4,5,6-
Pentafluorotoluene is preferred.

In the present invention, one or more pentafluorophenyl compounds selected from the pentafluorophenyl ether compound, pentafluorobiphenyl compound and pentafluorophenylalkane compound are contained in the non-aqueous electrolyte. If the content of the pentafluorophenyl compound contained in the non-aqueous electrolyte is excessively large, the battery performance may deteriorate, and if it is excessively small, the expected sufficient battery performance cannot be obtained. Therefore, its content is 0.0 with respect to the weight of the non-aqueous electrolyte.
The range of 1 to 20% by weight, preferably 0.05 to 10% by weight, and particularly preferably 0.1 to 5% by weight may improve the cycle characteristics.

Examples of the non-aqueous solvent used in the present invention include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC) and vinylene carbonate (VC), and γ-butyrolactone. Such as lactones, dimethyl carbonate (DMC), methyl ethyl carbonate (M
EC), chain carbonates such as diethyl carbonate (DEC), tetrahydrofuran, 2-methyltetrahydrofuran, 1,4-dioxane, 1,2-dimethoxyethane, 1,2-diethoxyethane, 1,2-dibutoxyethane And ethers, nitriles such as acetonitrile, esters such as methyl propionate, methyl pivalate and octyl pivalate, and amides such as dimethylformamide.

These non-aqueous solvents may be used alone or in combination of two or more.
The combination of non-aqueous solvents is not particularly limited, for example,
A combination of cyclic carbonates and chain carbonates, a combination of cyclic carbonates and lactones,
Various combinations such as a combination of three types of cyclic carbonates and chain carbonates can be mentioned.

Examples of the electrolyte salt used in the present invention include LiPF ₆ , LiBF ₄ , LiClO ₄ , and LiN.
(SO ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC
(SO ₂ CF ₃ ) ₃ , LiPF ₄ (CF ₃ ) ₂ , LiPF
₃ (C ₂ F ₅ ) ₃ , LiPF ₃ (CF ₃ ) ₃ , LiPF ₃ (is
_{o-C 3 F 7) 3} , LiPF 5 (iso-C 3 F 7) , and the like. These electrolyte salts may be used alone or in combination of two or more. These electrolyte salts are used after being dissolved in the above-mentioned non-aqueous solvent at a concentration of usually 0.1 to 3M, preferably 0.5 to 1.5M.

The electrolytic solution of the present invention can be obtained, for example, by mixing the above-mentioned non-aqueous solvent, dissolving the above electrolyte salt therein, and dissolving at least one of the pentafluorophenyl compounds.

The non-aqueous electrolyte of the present invention is used as a constituent member of a secondary battery, especially a lithium secondary battery. The constituent members other than the non-aqueous electrolyte that constitute the secondary battery are not particularly limited, and various conventionally used constituent members can be used.

For example, a composite metal oxide with lithium containing cobalt or nickel is used as the positive electrode active material. Only one kind of these positive electrode active materials may be selected and used, or two or more kinds thereof may be used in combination. Examples of such a composite metal oxide include:
LiCoO ₂ , LiNiO ₂ , LiCo _1-x Ni _x O
₂ (0.01 <x <1) and the like. Also, Li
CoO ₂ and LiMn ₂ O ₄ , LiCoO ₂ and LiNiO ₂ ,
It may be used by suitably combined as LiMn ₂ O ₄ and LiNiO _2.

In the positive electrode, a conductive agent such as acetylene black or carbon black and polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVDF), a copolymer of styrene and butadiene (S) is used as the positive electrode active material.
BR), a copolymer of acrylonitrile and butadiene (N
BR), carboxymethyl cellulose (CMC) and other binders to be kneaded into a positive electrode mixture, and the positive electrode material is rolled into an aluminum foil or a stainless steel lath plate as a current collector and then heated at 50 ° C to 250 ° C. It is produced by heat treatment under vacuum at a temperature of about C for about 2 hours.

As the negative electrode (negative electrode active material), lithium metal, lithium alloy, and carbon materials capable of inserting and extracting lithium (pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite, etc.), organic materials Materials such as molecular compound combustor, carbon fiber], or composite tin oxide are used. In particular, the plane spacing (d ₀₀₂ ) of the lattice plane ( ₀₀₂ )
It is preferable to use a carbon material having a graphite-type crystal structure having a value of 0.335 to 0.340 nm. Only one of these negative electrode active materials may be selected and used,
You may use in combination of 2 or more types. Powder materials such as carbon materials include ethylene propylene diene terpolymer (EPDM) and polytetrafluoroethylene (PDM).
TFE, polyvinylidene fluoride (PVDF), copolymer of styrene and butadiene (SBR), copolymer of acrylonitrile and butadiene (NBR), carboxymethyl cellulose (CMC), etc., and kneaded as a negative electrode mixture. used. The method for producing the negative electrode is not particularly limited, and the negative electrode can be produced by the same method as the above-described method for producing the positive electrode.

The structure of the lithium secondary battery is not particularly limited, and is a coin type battery having a positive electrode, a negative electrode and a single-layer or multi-layer separator, and a cylindrical battery having a positive electrode, a negative electrode and a roll-shaped separator. An example is a square battery or the like. As the separator, a well-known polyolefin microporous film, woven fabric, non-woven fabric, or the like is used.

[0021]

EXAMPLES Next, the present invention will be specifically described with reference to Examples and Comparative Examples. Example 1 [Preparation of non-aqueous electrolyte] PC: DMC (volume ratio) = 1: 2
A non-aqueous solvent to prepare the, LiPF ₆ thereto as an electrolyte salt
Was dissolved to a concentration of 1M to prepare a non-aqueous electrolyte, and then 2,3,4,5,6-pentafluoroanisole was added to 0.5 wt% with respect to the non-aqueous electrolyte. Added to.

[Preparation of Lithium Secondary Battery and Measurement of Battery Characteristics] 90% by weight of LiCoO ₂ (positive electrode active material), 5% by weight of acetylene black (conductive agent), and 5% by weight of polyvinylidene fluoride (binder). %, And a mixture of 1-methyl-2-pyrrolidone solvent added and mixed was applied onto an aluminum foil, dried, pressure-molded, and heat-treated to prepare a positive electrode. 90% artificial graphite (negative electrode active material)
Wt%, polyvinylidene fluoride (binder) 10 wt%
1-Methyl-2-pyrrolidone solvent was added to this, and the mixture was applied onto a copper foil, dried, pressure-molded, and heat-treated to prepare a negative electrode. Then, using a polypropylene microporous film separator, the above non-aqueous electrolyte was injected to prepare a coin battery (diameter 20 mm, thickness 3.2 mm). Using this coin battery, at room temperature (20 ° C.), constant current and constant voltage of 1.1 mA,
The battery was charged to a final voltage of 4.2 V for 5 hours, then discharged to a final voltage of 2.7 V under a constant current of 1.1 mA, and this charging / discharging was repeated. The initial charge and discharge capacity was 1M LiPF ₆ -EC / DEC without addition of pentafluorophenyl compound.
It was 0.99 as compared with the case where (capacity ratio 3/7) was used as the non-aqueous electrolyte (the initial charge / discharge capacity of Comparative Example 2 was 1), and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate is 9 when the initial discharge capacity is 100%.
It was 0.1%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 2 A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that 2,3,4,5,6-pentafluoroanisole was used as an additive in an amount of 1% by weight based on the nonaqueous electrolytic solution. Then, a coin battery was produced, and the battery characteristics after 50 cycles were measured, whereupon the discharge capacity retention rate was 90.5%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 3 A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that 2,3,4,5,6-pentafluoroanisole was used as an additive in an amount of 2% by weight based on the nonaqueous electrolytic solution. Then, a coin battery was produced, and the battery characteristics after 50 cycles were measured, whereupon the discharge capacity retention rate was 89.5%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 4 A coin battery was prepared in the same manner as in Example 1 except that decafluorobiphenyl was used as an additive in an amount of 1% by weight with respect to the non-aqueous electrolyte solution. After that, when the battery characteristics were measured, the discharge capacity retention rate was 90.3.
%Met. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 5 A nonaqueous electrolytic solution was prepared in the same manner as in Example 1 except that 2,3,4,5,6-pentafluorotoluene was used as an additive in an amount of 1% by weight based on the nonaqueous electrolytic solution. Then, a coin battery was produced, and the battery characteristics after 50 cycles were measured, whereupon the discharge capacity retention rate was 89.9%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 6 EC / MEC (volume ratio 3/7) was used as the non-aqueous solvent, and 2,3,4,5,6-pentafluoroanisole was added as an additive to the non-aqueous electrolyte. A non-aqueous electrolytic solution was prepared in the same manner as in Example 1 except that 1% by weight was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 91.7%. . Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 7 Example 1 was repeated except that EC / MEC (volume ratio 3/7) was used as the non-aqueous solvent and decafluorobiphenyl was used as an additive in an amount of 1% by weight based on the non-aqueous electrolyte. A non-aqueous electrolyte was prepared in the same manner as in 1. to produce a coin battery, and the battery characteristics after 50 cycles were measured.
It was 9.1%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 8 EC / MEC (volume ratio 3/7) was used as the non-aqueous solvent, and 2,3,4,5,6-pentafluorotoluene was added as an additive to the non-aqueous electrolyte. A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that 1% by weight was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured.
The discharge capacity retention rate was 90.7%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 9 EC / DEC (volume ratio 1/2) was used as the non-aqueous solvent, and 2,3,4,5,6-pentafluoroanisole was added as an additive to the non-aqueous electrolyte. A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that 1% by weight was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured. The discharge capacity retention rate was 90.4%. . Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 10 As a positive electrode active material, LiMn ₂ O was used instead of LiCoO _2.
4 was used, and 2,3,4,5,6-pentafluoroanisole was used as an additive in an amount of 1% by weight with respect to the nonaqueous electrolytic solution, and a nonaqueous electrolytic solution was prepared in the same manner as in Example 1. A coin battery was prepared by using the above method, and the battery characteristics after 50 cycles were measured, whereupon the discharge capacity retention rate was 89.2%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 11 As the negative electrode active material, natural graphite was used in place of artificial graphite, and 2,3,4,5,6-pentafluoroanisole was used as an additive in an amount of 1% by weight based on the non-aqueous electrolyte. A non-aqueous electrolyte was prepared in the same manner as in Example 1 except that it was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured, whereupon the discharge capacity retention ratio was 89.8%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Comparative Example 1 A non-aqueous solvent of PC: DMC (volume ratio) = 1: 2 was prepared,
LiPF ₆ was dissolved in this to a concentration of 1M.
At this time, no pentafluorophenyl compound was added. Using this non-aqueous electrolyte, a coin battery was prepared in the same manner as in Example 1 and the battery characteristics were measured, but no charge / discharge was performed.

Comparative Example 2 A non-aqueous solvent of EC: DEC (volume ratio) = 3: 7 was prepared,
LiPF ₆ was dissolved in this to a concentration of 1M.
At this time, no pentafluorophenyl compound was added. Using this non-aqueous electrolytic solution, a coin battery was produced in the same manner as in Example 1 and the battery characteristics were measured. The discharge capacity retention rate after 50 cycles is 82.1% of the initial discharge capacity.
Met. Table 1 shows the coin battery manufacturing conditions and battery characteristics.
Shown in.

Comparative Example 3 A coin battery was prepared by preparing a non-aqueous electrolyte in the same manner as in Example 1 except that 4-fluoroanisole was used as an additive in an amount of 1% by weight based on the non-aqueous electrolyte. The battery characteristics after the cycle were measured, but they were not charged or discharged at all. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Comparative Example 4 A coin battery was prepared by preparing a non-aqueous electrolytic solution in the same manner as in Example 1 except that 1% by weight of 2,4-difluoroanisole was used as an additive in the non-aqueous electrolytic solution. The battery characteristics were measured after 50 cycles, but no charge / discharge was performed.
Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Comparative Example 5 A coin battery was prepared in the same manner as in Example 1, except that 1% by weight of 2,4,5-trifluoroanisole was used as an additive in the non-aqueous electrolyte. And make 50
The battery characteristics after the cycle were measured, but they were not charged or discharged at all. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

[0038]

[Table 1]

The present invention is not limited to the embodiments described above, and various combinations that can be easily inferred from the spirit of the invention are possible. In particular, the combination of solvents in the above examples is not limited. Furthermore, although the above embodiments relate to coin batteries, the present invention is also applicable to cylindrical and prismatic batteries.

[0040]

According to the present invention, the cycle characteristics of the battery,
A lithium secondary battery having excellent battery characteristics such as electric capacity and storage characteristics can be provided.

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Claims (8)

[Claims] 1. A nonaqueous electrolytic solution in which an electrolyte salt is dissolved in a nonaqueous solvent, wherein the nonaqueous electrolytic solution contains at least one selected from a pentafluorophenyl ether compound, a pentafluorobiphenyl compound and a pentafluorophenylalkane compound. A non-aqueous electrolytic solution containing a pentafluorophenyl compound of a kind. 2. The pentafluorophenyl ether compound is represented by the following general formula (I): (In the formula, R ₁ represents an alkyl group having 1 to 12 carbon atoms or a haloalkyl group having 1 to 12 carbon atoms.) The nonaqueous electrolytic solution according to claim 1. 3. The pentafluorobiphenyl compound has the following general formula (II): _(Wherein, shows the _{R 2, R 3, R 4} , R 5, R 6 are each independently a hydrogen atom, a halogen atom, an alkoxy group having 1 to 12 carbon atoms, an acyloxy group having 2 to 12 carbon atoms.) The nonaqueous electrolytic solution according to claim 1, which is represented by: 4. The pentafluorophenylalkane compound has the following general formula (III): (In the formula, R ₇ represents an alkyl group having 1 to 12 carbon atoms.)
The nonaqueous electrolytic solution according to claim 1, which is represented by: 5. A lithium secondary battery comprising a positive electrode, a negative electrode and a non-aqueous electrolytic solution in which an electrolyte salt is dissolved in a non-aqueous solvent, wherein the non-aqueous electrolytic solution contains a pentafluorophenyl ether compound, a pentafluorobiphenyl compound, and A lithium secondary battery containing at least one pentafluorophenyl compound selected from pentafluorophenylalkane compounds. 6. The pentafluorophenyl ether compound has the following general formula (I): (In the formula, R ₁ represents an alkyl group having 1 to 12 carbon atoms or a haloalkyl group having 1 to 12 carbon atoms.) The lithium secondary battery according to claim 5. 7. The pentafluorobiphenyl compound has the following general formula (II): _(Wherein, shows the _{R 2, R 3, R 4} , R 5, R 6 are each independently a hydrogen atom, a halogen atom, an alkoxy group having 1 to 12 carbon atoms, an acyloxy group having 2 to 12 carbon atoms.) The lithium secondary battery according to claim 5, represented by: 8. The pentafluorophenylalkane compound has the following general formula (III): (In the formula, R ₇ represents an alkyl group having 1 to 12 carbon atoms.)
The lithium secondary battery according to claim 6, represented by: