JPH11329490A - Electrolytic solution for lithium secondary battery and lithium secondary battery using same - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide a lithium secondary battery excellent in battery characteristics such as cycle characteristics, electrical capacity, and preservation characteristics of the battery. SOLUTION: As this electrolytic solution for a lithium secondary battery, an eletrolytic solution having an electrolyte dissolved in a nonaqueous solvent is provided that contains penta-fluoro-benzene derivatives containing electron- attractive substitution groups expressed by a general formula (where, Y stands for an ester group containing an alkyl group or aryl group having the number of carbon atoms of 1 to 12, an acyl group containing an alkyl group or aryl group having the number of carbon atoms of 1 to 12, or a tri-fluoromethyl group. A halogen atom may be substituted for at least one of hydrogen atoms of the alkyl group or aryl group.), and a lithium secondary battery using this is also provided.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a novel electrolyte for a lithium secondary battery which can provide a lithium secondary battery having excellent battery characteristics such as cycle characteristics, electric capacity and storage characteristics of the battery, and The present invention relates to a lithium secondary battery using the same.

[0002]

2. Description of the Related Art In recent years, lithium secondary batteries have been widely used as power sources for driving small electronic devices and the like. A lithium secondary battery is mainly composed of a positive electrode, a non-aqueous electrolyte, 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 used. It is preferably used. And
Examples of the electrolyte for the lithium secondary battery include ethylene carbonate (EC) and propylene carbonate (PC).
Such carbonates are preferably used.

[0003]

However, regarding the battery characteristics such as the cycle characteristics and the electric capacity of the battery,
There is a demand for a secondary battery having more excellent characteristics.
In a lithium secondary battery using a highly crystallized carbon material such as natural graphite or artificial graphite as the negative electrode, peeling of the carbon material is observed, and the capacity may be irreversible depending on the degree of the phenomenon. This peeling is caused by the decomposition of the solvent in the electrolyte during charging, and is caused by the electrochemical reduction of the solvent at the interface between the carbon material and the electrolyte. Among them, PC having a low melting point and a high dielectric constant has high electric conductivity even at a low temperature, but when a graphite negative electrode is used, there is a problem that PC is decomposed and cannot be used for a lithium secondary battery. . EC also partially decomposes during repeated charge and discharge, resulting in a decrease in battery performance. Therefore, at present, the battery characteristics such as the cycle characteristics and the electric capacity of the battery are not always satisfactory.

The present invention solves the above-mentioned problems relating to the electrolyte solution for a lithium secondary battery, and provides a lithium battery having excellent cycle characteristics of a battery, and excellent battery characteristics such as electric capacity and storage characteristics in a charged state. An object of the present invention is to provide an electrolyte for a lithium secondary battery that can constitute a secondary battery, and a lithium secondary battery using the same.

[0005]

According to the present invention, there is provided an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, wherein the electrolyte has the following general formula (I):

[0006]

Embedded image

Wherein Y is an ester group containing an alkyl or aryl group having 1 to 12 carbon atoms,
Represents an acyl group containing an alkyl group or an aryl group, or a trifluoromethyl group. However, at least one of the hydrogen atoms of the alkyl group or the aryl group may be substituted with a halogen atom. The present invention relates to an electrolyte solution for a lithium secondary battery, which comprises a pentafluorobenzene derivative containing an electron-withdrawing substituent represented by the formula (1).

In a lithium secondary battery comprising a positive electrode, a negative electrode and an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent,
In the electrolyte, the following general formula (I)

[0009]

Embedded image

Wherein Y is an ester group containing an alkyl or aryl group having 1 to 12 carbon atoms,
Represents an acyl group containing an alkyl group or an aryl group, or a trifluoromethyl group. However, at least one of the hydrogen atoms of the alkyl group or the aryl group may be substituted with a halogen atom. The present invention relates to a lithium secondary battery containing a pentafluorobenzene derivative containing an electron-withdrawing substituent represented by the formula (1).

The pentafluorobenzene derivatives containing an electron-withdrawing substituent contained in the electrolytic solution are reduced during charging to form a passive film on the surface of the carbon material, and are used as natural graphite or artificial graphite. It is considered that a carbon material that has been highly crystallized due to such activity is coated with a passive film to have an effect of suppressing the decomposition of the electrolytic solution without impairing the normal reaction of the battery.

[0012]

BEST MODE FOR CARRYING OUT THE INVENTION In the pentafluorobenzene derivatives containing an electron-withdrawing substituent represented by the above formula (I) contained in an electrolytic solution in which an electrolyte is dissolved in a non-aqueous solvent, Y is At least hydrogen atoms such as methyl, ethyl, propyl, isopropyl, butyl, isobutyl, 2,2,2-trifluoroethyl, 2,2,3,3,3-pentafluoropropyl An alkyl group having 1 to 12 carbon atoms, one of which may be substituted with a halogen atom, or an aryl, wherein at least one of hydrogen atoms such as a phenyl group, a benzyl group and a pentafluorophenyl group may be substituted with a halogen atom Ester groups containing groups are preferred. Y represents a methyl group, an ethyl group, a propyl group, an isopropyl group, a butyl group, an isobutyl group, a 2,2,2-trifluoroethyl group,
At least one hydrogen atom such as a 2,3,3,3-pentafluoropropyl group or the like may be substituted with a halogen atom, or an alkyl group having 1 to 12 carbon atoms, or a phenyl group, a benzyl group, a pentafluorophenyl group, etc. And an acyl group containing an aryl group in which at least one of the hydrogen atoms may be substituted with a halogen atom. Further, Y is preferably a halogen-containing alkyl group such as a trifluoromethyl group.

Specific examples of the pentafluorobenzene derivatives represented by the above formula (I) include, for example, methyl pentafluorobenzenecarboxylate [Y = methoxycarbonyl group], ethyl pentafluorobenzenecarboxylate [Y
= Ethoxycarbonyl group], propyl pentafluorobenzenecarboxylate [Y = propoxycarbonyl group], isopropyl pentafluorobenzenecarboxylate [Y = isopropoxycarbonyl group], 2,2,2-trifluoroethyl pentafluorobenzenecarboxylate [ Y = 2,2,2-trifluoroethoxycarbonyl group], phenyl pentafluorobenzenecarboxylate [Y = phenoxycarbonyl group], benzyl pentafluorobenzenecarboxylate [Y
= Benzyloxycarbonyl group], esters such as pentafluorophenyl pentafluorobenzenecarboxylate [Y = pentafluorophenoxycarbonyl group],
2 ', 3', 4 ', 5', 6'-pentafluoroacetophenone [Y = acetyl group], octafluoroacetophenone [Y = trifluoroacetyl group], 2,3,4,5,6-pentafluorobenzophenone Acyls such as [Y = phenoxy group], octafluorotoluene [Y = trifluoromethyl group] and the like. In addition, the pentafluorobenzene derivatives in the present invention are not limited to the above specific examples as long as they contain an electron-withdrawing substituent.

When the content of the pentafluorobenzene derivative having an electron-withdrawing substituent represented by the above formula (I) is excessively large, the conductivity of the electrolytic solution is changed and the battery performance is lowered. If the amount is too small, a sufficient film is not formed and the expected battery characteristics cannot be obtained. Therefore, 0.01 to 20% by weight, particularly 0.1 to 10% by weight based on the weight of the electrolytic solution. % Is preferred.

The non-aqueous solvent used in the present invention is preferably a solvent composed of a high dielectric constant solvent and a low viscosity solvent. Preferred examples of the high dielectric constant solvent include cyclic carbonates such as ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC). These high dielectric constant solvents may be used alone or in combination of two or more.

As the low-viscosity solvent, for example, 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 lactones such as γ-butyrolactone, nitriles such as acetonitrile, esters such as methyl propionate, and amides such as dimethylformamide. These low-viscosity solvents may be used alone or in combination of two or more. The high dielectric constant solvent and the low viscosity solvent are arbitrarily selected and used in combination. The high dielectric constant solvent and the low viscosity solvent are used in a volume ratio (high dielectric constant solvent: low viscosity solvent) of usually 1: 9 to 4: 1, preferably 1: 4 to 7: 3. You.

As the electrolyte used in the present invention, for example, LiPF ₆ , LiBF ₄ , LiClO ₄ , LiN (S
O ₂ CF ₃ ) ₂ , LiN (SO ₂ C ₂ F ₅ ) ₂ , LiC (SO ₂
CF ₃ ) _{3 and the} like. These electrolytes may be used alone or in combination of two or more. These electrolytes are usually 0.1 to
It is used after being dissolved at a concentration of 3M, preferably 0.5 to 1.5M.

The electrolytic solution of the present invention is prepared, for example, by mixing the above-mentioned high dielectric constant solvent or low-viscosity solvent, dissolving the above-mentioned electrolyte, and adding the pentafluorobenzene derivative represented by the above formula (I). Obtained by dissolving.

The electrolytic solution of the present invention comprises a constituent member of a secondary battery,
In particular, it is suitably used as a component of a lithium secondary battery. The constituent members other than the electrolytic solution constituting the secondary battery are not particularly limited, and various conventionally used constituent members can be used.

For example, a composite metal oxide of lithium and at least one metal selected from the group consisting of cobalt, manganese, nickel, chromium, iron and vanadium is used as the positive electrode material (positive electrode active material). Examples of such a composite metal oxide include LiCoO ₂ , L
iMn ₂ O ₄ , LiNiO _{2 and the} like can be mentioned.

The positive electrode is prepared by kneading the above positive electrode material with a conductive agent such as acetylene black and carbon black and a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) to form a positive electrode mixture. The positive electrode material is rolled into a foil or lath plate made of aluminum or stainless steel as a current collector and heat-treated under a vacuum at a temperature of about 50 ° C. to 250 ° C. for about 2 hours.

As the negative electrode (negative electrode active material), lithium metal, a lithium alloy, and a carbon material having a graphite type crystal structure capable of occluding and releasing lithium [pyrolytic carbons, cokes, graphites (artificial graphite, natural graphite) Graphite, etc.),
Organic polymer compound combustion body, carbon fiber] and composite tin oxide. In particular, spacing of lattice planes ₍₀₀₂₎ (d 002) it is preferable to use a carbon material having a graphite-type crystal structure is 3.35 to 3.40 angstroms (Å). Powder materials such as carbon materials are ethylene propylene diene terpolymer (EPDM),
It is kneaded with a binder such as polytetrafluoroethylene (PTFE) and polyvinylidene fluoride (PVDF) and used as a negative electrode mixture.

The structure of the lithium secondary battery is not particularly limited. 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 And a prismatic battery. As the separator, a known microporous polyolefin membrane, woven fabric, nonwoven fabric, or the like is used.

[0024]

EXAMPLES Next, the present invention will be described in detail with reference to Examples and Comparative Examples, but these do not limit the present invention in any way. Example 1 [Preparation of electrolyte solution] A non-aqueous solvent having a PC-DMC (volume ratio) of 1: 2 was prepared, and LiPF ₆ was dissolved therein to a concentration of 1 M to prepare an electrolyte solution. Further, as a pentafluorobenzene derivative (additive) containing an electron-withdrawing substituent, methyl pentafluorobenzenecarboxylate [Y = methoxycarbonyl group] is added to the electrolyte solution.
0% by weight was added.

[Preparation of Lithium Secondary Battery and Measurement of Battery Characteristics] LiCoO ₂ (cathode active material) was 80% by weight, acetylene black (conductive agent) was 10% by weight, and polyvinylidene fluoride (binder) was 10% by weight. %, And N-methylpyrrolidone was added thereto to form a slurry, which was applied on an aluminum foil. Thereafter, it was dried and pressed to prepare a positive electrode. 90% by weight of natural graphite (negative electrode active material) and 10% by weight of polyvinylidene fluoride (binder) were mixed, and N-methylpyrrolidone was added thereto to form a slurry, which was coated on a copper foil. Thereafter, this was dried and molded under pressure to prepare a negative electrode. Then, using a separator made of a polypropylene microporous film, the above-mentioned electrolytic solution was injected into the coin battery (diameter 20 mm, thickness 3.2 mm).
Was prepared. Room temperature (20 ° C) using this coin battery
The battery was charged with a constant current and constant voltage of 0.8 mA to a final voltage of 4.2 V in 5 hours, and then discharged under a constant current of 0.8 mA to a final voltage of 2.7 V. This charge / discharge was repeated. The initial charge / discharge capacity was almost the same as in the case where EC-DMC (1/2) was used as the electrolytic solution (Comparative Example 2), and the battery characteristics after 50 cycles were measured.
The discharge capacity retention rate when it was 0% was 80.2%. Also, the low-temperature characteristics were good. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 2 As an additive, pentafluorobenzenecarboxylic acid 2,2,
An electrolytic solution was prepared in the same manner as in Example 1 except that 2-trifluoroethyl [Y = 2,2,2-trifluoroethoxycarbonyl group] was used in an amount of 2.0% by weight based on the electrolytic solution. When the battery characteristics were measured after 50 cycles, the discharge capacity retention ratio was 80.2%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 3 2 ', 3', 4 ', 5', 6'-pentafluoroacetophenone [Y = acetyl group] was added to the electrolyte as an additive.
Except for using 0% by weight, an electrolytic solution was prepared in the same manner as in Example 1 to prepare a coin battery, and the battery characteristics after 50 cycles were measured. As a result, the discharge capacity retention ratio was 79.3%. 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 octafluorotoluene [Y = trifluoromethyl group] was used as an additive in an amount of 2.0% by weight based on the electrolyte. And battery characteristics after 50 cycles were measured.
The discharge capacity retention ratio was 82.6%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 5 A coin battery was prepared by preparing an electrolyte in the same manner as in Example 1 except that octafluorotoluene [Y = trifluoromethyl group] was used as an additive in an amount of 0.2% by weight based on the electrolyte. And battery characteristics after 50 cycles were measured.
The discharge capacity retention ratio was 79.2%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 6 A coin battery was prepared in the same manner as in Example 1 except that octafluorotoluene [Y = trifluoromethyl group] was used as an additive at 5.0% by weight based on the electrolyte. And battery characteristics after 50 cycles were measured.
The discharge capacity retention ratio was 82.5%. 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 therein to a concentration of 1M.
At this time, pentafluorobenzene derivatives containing an electron-withdrawing substituent were not added at all. Using this electrolytic solution, a coin battery was fabricated in the same manner as in Example 1, and the battery characteristics were measured. As a result, PC was decomposed at the time of the first charge, and no discharge was possible. As a result of disassembling and observing the battery after the first charge, peeling was observed in the graphite negative electrode. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 7 A non-aqueous solvent of EC-DMC (volume ratio) = 1: 2 was prepared.
LiPF ₆ was dissolved therein to a concentration of 1 M to prepare an electrolytic solution, and octafluorotoluene [Y = trifluoromethyl group] was further added to the electrolytic solution as a pentafluoro derivative (additive). 0.0% by weight. Using this electrolytic solution, a coin battery was manufactured in the same manner as in Example 1, and the battery characteristics were measured.
It is almost the same as when only MC (1/2) was used as the electrolytic solution (Comparative Example 2). When the battery characteristics after 50 cycles were measured, the discharge capacity retention ratio when the initial discharge capacity was 100%. Was 91.2%. Also, the low-temperature characteristics were good. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 8 As an additive, methyl pentafluorobenzenecarboxylate [Y = methoxycarbonyl group] was added to the electrolyte solution.
An electrolyte was prepared in the same manner as in Example 7 except that 0% by weight was used to prepare a coin battery, and the battery characteristics after 50 cycles were measured. As a result, the discharge capacity retention ratio was 90.7%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Example 9 As a positive electrode active material, LiMn ₂ O ₄ was used instead of LiCoO _2.
Octafluorotoluene [Y
= Trifluoromethyl group] was used in the same manner as in Example 8, except that 3.0% by weight of the electrolytic solution was used to prepare a coin battery. The battery characteristics after 50 cycles were measured. The discharge capacity retention ratio was 91.8%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

Comparative Example 2 A non-aqueous solvent of EC: DMC (volume ratio) = 1: 2 was prepared.
LiPF ₆ was dissolved therein to a concentration of 1M.
At this time, pentafluorobenzene derivatives containing an electron-withdrawing substituent were not added at all. Using this 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 with respect to the initial discharge capacity was 83.8%. Table 1 shows the manufacturing conditions and battery characteristics of the coin battery.

[0036]

[Table 1]

It should be noted that the present invention is not limited to the embodiments described above, and various combinations that can be easily inferred from the gist of the invention are possible. In particular, the combinations of the solvents in the above examples are not limited. Further, while the above embodiments relate to coin batteries, the present invention is also applicable to cylindrical and prismatic batteries.

[0038]

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.

Claims (2)

[Claims] 1. An electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, wherein the electrolyte has the following general formula (I): (In the formula, Y represents an ester group containing an alkyl group or an aryl group having 1 to 12 carbon atoms, an acyl group containing an alkyl group or an aryl group having 1 to 12 carbon atoms, or a trifluoromethyl group. And at least one of the hydrogen atoms of the alkyl group or the aryl group may be substituted with a halogen atom.). An electrolytic solution for a lithium secondary battery, comprising: 2. A lithium secondary battery comprising a positive electrode, a negative electrode and an electrolyte in which an electrolyte is dissolved in a non-aqueous solvent, wherein the electrolyte has the following general formula (I): (In the formula, Y represents an ester group containing an alkyl group or an aryl group having 1 to 12 carbon atoms, an acyl group containing an alkyl group or an aryl group having 1 to 12 carbon atoms, or a trifluoromethyl group. And at least one of the hydrogen atoms of the alkyl group or the aryl group may be substituted with a halogen atom.). A rechargeable lithium battery.