KR100277786B1 - Electrolyte for Lithium Secondary Battery - Google Patents

Abstract

The present invention relates to an electrolyte solution for a lithium secondary battery, and provides an electrolyte solution for a lithium secondary battery including a non-ionic surfactant, an organic solvent, and a lithium salt. In the battery manufactured using this electrolyte solution, a uniform and thick electrolyte protective layer is formed on the surface of the negative electrode active material, thereby suppressing side reactions such as decomposition of the electrolyte generated from the negative electrode, thereby increasing the reversible capacity of the battery and the battery life.

Description

Electrolyte for Lithium Secondary Battery

[Industrial use]

The present invention relates to an electrolyte solution for lithium ion secondary batteries, and more particularly, to an electrolyte solution for lithium ion secondary batteries capable of producing a battery having a high reversible capacity and a long lifespan.

[Prior art]

A battery is a device that converts chemical energy of positive and negative electrode active materials into electrical energy through electrochemical oxidation and reduction reactions. In particular, a material capable of intercalations and deintercalations of lithium ions. Is used as a cathode and an anode, and an organic electrolyte or polymer electrolyte capable of moving lithium ions is charged between the anode and the cathode to oxidize when lithium ions are intercalated / deintercalated at the anode and the cathode. A battery that generates electrical energy by a reduction reaction is called a lithium ion battery. As the organic electrolyte, lithium salt dissolved in 1,2-dimethoxyethane, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate and diethoxy ethane and a mixture thereof is mainly used. The thing which melt | dissolved lithium salt in the polymers, such as an oxide or polyacrylonitrile, is mainly used. As the positive electrode of the lithium ion battery, a transition metal compound capable of inserting and detaching lithium ions is mainly used. Representatively, lithium cobalt oxide (LiCoO ₂ ), lithium nickel oxide (LiNiO ₂ ), lithium manganese oxide (LiMnO ₂ ), and the like. In this practical application, as a negative electrode active material, lithium ions are reversibly received or supplied while maintaining structural and electrical properties, and carbon-based materials having a chemical potential similar to that of metallic lithium upon insertion and desorption of lithium ions are mainly used. In general, the carbon-based negative electrode active material is classified into crystalline carbon and amorphous carbon. Graphite is typically used as crystalline carbon, and hard carbon obtained by carbonizing soft carbon and polymer resin obtained by heat treatment of pitch at about 1000 ° C as amorphous carbon. ) Is used. Although crystalline carbon has good dislocation flatness and relatively good reversibility of charging / discharging process, but has a small charging capacity, amorphous carbon has a relatively large capacity but has a disadvantage of large irreversibility in charging / discharging process. That is, in the case of amorphous carbon, only 70-80% of lithium ions inserted into the carbon lattice during the initial charging process are used for the next discharge process (20-30% of irreversible capacity). Only 85-90% of the inserted lithium ions are used for the next discharge process (10-15% of irreversible capacity).

The irreversible capacity that occurs when using the carbon-based negative electrode active material and the organic electrolyte is generated based on the structural characteristics of the carbon used primarily, and the degree of reduction reaction of the electrolyte at the interface between lithium and carbon and the electrolyte protective layer formed on the carbon surface It is known to vary depending on the degree of formation. That is, when the electrolyte protective layer formed on the surface of the negative electrode active material is unevenly formed, electrons in the negative electrode active material flow out through the non-uniform protective layer, thereby reducing the electrolyte and causing the electrolyte decomposition reaction. Such decomposition of the electrolyte interferes with the process of intercalation / deintercalation of lithium into the carbon lattice, thereby increasing the irreversible capacity of the negative electrode active material. FIG. 1 schematically shows a porous electrolyte film 2 formed unevenly on the negative electrode active material 1 as described above. As such, an increase in irreversible capacity not only causes a decrease in battery capacity and a decrease in lifespan, but also a disadvantage in that a battery having a minimum weight and maximum capacity cannot be manufactured.

The present invention is to solve the above problems, an object of the present invention is to provide an electrolyte solution for a lithium secondary battery that can increase the reversible capacity and increase the life of the battery by reducing the decomposition reaction of the electrolyte at the negative electrode. Another object of the present invention is to provide an electrolyte solution capable of forming a uniform and thick electrolyte protective layer on the surface of the negative electrode active material so that the negative electrode active material reversibly accepts or releases lithium ions. In addition, an object of the present invention is to provide an electrolyte solution capable of suppressing side reactions occurring at the negative electrode when the battery is initially charged at a high voltage, and forming an electrolyte coating film having a stable and high ionic conductivity in the negative electrode active material.

1 is a cross-sectional view schematically showing a negative electrode of a lithium ion battery using a conventional electrolyte solution.

2 is a cross-sectional view schematically showing a negative electrode of a lithium ion battery using the electrolyte solution of the present invention.

Explanation of symbols on the main parts of the drawings

1: cathode 2: electrolyte film

In order to achieve the above object, the present invention is a non-ionic surfactant; Organic solvents; And it provides a lithium secondary battery electrolyte containing a lithium salt.

Hereinafter, the present invention will be described in more detail. The non-ionic surfactant can be used as long as it can be included in the organic electrolyte solution to form a uniform electrolyte coating on the negative electrode active material, preferably polyethylene glycol dimethylether (Chemical Formula: CH ₃ O- (CH ₂ CH ₂ O) _n -CH ₃ ) or SiPPO (Silicon Polypara Phenylene Oxide) is preferable. The amount of the surfactant added is preferably 0.01 to 5% by weight, more preferably 0.05 to 3% by weight based on the organic electrolyte.

The organic solvent may also use all of the usual organic solvents to form an organic electrolyte, and representative organic solvents that can be used are 1,2-dimethoxyethane, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, diethoxy Ethane and mixtures thereof, and most preferably, a 2: 1 or 1: 1 mixture of ethylene carbonate and dimethyl carbonate, which is most stable for electrolyte decomposition reaction. The lithium salt may be used as long as it can promote the movement of lithium ions between the positive electrode and the negative electrode, and typically, LiPF ₆ , LiASF ₆ , LiCF ₃ SO ₃ , LiN (CF ₃ SO ₂ ) ₃ , LiBF ₆ and LiClO _{4 It} is selected from the group consisting of, usually used to mix to achieve a concentration of about 1M in the electrolyte. The electrolyte solution for a lithium secondary battery of the present invention can be prepared by the following method. The lithium salt is added to the organic solvent to a concentration of about 1 M. The organic electrolyte is prepared by adding 0.01 to 5% by weight, more preferably 0.05 to 3% by weight, of the nonionic surfactant with respect to the total organic electrolyte. At this time, when the content of the surfactant is less than 0.01% by weight, the film formation on the negative electrode active material is incomplete, and when the content of the surfactant is more than 5% by weight, the film is excessively formed, thereby deteriorating ion conductivity.

It is preferable that the electrolyte solution of this invention is used for the battery which uses amorphous carbon as a negative electrode. Examples of amorphous carbons in which the electrolyte of the present invention can be usefully used include soft carbon obtained by heat treatment of a precursor such as pitch at about 1000 ° C. and hard carbon obtained by carbonizing a polymer resin. It can be used either. The hard carbon precursor may include at least one selected from the group consisting of polyimide resin, furan resin, phenol resin, polyvinyl alcohol resin, cellulose resin, epoxy resin and polystyrene resin, and the soft carbon precursor may be petroleum-based. Pitch, coal-based pitch and oil-based raw material is preferably selected from the group consisting of. As the oil-based raw material, low molecular weight heavy oil is preferable.

When the electrolyte solution of the present invention is used together with such an amorphous carbon anode, a uniform and thick electrolyte film is formed during the initial charging of the battery, that is, when lithium ions are first intercalated into the lattice structure of the anode active material. As formed on the contact surface of, the electrolyte decomposition reaction at the carbon surface is suppressed and the reversibility of the negative electrode is increased. Thus, the electrolyte film formed by impregnating the negative electrode active material in the electrolyte solution of this invention is shown in FIG. As shown in Fig. 2, the electrolyte coating film 2 formed by the electrolyte solution of the present invention is formed evenly and densely on the negative electrode active material 1, so that side reactions such as decomposition of the electrolyte solution hardly occur. The electrolyte coating film 2 serves as a protective layer of the negative electrode active material, and is formed uniformly and thick enough to prevent the flow of electrons (e ⁻ ), thereby reducing the reduction of the electrolyte by the electrons flowing out of the negative electrode active material, Since the electrolyte coating 2 has a high ion conductivity, the negative electrode active material coated with the electrolyte coating 2 may reversibly accept or release lithium ions.

EXAMPLE

Hereinafter, preferred examples and comparative examples of the present invention are described. However, the following examples are not intended to limit the invention as a preferred embodiment of the present invention.

(Example 1)

Ethylene carbonate and dimethylene carbonate were mixed in a volume ratio of 2: 1, and 1M LiPF ₆ was added. 0.1% polyethylene glycol dimethyl ether was added to the mixture to prepare a lithium secondary battery electrolyte. Next, the phenol-based resin precursor was calcined at 1000 ° C. at low temperature to prepare a carbon electrode, which was used as a cathode, and a cathode plate was manufactured using lithium metal as a counter electrode. A coin-type battery was manufactured using the negative electrode, the lithium metal counter electrode, and the electrolyte solution, and the charge and discharge efficiency and the discharge capacity after 50 cycles were measured.

(Comparative Example 1)

Coin-type batteries were manufactured in the same manner as in Example 1, except that polyethylene glycol dimethyl ether was not used in the preparation of the electrolyte solution, and the charge and discharge efficiency and the discharge capacity after 50 cycles were measured. It was.

Initial charge capacity (mAh / g) Initial discharge capacity (mAh / g) efficiency(%) Discharge capacity after 50 cycles (mAh / g) Comparative Example 1 451 334 74 298 1 to implementation 435 361 83 337

As shown in Table 1, when the half cell manufactured by Example 1 was tested in comparison with Example 1, the battery of Example 1 had an initial discharge capacity of about 7 to 10% higher than that of Comparative Example 1, resulting in a negative electrode. It was found that the reversibility of the active material was increased, and the capacity after 50 cycles was also increased by about 10 to 15%.

As described above, a battery manufactured using an electrolyte solution containing a non-ionic surfactant has a uniform and dense electrolyte protective layer formed on the surface of the negative electrode active material as compared with a conventional battery, and thus may cause side reactions such as electrolyte decomposition. It can suppress and increase the reversible capacity and the lifetime of a battery.

Claims (5)

Non-ionic surfactants; Organic solvents; And The electrolyte solution for lithium secondary batteries containing a lithium salt. The electrolyte of claim 1, wherein the non-ionic surfactant is polyethylene glycol dimethyl ether or SiPPO. The electrolyte of claim 1, wherein the non-ionic surfactant is included in an amount of 0.01 to 5% by weight based on the electrolyte. The electrolyte of claim 1, wherein the organic solvent is selected from the group consisting of 1,2-dimethoxyethane, propylene carbonate, ethylene carbonate, dimethyl carbonate, diethyl carbonate, diethoxy ethane, and mixtures thereof. . The electrolyte solution for lithium ion secondary batteries according to claim 1, wherein the electrolyte solution is used for a battery using amorphous carbon as a negative electrode.