KR100309777B1 - A non-aqueous electrolyte and a lithium secondary battery made thereof - Google Patents

Abstract

The electrolyte solution for a lithium secondary battery according to the present invention comprises a lithium salt and an organic solvent, and the organic solvent is a non-aqueous organic solvent in which a halogenated benzene is added to a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate. The halogenated benzene is represented by the following formula (1):

Wherein X is F, Cl, Br, or I and n is an integer from 1 to 3.

The electrolyte solution of the present invention is applied to a lithium secondary battery using crystalline graphite as a negative electrode and lithium oxide as a positive electrode. The lithium secondary battery to which the electrolyte is applied has excellent life characteristics of a carbonate solvent and excellent low temperature discharge characteristics and high temperature storage characteristics by use of halogenated benzene.

Description

A non-aqueous electrolyte and a lithium secondary battery comprising the same {A NON-AQUEOUS ELECTROLYTE AND A LITHIUM SECONDARY BATTERY MADE THEREOF}

Field of invention

The present invention relates to a non-aqueous electrolyte and a lithium secondary battery comprising the same. More particularly, in order to improve low-temperature discharge characteristics of a battery, a ratio of adding halogenated benzene to a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate The present invention relates to an electrolyte solution containing lithium salt in an aqueous organic solvent and a lithium secondary battery including the same.

Prior art

Recently, with the development of the high-tech electronic industry, it is possible to reduce the weight and weight of electronic equipment, and thus the use of portable electronic devices is increasing. As a power source for such portable electronic devices, the need for a battery having a high energy density has been increased, and research on lithium secondary batteries has been actively conducted. Lithium metal or carbon material is used as a negative electrode material of a lithium secondary battery, and lithium oxide is used as a positive electrode material. When lithium metal is used as a negative electrode material, there is a risk of explosion due to battery shortage due to the formation of dendritic crystals (dentrite), which is being replaced by carbon material instead of lithium metal as a negative electrode material. Composite metal oxides such as LiMn ₂ O ₄ , LiMnO ₂ , LiCoO ₂ , LiNiO ₂ , LiNi _1-x Co _x O ₂ (0 <x <1) are used as the anode material. Mn-based electrode materials, such as LiMn ₂ O ₄ , LiMnO ₂ , are easy to synthesize, relatively inexpensive, and have low environmental pollution but have a small capacity. In particular, LiMn ₂ O ₄ has a lower discharge capacity than other active materials such as LiCoO ₂ and LiNiO ₂ , a rapid decrease in discharge capacity at high rates of charge and discharge, and battery life due to elution of manganese during continuous charge and discharge at high temperatures. There is a problem of this deterioration rapidly. LiCoO ₂ has good electrical conductivity, high battery voltage, and excellent electrode characteristics, and is a representative anode electrode material commercialized and sold by SONY, and has a disadvantage of being expensive. LiNiO ₂ is relatively inexpensive among the above-mentioned cathode electrode materials and exhibits the highest discharge capacity. However, LiNiO ₂ is difficult to synthesize and has a high discharge capacity.

In addition, since a battery is characterized by a complex reaction such as an anode / electrolyte and a cathode / electrolyte, the use of an appropriate electrolyte is also one of the important variables for improving the performance of the battery. Conventional electrolyte systems have expected and only acted as the mediators to move lithium ions. Recently, these electrolytes can be decomposed during charging and discharging, and these decomposition reactions have been found to be fatal to battery performance. Accordingly, carbonate-based solvents are known to be the most suitable solvents for high-potential lithium secondary batteries, and the applicability of other solvents is slim.

As the carbonate solvent, a mixed solvent of cyclic carbonate having high dielectric constant and chain carbonate having low dielectric constant but low viscosity is used. Ethylene carbonate is mainly used as the cyclic carbonate, and dimethyl carbonate, diethyl carbonate or methylethyl carbonate is mainly used as the chain carbonate. When a mixed solvent of ethylene carbonate and dimethyl carbonate is applied to a secondary battery using crystalline graphite as a negative electrode, excellent life characteristics and discharge characteristics at different rates are obtained. However, when an electrolyte solution in which lithium salt is added to a mixed solvent of ethylene carbonate and dimethyl carbonate is applied to a secondary battery, the freezing point of ethylene carbonate is high, and thus the low temperature performance of the battery is sharply lowered. In practice, the discharge capacity at low temperature does not reach the level of about 20 to 30 with respect to the nominal capacity. In addition, diethyl carbonate and methylethyl carbonate in the chain carbonate has a relatively higher viscosity than dimethyl carbonate, which weakens the conductivity of the electrolyte and degrades the charge / discharge characteristics of the battery, particularly the life characteristics.

An object of the present invention is to provide a non-aqueous electrolyte having excellent conductivity characteristics even at low temperatures by adding halogenated benzene to a carbonate solvent.

Another object of the present invention is to provide a lithium secondary battery having excellent charge / discharge characteristics and lifetime characteristics, as well as excellent low temperature discharge characteristics and high temperature storage characteristics.

1 is a graph showing the life characteristics of the cycle of Examples 1 to 5 and Comparative Examples 1 and 2.

In order to achieve the object of the present invention, as an electrolyte solution containing an organic solvent and a lithium salt, the organic solvent is a lithium secondary non-aqueous organic solvent in which a halogenated benzene is added to a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate It provides a battery electrolyte.

Hereinafter, the present invention will be described in more detail.

The non-aqueous electrolyte solution of the present invention is composed of an organic solvent and a lithium salt, and the organic solvent is a halogenated benzene in order to improve low temperature characteristics, life characteristics and high temperature storage characteristics of a battery in a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate. Add. The mixing ratio of the ethylene carbonate: dimethyl carbonate: diethyl carbonate may be mixed in a ratio of 30 to 50:30 to 50:10 to 30. Out of this mixing ratio does not sufficiently improve the performance of the battery.

The halogenated benzene added to the mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate is represented by the following general formula (1):

Formula 1

Wherein X is F, Cl, Br, or I and n is an integer from 1 to 3. The halogenated benzene has a high freezing point, is electrochemically stable in the operating voltage range of the battery, and exhibits high conductivity at low temperatures. Halogenated benzene can be added in an amount of 5 to 40 volumes with respect to the total solvent, preferably in an amount of 10 to 30 volumes. If the amount of halogenated benzene is less than 5 volumes, the low temperature conductivity is lowered, and if it exceeds 40 volumes, the room temperature conductivity is lowered.

Lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroacetate (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) or mixtures of two or more thereof can be used. These salts are added at a concentration of 0.7-2.0 M. If the salt concentration is less than 0.7M, the conductivity of the electrolyte is lowered, so that the performance of the electrolyte is lowered, and if it exceeds 2.0M, there is a problem that the low temperature performance is lowered as the viscosity increases at a lower temperature.

The nonaqueous electrolyte solution of the present invention is applied to a lithium secondary battery using crystalline graphite as a negative electrode and lithium oxide as a positive electrode. The crystalline graphite is made of a graphite carbonaceous material capable of intercalating / deintercalating lithium ions. The crystalline graphite preferably has an d ₀₀₂ interplanar distance of less than 3.4 kPa. Graphite or natural graphite prepared by heat-treating mesophase pitch cokes at a high temperature of 2500 to 3000 ° C may be preferably used.

The lithium oxide constituting the anode may also insert / deinsert lithium ions, and specific examples thereof may include LiCoO.₂, LiNi_1-xyCo_xM_yO₂(0 <x <1, 0 <y <1, 0 <x + y <1, M is metal such as Al, Sr, Mg, La), LiMnO₂, LiMn₂O₄Etc.

The following presents a preferred embodiment to aid the understanding of the present invention. However, the following examples are merely provided to more easily understand the present invention, and the present invention is not limited to the following examples.

Examples and Comparative Examples

Examples 1-5

In Examples 1 to 5, fluorobenzene was added to an organic solvent in which ethylene carbonate / dimethyl carbonate / diethyl carbonate (EC / DMC / DEC) was mixed in 1/1/1, and 1.15 M of LiPF ₆ was added to the non-aqueous solution. An electrolyte solution was prepared and used. The type and amount of fluorobenzene used in Examples 1 to 5 are listed in Table 1. An 18650 cylindrical battery was fabricated using LiCoO ₂ , a cathode active material, a cathode composed of crystalline graphite, an anode active material, polyvinylidene fluoride (PVDF) as a binder, and acetylene black as a conductive agent.

Comparative Examples 1 and 2

Comparative Example 1 used a mixed solvent of ethylene carbonate / dimethyl carbonate (EC / DMC) and Comparative Example 2 used a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate (EC / DMC / DEC). Comparative Examples 1 and 2 were prepared in the same manner as in Examples 1 to 5 except that fluorobenzene was not added.

Organic solvent Halogenated benzene Usage of Halogenated Benzene (Volume) Example One EC / DEC / DMC Fluorobenzene 10 2 EC / DEC / DMC 1,2-difluorobenzene 10 3 EC / DEC / DMC 1,3-difluorobenzene 10 4 EC / DEC / DMC 1,4-difluorobenzene 10 5 EC / DEC / DMC 1,2,4-trifluorobenzene 10 Comparative example One EC / DMC - - 2 EC / DMC / DEC - -

For the lithium secondary batteries of Examples 1 to 5 and Comparative Examples 1 to 2 prepared above, the life characteristics of the cycles of the batteries were measured and shown in FIG. 1. The discharge capacity of the battery was measured by charging at a constant current of 1C and a constant voltage of 4.1V, then discharging at cut-off voltages of 1C and 2.75V. It can be seen from FIG. 1 that the batteries according to Examples 1 to 5 containing fluorobenzene have superior life characteristics according to cycles as compared to the batteries of Comparative Examples 1 to 2 composed of only a carbonate solvent.

Charged at a constant current of 0.2C and a constant voltage of 4.1V, and then allowed to stand at -20 ℃ for 16 hours to discharge at a cut-off voltage of 0.2C and 2.75V to measure the low-temperature discharge capacity. The ratio of the discharge capacity at -20 ° C to the nominal capacity is shown in Table 2 below. After charging at a constant current of 0.5C and a constant voltage of 4.1V, it was left for 30 days at 60 ℃ and discharged under the cut-off voltage conditions of 0.2C and 2.75V at room temperature to determine the high temperature storage characteristics. The ratio of the discharge capacity at room temperature to the nominal capacity is shown in Table 2 below.

Example Comparative example One 2 3 4 5 One 2 Low Temperature Discharge Capacity () ^* 84.5 84.2 83.3 84.1 81.3 23.7 83.1 High Temperature Discharge Capacity () ^** 89.20 88.98 89.01 89.13 88.23 87.50 85.46

Note) Ratio of initial discharge capacity to nominal capacity

** Ratio of discharge capacity at -20 ℃ to nominal capacity

As shown in Table 2, it can be seen that Examples 1 to 5 to which fluorobenzene was added are excellent in both low-temperature discharge characteristics and high-temperature storage characteristics as compared to Comparative Examples 1 and 2.

The lithium secondary battery electrolyte prepared according to the present invention has excellent ion conductivity at low temperatures by adding halogenated benzene to a carbonate solvent. When the nonaqueous electrolyte solution of the present invention is applied to a lithium secondary battery using crystalline graphite as a negative electrode and lithium oxide as a positive electrode, not only the charge and discharge characteristics and life characteristics are excellent, but also the low temperature discharge characteristics and the high temperature storage characteristics are improved. do.

Simple modifications or changes of the present invention can be easily carried out by those skilled in the art, and all such modifications or changes can be seen to be included in the scope of the present invention.

Claims (6)

An electrolyte solution containing an organic solvent and a lithium salt, wherein the organic solvent is a non-aqueous organic solvent in which a halogenated benzene of Formula 1 is added to a mixed solvent of ethylene carbonate / dimethyl carbonate / diethyl carbonate: Formula 1 Wherein X is F, Cl, Br, or I and n is an integer from 1 to 3. The electrolyte solution for a lithium secondary battery according to claim 1, wherein the volume ratio of said ethylene carbonate: dimethyl carbonate: diethyl carbonate is 30-50: 30-50: 10-10. The electrolyte of claim 1, wherein the amount of the halogenated benzene added is 5 to 40 vol. The method of claim 1, wherein the lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium hexafluoroacetate (LiAsF ₆ ), lithium perchlorate (LiClO ₄ ), lithium tri An electrolytic solution for a lithium secondary battery selected from the group consisting of fluoromethanesulfonate (LiCF ₃ SO ₃ ) and a mixture of two or more thereof. The electrolyte solution for lithium secondary batteries according to claim 4, wherein the lithium salt is added at a concentration of 0.7 to 2.0 M. Electrolyte according to any one of claims 1 to 5; A cathode composed of crystalline graphite; And A positive electrode composed of lithium oxide; Lithium secondary battery comprising a.