KR20110083930A - Electrolyte for lithium secondary battery having excellent stability against overcharge and lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: An electrolyte for a lithium secondary battery is provided to minimize the degradation of other battery properties and to ensure excellent safety by suppressing gas generation in an overcharging condition. CONSTITUTION: An electrolyte for a lithium secondary battery in which lithium salts are dissolved in an electrolyte solution comprises a fluoro-substituted aromatic compound having oxidation potential of 4.5~5.0V in order to improve safety against overcharging. The fluoro-substituted aromatic compound is one or more selected from group consisting of 2,5-difluoro toluene and 3,4-difluoro toluene. The content of the fluoro-substituted aromatic compound is 0.05 ~10 weight%.

Description

Electrolyte for lithium secondary battery with excellent safety against overcharge and lithium secondary battery comprising same {Electrolyte for Lithium Secondary Battery Having Excellent Stability against Overcharge and Lithium Secondary Battery Comprising the Same}

The present invention relates to an electrolyte for lithium secondary batteries having excellent safety against overcharging. More particularly, the present invention relates to a lithium secondary battery electrolyte in which lithium salts are dissolved in an electrolyte of a carbonate compound, so as to improve safety against overcharging. A fluoro-substituted aromatic compound having an oxidation potential of 5.0 V is included, and the fluorine-substituted aromatic compound is at least one selected from the group consisting of 2,5-difluoro toluene and 3,4-difluoro toluene. It relates to an electrolyte for lithium secondary batteries.

Due to the rapid increase in the use of fossil fuels, the demand for the use of alternative energy or clean energy is increasing. As a part of this, the most actively researched fields are power generation and storage using electrochemistry.

A representative example of an electrochemical device using such electrochemical energy is a secondary battery, and its use area is gradually increasing. Among such secondary batteries, many studies have been conducted on lithium secondary batteries that exhibit high energy density and operating potential, have a long cycle life, and have a low self discharge rate, and have also been commercialized and widely used.

In addition, as interest in environmental issues grows, researches on electric vehicles and hybrid electric vehicles, which can replace vehicles using fossil fuel, such as gasoline and diesel vehicles, which are one of the main causes of air pollution, are being conducted. . As a power source of such electric vehicles and hybrid electric vehicles, nickel-metal hydride secondary batteries are mainly used, but researches using lithium secondary batteries with high energy density and discharge voltage have been actively conducted and some commercialization stages are in progress.

In general, a lithium secondary battery mainly uses a carbon material as a negative electrode active material, and use of lithium metal, sulfur compounds, and the like is also considered. Lithium cobalt oxide (LiCoO ₂ ) is mainly used as a positive electrode active material. Lithium transitions such as lithium manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, and lithium nickel oxide (LiNiO ₂ ) Metal oxides are used.

However, since a high energy density means that it may be exposed to a high risk at the same time, the higher the energy density, there is a problem that the risk of ignition, explosion, etc. increases. However, lithium secondary batteries, which are currently the mainstream of secondary batteries, have the disadvantage of low safety. For example, when the battery is overcharged to about 4.2 V or more, decomposition reaction of the positive electrode active material occurs, dendrite growth of lithium metal at the negative electrode, decomposition reaction of electrolyte solution, and the like occur.

In addition, when the temperature of the battery rises, the reaction between the electrolyte and the electrode is accelerated. As a result, a thermal runaway phenomenon occurs in which the temperature of the battery rises sharply, and when the temperature rises above a predetermined temperature, the battery may ignite. As a result of the reaction between the electrolyte and the electrode, gas may be generated to increase the internal pressure of the battery. And the lithium secondary battery explodes above a predetermined pressure. The risk of ignition / explosion can be said to be the most fatal drawback of lithium secondary batteries.

Therefore, essential considerations for the development of a lithium secondary battery are to ensure safety. As part of efforts to secure such safety, one method of using a material inside a cell is to add an additive that improves safety to an electrolyte or an electrode.

For example, 4CT (4-Chlorotoluene) was added to the electrolyte to reduce the generation of gas during overcharge. However, since the material has a problem that it is difficult to commercialize because it is designated as an environmental regulatory substance, an additive is required to replace it.

In this regard, some prior art has proposed fluorine-substituted compounds as additives to improve safety against overcharging. However, it has been confirmed by the inventors that most of the fluorine-substituted compounds do not inhibit gas generation to a desired level in an overcharged state. In addition, although a technique of using non-halogenated compounds as an electrolyte additive has been proposed, these compounds have a problem of causing performance degradation of the battery.

Therefore, there is a high need for the development of a more effective technology capable of securing the safety of the battery by minimizing the degradation of the battery performance while inducing less gas generation in overcharge conditions.

SUMMARY OF THE INVENTION It is an object of the present invention to solve the above-described problems of the prior art and the technical problems required from the past.

After extensive research and extensive experiments, the inventors of the present application have found that certain fluorine-substituted aromatic compounds, as described below, minimize gas deterioration in other cell performances, while surprisingly reducing gas evolution over other electrolyte additives. It confirmed that it suppressed greatly and came to complete this invention.

Accordingly, the present invention provides a lithium secondary battery electrolyte in which lithium salt is dissolved in an electrolyte of a carbonate compound, and includes a fluoro-substituted aromatic compound having an oxidation potential of 4.5 to 5.0 V to improve safety against overcharging. It is included, the fluorine-substituted aromatic compound provides an electrolyte of a lithium secondary battery according to one or more selected from the group consisting of 2,5-difluoro toluene and 3,4-difluoro toluene.

In the present invention, the specific compounds, while minimizing the degradation of the battery performance, it can greatly improve the safety of the battery by generating only a small amount of gas under overcharge conditions than the known electrolyte additives.

The present invention is described in more detail below.

In general, lithium secondary batteries are capable of charging and discharging at 3.1 to 4.2 V, and when the overvoltage is applied to the battery by overcharging, the electrolyte of the carbonate compound starts to decompose. That is, there is a problem that a large amount of gas is generated by decomposition due to the oxidation reaction of the electrolyte solution, resulting in a swelling phenomenon in which the battery swells, and the performance of the battery is degraded. In addition, the rapid generation of gas increases the likelihood of explosion, greatly deteriorating the safety of the battery.

Therefore, by adding a predetermined compound having an oxidation potential of 4.5 to 5.0 V to the electrolyte, the additive reacts first before the electrolyte is oxidized during overcharging, thereby preventing decomposition of the electrolyte and preventing gas generation. .

The oxidation potential refers to a potential at which the oxidation reaction starts, that is, the starting voltage of the electrolyte decomposition reaction. The oxidation potential may be changed according to the type of organic solvent of the electrolyte solution used with the additive. In the present invention, the value indicated when using a carbonate-based compound as the organic solvent of the electrolyte solution. That is, even if other organic solvents are used in the actual electrolyte, the above value is satisfied when the oxidation reaction potential is measured using the additive of the present invention and the carbonate-based compound.

Conventionally, halogen substituted aromatic compound additives composed of two aromatic ring structures, such as biphenyl, have also been used. However, when a relatively high voltage occurs locally at a normal operating voltage, biphenyl gradually increases during charging and discharging. There is a problem of decomposition.

Recently, halogen-substituted aromatic compounds composed of one aromatic ring structure that can ensure safety even in high voltage batteries have been used. These compounds are additives having a relatively high oxidation potential, and heat generation occurs due to heat generation due to oxidation of the electrode material and the electrolyte due to overcharging as the temperature of the electrolyte is rapidly increased due to the progress of the heat of oxidation at 4.5 V or higher. Before, the membrane is shut down only by the temperature of the electrolyte, so that thermal runaway is controlled and is also electrochemically and thermally stable.

Among the halogen-substituted aromatic compounds composed of one aromatic ring structure, toluene is preferable as a compound capable of securing battery safety as an overcharge prevention additive. Halogens such as fluorine are generally highly reactive and suitable for performing the role of oxidation reaction. Moreover, it is also preferable as a substance which can be added to the electrolyte solution of a secondary battery.

However, the inventions of the present application have confirmed that most of the fluorine-substituted aromatic compounds based on one aromatic structure do not suppress gas generation to a desired level in an overcharged state. That is, it is true that the compounds inhibit gas generation under overcharge conditions, but the amount of gas generated when the overcharge state persists has been found to reach the degree of opening the vent structure of the battery. Therefore, it does not provide the desired level of safety against overcharging.

On the other hand, even in many fluorine-substituted aromatic compounds based on one aromatic structure, 2,5-difluoro toluene and 3,4-difluoro toluene significantly inhibit gas generation compared to other fluorine-substituted aromatic compounds. It was confirmed. This means that the safety of overcharging is greatly changed only by the difference in the number of substitution and the position of substitution of the fluorine element, and it is not known in the art at all and is an unexpected result. This fact can be confirmed in the experimental content, such as examples provided later.

In the present invention, the content of the fluorine-substituted aromatic compound may be included in 0.05 to 10% by weight based on the total weight of the electrolyte. If the additive is added too little relative to the total weight of the electrolyte, the effect of suppressing gas generation in the overcharged state is insignificant. On the contrary, if the additive is added in an excessively large amount, the amount of the electrolyte may be relatively decreased, thereby degrading the performance of the battery. It is not preferable because the deterioration of its life can occur. More preferred content of the mixed additive may be 0.1 to 5% by weight.

In some cases, xylene may be further included in addition to the specific fluorine-substituted aromatic compound. Conventionally, xylene is generally recognized as not a non-polar solvent to oxidize, but the inventors have found that when xylene is reacted at a high temperature, the methyl (CH ₃ ) group, which is a substituent of xylene, is oxidized to aldehyde, and even a non-polar solvent can be oxidized. Confirmed. Specifically, xylene can be oxidized at a high temperature, thereby acting as an oxidation reaction that is oxidized first before the electrolyte is oxidized during overcharging, thereby inducing low gas generation and fundamentally improving the overcharge preventing property.

The xylene may be at least one selected from the group consisting of m-xylene, o-xylene and p-xylene.

Even when xylene is further included, the total amount of the additive is preferably in the range of 0.05 to 10% by weight based on the total weight of the electrolyte. In addition, the content ratio of xylene to the fluorine-substituted aromatic compound is preferably in the range of 0.1 to 0.6 in molar ratio.

The electrolyte for a lithium secondary battery of the present invention may be a lithium-containing non-aqueous electrolyte, which consists of a non-aqueous electrolyte and a lithium salt.

Examples of the nonaqueous electrolyte include N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butylo lactone, and 1,2-dimethoxy ethane. , Tetrahydroxy franc, 2-methyl tetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolon, formamide, dimethylformamide, dioxolon, acetonitrile, nitromethane, methyl formate, methyl acetate , Phosphoric acid triester, trimethoxy methane, dioxolon derivative, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivative, tetrahydrofuran derivative, ether, methyl pyroionate One or two or more selected from the group consisting of aprotic organic solvents, such as ethyl propionate, may be used. More preferably, the electrolyte of the carbonate compound may be composed of a mixture of a cyclic carbonate compound and a linear carbonate compound.

The lithium salt is a material that is easy to dissolve in the non-aqueous electrolyte, for example, LiCl, LiBr, LiI, LiClO ₄ , LiBF ₄ , LiB ₁₀ Cl ₁₀ , LiPF ₆ , LiCF ₃ SO ₃ , LiCF ₃ CO ₂ , LiAsF _{6, LiSbF 6, LiAlCl 4,} CH 3 SO 3 Li, CF 3 SO 3 Li, (CF 3 SO 2) 2 NLi, chloroborane lithium, lower aliphatic carboxylic acid lithium, lithium tetraphenyl borate and imide have.

In some cases, an organic solid electrolyte, an inorganic solid electrolyte, or the like may be used. As the organic solid electrolyte, for example, a polyethylene derivative, a polyethylene oxide derivative, a polypropylene oxide derivative, a phosphate ester polymer, a polyedgetion lysine (agitation) lysine), polyester sulfide, polyvinyl alcohol, polyvinylidene fluoride, polymers containing ionic dissociating groups, and the like.

Examples of the inorganic solid electrolyte include Li ₃ N, LiI, Li ₅ NI ₂ , Li ₃ N-LiI-LiOH, LiSiO ₄ , LiSiO ₄ -LiI-LiOH, Li ₂ SiS ₃ , Li ₄ SiO ₄ , Nitrides, halides, sulfates and the like of Li, such as Li ₄ SiO ₄ -LiI-LiOH, Li ₃ PO ₄ -Li ₂ S-SiS ₂ , and the like, may be used.

In addition, for the purpose of improving charge / discharge characteristics, flame retardancy, etc., the non-aqueous electrolyte solution includes, for example, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, and hexaphosphate triamide. , Nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N, N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxy ethanol, aluminum trichloride, etc. are added May be In some cases, in order to impart nonflammability, a halogen-containing solvent such as carbon tetrachloride or ethylene trifluoride may be further included, and carbon dioxide gas may be further included to improve high temperature storage characteristics, and FEC (Fluoro-Ethylene) may be further included. carbonate), PRS (propene sultone), FEC (Fluoro-Ethlene carbonate) and the like may be further included.

The present invention also provides a lithium secondary battery comprising the electrolyte for a lithium secondary battery.

The lithium secondary battery generally includes an electrode assembly having a positive electrode / separation membrane / cathode structure in addition to a lithium salt-containing nonaqueous electrolyte. It is composed of a structure.

The positive electrode is prepared by, for example, applying a mixture of a positive electrode active material, a conductive material, and a binder to a positive electrode current collector, followed by drying. If necessary, a viscosity modifier and a filler may be further added to the mixture.

The positive electrode active material is a lithium transition metal oxide, and includes two or more transition metals, and for example, layered compounds such as lithium cobalt oxide (LiCoO ₂ ) and lithium nickel oxide (LiNiO ₂ ) substituted with one or more transition metals. ; Lithium manganese oxide substituted with one or more transition metals; Formula LiNi _1-y M _y O ₂ , wherein M = Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn or Ga and comprises at least one of the above elements, wherein 0.01 ≦ _y ≦ 0.7 Lithium nickel-based oxide represented by; Li _{1 + z} Ni _1/3 Co _1/3 Mn _1/3 O ₂ , Li _{1 + z} Ni _0.4 Mn _0.4 Co _0.2 O _2, etc. Li _{1 + z} Ni _b Mn _c Co _{1- (b + c + d )} M _d O _(2-e) A _e (where -0.5≤z≤0.5, 0.1≤b≤0.8, 0.1≤c≤0.8, 0≤d≤0.2, 0≤e≤0.2, b + c + d Lithium nickel cobalt manganese composite oxide represented by <1, M = Al, Mg, Cr, Ti, Si or Y, and A = F, P or Cl; Although these etc. are mentioned, it is not limited only to these.

The cathode current collector generally has a thickness of 3 to 500 mu m. Such a positive electrode current collector is not particularly limited as long as it has high conductivity without causing chemical changes in the battery. For example, the surface of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel Surface treated with carbon, nickel, titanium, silver or the like can be used. The current collector may form fine irregularities on its surface to increase the adhesion of the positive electrode active material, and may be in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

The conductive material is a component for further improving the conductivity of the electrode active material, and may be added in an amount of 1 to 30 wt% based on the total weight of the electrode mixture. Such a conductive material is not particularly limited as long as it has conductivity without causing chemical change in the battery, and examples thereof include graphite such as natural graphite and artificial graphite; Carbon blacks such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and summer black; Carbon derivatives such as carbon nanotubes and fullerenes, conductive fibers such as carbon fibers and metal fibers; Metal powders such as carbon fluoride powder, aluminum powder and nickel powder; Conductive whiskeys such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The binder is a component that assists in bonding the active material and the conductive material to the current collector, and is generally added in an amount of 1 to 50 wt% based on the total weight of the mixture including the positive electrode active material. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene , Polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene butylene rubber, fluorine rubber, various copolymers and the like.

The viscosity adjusting agent may be added up to 30% by weight based on the total weight of the electrode mixture, so as to control the viscosity of the electrode mixture so that the mixing process of the electrode mixture and the coating process on the collector may be easy. Examples of such viscosity modifiers include carboxymethylcellulose, polyvinylidene fluoride and the like, but are not limited thereto. In some cases, the above-described solvent may play a role as a viscosity adjusting agent.

The filler is optionally used as a component for suppressing the expansion of the anode, and is not particularly limited as long as it is a fibrous material without causing a chemical change in the battery. Examples of the filler include olefin polymers such as polyethylene and polypropylene; Fibrous materials such as glass fibers and carbon fibers are used.

The negative electrode is manufactured by coating and drying a negative electrode active material on a negative electrode current collector, and if necessary, the components as described above may be further included.

The negative electrode collector is generally made to have a thickness of 3 to 500 mu m. Such an anode current collector is not particularly limited as long as it has conductivity without causing chemical change in the battery, and may be formed of a material such as copper, stainless steel, aluminum, nickel, titanium, fired carbon, surface of copper or stainless steel A surface treated with carbon, nickel, titanium, silver or the like, an aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode current collector, fine concavities and convexities may be formed on the surface to enhance the bonding strength of the negative electrode active material, and may be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a nonwoven fabric.

Examples of the negative electrode active material include carbon and graphite materials such as natural graphite, artificial graphite, expanded graphite, carbon fiber, non-graphitizable carbon, carbon black, carbon nanotube, fullerene, and activated carbon; Metals such as Al, Si, Sn, Ag, Bi, Mg, Zn, In, Ge, Pb, Pd, Pt and Ti which can be alloyed with lithium and compounds containing these elements; Complexes of metals and their compounds and carbon and graphite materials; Lithium-containing nitrides, and the like. Among them, a carbon-based active material, a silicon-based active material, a tin-based active material, or a silicon-carbon based active material is more preferable, and these may be used singly or in combination of two or more.

The separator is interposed between the anode and the cathode, and an insulating thin film having high ion permeability and mechanical strength is used. The pore diameter of the separator is generally from 0.01 to 10 ㎛ ㎛, thickness is generally 5 ~ 300 ㎛. As such a separator, for example, olefin polymers such as chemical resistance and hydrophobic polypropylene; Sheets or non-woven fabrics made of glass fibers or polyethylene are used. When a solid electrolyte such as a polymer is used as the electrolyte, the solid electrolyte may also serve as a separator.

The secondary battery according to the present invention may be preferably used as a power source of an electric vehicle, a hybrid electric vehicle, a plug-in hybrid electric vehicle, etc., in which particularly excellent safety and high rate characteristics are required.

As described above, the electrolyte for a lithium secondary battery according to the present invention includes a specific fluorine-substituted aromatic compound, thereby minimizing deterioration of battery performance, and restraining gas generation under overcharging conditions as much as possible, thereby improving battery safety. .

1 is a graph showing the results for the battery of Example 1 in Experimental Example 1 of the present invention;
Figure 2 is a graph showing the results for the battery of Example 2 in Experimental Example 1 of the present invention;
3 is a graph showing the results for the battery of Comparative Example 1 in Experimental Example 1 of the present invention;
Figure 4 is a graph showing the results for the battery of Comparative Example 2 in Experimental Example 1 of the present invention;
5 is a graph showing the results of the battery of Comparative Example 3 in Experimental Example 1 of the present invention.

Hereinafter, the present invention will be described in more detail with reference to Examples, but the following Examples are provided to illustrate the present invention, and the scope of the present invention is not limited thereto.

Example 1

A lithium secondary battery having a capacity of 870 mA was prepared by inserting a polyethylene (PE) film separator (separation membrane) between electrodes prepared by manufacturing a positive electrode and a negative electrode and injecting a lithium salt-containing electrolyte.

Specifically, in a mixed solvent (3: 4: 3 volume ratio) of EC (ethylene carbonate) / EMC (ethylmethyl carbonate) / DEC (diethyl carbonate) as an electrolyte solution, LiPF ₆ and VC (Vinylene Carbonate) / VEC (Vinyl Ethylene Carbonate) / FA (FEC) (Florinated Ethylene Carbonate) / PS (Propane Sulfone) was added at a weight of 0.1% by weight with LiBF ₄ at 2: 0.5: 2: 1 (volume ratio), and 2, 5-difluoro toluene was added.

[Example 2]

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 3,4-difluoro toluene was added instead of 2,5-difluoro toluene as an electrolyte additive to prepare an electrolyte.

Comparative Example 1

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 2,6-difluoro toluene was added instead of 2,5-difluoro toluene as an electrolyte additive to prepare an electrolyte.

Comparative Example 2

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 3-fluoro toluene was added instead of 2,5-difluoro toluene as an electrolyte additive to prepare an electrolyte.

Comparative Example 3

A lithium secondary battery was manufactured in the same manner as in Example 1, except that 4-fluoro toluene was added instead of 2,5-difluoro toluene as an electrolyte additive to prepare an electrolyte.

[Experimental Example]

For each of the batteries manufactured in Examples 1 to 2 and Comparative Examples 1 to 3, a constant current of 550 mA was continuously applied for 1440 minutes to make an overcharge state, and the voltage and temperature change and the thickness change of the battery were measured over time. It was. The change in thickness of the cell (delta T: ΔT) refers to the difference in thickness of the expanded cell relative to the thickness of the initial cell.

The experimental results are disclosed in FIGS. 1 to 5, respectively.

1 to 5, the batteries of Examples 1 and 2 (FIGS. 1 and 2) comprising 2,5-difluoro toluene and 3,4-difluoro toluene according to the present invention have a thickness of the battery under overcharge conditions. The change delta T (ΔT) does not exceed about 4, and it can be seen that no change in thickness occurs over time.

On the other hand, in the battery of Comparative Example 1 containing 2,6-difluoro toluene (FIG. 3), the delta T (ΔT) was close to 6, even though the compound having a structure in which two fluorine elements were substituted with toluene was added. In addition, it can be seen that the increase in thickness over time. When the delta T (ΔT) exceeds about 6 in the cell used in this experiment, the vent structure of the cell is opened, and the cell of Comparative Example 1 shows that a large amount of gas is generated under overcharge conditions.

In addition, the cells of Comparative Examples 2 and 3 (Figs. 4 and 5) to which a compound having a structure in which toluene was substituted by one fluorine element (Figs. 4 and 5) had a delta T (ΔT) of more than 6, particularly in Comparative Example 3 The battery can confirm that the vent of the battery has been opened at 960 minutes.

Although described above with reference to embodiments of the present invention, those skilled in the art will be able to perform various applications and modifications within the scope of the present invention based on the above contents.

Claims (8)

A lithium secondary battery electrolyte in which lithium salt is dissolved in an electrolyte of a carbonate compound, and includes a fluoro-substituted aromatic compound having an oxidation potential of 4.5 to 5.0 V so as to improve safety against overcharging. The substituted aromatic compound is at least one selected from the group consisting of 2,5-difluoro toluene and 3,4-difluoro toluene, the electrolyte for a lithium secondary battery. According to claim 1, wherein the content of the fluorine-substituted aromatic compound is a lithium secondary battery electrolyte, characterized in that 0.05 to 10% by weight based on the total weight of the electrolyte. The method of claim 2, wherein the content of the fluorine-substituted aromatic compound is a lithium secondary battery electrolyte, characterized in that 0.1 to 5% by weight based on the total weight of the electrolyte. The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte further comprises xylene. The electrolyte of claim 4, wherein the xylene is at least one selected from the group consisting of m-xylene, o-xylene, and p-xylene. The electrolyte for a lithium secondary battery according to claim 4, wherein the content ratio of xylene on the basis of the fluorine-substituted aromatic compound is in the range of 0.1 to 0.6 in molar ratio. The electrolyte for a lithium secondary battery according to claim 1, wherein the electrolyte of the carbonate compound is a mixture of a linear carbonate compound and a cyclic carbonate compound. A lithium secondary battery according to any one of claims 1 to 7, wherein the electrolyte for a lithium secondary battery is impregnated in an electrode assembly having a cathode / separator / cathode structure.

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