KR20020086069A - A non-aqueous electrolyte and a lithium secondary battery comprising the same - Google Patents

Abstract

PURPOSE: Provided are a non-aqueous electrolyte capable of improving an electrochemical property, and a lithium secondary cell excellent in electrochemical property. CONSTITUTION: The electrolyte comprises a lithium salt; a non-aqueous, organic solvent; and an organic compound of formula 1 or 2, or a mixture thereof. In the formulae, each of R1 to R3 represents primary, secondary, or tertiary alkyl group, alkenyl group or aryl group, X is hydrogen or halogen atom, and each of n and m is 0-3. The lithium salt is selected from the group consisting of lithium hexafluorophosphate(LiPF6), lithium tetrafluoroborate(LiBF4), lithium perchlorate(LiClO4), lithium trifluoromethanesulfonate(LiCF3SO3), lithium hexafluoroarsenate(LiAsF6), and a mixture thereof.

Description

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

Field of invention

The present invention relates to a nonaqueous electrolyte and a lithium secondary battery comprising the same, and more particularly, to a nonaqueous electrolyte and a lithium secondary battery comprising the same to improve the low temperature characteristics of the battery.

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 such as notebook computers, camgoders, and mobile phones 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 oxides capable of inserting and detaching lithium ions are used as positive electrode active materials of lithium secondary batteries, and lithium metals, lithium alloys, (crystalline or amorphous) carbon or carbon composite materials are used as negative electrode active materials. The active material is applied to a current collector with a suitable thickness and length, or the active material itself is applied in a film shape to form an electrode group by winding or laminating together with a separator, which is an insulator, and then placing the active material in a can or a similar container, and then lithium in an organic solvent. It is prepared by injecting an electrolyte solution in which a salt is dissolved.

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, to produce such a high driving voltage, an electrochemically stable electrolyte is required in the charge and discharge voltage range of 0 to 4.2 V. For this reason, a non-aqueous electrolyte solution in which lithium salts such as LiPF ₆ , LiBF ₄ , LiClO _4, and the like are added to non-aqueous carbonate solvents such as ethylene carbonate, dimethyl carbonate and diethyl carbonate as a source of lithium ions. However, such a non-aqueous electrolyte has a problem of significantly lower ion conductivity compared to an aqueous electrolyte used in a Ni-MH battery or a Ni-Cd battery, thereby degrading battery characteristics at high rates of charge and discharge.

Since the ion conductivity of the electrolyte greatly affects the electrochemical characteristics of the battery such as charge and discharge performance and rapid discharge performance, it is preferable that the ion conductivity is high. In order to do this, the number of free ions must be high, so the dielectric constant must be high and the viscosity of the electrolyte must be low in order to facilitate the movement of free ions. In addition, when the electrolyte is solidified at low temperature, the movement of ions is restricted, and thus the charging and discharging of the battery becomes impossible. Therefore, it should have a low freezing point (Makoto Ue, Solution Chemistry of Organic Electrolytes, Progress in Battery Materials , Vol.

As a method for increasing the ionic conductivity of the electrolyte, US Patent Nos. 5,639,575 and 5,525,443 include a method of improving the electrochemical properties of lithium ion batteries by mixing a high dielectric constant solvent and a low viscosity solvent, particularly a low freezing solvent. A method of improving the performance at low temperatures of a lithium ion battery is described. However, by changing the solvent composition alone, the mobility of lithium ions at low temperatures, especially at -20 ° C, is very low, so when discharging at a high rate (IC), there is a problem that the discharge characteristics rapidly decrease due to a rapid increase in internal resistance. have. Therefore, in order to improve high-rate discharge characteristics at low temperatures, the electrolyte must have high ion conductivity and at the same time reduce resistance inside the battery when lithium ions move during high-rate discharges.

During the initial charging of the lithium secondary battery, lithium ions from the lithium metal oxide as the positive electrode move to the carbon electrode as the negative electrode and are intercalated with carbon. At this time, lithium is highly reactive and reacts with the carbon electrode to generate Li ₂ CO ₃ , LiO, LiOH and the like to form a film on the surface of the negative electrode. Such a coating is called a solid electrolyte interface (SEI) film. The SEI film formed at the beginning of charging prevents the reaction of lithium ions with a carbon anode or other material during charging and discharging. It also acts as an ion tunnel, allowing only lithium ions to pass through. The ion tunnel serves to prevent the large molecular weight organic solvents that solvate lithium ions and move together to be co-intercalated with the carbon anode to disrupt the structure of the carbon anode. Once the SEI film is formed, the lithium ions again do not react sideways with the carbon anode or other materials so that the amount of lithium ions is reversibly maintained. That is, the carbon of the cathode reacts with the electrolyte at the initial stage of charging to form a passivation layer such as an SEI film on the surface of the cathode to maintain stable charge and discharge without further decomposition of the electrolyte ( J. Power Sources , 51 (1994), 79-104. For this reason, the lithium secondary battery can maintain a stable cycle life after showing no reaction of forming an irreversible passivation layer after the initial charging reaction.

The characteristics of the SEI film formed on the surface of the anode by reacting with the carbon of the cathode are known as the main factors that affect the ion and charge transfer due to the change in properties depending on the type of solvent or additive used as the electrolyte. Shoichiro Mori, Chemical Properties of Various Organic Electrolytes for Lithium Rechargeable Batteries, J. Power Sources , 68 (1997).

In addition, efforts have been made to change the SEI film formation reaction by adding an additive to the electrolyte in order to produce a better SEI film produced by reacting with the carbon constituting the negative electrode of the lithium ion battery. As such an example, Japanese Patent Laid-Open No. 7-176323 discloses a method of adding CO ₂ to an electrolyte, and Japanese Patent Laid-Open No. 7-320779 describes a method of suppressing decomposition of an electrolyte by adding a sulfide compound to the electrolyte. It is.

An object of the present invention is to provide a non-aqueous electrolyte that can improve the electrochemical properties of the battery.

Another object of the present invention is to provide a lithium secondary battery having excellent electrochemical properties.

1 is a graph showing the discharge capacity of a lithium secondary battery according to an embodiment of the present invention and a comparative example.

In order to achieve the above object of the present invention, the present invention is a lithium salt; Non-aqueous organic solvents; And it provides a non-aqueous electrolyte comprising an organic compound of formula 1 or 2, or a mixture thereof:

[Formula 1]

[Formula 2]

In the above formula, R ¹ to R ³ are each independently a primary, secondary or tertiary alkyl group, alkenyl group or aryl group, X is hydrogen or a halogen atom, n and m are each 0 to 3. Wherein R ¹ to R ³ is preferably an alkenyl group or an aryl group of C ₆ ~C ₁₄ alkyl group of _{C 1 ~C 11, C 2 ~C} 11. In a preferred embodiment of the present invention, R ¹ to R ³ is a vinyl, benzyl or phenyl group.

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

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

The electrolyte solution of the present invention is a lithium salt; Non-aqueous organic solvents; And organic compounds of Formula 1 or 2 or mixtures thereof.

The lithium salt used as the solute of the electrolyte is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ), And lithium hexafluoroacenate (LiAsF ₆ ), or a mixture of two or more thereof may be used. At this time, the concentration of the salt is preferably used in the range of 0.7 to 2.0M. If the salt concentration is less than 0.7M, the conductivity of the electrolyte is lowered, the performance of the electrolyte is lowered. If the salt is more than 2.0M, the viscosity of the electrolyte is increased, thereby reducing the mobility of lithium ions. These lithium salts act as a source of lithium ions in the battery to enable operation of the basic lithium secondary battery.

As the non-aqueous carbonate organic solvent, an organic solvent such as cyclic or chain carbonate may be used, or two or more may be mixed. In the present invention, the cyclic carbonate and the chain carbonate are preferably used by mixing in a volume ratio of 1: 1 to 1: 9. Specific examples of the cyclic carbonates include ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), and γ-butyrolactone (γ-BL). Specific examples of the linear carbonate include dimethyl carbonate (DMC). ), Diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC) and the like.

The nonaqueous electrolyte solution of the present invention is prepared by adding the organic compound represented by Chemical Formula 1 or 2 to the non-aqueous carbonate organic solvent containing the lithium salt.

Specific examples of the organic compound added to the electrolyte in the present invention include methylmethane sulfonate, methyl p-toluenesulfonate, benzenesulfonyl fluoride, p-toluenesulfonyl fluoride and the like.

The organic compound is added in an amount of 0.1 to 10% by weight, preferably 0.1 to 5% by weight, based on the electrolyte. If the addition amount is less than 0.1% by weight does not appear the addition effect, when it exceeds 10% by weight there is a problem that the life performance is lowered.

The organic compound is decomposed before the carbonate-based organic solvent at high rate discharge at a low temperature of the battery to form an SEI film on the surface of the negative electrode, thereby inhibiting decomposition of the carbonate-based organic solvent. In addition, the SEI film formed on the surface of the negative electrode enables lithium ions to be easily inserted at the surface of the electrode, thereby reducing the internal resistance of the battery and thus improving the electrochemical characteristics of the battery.

The electrolyte solution of the lithium secondary battery of the present invention is usually stable in the temperature range of -20 to 60 ℃ to maintain stable characteristics even at a voltage of 4V. The electrolyte solution of the present invention can be applied to all lithium secondary batteries such as lithium ion batteries and lithium polymer batteries.

In the present invention, a cathode material of a lithium secondary battery is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , or LiNi _1-xy Co _x M _y O ₂ (0 ≦ x ≦ 1, 0 ≦ _y ≦ 1, 0 ≤ x + y ≤ 1, M is a lithium metal oxide such as Al, Sr, Mg, La (metals such as La, etc.), and as the negative electrode material crystalline or amorphous carbon, carbon composite, lithium metal, or lithium alloy do. The active material is applied to a current collector of a thin plate with a suitable thickness and length, or the active material itself is applied in a film form, wound or laminated with a separator as an insulator to make an electrode group, and then placed in a can or a similar container, and then the chemical formula A lithium secondary battery is manufactured by injecting a non-aqueous electrolyte solution containing 1 or 2 organic compounds. As the separator, a resin such as polyethylene or polypropylene may be used.

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 to 8)

1 M of LiPF ₆ was added to a non-aqueous organic solvent in which ethylene carbonate / dimethyl carbonate (EC / DMC) was mixed in 1/1, and an organic compound was added as described in Table 1 below to prepare an electrolyte solution.

TABLE 1

Organic compounds Addition amount (% by weight) Example 1 Methylmethanesulfonate 2 Example 2 Methylmethanesulfonate 5 Example 3 Methyl p-toluenesulfonate 2 Example 4 Methyl p-toluenesulfonate 5 Example 5 Benzenesulfonyl fluoride 2 Example 6 Benzenesulfonyl fluoride 5 Example 7 p-toluenesulfonyl fluoride 2 Example 8 p-toluenesulfonyl fluoride 5

(Comparative Example 1)

An electrolytic solution was prepared in the same manner as in Example, except that no organic compound was added.

Decomposition Voltage Measurement

The decomposition voltages of the electrolyte solutions of Examples 1, 3, 5, 7 and Comparative Example 1 were measured by cyclic voltametry, and are shown in Table 2 below.

TABLE 2

Resolution voltage (V) Example 1 0.54 Example 3 1.05 Example 5 0.90 Example 7 1.33 Comparative Example 1 0.5

The cycle voltametry measurement conditions are as follows:

Working electrode: MCF, reference electrode: Li-metal, counter electrode: Li-metal

Voltage range: 3 V to 0 V, scan rate: 0.1 mV / s

The electrolyte solution of the example in which the organic compound is added has a higher decomposition voltage than the electrolyte solution of Comparative Example 1 in which the compound is not added, and thus decomposes earlier than the carbonate-based organic solvent during initial charging, and the SEI film formation reaction occurs at the decomposition voltage.

Fabrication of Lithium Secondary Battery

LiCoO ₂ 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. It was. The slurry was coated on an aluminum foil having a thickness of 20 μm, dried, and rolled to prepare a positive electrode. Crystalline artificial graphite as a negative electrode active material and PVDF as a binder were mixed in a weight ratio of 92: 8, and then dispersed in N-methyl-2-pyrrolidone to prepare a negative electrode slurry. The slurry was coated on a copper foil having a thickness of 15 μm, dried, and rolled to prepare a negative electrode. The prepared electrodes were wound and compressed using a polyethylene separator having a thickness of 25 μm, placed in a rectangular can of 30 mm × 48 mm × 6 mm, and then the electrolyte solutions of Examples 1 to 8 and Comparative Example 1 were injected. The battery was prepared.

Measurement of discharge capacity of lithium secondary battery

The produced batteries were charged under a constant current-constant voltage (CC-CV) condition to a charging voltage of 4.2V at a current of 160mA, and then discharged to 2.5V with a current of 160mA and left for 1 hour after being left for 1 hour. After repeating this process three times, it was charged with a current of 800 mA for 2.5 hours to a charging voltage of 4.2 V. After charging, the battery was placed in a cooling furnace at −20 ° C. and left for 16 hours, and then the discharge capacity when the battery was discharged to 2.75 V at a current of 160 mA was measured and described in Table 3 below.

TABLE 3

Discharge capacity (%) Example 1 75.4 Example 2 71.9 Example 3 73.3 Example 4 69.8 Example 5 71.0 Example 6 75.3 Example 7 76.8 Example 8 73.5 Comparative Example 1 69.5

The batteries were charged again under CC-CV conditions to 4.2V charging voltage at a current of 160mA at room temperature, and placed in a cooling furnace at -20 ° C for 5 hours, and then discharged to 2.75V at a current of 800mA. 4 is shown.

TABLE 4

Initial discharge voltage (V) Discharge capacity (%) Example 1 3.55 24.4 Example 2 3.35 11.9 Example 3 3.25 18.2 Example 4 3.20 7.9 Example 5 3.52 12.4 Example 6 3.55 17.5 Example 7 3.25 26.8 Example 8 3.31 18.9 Comparative Example 1 3.11 2

As can be seen from Table 4, the battery containing the electrolyte solution of Examples 1 to 8 of the present invention is significantly improved discharge characteristics at a low temperature of -20 ℃ compared to the battery containing the electrolyte solution of Comparative Example 1.

The cells of the embodiment including the electrolyte solution to which the organic compound was added showed an improvement of about 2 to 5% at low rate (0.2C) discharge at a low temperature of -20 ° C, but a significant improvement of about 20% or more at high rate (1C) discharge. Seemed.

The batteries were again charged to 4.2V at 800mA current at 800mA under CC-CV conditions at room temperature and discharged to 2.75V at a current of 800mA again. The results of repeating the charge / discharge 300 times with respect to the battery containing the electrolyte solutions of Examples 5 to 8 are shown in FIG. 1, respectively. As shown in FIG. 1, the battery using the electrolyte solution of the example including the organic compound of the present invention showed better life characteristics than the battery using the electrolyte solution of Comparative Example 1 in which the organic compound was not added.

The organic compound of the present invention can inhibit decomposition of the carbonate-based organic solvent by decomposing before the carbonate-based organic solvent at high rate discharge at low temperature to form an SEI film on the surface of the negative electrode. In addition, the SEI film formed on the surface of the negative electrode enables lithium ions to be easily inserted at the electrode surface, thereby reducing the internal resistance of the battery, thereby improving the electrochemical characteristics of the battery. The lithium secondary battery of the present invention is excellent in not only charging and discharging characteristics but also life characteristics, temperature characteristics, and particularly high rate discharge characteristics at low temperatures.

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 (7)

Lithium salts; Non-aqueous organic solvents; And A non-aqueous electrolyte comprising an organic compound of Formula 1 or 2, or a mixture thereof: [Formula 1] [Formula 2] Wherein R ¹ to R ³ are each independently a primary, secondary or tertiary alkyl group, alkenyl group or aryl group, X is hydrogen or a halogen atom, and n and m are each 0 to 3; The method of claim 1, wherein the lithium salt is lithium hexafluorophosphate (LiPF ₆ ), lithium tetrafluoroborate (LiBF ₄ ), lithium perchlorate (LiClO ₄ ), lithium trifluoromethanesulfonate (LiCF ₃ SO ₃ ) , And lithium hexafluoroacenate (LiAsF ₆ ), a non-aqueous electrolyte solution of one or two or more selected from the group consisting of. The nonaqueous electrolyte of claim 1, wherein the nonaqueous organic solvent is a cyclic carbonate and a chain carbonate in a volume ratio of 1: 1 to 1: 9. The cyclic carbonate is one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), γ-butyrolactone (γ-BL), and mixtures thereof. Carbonate, the chain carbonate is dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methylethyl carbonate (MEC) and A non-aqueous electrolyte solution, which is at least one carbonate selected from the group consisting of mixtures thereof. The non-aqueous electrolyte solution of claim 1, wherein the organic compound of Formula 1 or 2 is selected from the group consisting of methylmethane sulfonate, methyl p-toluenesulfonate, benzenesulfonyl fluoride, and p-toluenesulfonyl fluoride . The non-aqueous electrolyte solution according to claim 1, wherein the organic compound is added in an amount of 0.1 to 10% by weight. Electrolyte according to any one of claims 1 to 6; A positive electrode including lithium metal oxide as a positive electrode active material; And Anode containing carbon, carbon composite, lithium metal or lithium alloy as anode active material The lithium secondary battery which consists of.