KR100408515B1 - Organic electrolyte and lithium secondary battery using the same - Google Patents

Abstract

The present invention relates to an organic electrolyte solution containing a mixed organic solvent and a lithium salt, wherein the mixed organic solvent comprises a high dielectric constant solvent, a low viscosity solvent, and a compound represented by the following Chemical Formula 1; The present invention relates to a lithium secondary battery employing the same. The organic electrolyte solution for a lithium secondary battery according to the present invention has an advantage of excellent ion conductivity and low temperature storage characteristics and a wide potential window region. In addition, the present invention also provides an organic electrolyte solution, characterized in that the lithium salt includes an inorganic lithium salt and an organic lithium salt, the lithium secondary battery employing the same has a large battery capacity and stable charge and discharge characteristics of the battery In addition to improving the lifespan characteristics, the high temperature and self discharge characteristics are also improved.

In the above formula,

R ₁ and R ₂ are independent of each other, a linear or cyclic alkyl group having 1 to 3 carbon atoms, and x is an integer of 1 to 4;

Description

Organic Electrolyte and Lithium Secondary Battery Using the Same

The present invention relates to a lithium secondary battery, and more particularly, an organic electrolyte and lithium using the same, which can improve battery capacity, low-temperature storage and charge-discharge cycle characteristics, as well as high-temperature characteristics and self-discharge characteristics. It relates to a secondary battery.

In recent years, the miniaturization, thinning, and lightening of electronic devices have been rapidly made. In particular, in the field of office automation, portable electronic devices such as camcorders, cellular phones, and the like have been rapidly reduced. Is spreading.

In accordance with the trend toward miniaturization, light weight, and thinness of such electronic devices, high performance is also required for secondary batteries that supply power to them. That is, it is possible to replace the existing lead acid battery or nickel-cadmium battery, etc., the development of a lithium secondary battery that can be charged and discharged repeatedly with a high energy density while being compact and lightweight.

A rechargeable lithium battery is an organic electrolyte or polymer electrolyte in which lithium ions can move between a positive electrode, a negative electrode, and a positive electrode and a negative electrode using a material capable of intercalation and deintercalation of lithium ions as an active material. A battery prepared by charging a lithium ion, which generates electrical energy by oxidation and reduction reactions when lithium ions are intercalated / deintercalated at the positive and negative electrodes.

As a cathode of a rechargeable lithium battery, a complex oxide of lithium and a transition metal capable of intercalating and deintercalating lithium ions, which is about 3-4.5 V higher than the electrode potential of Li / Li ⁺ , is mainly used. Examples thereof include lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), and the like.

In addition, as a cathode, the chemical potential at the time of intercalation / deintercalation of lithium metal or lithium alloy or lithium ion capable of reversibly accepting or supplying lithium ions while maintaining structural and electrical properties is almost similar to that of metallic lithium. Carbon-based materials are mainly used.

The use of lithium metal or an alloy thereof as a negative electrode active material is called a lithium metal battery, and the use of a carbon material is called a lithium ion battery. A lithium metal battery using lithium metal or an alloy as a negative electrode has a short battery life and low stability, such as a volume change of the lithium metal during charging and discharging, and a short circuit due to local deposition of lithium on the surface of the lithium metal. In order to solve this problem, a lithium ion battery using a carbon material as a negative electrode active material has been developed. Lithium ion batteries have only the movement of lithium ions during charging and discharging, and thus the electrode active materials remain intact, improving battery life and stability compared to lithium metal batteries.

In addition, lithium secondary batteries may be classified according to the type of electrolyte. In particular, lithium secondary batteries using a solid polymer electrolyte are called lithium polymer batteries, and lithium polymer batteries do not contain any organic electrolyte according to the type of polymer electrolyte. It can be divided into a fully solid lithium polymer battery using a completely solid electrolyte, and a gel lithium polymer battery using a gel polymer electrolyte in which an organic electrolyte is impregnated into a polymer. In addition, the lithium polymer battery may be classified into a lithium ion polymer battery and a lithium metal polymer battery according to a material used as a negative electrode active material as described above.

The organic electrolyte is an important factor for determining the performance of the lithium polymer battery as well as the lithium ion battery. The organic electrolyte is an ion conductor in which lithium salt is dissolved in an organic solvent, and should be excellent in conductivity of lithium ions and chemical and electrochemical stability of the electrode. In addition, the available temperature range should be wide and the manufacturing cost should be low. Therefore, it is preferable to use an organic solvent having high ionic conductivity and low dielectric constant and low viscosity.

However, since there is no single organic solvent that can satisfy the above conditions, a mixed organic solvent system in which a low viscosity organic solvent is mixed with a high dielectric constant organic solvent is used. Examples thereof include propylene carbonate and diethyl carbonate. Carbonate ester mixed solvents; Examples thereof include a mixed solvent of ethylene carbonate, dimethyl carbonate, and diethyl carbonate.

The use of such a mixed organic solvent has the advantage of greatly improving the ion conductivity and increasing the initial capacity of the battery by increasing the mobility of lithium ions. However, since the electrolyte reacts with the negative electrode active material as the cycle progresses, the battery There is a problem that the capacity of is decreased, and at low temperatures, the mobility of lithium ions decreases due to freezing of the organic solvent, and thus there is a fear that the ionic conductivity drops rapidly.

Japanese Patent Application Laid-open No. Hei 7-169504 discloses a methylpropionate and ethylpropionate having a very low freezing point in a two-component organic solvent composed of a conventional high dielectric constant solvent and a low viscosity solvent in order to improve ionic conductivity at low temperature. An organic electrolyte solution to which a third component solvent such as is added is described. In this case, however, the low-temperature discharge characteristics are improved, but the lifespan characteristics at room temperature are deteriorated, and the electrolyte is contaminated due to the product by spontaneous reaction with the current collector, thereby adversely affecting the battery characteristics.

In addition, lithium salts currently used in the organic electrolyte of lithium secondary batteries include LiClO ₄ , LiBF ₄ , LiPF ₆ , LiCF ₃ SO ₃ or LiN (CF ₃ SO ₂ ) ₂ , but these have poor thermal stability and ions There is a problem of low conductivity. On the other hand, LiPF ₆ has excellent ion conductivity, but has a problem in that the electrolyte itself is easily decomposed because it is very sensitive to moisture.

SUMMARY OF THE INVENTION The present invention has been made in an effort to provide an organic electrolyte solution that does not easily react with a negative electrode active material and thus has excellent charge / discharge cycle characteristics and low temperature characteristics.

Another object of the present invention is to provide an organic electrolyte solution capable of improving high temperature characteristics and self discharge characteristics of a lithium secondary battery.

Another technical problem to be achieved by the present invention is to provide a lithium secondary battery having improved charge / discharge cycle characteristics, low temperature characteristics, high temperature characteristics, and self discharge characteristics by employing the organic electrolyte.

1 is a graph showing the potential window characteristics of an organic electrolyte according to an embodiment of the present invention,

Figure 2 is a graph showing the potential window characteristics of the organic electrolytic solution according to another embodiment of the present invention.

3 is a cross-sectional view of a coin-type battery employing an organic electrolyte according to an embodiment of the present invention.

4 is a cross-sectional view of a rectangular lithium ion polymer battery employing an organic electrolyte according to another embodiment of the present invention.

Technical problem of the present invention is a lithium secondary battery electrolyte comprising a mixed organic solvent and a lithium salt, the mixed organic solvent comprises a high dielectric constant solvent, a low viscosity solvent and a compound represented by the following formula (1) It can be done by organic electrolyte:

[Formula 1]

In the above formula, R ₁ and R ₂ are independent of each other, a linear or cyclic alkyl group having 1 to 3 carbon atoms, and x is an integer of 1 to 4.

Another technical problem of the present invention is a lithium secondary battery electrolyte comprising a mixed organic solvent and a lithium salt, wherein the mixed organic solvent includes a high dielectric constant solvent, a low viscosity solvent, and a compound represented by Formula 1, wherein the lithium The salt may be made of an organic electrolyte solution for a lithium secondary battery, which is a mixture of an inorganic lithium salt and an organic lithium salt.

In addition, another technical problem of the present invention is an anode including an oxide or sulfide of a lithium-containing metal; A negative electrode comprising a lithium metal, a lithium alloy or a carbon material; And it can be made by a lithium secondary battery comprising the organic electrolyte of the present invention as described above.

The organic electrolyte solution for achieving the first object of the present invention is characterized in that it comprises a compound represented by the formula (1), the compound inhibits the oxidation reaction between the electrolyte and the negative electrode active material and is not easily decomposed under high voltage battery It can contribute to improve the charge / discharge cycle characteristics of In addition, since the melting point is very low, it can contribute to the improvement of the low temperature characteristic, which is pointed out as a serious drawback of the lithium battery.

Specific examples of the compound represented by Formula 1 include dimethyl malonate, diethyl malonate, dimethyl succinate, dimethyl glutarate, and dimethyl adipate.

The high dielectric constant solvent used in the organic electrolytic solution of the present invention preferably has a dielectric constant of 30 or more, and the low viscosity solvent preferably has a viscosity of 1.5 cP or less.

As the high dielectric constant solvent used in the organic electrolytic solution of the present invention, at least one compound selected from ethylene carbonate, propylene carbonate or gamma butyrolactone is preferable, and as the low viscosity solvent, dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethicone Preference is given to at least one compound selected from oxyethane and tetrahydrofuran.

In the mixed organic solvent used in the organic electrolytic solution of the present invention, the mixed volume ratio of the high dielectric constant solvent, the low viscosity solvent, and the compound represented by the formula (1) is preferably 30-50: 30-40: 20-30.

Lithium salt used in the organic electrolyte solution for achieving the first technical problem of the present invention may include one or more selected from the group consisting of inorganic and organic-based lithium salts known in the art.

The inorganic lithium salt is a lithium salt which does not contain carbon in the chemical structure, and examples thereof include LiClO ₄ , LiBF ₄ , LiPF _6, and the like.

Organic-based lithium salts are lithium salts containing carbon in the chemical formula, for example, LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) ₃ , LiN (C ₂ F ₅ SO ₂ ) ₂ and so on.

It is preferable that the total content of the lithium salt contained in an organic electrolyte solution is 1 mol-1.5 mol per liter of organic electrolyte solution. This is because the ionic conductivity of the lithium salt in the organic electrolyte is the highest in the above range.

On the other hand, the organic electrolyte for achieving the second technical problem of the present invention is a low-temperature discharge characteristics and self-discharge characteristics by mixing an inorganic lithium salt and an organic lithium salt having excellent thermal stability and less influence of moisture in a certain ratio Characterized in that improved.

The inorganic lithium salt that can be used in the mixture of the inorganic lithium salt and the organic lithium salt is not particularly limited as long as it is a lithium compound that dissociates in an organic solvent to give lithium ions, and among these, a fluorine lithium salt having excellent electrical characteristics is preferable. .

The organic lithium salt that may be used in the mixture of the inorganic lithium salt and the organic lithium salt includes LiN (CF ₃ SO ₂ ) ₂ , LiC (CF ₃ SO ₂ ) _3, and LiN (C ₂ F ₅ SO ₂ ) ₂ . At least one compound selected from the group is preferred. Since the compounds have a high pyrolysis temperature and are less affected by moisture, the compounds serve to improve high temperature and self discharge characteristics of the organic electrolyte.

The ratio of the inorganic lithium salt and the organic lithium salt is preferably in a molar ratio of 0.7 to 0.9: 0.3 to 0.1. When the molar ratio of the organic lithium salt is 0.3 or more, the ion conductivity of the electrolyte is sharply lowered so that the capacity characteristics of the battery are reduced. This becomes poor, and when it is 0.1 or less, the self discharge characteristic does not improve.

In addition, the organic electrolyte of the present invention may further include an inorganic additive in order to further improve the self-discharge characteristics. LiBO ₂ , Li ₂ CO ₃ , Li ₃ PO ₄ , Li ₃ N, SnO ₂ , and the like may be used as the inorganic additive. Among them, a lithium boric acid compound (LiBO ₂ ) is most preferable. For example, the boron atom has a valence of 3 and has unbound electrons, and thus acts as an electron acceptor to withdraw electrons from carbon as a negative electrode active material. Therefore, the bonding force between carbon and boron becomes considerably large, and the chemical potential of the negative electrode increases by the increase of the bonding force, which tends to accept lithium ions more. As a result, the concentration of lithium ions in the negative electrode increases and the capacity of the battery increases. As for the addition amount of the inorganic additive to exhibit such an effect, 1 * 10 <^-4> -5 * 10 <^-2> mol per 1 liter of organic electrolytes is preferable.

The organic electrolyte of the present invention can also be applied to a lithium ion polymer battery having a gel polymer solid electrolyte. That is, the present invention can be applied to all lithium secondary batteries except for fully solid lithium polymer batteries.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the present invention is not limited only to the following examples.

LiPF ₆ used in Examples and Comparative Examples below was used as a battery reagent grade product of Hashimoto, Japan, without purification, LiC (CF ₃ SO ₂ ) ₃ , LiN (C ₂ F ₅ SO ₂ ) ₂ and LiN (CF ₃ SO ₂ ) ₂ was used as a 3M battery reagent grade product without purification. The solvent used in the preparation of the organic electrolyte solution was Merck's battery reagent grade product, and all experiments were conducted in an argon gas (99.9999% or more) atmosphere.

<Example 1>

A reagent bottle containing ethylene carbonate in a solid state was placed in an electric mantle, and then slowly heated to 70-80 ° C. to liquefy. Then, insert the content of LiPF ₆ to make a 1M LiPF ₆ solution in a plastic bucket to hold the electrolyte solution and then, given vigorous shaking into the dimethyl carbonate was dissolved in the lithium metal. The liquefied ethylene carbonate solution was added to the mixture using a pipette to shake the mixture, and dimethylmalonate was added thereto as a compound of Formula 1, and the mixture was shaken vigorously to mix the solution evenly. At this time, the volume ratio of ethylene carbonate, dimethyl carbonate and dimethyl malonate was 40:40:30.

The organic electrolyte solution prepared as described above was stored for 20 days in a dry box, and then the amount of water in the organic electrolyte solution was measured using a Karl-Fisher titration method (used device: 737KF coolometer manufactured by Metrom, Switzerland). As a result of moisture measurement, the water content in the organic electrolyte solution was approximately 20 ppm.

<Example 2>

It was prepared in the same manner as in Example 1 except that diethylmalonate was used instead of dimethylmalonate as the compound of Formula 1.

<Example 3>

It was prepared in the same manner as in Example 1 except for using dimethyl succinate instead of dimethylmalonate as the compound of Formula 1.

<Example 4>

It was prepared in the same manner as in Example 1 except for using dimethyl glutarate instead of dimethylmalonate as the compound of Formula 1.

Example 5

It was prepared in the same manner as in Example 1 except for using dimethyladipate instead of dimethylmalonate as the compound of Formula 1.

<Example 6>

It was prepared in the same manner as in Example 1 except that the mixing volume ratio of ethylene carbonate, dimethyl carbonate and dimethyl malonate was 40:40:20.

<Example 7>

It was prepared in the same manner as in Example 1 except that the mixing volume ratio of ethylene carbonate, dimethyl carbonate and dimethyl malonate was 50:30:20.

Comparative Example 1

An electrolyte solution was prepared in the same manner as in Example 1, except that dimethyl malonate was not added to the mixed organic solvent. At this time, the volume ratio of ethylene carbonate and dimethyl carbonate was 1: 1.

Comparative Example 2

An electrolyte solution was prepared in the same manner as in Example 1 except that the mixing volume ratio of ethylene carbonate, dimethyl carbonate, and dimethyl malonate was 60:20:20.

Comparative Example 3

It was prepared in the same manner as in Example 1 except that the mixing volume ratio of ethylene carbonate, dimethyl carbonate and dimethyl malonate was 30:30:40.

Ion conductivity, low temperature storage characteristics and potential window characteristics of the organic electrolyte prepared according to Examples 1-7 and Comparative Examples 1-3 were evaluated. At this time, the characteristic evaluation was performed according to the following method.

1) Ion Conductivity of Electrolyte

Non-blocking measuring cells were assembled in a dry box. About 15 mL of the electrolyte to be measured and a platinum electrode were used, and the cell was stored in a metal thin case for reagent storage to prevent the cell from contacting with air. The cell stored in the metal thin film case was taken out and stored in a constant temperature and humidity chamber for 1 hour, and then the conductivity was evaluated by measuring impedance with an impedance meter.

2) Low Temperature Storage Characteristics

Prepare two 30 ml plastic containers, add 15 ml of electrolyte to each container, close the lid and wrap it around with a paraffin film to completely block air contact, and then place the container in the TABAI thermo-hygrostat. The mixture was allowed to stand for 24 hours at temperatures of -30 ° C and -40 ° C, respectively, and then visually observed whether the electrolyte was frozen.

3) potential window

For the organic electrolyte solution of Example 1-7 and Comparative Example 1-3, the range of the potential window was measured using cyclic voltammography. A tripolar measuring cell was used, and a carbon electrode and a lithium metal electrode were used, respectively. The scanning speed was 1 mV / sec at a frequency of 1 MHz, and the measurement results are shown in FIG. 1.

4) Charge and discharge life characteristics

In order to evaluate the charge and discharge life characteristics of the battery using the organic electrolyte prepared according to Examples 1-7 and Comparative Examples 1-3, a 2016 type coin-type battery (see FIG. 3) was prepared as follows.

A paste-type cathode active material was prepared by mixing LiCoO ₂ , Super-P carbon (manufactured by MMM Carbon Co.) and polytetrafluoroethylene dissolved in N-methylpyrrolidone, and then, paste it onto an aluminum foil having a thickness of 200 μm. After casting, drying, pressing, and cutting to prepare a positive electrode 34 for a coin-type battery.

In addition, a paste-type negative electrode active material was prepared by mixing graphite powder (MCMB 2528, manufactured by Osaka Gas Co.), Super-P carbon (made by MMM Carbon Co.), and polytetrafluoroethylene dissolved in N-methylpyrrolidone. After manufacturing, it was cast on an aluminum foil having a thickness of 200 μm, dried, pressed, and cut to prepare a negative electrode 33 for a coin-type battery.

As the separator 35, Cellgard 2400 manufactured by Hoechst Cellanese Co. was used. The separator was placed between the cathode and the anode, and the organic electrolyte solution of Examples 1-7 and Comparative Examples 1-3 was used. Soak for 10 minutes. After 10 minutes, it was removed, and a 2016 type coin-type battery (FIG. 3) was completely sealed with a stainless steel case 31, a stainless steel lid 32, and an insulating gasket 36 using a cramping machine. At this time, the battery had a capacity of 3.15 mAh.

For the coin-type battery manufactured as described above, the initial capacity and the capacity after the 100 and 200 cycle charge and discharge experiments were measured and shown in relation to the initial capacity. A charge / discharger (manufactured by Maccor) with a capacity of 1 A was used, and charging and discharging were performed at 0.2 C at 25 ° C., respectively, and the charging voltage was 3.0-4.2 V.

Table 1 shows the ion conductivity, low temperature storage characteristics, and charge and discharge life characteristics of the organic electrolyte prepared according to Example 1-7 and Comparative Examples 1-3 according to the above-described method.

division Ion Conductivity (25 ℃, s / cm) Low Temperature Storage Characteristics Charge and discharge life characteristics -30 ℃ -40 ℃ Initial capacity (mAh) After initial cycle after 100 cycles Of initial capacity after 200 cycles Example 1 1.124 × 10 ^-2 Non-freezing Non-freezing 2.89 92% 87% Example 2 1.226 × 10 ^-2 〃 〃 2.91 90% 85% Example 3 1.137 × 10 ^-2 〃 〃 2.88 90% 86% Example 4 1.123 × 10 ^-2 〃 〃 2.86 91% 87% Example 5 1.118 × 10 ^-2 〃 〃 2.84 90% 85% Example 6 1.116 × 10 ^-2 〃 〃 2.85 90% 86% Example 7 1.133 × 10 ^-2 〃 〃 2.87 91% 85% Comparative Example 1 1.420 × 10 ^-2 freezing freezing 2.88 93% 87% Comparative Example 2 1.475 × 10 ^-2 〃 〃 2.91 85% 79% Comparative Example 3 1.112 × 10 ^-2 Non-freezing Non-freezing 2.77 82% 75%

From the results of Table 1 and FIG. 1, it can be seen that the organic electrolyte solution and the lithium ion battery employing the same according to the present invention have high ion conductivity values of 1 × 10 ⁻² S / cm or more and excellent low temperature storage characteristics. . In addition, since the capacity of the battery is less than 90% of the initial capacity after 100 cycles, and 85% or more after 200 cycles, it can be seen that the lifespan characteristics are very excellent in view of the small capacity change rate due to the cycle progression. The large potential window area ensures stable battery characteristics over a wide range of voltages.

<Example 8>

As the lithium salt LiPF ₆ and _{0.9M-0.1M-LiC (CF 3 SO 2} ) the content to create a _third solution, LiPF ₆ and LiC (CF ₃ SO _{2) 3,} and using, as the high dielectric constant solvents, ethylene carbonate, low viscosity An organic electrolyte solution was prepared in the same manner as in Example 1, except that ethyl methyl carbonate was used as a solvent and dimethyl succinate was used as a third solvent.

Example 9

An organic electrolyte solution was prepared in the same manner as in Example 8, except that dimethyl glutarate was used as the third solvent.

<Example 10>

An organic electrolyte solution was prepared in the same manner as in Example 8, except that dimethyl adipate was used as the third solvent.

<Example 11>

An organic electrolyte was prepared in the same manner as in Example 8 except that the contents of LiPF ₆ and LiC (CF ₃ SO ₂ ) ₃ were 0.8M and 0.2M, respectively.

<Example 12>

An organic electrolyte was prepared in the same manner as in Example 8 except that the contents of LiPF ₆ and LiC (CF ₃ SO ₂ ) ₃ were set to 0.7M and 0.3M, respectively.

Example 13

It was prepared in the same manner as in Example 8 except that 2 × 10 ⁻² M of LiBO ₂ was further added.

<Comparative Example 4>

The same method as in Example 8 except that no ethyl methyl carbonate was added and the volume ratio of ethylene carbonate and dimethyl carbonate was 2: 1, and only 1M-LiPF _{6 was} used without adding LiC (CF ₃ SO ₂ ) ₃ . An organic electrolyte was prepared.

Comparative Example 5

Ethyl methyl carbonate was used instead of dimethyl carbonate, and an organic electrolyte was prepared in the same manner as in Comparative Example 4 except that the volume ratio of ethylene carbonate and ethyl methyl carbonate was 1: 1.

Comparative Example 6

An organic electrolyte was prepared in the same manner as in Example 8 except that the contents of LiPF ₆ and LiC (CF ₃ SO ₂ ) ₃ were set to 0.6M and 0.4M, respectively.

The ion conductivity and the potential window characteristics of the organic electrolyte prepared according to Example 8-13 and Comparative Example 4-6 were measured by the same method as described above, and are shown in Table 2 and FIG. 2.

division Ion Conductivity 20 ℃, S / cm Potential window area, V Example 8 1.124 × 10 ^-2 5.12 Example 9 1.126 × 10 ^-2 5.04 Example 10 1.137 × 10 ^-2 4.98 Example 11 1.223 × 10 ^-2 5.19 Example 12 1.118 × 10 ^-2 5.02 Example 13 1.216 × 10 ^-2 5.06 Comparative Example 4 1.133 × 10 ^-2 4.89 Comparative Example 5 1.220 × 10 ^-2 4.85 Comparative Example 6 1.112 × 10 ^-2 4.87

The organic electrolyte solution (Examples 8-13 and Comparative Examples 4-6) of the present invention was applied to a lithium ion polymer battery (see FIG. 4). Kinar 2801 (trade name; Kynar2801, manufactured by Altochem. Co.), a copolymer of polyvinylidene fluoride and propylene hexafluoride (PVdF-HFP), was used.

In the positive electrode 46, 65 parts by weight of LiCoO ₂ , a positive electrode active material, 20 parts by weight of dibutyl phthalate, and 15 parts by weight of Kinar 2801, were shaken in 450 ml of acetone, and the Kinar 2801 was sufficiently dissolved in an oven at 50 to 60 ° C. I was. It was prepared by mixing for 48 hours in a ball mill, then cast to a thickness of 120㎛ using a doctor blade and dried in the air.

In the case of the negative electrode 42, 65 parts by weight of the graphite-based carbon active material as the negative electrode active material, instead of the positive electrode active material, was manufactured in the same manner as in the preparation of the positive electrode.

The separator 43 is a mixture of 250 g of acetone, 30 g of Kinar 2801, 40 g of dibutyl phthalate, and 30 g of silicon oxide, and then sufficiently dissolved in an oven at 50 to 60 ° C. After casting to 50 to 55㎛ dried in air to prepare acetone by volatilization.

A lithium ion polymer battery (FIG. 4) was manufactured using the positive electrode 46, the negative electrode 42, the separator 43, the copper current collector 41, and the aluminum current collector 45 prepared as described above.

1) Charge and discharge life characteristics

When the positive electrode active material is LiCoO ₂ , the theoretical capacity of the battery was calculated based on 130 mAh / g, and the capacity ratio of the positive electrode and the negative electrode was 1: 2.1 to 2.2. At this time, the battery capacity was made 170mAh, the initial capacity was 177mAh. Charging and discharging were carried out twice at a rate of 10 hours in the range of 2.8 to 4.2 V for chemical formation.

The charge / discharge cycle of the battery was carried out at 2.8 to 4.2 V at a rate of 2 hours under constant current / constant voltage conditions, and the constant voltage section was 1/10 of the constant current section. The capacity and charge / discharge cycle life characteristics of the battery are shown in Table 3.

2) Low temperature discharge characteristics

The battery produced using the organic electrolyte solution of Example 8-13 and Comparative Example 4-6 was charged under constant current / constant voltage conditions to 4.2 V at a rate of 2 hours. The charged battery was left at −20 ° C. for 17 hours and then discharged to 2.75 V at a rate of 0.5 hours. The results are shown in Table 3.

3) Self discharge characteristic

In order to evaluate the capacity reduction rate due to self discharge of the battery using the organic electrolyte solution of Examples 8-13 and Comparative Examples 4-6, the battery having been completed in the formation step was charged under constant current / constant voltage conditions up to 4.2 V at a rate of 5 hours. Discharged at a rate of 5 hours. Again, the battery was charged at a constant current / constant voltage condition up to 4.2 V at a rate of 2 hours, then left at 20 ° C. for 30 days, and then discharged at a rate of 2 hours up to 2.75 V. The test results are shown in Table 3.

division Low temperature discharge characteristic (-20 ℃) Charge / discharge life characteristics (20 ℃) Self discharge rate (20 ℃,%) Discharge Capacity (mAh) Discharge rate (%) Battery capacity after 100 cycles (mAh) % Of initial capacity after 100 cycles Example 8 149.39 84.4 166.03 93.8 10.7 Example 9 145.86 82.4 161.96 91.5 11.5 Example 10 139.21 78.6 158.59 89.6 12.6 Example 11 147.65 83.4 167.62 94.7 9.1 Example 12 145.45 82.18 166.38 94.0 11.98 Example 13 151.69 85.7 168.46 95.14 10.87 Comparative Example 4 25.134 14.2 164.08 92.7 14.1 Comparative Example 5 115.23 65.1 154.34 87.2 13.8 Comparative Example 6 97.03 54.8 145.67 82.3 12.9

From Table 3, when the organic electrolyte solution of Example 8-13 is used in comparison with the case of using the organic electrolyte solution of Comparative Example 4-6, the life characteristics are similar to or better than those of using only inorganic lithium salts. Low temperature discharge characteristics and self discharge characteristics were improved, and high temperature self discharge characteristics of 60 ° C. were also improved by using an organic lithium salt and an inorganic lithium salt together.

The organic electrolyte solution for a lithium secondary battery according to the present invention has the advantages of excellent ion conductivity, low temperature storage, wide potential window area, and excellent charge / discharge life characteristics by including the compound represented by the formula (1). In addition, the organic electrolyte for lithium secondary batteries of the present invention containing inorganic lithium salts and organic lithium salts at a constant ratio has improved capacity characteristics due to a decrease in the self discharge rate, and excellent thermal stability to discharge characteristics at high temperatures. Is improved. Therefore, the lithium secondary battery employing the electrolyte solution of the present invention exhibits stable charge and discharge characteristics even when the battery capacity is large and the cycle progresses, and also has good low temperature storage characteristics, high temperature characteristics, and battery life characteristics.

Claims (14)

In an organic electrolyte solution containing a mixed organic solvent and a lithium salt, The mixed organic solvent comprises a high dielectric constant solvent, a low viscosity solvent and a compound represented by the formula (1). [Formula 1] In the above formula, R_OneAnd R₂Are independent of each other, a linear or cyclic alkyl group having 1 to 3 carbon atoms, x is a 1 to 4 Is an integer. The organic electrolyte solution of claim 1, wherein the mixing volume ratio of the high dielectric constant solvent, the low viscosity solvent, and the compound of Formula 1 is 30-50: 30-40: 20-30. The organic electrolyte solution according to claim 1, wherein the high dielectric constant solvent is at least one compound selected from the group consisting of ethylene carbonate, propylene carbonate, and gamma butyrolactone. The organic electrolyte of claim 1, wherein the low viscosity solvent is at least one compound selected from the group consisting of dimethyl carbonate, diethyl carbonate, ethyl methyl carbonate, dimethoxyethane and tetrahydrofuran. The organic electrolyte solution of claim 1, wherein the compound of Formula 1 is at least one compound selected from the group consisting of dimethylmalonate, diethylmalonate, dimethyl succinate, dimethylglutarate, and dimethyl adipate. . The organic electrolytic solution according to claim 1, wherein the content of the lithium salt is 1 mol to 1.5 mol per liter of the organic electrolytic solution. The organic electrolytic solution according to claim 6, wherein the lithium salt is at least one compound selected from the group consisting of inorganic lithium salts and organic lithium salts. The organic electrolytic solution according to claim 7, wherein the lithium salt is a mixture of an inorganic lithium salt and an organic lithium salt. The organic electrolytic solution according to claim 8, wherein the inorganic lithium salt is a fluorine lithium salt. The method of claim 8, wherein the organic lithium salt is at least one compound selected from the group consisting of LiC (CF ₃ SO ₂ ) ₃ , LiN (C ₂ F ₅ SO ₂ ) ₂ and LiN (CF ₃ SO ₂ ) ₂ An organic electrolyte solution, characterized in that. The organic electrolyte solution according to claim 8, wherein the molar ratio of the inorganic lithium salt and the organic lithium salt is 0.7 to 0.9: 0.3 to 0.1. The organic electrolytic solution according to claim 1, further comprising an inorganic additive having a molar concentration of 1 × 10 ^{−4 to} 5 × 10 ⁻² . The organic electrolyte of claim 12, wherein the inorganic additive is at least one compound selected from the group consisting of LiBO ₂ , Li ₂ CO ₃ , Li ₃ PO _4, Li ₃ N, and SnO ₂ . An anode comprising an oxide or sulfide of a lithium-containing metal; A negative electrode comprising a metal lithium, a lithium alloy or a carbon material; And A lithium secondary battery comprising the organic electrolyte of any one of claims 1 to 13.