KR20010011907A - A non-aqueous electrolyte and a lithium secondary battery made thereof - Google Patents

Abstract

PURPOSE: A non-aqueous electrolyte solution and secondary lithium battery having the same are provided to have a good conductive feature at a lower temperature by adding a halogenation benzene to a carbonate system solution. CONSTITUTION: A non-aqueous electrolyte solution includes an organic solvent and a lithium salt. The organic solvent is obtained by adding a halogenation benzene to a mixing solvent of an ethylene carbonate/dimethyl carbonate/propylen carbonate. Thereby, the secondary lithium battery with the non-aqueous electrolyte solution has a good conductivity at a lower temperature.

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, and more particularly, to a non-aqueous solution in which a halogenated benzene is added to a mixed solvent of ethylene carbonate / dimethyl carbonate / propylene carbonate in order to improve low-temperature discharge characteristics of the battery. The present invention relates to an electrolyte solution containing lithium salt in an 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 necessity of 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 temperatures is less than 20-30% of 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.

Propylene carbonate is mixed to improve low temperature performance and lifetime characteristics, which are disadvantages of the mixed solvent of ethylene carbonate and chain carbonate. However, propylene carbonate tends to decompose during initial charge and discharge, resulting in a decrease in the initial capacity of the battery.

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 lifespan characteristics, as well as excellent low-temperature discharge characteristics.

1 is a graph showing the life characteristics of the cycle of Examples 1 to 4 and Comparative Examples 1 to 3.

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 non-aqueous organic solvent in which a halogenated benzene is added to a mixed solvent of ethylene carbonate / dimethyl carbonate / propylene carbonate for lithium secondary battery Provide an electrolyte solution.

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

The non-aqueous electrolyte solution of the present invention consists of an organic solvent and a lithium salt, and the organic solvent essentially includes propylene carbonate in a mixed solvent of ethylene carbonate and dimethyl carbonate to improve low temperature performance and life characteristics. Halogenated benzene is added to the mixed solvent of ethylene carbonate / dimethyl carbonate / propylene carbonate to improve the initial capacity reduction caused by the decomposition of propylene carbonate. The mixing ratio of the ethylene carbonate: dimethyl carbonate: propylene carbonate may be mixed in a ratio of 30 to 50: 30 to 50: 0 to 20, if out of this range can sufficiently improve the charge and discharge characteristics and life characteristics of the battery. none.

The halogenated benzene added to the mixed solvent of ethylene carbonate / dimethyl carbonate / propylene 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 5-40% by volume with respect to the total solvent, preferably in an amount of 10-20% by volume. When the amount of halogenated benzene is less than 5% by volume, low temperature conductivity is lowered, and when it exceeds 40% by volume, 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. Preferably, the crystalline graphite has a d ₂ 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.

Lithium oxide constituting the positive electrode may also be inserted / deinserted lithium ions, specific examples thereof include LiCoO ₂ , LiNi _1-xy Co _x M _y O ₂ (0 <x <1, 0 <y <1, 0 <x + y <1, M may include Al, Sr, Mg, a metal such as _{La), LiMnO 2, LiMn 2} O 4.

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 1-4

In Examples 1 to 4, fluorobenzene was added to a mixed solvent of ethylene carbonate / dimethyl carbonate / propylene carbonate, and 1.15 M of LiPF ₆ was added to prepare a non-aqueous electrolyte solution. The amount of the solvent used in Examples 1 to 4 is as described in Table 1 below. 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 to 3

Comparative Examples 1 to 3 were prepared in the same manner as in Example 1 except for changing the type and amount of the linear carbonate contained in the organic solvent and not adding fluorobenzene as shown in Table 1 below.

Ethylene carbonate Chain carbonate Propylene carbonate Fluorobenzene Dimethyl carbonate Diethyl carbonate Methyl ethyl carbonate Example One 40 40 - - 5 15 2 40 35 - - 10 15 3 35 40 - 5 20 4 35 35 - - 10 20 Comparative example One 45 45 - - 10 - 2 45 - 45 - 10 - 3 45 - - 45 10 -

Note) Unit: Volume%

For the lithium secondary batteries of Examples 1 to 4 and Comparative Examples 1 to 3, 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 4 containing fluorobenzene have excellent initial performance and excellent life characteristics according to cycles, compared to the batteries of Comparative Examples 1 to 3 composed of only carbonate solvents.

After 1 cycle, it was charged at a constant current of 0.5C and a constant voltage of 4.1V, and then discharged at a cut-off voltage of 0.2C and 2.75V to measure the initial capacity, and the ratio of the initial capacity to the nominal capacity is shown in Table 2 below. It was. After charging at a constant current of 0.5C and a constant voltage of 4.1V, it was left for 16 hours at -20 ℃ and discharged at 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.

Example Comparative example One 2 3 4 One 2 3 Initial discharge capacity (%) ^* 100.5 99.6 100.7 100.1 97.5 95.3 96.5 Low Temperature Discharge Capacity (%) ^** 84.3 80.9 85 82.2 65.1 47.3 67.4

Note) Ratio of initial discharge capacity to nominal capacity

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

As shown in Table 2, Examples 1 to 4 to which fluorobenzene was added are superior to Comparative Examples 1 to 3, and both of the initial capacity characteristics and the low temperature discharge characteristics were excellent.

In the lithium secondary battery electrolyte prepared according to the present invention, halogenated benzene is added to the carbonate-based solvent, and thus the ion conductivity is excellent at low temperatures. 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 the life characteristics are excellent, but also the low temperature discharge characteristics and the initial capacity 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 / propylene 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: propylene carbonate is 30-50: 30-50: 0-20. The electrolyte of claim 1, wherein the amount of the halogenated benzene added is 5 to 40% by volume. 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 electrolyte 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.