KR101294764B1 - Electrolyte for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, the electrolyte is a non-aqueous organic solvent; Lithium salts; And an additive, wherein the additive is [1,3] dichiolo [4,5-d] [1,3] dithiol-2,5-dione ([1,3] dithiolo [4] Of a carbonate derivative having a substituent selected from the group consisting of a first additive of a, 5-d] [1,3] dithiole-2,5-dione) compound and a halogen, a cyano group (CN) and a nitro group (NO ₂ ) It is characterized by including a second additive.

[Formula 1]

The electrolyte solution of the present invention can provide a lithium secondary battery having excellent life characteristics and excellent low temperature discharge capacity characteristics.

Electrolyte, Life, Low Temperature Discharge, TPDONE

Description

Electrolyte for lithium secondary battery and lithium secondary battery comprising same {Electrolyte for lithium secondary battery and lithium secondary battery comprising the same}

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, and more particularly, to a lithium secondary battery electrolyte having excellent life characteristics and low temperature discharge characteristics, and a lithium secondary battery comprising the same.

With the recent rapid development of the electronics, telecommunications, and computer industries, the use of portable electronic products such as camcorders, mobile phones, notebook PCs, etc., as well as the compactness, light weight, and high functionality of the devices are becoming common. The demand for it is increasing. In particular, the rechargeable lithium secondary battery has a high energy density per unit weight of about 3 times higher than conventional lead acid batteries, nickel-cadmium batteries, nickel-hydrogen batteries, and nickel-zinc batteries, and can be rapidly charged. Development is underway.

Lithium metal oxide is used as a positive electrode active material of a lithium secondary battery, and lithium metal, a lithium alloy, (crystalline or amorphous) carbon or a carbon composite material is used as a negative electrode active material. A lithium secondary battery may be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery according to the type of separator and electrolyte used, and may be cylindrical, square or pouch type depending on the form.

The average discharge voltage of the lithium secondary battery is about 3.6 to 3.7 V, and high power can be obtained as compared with other alkaline batteries, Ni-MH batteries, Ni-Cd batteries and the like. However, in order to produce such a high driving voltage, an electrochemically stable electrolyte composition is required in the charge and discharge voltage range of 0 to 4.2 V.

For this reason, an organic electrolyte solution in which lithium salts are dissolved in a non-aqueous organic solvent is used as an electrolyte for a lithium secondary battery. In this case, it is preferable to use an organic solvent having high ionic conductivity and high dielectric constant and low viscosity. However, since there is no single non-aqueous organic solvent that satisfies all of these conditions, a mixed solvent of a high dielectric constant organic solvent and a low dielectric constant organic solvent or a mixture of a high dielectric constant organic solvent and a low viscosity organic solvent is used. Solvent is used.

US Patent No. 6,114,070 discloses a method of improving ionic conductivity of an organic solvent by mixing dimethyl carbonate or diethyl carbonate and ethylene carbonate or propylene carbonate as a mixed solvent of chain carbonate and cyclic carbonate. It is. However, these mixed solvents can be used usually below 120 ℃ but when the temperature is higher than the gas generated by the vapor pressure there is a problem that the battery is swelling and can not be used.

U.S. Patent 5,352,548 discloses an electrolyte comprising an organic solvent having a vinylene carbonate (VC) content of at least 20%. However, vinylene carbonate has a low dielectric constant compared to ethylene carbonate, propylene carbonate, and gamma butyrolactone, and thus has a problem in that charge and discharge characteristics and efficiency characteristics of a battery are reduced when used as a main solvent.

In order to solve the problems of the non-aqueous organic solvent, researches have been continuously conducted to further improve battery characteristics by adding an additive having excellent performance to the electrolyte, but in the case of adding a specific additive to the electrolyte, You can expect some items to improve performance, but they often reduce the performance of others.

Accordingly, the present invention is to solve the above-mentioned disadvantages and problems of the prior art, and an object of the present invention is to provide an electrolyte solution for a lithium secondary battery having excellent life characteristics among battery characteristics and excellent low temperature discharge characteristics.

In order to achieve the object as described above, the present invention is a non-aqueous organic solvent; Lithium salts; And an additive, wherein the additive is [1,3] dichiolo [4,5-d] [1,3] dithiol-2,5-dione ([1,3] dithiolo [4] Of a carbonate derivative having a substituent selected from the group consisting of a first additive of a, 5-d] [1,3] dithiole-2,5-dione) compound and a halogen, a cyano group (CN) and a nitro group (NO ₂ ) It provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the same, characterized in that it comprises a second additive.

[Formula 1]

In addition, the present invention provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the content of the first additive is 0.1 to 3% by weight based on 100% by weight of the total electrolyte.

In addition, the present invention provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the content of the second additive is 0.1 to 10% by weight relative to the total 100% by weight of the electrolyte.

In another aspect, the present invention provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the second additive is fluoroethylene carbonate.

Accordingly, the lithium secondary battery electrolyte of the present invention and a lithium secondary battery comprising the same are 0.1 wt% to 3 wt% of [1,3] dichiol [4,5-d] [1,3] dithiol as an additive to the electrolyte. Lifetime by adding -2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2,5-dione) and 0.1% to 10% by weight of fluoroethylene carbonate It is effective in providing a secondary battery having excellent characteristics and excellent low-temperature discharge capacity characteristics.

Details of the above object and technical configuration of the present invention and the effects thereof will be more clearly understood by the following detailed description of the present invention.

The present invention, as an additive to the electrolyte containing a non-aqueous organic solvent and lithium salt for the purpose of improving the life characteristics and low-temperature discharge characteristics, [1,3] dichilo [4,5-d] First additives of [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2,5-dione) compounds and halogen, cyano groups It provides a lithium secondary battery electrolyte and a lithium secondary battery comprising the second additive of a carbonate derivative having a substituent selected from the group consisting of (CN) and nitro group (NO ₂ ).

[Formula 1]

Hereinafter, a lithium secondary battery including an electrolyte according to the present invention will be described.

First, the electrolyte according to the present invention includes a non-aqueous organic solvent, and the non-aqueous organic solvent may be carbonate, ester, ether or ketone. The carbonates include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC) ethylene carbonate (EC), Propylene carbonate (PC), butylene carbonate (BC) and the like can be used, and the ester is butyrolactone (BL), decanolide (decanolide), valerolactone, mevalonolactone (mevalonolactone) , Caprolactone, n-methyl acetate, n-ethyl acetate, n-propyl acetate, and the like may be used. The ether may be dibutyl ether, and the like, and the ketone may include polymethylvinyl ketone. The present invention is not limited to the type of nonaqueous organic solvent.

When the non-aqueous organic solvent is a carbonate-based organic solvent, it is preferable to use a mixture of a cyclic carbonate and a chain carbonate. In this case, the cyclic carbonate and the chain carbonate are preferably used by mixing in a volume ratio of 1: 1 to 1: 9, and more preferably used by mixing in a volume ratio of 1: 1.5 to 1: 4. The performance of the electrolyte is preferable when mixed in the above volume ratio.

The electrolyte solution of the present invention may further include an aromatic hydrocarbon organic solvent in the carbonate solvent. An aromatic hydrocarbon compound may be used as the aromatic hydrocarbon organic solvent.

Specific examples of the aromatic hydrocarbon-based organic solvent include benzene, fluorobenzene, chlorobenzene, nitrobenzene, toluene, fluorotoluene, trifluorotoluene, xylene and the like. In the electrolyte containing an aromatic hydrocarbon-based organic solvent, the volume ratio of the carbonate solvent / aromatic hydrocarbon solvent is preferably 1: 1 to 30: 1. The performance of the electrolyte is preferable when mixed in the above volume ratio.

Next, the electrolyte according to the present invention comprises a lithium salt, the lithium salt acts as a source of lithium ions in the battery to enable the operation of the basic lithium battery, for example LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiClO ₄ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiN (C ₂ F ₅ SO ₂ ) ₂ , LiAlO ₄ , LiAlCl ₄ , LiN (C _x F _{2x +1} SO ₂ ) ( CyF _{2x +1} SO ₂ ), where x and y are natural water and LiSO ₃ CF ₃ and include one or more or mixtures thereof.

At this time, the concentration of the lithium salt can be used in the range of 0.6 to 2.0M, preferably in the range of 0.7 to 1.6M. If the concentration of the lithium salt is less than 0.6M, the conductivity of the electrolyte is lowered and the performance of the electrolyte is lowered. If the lithium salt is more than 2.0M, the viscosity of the electrolyte is increased, thereby reducing the mobility of lithium ions.

Next, the electrolyte according to the present invention is an additive of [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] as an additive. dithiolo [4,5-d] [1,3] dithiole-2,5-dione) compounds.

[Formula 1]

Although it is not common to include additives in battery manufacturing, additives may be added depending on the purpose, especially in order to improve the characteristics of each sector, for example, life characteristics, low temperature high rate discharge characteristics, high temperature safety, overcharge prevention, and high temperature swelling. As added, in the present invention, it can be said that the additive is used to improve the life characteristics and low temperature discharge characteristics.

The content of the additive of [Formula 1] is preferably added in 0.1 to 3% by weight relative to 100% by weight of the total electrolyte.

When the content is less than 0.1% by weight, there is no effect of improving the low temperature discharge characteristics, and when the content is more than 3% by weight, there is no effect of improving the low temperature discharge characteristics. There is a tendency that the amount contained is reduced and other battery characteristics are lowered, and in particular, the present invention has a problem that the life characteristics are not good.

Next, the electrolyte according to the present invention further comprises a carbonate derivative having a substituent selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ) as an additive. Adding an additive of the carbonate derivative is preferable because it can provide a battery having excellent electrochemical characteristics such as high temperature swelling characteristics, capacity, lifespan, and low temperature characteristics. As such an additive, an ethylene carbonate derivative represented by the following Chemical Formula 2 is preferable, and fluoroethylene carbonate is most preferred.

[Formula 2]

(Wherein X is selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ).)

The additive is preferably included in 0.1 to 10 parts by weight based on 100 parts by weight of the total electrolyte. When the amount of the additive is less than 0.1 parts by weight, it is difficult to expect the effect of suppressing gas generation inside the battery, and there is a problem in that the life characteristics are not good, and when it exceeds 10 parts by weight, a problem of swelling at high temperature occurs. , Low temperature discharge capacity is not good.

Next, the lithium secondary battery including the electrolyte solution of the present invention includes a positive electrode and a negative electrode.

The positive electrode includes a positive electrode active material capable of reversibly intercalating and deintercalating lithium ions. Representative examples of the positive electrode active material include LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , or LiNi _{1 −} Lithium-transition metal oxides such as _{x- y} Co xMyO ₂ (0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ x + y ≦ 1, M is a metal of Al, Sr, Mg, La, etc.) are used .

The negative electrode includes a negative electrode active material capable of intercalating and deintercalating lithium ions, and the negative electrode active material uses crystalline or amorphous carbon or a carbon-based negative electrode active material of a carbon composite.

Applying the positive electrode and the negative electrode active material to the current collector of a thin plate with a suitable thickness and length, respectively, wound or laminated with a separator as an insulator to make an electrode group, and then put it in a can or a similar container, and then inject the electrolyte solution of the present invention A lithium secondary battery is manufactured. As the separator, a resin such as polyethylene or polypropylene may be used.

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are only preferred embodiments of the present invention and the present invention is not limited to the following examples.

Example 1

LiCoO2 as a positive electrode active material, polyvinylidene fluoride (PVDF) as a binder and carbon as a conductive agent were mixed in a weight ratio of 92: 4: 4, and then dispersed in N-methyl-2-pyrrolidone to prepare a positive electrode slurry. . The slurry was coated on an aluminum foil having a thickness of 20 μm, dried, and rolled to prepare a positive electrode. Synthetic graphite as a negative electrode active material, styrene-butadiene rubber as a binder, and carboxymethyl cellulose as a thickener were mixed in a weight ratio of 96: 2: 2, and then dispersed in water to prepare a negative electrode active material slurry. This slurry was coated on a copper foil having a thickness of 15 mu m, followed by drying and rolling to prepare a negative electrode.

A film separator made of a polyethylene (PE) material having a thickness of 20 μm was put between the prepared electrodes and then wound and compressed, and inserted into a rectangular can.

An electrolyte was injected into the square can to prepare a lithium secondary battery. The electrolyte solution was dissolved 1M LiPF ₆ in ethylene carbonate / ethylmethylcarbonate / dimethyl carbonate mixed solvent (1: 1: 1), and then added as [1,3] dichiol [4,5-d] [1.3] as an additive. Prepared by addition of a dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1.3] dithiole-2,5-dione) (TPDONE) compound and fluoro ethylene carbonate (FEC), At this time, the [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] 0.5 wt% of dithiole-2,5-dione) (TPDONE) compound was added, and 3 wt% of the fluoroethylene carbonate (FEC) was added.

[Example 2]

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 , 5-dione) (TPDONE) was carried out in the same manner as in Example 1, except that 1% by weight of the compound was added.

[Example 3]

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 , 5-dione) (TPDONE) was added in the same manner as in Example 1 except that 1% by weight of the compound and 5% by weight of fluoroethylene carbonate (FEC).

Example 4

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 , 5-dione) (TPDONE) was added in the same manner as in Example 1, except that 3% by weight of the compound and 5% by weight of fluoroethylene carbonate (FEC).

[Example 5]

It carried out similarly to Example 1 except adding 10 weight% of fluoro ethylene carbonate (FEC).

Comparative Example 1

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 , 5-dione) (TPDONE) was added in the same manner as in Example 1 except that 3% by weight of the compound was not added and fluoroethylene carbonate (FEC) was not added.

Comparative Example 2

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 The same procedure as in Example 1 was carried out except that 5 wt% of fluoroethylene carbonate (FEC) was added without addition of, 5-dione) (TPDONE) compound.

[Comparative Example 3]

[1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2 , 5-dione) (TPDONE) was added in the same manner as in Example 1 except that 5% by weight of the compound and 5% by weight of fluoroethylene carbonate (FEC).

[Comparative Example 4]

It carried out similarly to Example 1 except having added 15 weight% of fluoro ethylene carbonate (FEC).

The lithium batteries of Examples 1 to 5 and Comparative Examples 1 to 4 were charged at 0.5 C charge / discharge rate for 3 hours at 4.2 V CC-CV, and the standard capacity was measured, and charged at 4.2 V CC-CV for 3 hours at 1 C charge / discharge rate. After 100 times of performing 3V CC discharge at 1C charge / discharge rate, the capacity retention rate of the 100th time was expressed as a percentage of 100 times based on the standard capacity.

In addition, the lithium battery of Examples 1 to 5 and Comparative Examples 1 to 4 was charged with 4.2V CC-CV for 3 hours at 0.5C charge and discharge rate at room temperature, and left at -20 ° C for 4 hours, and then charged with 0.5C charge and discharge rate. The low-temperature discharge was measured by 3.2V CC discharge, and the low-temperature discharge was expressed as a percentage of the discharge capacity at low temperature based on the room temperature discharge.

The measurement results are shown in Table 1 below.

[Table 1]

division TPDONE (% by weight) FEC (% by weight) 100 times capacity (%) -20 ℃ discharge capacity (%) Example 1 0.5 3 93.0 36 Example 2 One 3 93.5 38 Example 3 One 5 94.0 34 Example 4 3 5 91 24 Example 5 0.5 10 91.0 18 Comparative Example 1 3 0 78.0 5 Comparative Example 2 0 5 90.0 15 Comparative Example 3 5 5 81.0 15 Comparative Example 4 0.5 15 88.0 8

From the results shown in Table 1, it can be seen that the 100-time capacity of Examples 1 to 5 is 90% or more, which is very good in life characteristics, and 24% or more except for Example 5 even at -20 ° C discharge capacity. It can be seen that very good.

However, in Example 5, the discharge capacity of -20 ° C was 18%, which is lower than that of the other examples, but it was higher than that of the comparative example, which was determined based on the numerical value of Comparative Example 5, so that the larger the content of FEC was, It can be seen that the -20 ° C discharge capacity is reduced, especially when the FEC content exceeds 15% by weight, that is, 10% by weight -20 ° C discharge capacity is significantly reduced.

In addition, in the case of Comparative Example 1 containing only TPDONE, it can be seen that both -20 ° C discharge capacity and 100 times capacity are not good.In Comparative Example 2 including only FEC, the capacity of 100 times is good, but -20 ° C discharge capacity is good. It can be seen that the characteristics are not good.

In addition, in the case of Comparative Example 3 containing more than 5% by weight, ie, 3% by weight of TPDONE, it can be seen that the discharge capacity characteristic of -20 ° C is not as good as that of Example 5 including 0.5% by weight, and 100 times the capacity. It can be seen that the decrease significantly.

Therefore, in the present invention, based on the measurement values in Table 1 described above, [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1,3] dithiole-2,5-dione) (TPDONE) comprises fluoroethylene carbonate (FEC), in the range of 0.1% to 3% by weight relative to 100% by weight of the total electrolyte. It is preferably included in the range of 0.1% to 10% by weight relative to 100% by weight of the total electrolyte.

Next, [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1, 3] The life characteristics of the lithium batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were measured to compare the properties of dithiole-2,5-dione (TPDONE) and fluoroethylene carbonate (FEC). It was.

The lithium batteries of Examples 1 to 3 and Comparative Examples 1 and 2 were charged with 4.2V CC-CV for 3 hours at 0.5C charge / discharge rate, and the initial capacity was measured, and 4.2V CC-CV for 3 hours at 1C charge / discharge rate. After 100 times of 3C CC discharge at 1C charge and discharge rate, the capacity according to each cycle number is shown in FIG. 1, and the capacity retention rate according to each cycle number is expressed as a percentage based on the initial capacity. 2 is shown.

1 is a graph showing a change in capacity according to the charge and discharge cycle, Figure 2 is a graph showing a change in capacity retention rate according to the charge and discharge cycle.

1, the initial capacities of Examples 1 to 3 and Comparative Examples 1 and 2 were 902 mAh, 898 mAh, 900 mAh, 885 mAh and 893 mAh, respectively, and the 100th capacities were 839 mAh, 840 mAh, 843 mAh, 690 mAh and 804 mAh, respectively. Therefore, the initial capacity does not show a difference, but as the number of charge / discharge cycles increases, it can be seen that the capacity of the comparative example 1 including only TPDONE significantly decreases, and in the case of the comparative example 2 including only FEC It can be seen that the capacity is reduced compared to the embodiment of.

In addition, referring to FIG. 2, the capacity retention ratios of the 100th to 100% initial capacities of Examples 1 to 3 and Comparative Examples 1 and 2 were 93%, 93.5%, 94%, 18%, and 90%, respectively. As the number of charge / discharge cycles increases, it can be seen that the capacity retention rate is significantly decreased in Comparative Example 1 including only TPDONE, and that in Comparative Example 2 including only FEC, capacity retention is reduced compared to the embodiment of the present invention. Able to know.

In other words, in the present invention, [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione ([1,3] dithiolo [4,5-d] [1, 3] The use of dithiole-2,5-dione (TPDONE) and fluoroethylene carbonate (FEC) at the same time is more effective in capacity retention characteristics than their respective use.

Therefore, the lithium secondary battery according to the present invention is 0.1 wt% to 3 wt% of [1,3] dichiolo [4,5-d] [1,3] dichiol-2,5-dione as an additive to an electrolyte solution. Excellent service life by adding ([1,3] dithiolo [4,5-d] [1,3] dithiole-2,5-dione) and 0.1 wt% to 10 wt% of fluoroethylene carbonate A secondary battery having excellent discharge capacity characteristics can be provided.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, Various changes and modifications will be possible.

1 is a graph showing a change in capacity according to a charge and discharge cycle,

2 is a graph showing a change in capacity retention rate according to charge and discharge cycles.

Claims (10)

Non-aqueous organic solvents; Lithium salts; And Contains additives, The additive is [1,3] dichiolo [4,5-d] [1,3] dithiol-2,5-dione ([1,3] dithiolo [4,5-d] [ 1,3] dithiole-2,5-dione) a first additive of the compound and a second additive of a carbonate derivative having a substituent selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ) The electrolyte solution for lithium secondary batteries characterized by the above-mentioned. [Formula 1]

The method of claim 1, The content of the first additive is a lithium secondary battery electrolyte, characterized in that 0.1 to 3% by weight relative to the total 100% by weight of the electrolyte. The method of claim 1, The content of the second additive is a lithium secondary battery electrolyte, characterized in that 0.1 to 10% by weight relative to the total 100% by weight of the electrolyte. The method of claim 1, The second additive is an ethylene carbonate derivative represented by the following formula (2) for a lithium secondary battery electrolyte. [Formula 2]

(Wherein X is selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ).) The method of claim 1, The second additive is fluoroethylene carbonate, characterized in that the electrolyte for lithium secondary batteries. An electrolyte comprising a non-aqueous organic solvent, a lithium salt, and an additive; A positive electrode including a positive electrode active material capable of intercalating and deintercalating the lithium; And A negative electrode including a negative electrode active material capable of intercalating and deintercalating the lithium; The additive is [1,3] dichiolo [4,5-d] [1,3] dithiol-2,5-dione ([1,3] dithiolo [4,5-d] [ 1,3] dithiole-2,5-dione) a first additive of the compound and a second additive of a carbonate derivative having a substituent selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ) A lithium secondary battery, characterized in that. [Formula 1]

The method of claim 6, The content of the first additive is a lithium secondary battery, characterized in that 0.1 to 3% by weight based on 100% by weight of the total electrolyte. The method of claim 6, The content of the second additive is a lithium secondary battery, characterized in that 0.1 to 10% by weight relative to the total 100% by weight of the electrolyte. The method of claim 6, The second additive is a lithium secondary battery, characterized in that the ethylene carbonate derivative of the formula (2). [Formula 2]

(Wherein X is selected from the group consisting of halogen, cyano group (CN) and nitro group (NO ₂ ).) The method of claim 6, The second additive is a lithium secondary battery, characterized in that fluoroethylene carbonate.

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