KR20020006800A - Electrolyte for lithium secondary rechargeable battery - Google Patents

Abstract

PURPOSE: An electrolyte solution for a lithium secondary battery is provided, to inhibit the degradation of an electrolyte solution at an electrode in overcharging and overdischarging or at a high temperature, thereby increasing the charging/discharging efficiency and the lifetime of a battery and to inhibit the impedance inside of a battery, thereby improving the self-discharging property of a battery. CONSTITUTION: The electrolyte solution comprises an aprotic organic solvent; a lithium salt; and 0.01-10 wt% of a nonionic surfactant. The nonionic surfactant is a fluoro aliphatic ester-based polymer or a fluoro aliphatic ether-based polymer, and whose one terminal is a lipophilic group and another terminal is a fluorocarbonyl group. Preferably the aprotic organic solvent is at least two compounds selected from the group consisting of ethylene carbonate, propylene carbonate, γ-butyrolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate and tetrahydrofuran. The lithium salt is selected from the group consisting of LiClO4, LiBF4, LiCF3SO3, LiPF6 and their mixtures.

Description

Electrolyte for lithium secondary battery {Electrolyte for lithium secondary rechargeable battery}

The present invention relates to an electrolyte solution for a lithium secondary battery containing an aprotic organic solvent, a lithium salt and a surfactant.

A device that converts chemical energy of positive and negative electrode active materials into electrical energy through electrochemical oxidation / reduction reactions is called a battery. In particular, carbon materials and lithium composite oxides that can insert and detach lithium ions are used as active materials. Lithium secondary batteries using a nonaqueous electrolyte as a medium of a negative electrode, a positive electrode, and lithium ions have been developed and widely used. In the negative electrode active material, lithium ions are reversibly intercalated while maintaining structural and electrical properties, and carbon-based materials similar to metallic lithium during desorption have been mainly used. Although the negative electrode active material has good potential flatness and is relatively large, it is divided into crystalline carbon having excellent charge / discharge reversibility but small capacity, and amorphous carbon having relatively large capacity but large charge / discharge irreversibility. The crystalline carbon has a low irreversible capacity with 85 to 92% of charge and discharge efficiency, while the amorphous carbon has a very high irreversible capacity of 20 to 30%.

The irreversible capacity generated during the charge / discharge process of the battery is primarily due to the structural characteristics of carbon, and greatly depends on the degree of reduction reaction of the electrolyte at the interface between the carbon and the electrolyte and the electrolyte decomposition layer formed on the carbon surface. If the electrolyte decomposition layer is uneven or impurities are present in the electrolyte, the decomposition reaction of the electrolyte occurs due to the battery provided by carbon, which impedes the insertion and desorption of lithium ions into the carbon grid, thereby degrading charge and discharge characteristics, thereby reducing battery life and capacity. Let's do it.

Particularly, when gas is overcharged, excess gas and heat are generated due to the decomposition reaction of the solvent and the structural destruction of the electrode active material on the surface of the activated electrode plate. As the battery side reaction proceeds, the generated heat increases the temperature inside the battery exponentially, and serves to promote battery side reaction. This phenomenon occurs especially when the battery is stored at high temperature after being fully charged, and in order to improve the lifespan, self-discharge characteristics, and stability of the lithium secondary battery, it is inevitable to control the non-aqueous electrolyte and electrode reaction.

The primary method for suppressing this phenomenon in a battery is to suppress the decomposition of the solvent and suppress the ignition as much as possible by the addition of a certain amount of the flame retardant solvent. U.S. Patent No. 5916708 has tried to increase the stability of the battery by using ether containing fluorine, chlorine or perfluoroalkyl group as a solvent of the electrolyte, but this solvent has a disadvantage of deteriorating the cycle characteristics of the battery due to low ionic conductivity have. Japanese Patent No. 07-249432 also used partially fluorinated ether as a solvent. However, this solvent has a high vapor pressure, a low boiling point, and a low dielectric constant, which makes it difficult to dissociate lithium salts.

In the second method, as shown in U.S. Patent No. 5169736, in order to suppress the flammability of the electrolyte, a polymer such as polypropylene oxide may be added to solidify the electrolyte, or a filler such as SiO2, Al ₂ O₃ may be added to increase the viscosity of the electrolyte. There is a method of improving the stability of the battery by increasing, in this case, there is a disadvantage in that the battery characteristics are deteriorated due to the high viscosity of the electrolyte and the mobility of low lithium ions.

The present invention has been made to solve the above-mentioned conventional problems, by suppressing the decomposition reaction of the electrolyte at the negative electrode to increase the charge and discharge efficiency of the battery, improve the life as well as electrochemically stable electrolyte protection on the surface of the negative electrode It provides an electrolyte solution that can form a layer, and also applies a surfactant to the surface of the negative electrode to block the direct contact between the solvent and the active electrode plate when the battery is left unattended or overcharged at a high potential for a long time to suppress the decomposition of the solvent, deterioration of the electrode plate It is an object of the present invention to provide an electrolyte solution capable of suppressing the reaction.

In order to achieve the above object, the present invention provides an electrolyte solution in which a nonionic surfactant is added to an electrolyte solution consisting of an aprotic solvent and a lithium salt.

Hereinafter, the present invention will be described in detail.

The nonionic surfactant of the present invention should be included in the non-aqueous electrolyte solution so that it can be uniformly applied to the surface of the negative electrode or the positive electrode, and the structure destruction of the active material due to the penetration of the solvent in the electrolyte into the electrode plate and the suppression of the battery at high temperature and high potential It should be capable of reducing the self-discharge capacity.

While the ionic surfactant promotes the irreversible reaction of the battery, the discharge capacity is small, while the nonionic surfactant has excellent chemical stability, thereby suppressing side reactions of lithium ions, and excellent stability against the electrolyte, thereby increasing the discharge capacity. Can be.

Exemplary nonionic surfactants that can be used in the present invention are fluoroaliphatic ester-based polymers, fluoroaliphatic ester-based polymers, and the like, in which a few atoms of hydrophobic groups are replaced by some or all of fluorine elements. The structure of such a nonionic surfactant is a structure as shown in the following formula (I), wherein R _F represents an insoluble fluorocarbon chain, and X represents a soluble group. Depending on the length and structure of the tail of the fluorocarbons, it is possible to improve thermally, chemically and electrically stable properties, in particular this part has a very low surface tension. Therefore, fluoroaliphatic ester-based polymers and fluoroaliphatic ester-based polymer surfactants having this structure can lower the surface tension even at very small concentrations compared to other surfactants, and in particular, lower the surface tension of non-aqueous solutions and solid organic polymers. have. In addition, the surfactant of the present invention increases the electrolyte impregnation of the polymer and the electrode plate, especially in a battery in which lithium ions move through micropores, such as a lithium polymer electrolyte, to facilitate the penetration of the electrolyte into the electrode plate and the polymer to facilitate the penetration of lithium ions. Increase mobility.

[Formula I]

In the present invention, it is preferable to add 0.01 to 10% by weight of the nonaqueous electrolyte for the lithium ion battery composed of the aprotic organic solvent and the lithium salt, and the polymer, ceramic filler, lithium salt and nonaqueous solvent of the lithium polymer battery. It can be used by adding 0.01 to 10% by weight in the polymer electrolyte consisting of.

The aprotic organic solvent may be any organic solvent usable in a lithium secondary battery. For example, ethylene carbonate, propylene carbonate, Butyrolactone ( Aprotic solvents comprising at least one of -butylrolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, tetrahydrofuran and mixtures thereof are used. The lithium salt can be used by selecting at least one of lithium perchlorate (LiClO '), lithium tetrafluoroborate (LiBF'), lithium trifluoromethane sulfate (LiCF ₃ SO3), lithium hexafluorophosphate (LiPF ₆ ), and a mixture.

For use in lithium ion batteries, the electrolyte according to the present invention is prepared by dissolving a lithium salt in two or more of the above organic solvents in a concentration of about 1 M, and using a nonionic surfactant in an amount of 0.01 to 10% by weight relative to the electrolyte. The negative electrode can be used as the active material of MCMB, MPCF, graphite-based and non-graphite-based carbon, which is made of coke, and the positive electrode is selected from lithium cobalt oxide, lithium manganese oxide, lithium nickel oxide and lithium nickel cobalt oxide. Lithium complex oxide containing at least one can be used.

In order to apply the present invention to a lithium polymer battery, the negative electrode and the positive electrode are the same as the lithium ion battery, and the polymer electrolyte includes at least one of a polymer made of PVDF, PAN, PEO, PMMA or a copolymer thereof, and silica and alumina. Or ceramic fillers containing at least one of zirconia, ethylene carbonate, propylene carbonate, Butyrolactone ( aprotic solvents and lithium perchlorates including at least two of -butylrolactone, diethyl carbonate, dimethyl carbonate, ethylmethyl carbonate, tetrahydrofuran and mixtures thereof ), 0.01 to 10% by weight of the electrolyte solution in an electrolyte solution containing at least one of lithium terafluroborate (LiPF₄), lithium trifluoromethanesulfate (LiCF ₃ SO ₃ ), lithium hexaflourophosphate (LiPF ₆ ) and mixtures thereof % Of nonionic surfactants can be added to the polymer electrolyte for use. The amount of the nonionic surfactant is more preferably 0.01 to 5% by weight. If the amount of the nonionic surfactant is less than 0.01% by weight, the surfactant is incompletely formed on the surface of the electrode plate. When the amount of the nonionic surfactant is more than 10% by weight, the electrolyte plate is excessively formed on the electrode plate, thereby increasing the resistance and increasing the ion conductivity of the lithium ion. There is a disadvantage of deterioration.

Hereinafter, the present invention will be described in more detail with reference to Examples and Comparative Examples. However, the following Examples are preferred examples of the present invention and do not limit the present invention.

Example 1

In the case of applying a nonionic surfactant to a lithium polymer battery, the negative electrode was prepared by dissolving 75 parts by weight of MCMB, 8 parts by weight of P (VDF-HFP) copolymer, and 17 parts by weight of DBP in acetone to form a slurry, and coating directly onto Cu exmet. It was dried by hot air. The positive electrode was dissolved in 75, 6, 4, and 15 parts by weight of LiCoO₂, acetylene black, P (VDF-HFP) copolymer, and DBP and acetone, respectively, to make a slurry, and directly applied on Al exemt, followed by hot air drying.

In the polymer electrolyte, 50, 30 and 20 parts by weight of P (VDF-HFP) copolymer, silica and DBP were dissolved in acetone to make a slurry, followed by direct application on PET, followed by hot air drying to extract DBP, followed by 1M lithium hexafluoride and ethylene An electrolyte solution containing 2 parts by weight of a fluoroaliphatic ester-based nonionic surfactant was prepared by injecting an electrolyte consisting of carbonate / ethylmethyl carbonate, and a battery was prepared by combining anode / polymer electrolyte / cathode / polymer electrolyte / anode.

Example 2

The negative electrode, the positive electrode, and the polymer electrolyte were the same as in Example 1, and 1 part by weight of a fluoroaliphatic ether-based nonionic surfactant was added to the electrolyte to prepare a battery as in Example 1.

Comparative Example 1

As in Example 1, a battery was prepared without adding a nonionic surfactant to the electrolyte solution, and battery characteristics were investigated.

The battery produced by the above Examples and Comparative Examples was applied to a current of 1㎃ / ㎠ up to 4.2V after charging for 4 hours at 90 ℃ after the internal impedance change and the self-discharge rate of the battery was investigated Shown in As can be seen from the case where the fluoroaliphatic ester-based nonionic surfactant was added to the polymer electrolyte, the self-discharge characteristics were improved because the change in the internal impedance of the battery and the self-discharge rate were very small.

Surfactant content (% by weight) Impedance increase (%) Self discharge rate (%) Example 1 0.01 470 8.2 One 190 4.3 2 110 1.5 10 490 9 Example 2 0.01 490 8.6 One 200 5.0 2 125 1.9 10 510 9.6 Comparative example 0 750 25.3

The present invention increases the charge and discharge efficiency of the battery by suppressing the decomposition reaction of the electrolyte in the electrode plate during overcharge, high potential or high temperature storage by adding a fluoroaliphatic ester-based or fluoroaliphatic ether-based nonionic surfactant to the electrolyte. In addition, the self-discharge characteristics of the battery may be improved by increasing the internal impedance of the battery, and thus, the present invention may be applied not only to lithium ion polymer batteries but also to lithium ion batteries and lithium polymer batteries.

Claims (5)

An aprotic organic solvent, a lithium salt and a nonionic surfactant, characterized in that the electrolyte solution for a lithium secondary battery. The electrolyte of claim 1, wherein the nonionic surfactant is a fluoroaliphatic ester-based or fluoroaliphatic ether-based polymer, and has a structure as follows. (Wherein R _F is a fluorocarbon group and X is a lipophilic soluble group.) The electrolyte of claim 1, wherein the nonionic surfactant is used in an amount of 0.01 to 10% by weight relative to the electrolyte. The method of claim 1, wherein the aprotic organic solvent is ethylene carbonate, propylene carbonate, An electrolyte solution for a lithium secondary battery, characterized in that it contains at least two of butyrolactone, diethyl carbonate, dimethyl carbonate, ethyl methyl carbonate, tetrahydrofuran, and mixtures thereof. The lithium salt of claim 1, wherein the lithium salt is at least one selected from lithium perchlorate (LiClO₄), lithium tetrafluoroborate (LiBF₄), lithium trifluromethane sulfate (LiCF ₃ SO₃), lithium hexafluorophosphate (LiPF ₆ ), and mixtures thereof. Electrolyte for lithium secondary battery comprising a.