KR20000041563A - Electrolyte of a lithium ion battery - Google Patents

Abstract

PURPOSE: An electrolyte of a lithium ion battery is provided to improve the low temperature discharging characteristics and the cycle life characteristics of a lithium ion battery by using an organic solvent and a lithium salt. CONSTITUTION: An electrolyte of a lithium ion battery uses a metal oxide as an anode active material, and uses a black lead as a cathode active material. The electrolyte of a lithium ion battery comprises an organic solvent and a lithium salt. The organic solvent comprises ethylene carbonate, dimethyl carbonate, diethyl carbonate, and propylene carbonate. According to the electrolyte of a lithium ion battery, the low temperature discharging characteristics and the cycle life characteristics of the lithium ion battery is improved.

Description

Electrolyte for Lithium Ion Battery

Industrial use field

The present invention relates to an electrolyte for lithium ion batteries, and more particularly to an electrolyte having excellent low temperature discharge characteristics and cycle life characteristics.

Prior art

The organic electrolyte used in the lithium ion battery is composed of a lithium salt and an organic solvent, the organic solvent is first, the reactivity with lithium is small, second, the internal resistance is small, the movement of lithium ions must be made smoothly, third, Fourth, it must be thermally stable over a wide range of temperatures. Fourth, it must be compatible with other cell components such as a negative electrode and a positive electrode, particularly a negative active material. Fifth, it must have a high dielectric constant to dissolve a large amount of lithium salt.

Such organic solvents include cyclic carbonates such as propylene carbonate (PC), ethylene carbonate (EC), dimethyl carbonate (DMC) and diethyl carbonate (DEC). Linear carbonates such as) are mainly used.

In the organic electrolyte, PC has a melting point (MP) of -49 ° C, excellent low temperature characteristics, good compatibility with amorphous carbon, and a large dielectric constant, which can dissolve a large amount of inorganic lithium salts. When used with a crystalline anode active material such as graphite and having a large viscosity, it is known to be inserted into the carbon layer of the anode during decomposition to decompose to form propylene gas and lithium carbonate, thereby reducing battery capacity and increasing irreversible capacity. . On the other hand, EC does not react with the graphite-based negative active material, so it can be easily applied to a battery using crystalline carbon as a negative electrode. Since EC has a large dielectric constant, it can dissolve a large amount of lithium salt. There is a disadvantage in that it can not secure the low temperature performance.

In addition, linear carbonates such as dimethyl carbonate (DMC) and diethyl carbonate (DEC) are small in viscosity and can be easily intercalated between the carbon layers to reduce the irreversible capacity of the battery and to react with lithium. Although small, in general, a low dielectric constant has a disadvantage in that a large amount of lithium salt cannot be dissolved. In particular, DMC is expected to be used in high current and high voltage batteries because of its high electrical conductivity, but its low melting point (M.P. = 4.6 ° C.) results in poor low temperature characteristics.

Therefore, in recent years, a method of mixing one or more solvents in order to compensate for the drawbacks of the respective electrolyte solvents has been studied.

In US Patent No. 5,422,203, a mixture of EC and DMC was used as an electrolyte for lithium batteries. Thus, using an EC: DMC system and using LiPF ₆ as a lithium salt, irreversible at the first reaction, which was previously a problem in secondary batteries, was used. It has been reported that the reversible cycle of up to 90% was achieved at 1C discharge by reducing the amount of lithium loss. This effect is caused by free insertion and desorption of linear carbonates or chain esters present in the electrolyte at least 50% by volume between the carbon layers. It is known to occur. However, in the case of EC: DMC system, since the melting point of EC is high, the low temperature discharge characteristic at -20 ° C is about 20% of the nominal capacity, which is very poor.

In order to solve the above problems, an object of the present invention is to provide a lithium ion battery electrolyte excellent in low-temperature discharge characteristics. It is another object of the present invention to provide an electrolyte solution for a lithium ion battery having excellent cycle life characteristics with relatively high electrical conductivity and solubility of lithium salts.

1 is a graph showing the cycle life characteristics of a battery employing an electrolyte according to an embodiment and a comparative example of the present invention.

In order to achieve the object of the present invention, the present invention is a lithium ion battery electrolyte using a metal oxide as a positive electrode active material, crystalline graphite as a negative electrode active material, ethylene carbonate (ethylene carbonate), dimethyl carbonate (dimethyl carbonate) It provides a non-aqueous organic solvent containing diethyl carbonate and propylene carbonate and a lithium ion battery electrolyte containing a lithium salt.

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

MEANS TO SOLVE THE PROBLEM In the lithium ion battery which uses a metal oxide as a positive electrode active material, and uses crystalline graphite as a negative electrode active material, by adding DEC to an EC: DMC system, the inventors can provide the electrolyte solution excellent in low-temperature characteristics, and here it determines The present invention has been found to solve the problem of deterioration of cycle life characteristics due to the addition of DEC by adding PC, which is known to be difficult to apply to the graphite.

The lithium ion battery suitable for applying the electrolyte according to the present invention uses a metal oxide such as LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ as a positive electrode active material, and has a good potential flatness as the negative electrode active material. Relatively crystalline graphite having good reversibility in the charge and discharge process is used.

The electrolyte according to the invention preferably comprises 5-50% by volume of ethylene carbonate, 3-40% by volume of dimethyl carbonate, 5-20% by volume of diethyl carbonate and 5-50% by volume of propylene carbonate. When diethyl carbonate is used in an amount of 20% by volume or more, a problem arises in that electrical conductivity is small and dissolution of lithium salt is difficult. If it is out of the composition range, it is difficult to exhibit desirable low-temperature discharge characteristics and cycle life characteristics.

The lithium salt may use LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF ₄ or a mixture thereof, and is preferably used at a concentration of 0.8-1.5M.

Those skilled in the art will be able to easily manufacture a lithium ion secondary battery according to a known battery manufacturing method using the negative electrode active material of the present invention.

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.

Example 1 and Comparative Example 1-2

An electrolyte solution containing a non-aqueous organic solvent and LiPF ₆ (1M) was prepared in the composition of Table 1.

LiCoO ₂ as a positive electrode active material, polyvinylidene fluoride as a binder, and acetylene black as a conductive agent were mixed with N-methyl pyrrolidone to prepare a positive electrode active material slurry, and then applied to an aluminum foil current collector to prepare a positive electrode plate. It was.

Crystalline graphite as a negative electrode active material and polyvinylidene fluoride as a binder were mixed with N-methyl pyrrolidone to prepare a negative electrode active material slurry, which was then applied to a copper foil current collector to prepare a negative electrode plate.

An 18650 type cylindrical battery was manufactured using the positive electrode plate, negative electrode plate, and electrolyte solution. The low temperature discharge efficiency of this battery was measured and shown in Table 1, and the cycle life characteristics were measured and shown in FIG. Low-temperature discharge efficiency was measured by 1C, 4.1V constant current constant voltage charging, 1C 2.75V cut-off discharge, the discharge capacity at -20 ℃ compared to the nominal capacity expressed as a percentage ratio. Cycle life characteristics were measured by 0.2C, 2.75V cut-off discharge after charging the battery to 0.2C, 4.1V constant current constant voltage and left at -20 ℃ 16 hours.

Electrolyte composition Low Temperature Discharge Efficiency (%) EC PC DMC DEC Example 1 36% by volume 16% by volume 36% by volume 12% by volume 82.9 Comparative Example 1 50% by volume 50% by volume 21.2 Comparative Example 2 42.5% by volume 42.5% by volume 15% by volume 72.1

As shown in Table 1, the low temperature characteristics are improved by adding DEC to the EC: DMC system, and it can be seen that the low temperature characteristics are further improved by adding PC thereto.

In addition, as shown in FIG. 1, the cycle life characteristics are lowered by adding DEC to the EC: DMC system, but it can be seen that the cycle life characteristics are significantly improved when the PC is added thereto.

The electrolyte according to the present invention can be suitably applied to lithium ion batteries using crystalline graphite as a negative electrode active material, and has excellent low-temperature discharge characteristics and cycle life characteristics, and includes relatively small amounts of DEC, thereby preventing electrical conductivity and lithium salts. It provides an electrolyte solution having a relatively high solubility.

Claims (3)

As an electrolyte for lithium ion batteries using a metal oxide as a positive electrode active material and crystalline graphite as a negative electrode active material, ethylene carbonate, dimethyl carbonate, diethyl carbonate, and propylene carbonate A non-aqueous organic solvent containing carbonate) and a lithium ion battery electrolyte containing a lithium salt. The method of claim 1, wherein the electrolyte comprises 5-50% ethylene carbonate, 3-40% dimethyl carbonate, 5-20% diethyl carbonate and 5-50% propylene carbonate. An electrolyte solution for lithium ion batteries. The electrolyte of claim 1, wherein the lithium salt is selected from the group consisting of LiPF ₆ , LiClO ₄ , LiAsF ₆ , LiBF _4, and mixtures thereof.