KR20150030853A - Electrolytes having a Excellent Charge-Discharge Characteristics at low-temperature and Lithium Secondary Battery using the Same - Google Patents

Abstract

The purpose of the present invention is to provide an electrolyte for a secondary battery with lifetime maintenance characteristics such as a charge-discharge capacity at very low temperature of -20 °C or lower and having a low melting point and low viscosity characteristics. The present invention relates to an electrolyte for a lithium secondary battery and to a lithium secondary battery using the same, the electrolyte for a lithium secondary battery comprising: a lithium salt; and a non-aqueous organic solvent including a cyclic carbonate compound and a nitrile-based compound, wherein the nitrile-based compound is included in an amount of 10 to 33 volume% with respect to the whole amount of the non-aqueous organic solvent.

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery having excellent cryogenic charging / discharging characteristics, and a lithium secondary battery using the electrolyte. 2. Description of the Related Art [0002]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an electrolyte for a lithium secondary battery, and more particularly, to an electrolyte for a lithium secondary battery excellent in cryogenic charging / discharging characteristics and a lithium secondary battery using the same.

As the electric, electronic, communication and computer industries rapidly develop, demand for high performance and high safety secondary batteries is rapidly increasing. In particular, secondary batteries, which are core components, are required to be light in weight and small in size in accordance with the trend of small size and portable use of electric and electronic products. In addition, the necessity of a new type of energy supply source due to environmental pollution problems such as air pollution and noise due to massive supply of automobiles and oil depletion, and the need for development of electric vehicles capable of solving the problems have been increased. It is required to develop a battery having high output and high energy density.

In accordance with this demand, lithium ion batteries are gaining popularity as energy storage devices widely used in hybrid vehicles, plug-in hybrid vehicles, and electric vehicles, and their application fields are gradually expanding in a large capacity direction.

However, a large-capacity lithium ion battery used in an electric automobile and the like has not only characteristics such as high voltage. It is required to have excellent lifetime characteristics even at an extreme temperature.

However, the electrolyte of the conventional lithium ion battery has a high ionic conductivity at room temperature as shown in FIG. 1, but the ionic conductivity drops sharply at a very low temperature of -20 ° C. or lower, I had a falling problem.

For example, Korean Patent Laid-Open Publication No. 2006-0119900 discloses the improvement of the swelling property while maintaining the low-temperature discharge capacity characteristic as compared with the existing carbonate-based electrolytic solution. However, this also shows the lifetime at a very low temperature of -20 캜 It is not able to improve the characteristics.

Korea Patent Publication No. 2006-0119900

DISCLOSURE Technical Problem Accordingly, it is an object of the present invention to provide an electrolyte solution for a lithium secondary battery having a low melting point and a low viscosity property while maintaining lifetime characteristics such as charge / discharge capacity even at a very low temperature of -20 캜 or lower .

The present invention relates to a lithium salt; And a non-aqueous organic solvent containing a cyclic carbonate compound and a nitrile-based compound, wherein the nitrile-based compound is contained in an amount of 10 to 33% by volume based on the total amount of the non-aqueous organic solvent.

The present invention also provides a lithium secondary battery comprising an electrode assembly comprising a cathode, a cathode, and a separator interposed between the anode and the cathode, and an electrolyte injected into the electrode assembly, wherein the electrolyte is an electrolyte solution for the lithium secondary battery A lithium secondary battery is provided.

The lithium secondary battery using the electrolyte for a lithium secondary battery of the present invention maintains lifetime characteristics such as charge / discharge capacity even at a very low temperature of -20 占 폚 or lower and has a low melting point and low viscosity, The effect can be exhibited.

1 is a graph showing temperature characteristics of an electrolyte according to the prior art.
2 is a graph showing ion conductivity differences according to an embodiment of the present invention.
3 is a graph illustrating normalized discharge capacity values according to an embodiment of the present invention.
4 is a graph illustrating a normalized discharge capacity value according to another embodiment of the present invention.
5 is a graph illustrating normalized discharge capacity values according to another embodiment of the present invention.
6 is a graph showing a difference in initial discharge capacity according to an embodiment of the present invention.
7 is a graph showing charge / discharge characteristics according to an embodiment of the present invention.
8 is a graph showing lifetime characteristics according to an embodiment of the present invention.

Hereinafter, the present invention will be described in detail.

The present invention relates to a lithium salt; And a non-aqueous organic solvent containing a cyclic carbonate compound and a nitrile-based compound, wherein the nitrile-based compound is contained in an amount of 10 to 33% by volume based on the total amount of the non-aqueous organic solvent, In particular, by controlling the content of the nitrile-based compound, the lifetime characteristics such as charge-discharge capacity and the like can be maintained even at a very low temperature of -20 ° C or lower, and low melting point and low viscosity characteristics can be obtained.

The electrolyte solution for a lithium secondary battery of the present invention includes a non-aqueous organic solvent.

The non-aqueous organic solvent includes a cyclic carbonate compound and a nitrile-based compound.

Examples of the cyclic carbonate compound include ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate (GBL), and more preferably a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC) can be used.

The cyclic carbonate compound may be contained in an amount of 67 to 90% by volume based on the total amount of the non-aqueous organic solvent.

The nitrile compound may be at least one selected from the group consisting of butyronitrile, propionitrile, and acetonitrile. The nitrile compound may be contained in an amount of 10 to 33% by volume based on the total amount of the non-aqueous organic solvent. When the nitrile-based compound is contained within the above range, lifetime characteristics such as the charge-discharge capacity of the secondary battery can be maintained at a very low temperature of -20 占 폚 or lower, and a low melting point and low viscosity characteristics can be maintained.

The electrolyte for a lithium secondary battery of the present invention may further comprise an acetate compound. The acetate compound is preferably contained in an amount of 10 to 30% by volume based on the total amount of the organic solvent. When the above-mentioned acetate compound is included in the above-mentioned range, there is an advantage that the room temperature lifetime characteristics are excellent.

The electrolyte for a lithium secondary battery of the present invention may further include fluoroethylene carbonate (FEC). The fluoroethylene carbonate (FEC) is preferably contained in an amount of 1 to 3% by weight based on the total weight of the electrolytic solution. When the fluorine ethylene carbonate (FEC) is included within the above range, the resistance is low due to the formation of the SEI layer having excellent characteristics such that the decomposition of the lithium salt is less than that of the SEI layer produced by decomposition of EC, resulting in excellent low temperature characteristics of the graphite anode.

The electrolyte solution for a lithium secondary battery of the present invention includes a lithium salt.

The lithium salt is not particularly limited, but LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO _{3, LiClO 4, LiAlO 2,} LiAlCl 4, LiN (C x F 2x + 1 SO 2) (C y F 2y + 1 SO 2) (x and y are natural numbers), LiCl, LiI, and LiB (C ₂ O ₄ ) ₂ (LiBOB) may be used.

The concentration of the lithium salt is preferably within the range of about 0.1M to about 2.0M. When the concentration of the lithium salt is within the above range, the electrolytic solution has an appropriate conductivity and viscosity, so that it can exhibit excellent electrolyte performance, and lithium ions can effectively move.

The electrolyte solution for a lithium secondary battery of the present invention described above is applied to a conventional lithium secondary battery having a cathode and an anode.

Specifically, the electrolyte solution for a lithium secondary battery of the present invention can be applied to an electrode assembly comprising a cathode, a cathode, and a separator interposed between the anode and the cathode, and a lithium secondary battery having an electrolyte injected into the electrode assembly.

The negative electrode can be any negative electrode capable of intercalating and deintercalating lithium ions. Examples of the negative electrode include a metallic material such as lithium metal and lithium alloy, and a carbonaceous material such as low crystalline carbon and highly crystalline carbon. Examples of the low crystalline carbon include soft carbon and hard carbon. Examples of highly crystalline carbon include natural graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch carbon fiber high temperature sintered carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches and petroleum or coal tar pitch derived cokes. Alloys containing silicon and oxides such as Li ₄ Ti ₅ O ₁₂ are also well known cathodes. The negative electrode may include a binder. Examples of the binder include vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, Various kinds of binder polymers such as polymethylmethacrylate and styrene-butadiene rubber (SBR) may be used. In addition, the cathode may include a cathode layer containing a lithium-containing oxide, and the lithium-containing oxide may be a lithium-containing transition metal oxide.

The positive electrode may be any positive electrode containing lithium and capable of intercalating and deintercalating lithium ions. For example, as the anode, a lithium-containing transition metal oxide can be preferably used. For example, LiCoO2, LiNiO2, LiMnO2, LiMn2O4, Li (NiaCobMnc) O2 (0 <a <1, 0 <b < LiNi1-yMnyO2 (O = y <1), Li (NiaCobMnc) O4 (where 0 <y <1, a + b + c = 1), LiNi1-yCoyO2 (0 <z <2), LiMn2-zCozO4 (0 <z <2), LiMn2-zNizO4 LiCoPO4 and LiFePO4, or a mixture of two or more of them may be used. The positive electrode may include a positive electrode active material layer containing a lithium metal, a carbon material, a metal compound, and a mixture thereof. The metal compound may be at least one selected from the group consisting of Si, Ge, Sn, Pb, P, Sb, , A compound containing at least one metal element selected from the group consisting of In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, Mg, Sr and Ba or a mixture thereof.

A conventional separator may be interposed between the positive electrode and the negative electrode. For example, porous polymer films made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers and ethylene / methacrylate copolymers, etc., Can be laminated and used as a separator. In addition, nonwoven fabrics made of porous nonwoven fabric, for example, glass fibers having a high melting point, polyethylene terephthalate fibers or the like can be used, but the present invention is not limited thereto.

The external shape of the lithium secondary battery of the present invention is not particularly limited, but may be a cylindrical shape, a square shape, a pouch shape, a coin shape, or the like using a can.

BEST MODE FOR CARRYING OUT THE INVENTION Hereinafter, the present invention will be described in detail with reference to examples. However, the embodiments according to the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. The embodiments of the present invention are provided to enable those skilled in the art to more fully understand the present invention.

Example

Preparation of electrolytic solution

Example 1

An electrolytic solution was prepared by dissolving LiPF ₆ at a concentration of 1 M in a solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC) and propionitrile were mixed at a volume ratio of 1: 1: 1.

Example 2

LiPF ₆ was dissolved at a concentration of 1 M in a solvent in which ethylene carbonate (EC), dimethyl carbonate (DMC), propionitrile and methyl acetate were mixed at a volume ratio of 1: 1: 1: 1 Thereby preparing an electrolytic solution.

Comparative Example 1

An electrolytic solution was prepared by dissolving LiPF ₆ at a concentration of 1 M in a solvent in which ethylene carbonate (EC) and dimethyl carbonate (DMC) were mixed at a volume ratio of 1: 1.

Comparative Example 2

An electrolytic solution was prepared in the same manner as in Example 1 except that propionitrile and methyl acetate were used instead of propionitrile.

Evaluation of ionic conductivity of electrolyte

The ion conductivity at -20 ° C to 60 ° C was measured for the electrolyte prepared in Example 1 and Comparative Example 1 using the SevenMulti equipment of Mettler-Toledo. The obtained results are shown in Fig. As shown in FIG. 2, the electrolyte of Example 1 according to the present invention exhibited a higher ionic conductivity at all temperatures than the electrolytes of Comparative Examples 1 and 2, and particularly had a high ionic conductivity of about three times at a cryogenic temperature of -20 ° C Lt; / RTI &

How to evaluate the initial performance of a battery

A battery was prepared by preparing a pouch battery using nickel cobalt aluminum (NCA) as an anode and artificial graphite as a cathode, and injecting non-aqueous electrolytes of Examples 1 and 2 and Comparative Examples 1 and 2.

Thereafter, the charge and discharge conditions of the battery prepared in Example 1 and Comparative Examples 1 and 2 were changed to 0.2C at 23 DEG C, 0.2C at -20 DEG C, 1C at -20 DEG C, , And the discharge capacity and the C-rate characteristic were measured. The results are shown in Table 1 and Figs. 3 to 6.

Discharge current
Temperature (℃) 0.2C
23 ^o C 0.2C
-20 ^o C 1C
-20 ^o C 3C
-20 ^o C Comparative Example 1 441.85 205.28 20.31 0.74 Comparative Example 2 602.78 499.18 351.47 131.21 Example 1 488.95 474.79 360.87 179.15

As shown in Table 1 and FIG. 6, it can be seen that the difference in discharge capacity between Example 1 and Comparative Example 1 and Comparative Example 2 was small even at a low temperature.

3 to 5 show the results of the C-rate characteristics of Example 1, Comparative Example 1 and Comparative Example 2. The normalized discharge capacity of Example 1 was at least twice as high as that of Comparative Example 1 (-20 C &lt; / RTI &gt; at 0.2 C).

Evaluation of battery life performance

A battery (battery capacity: 3.5 mAh) made from the electrolytes of Examples 1 and 2 and Comparative Example 1 was charged at a constant current of 2.0 C rate at 25 캜 until the voltage reached 4.3 V.

Thereafter, it was left for 10 minutes and then discharged at a constant current of 1.0 C rate until the voltage reached 3 V. The battery capacity was measured after charging and discharging for 20 cycles or more, and it is shown in Fig. As shown in Fig. 7, it can be seen that the lifetime characteristics of the nitrile compound-containing examples 1 and 2 are better than those of the comparative example 1.

Further, the battery was charged to 4.3 V at a constant current of 2.0 C rate to the same battery. Thereafter, it was left to stand for 10 minutes and then discharged at a constant current of 2.0 C rate until the voltage reached 3 V. After performing the charge and discharge for 150 cycles or more, the discharge efficiency was measured and shown in FIG. The batteries manufactured from the electrolytes of Example 1 and Example 2 exhibited discharging efficiencies of 91.3% and 93.6%, respectively, as compared with those of Comparative Example 1, which exhibited discharge efficiency of 87.9%.

Claims (14)

Lithium salts; And
A non-aqueous organic solvent containing a cyclic carbonate compound and a nitrile-based compound,
Wherein the nitrile-based compound is contained in an amount of 10 to 33% by volume based on the total amount of the non-aqueous organic solvent.
The method according to claim 1,
Wherein the nitrile compound is at least one selected from the group consisting of butyronitrile, propionitrile, and acetonitrile.
The method according to claim 1,
The cyclic carbonate compound may be at least one selected from the group consisting of ethylene carbonate (EC), dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), propylene carbonate (PC), butylene carbonate GBL). &Lt; RTI ID = 0.0 &gt; 1 &lt; / RTI &gt;
The method of claim 3,
Wherein the cyclic carbonate compound is a mixture of ethylene carbonate (EC) and dimethyl carbonate (DMC).
The method according to claim 1,
The electrolyte solution for a lithium secondary battery according to claim 1, further comprising fluoroethylene carbonate (FEC).
6. The method of claim 5,
Wherein the fluoroethylene carbonate (FEC) is contained in an amount of 1 to 3 wt% based on the total weight of the electrolytic solution.
The method according to claim 1,
Wherein the electrolytic solution further comprises an acetate compound.
6. The method of claim 5,
Wherein the acetate compound is contained in an amount of 10 to 33% by volume based on the total amount of the non-aqueous organic solvent.
The method according to claim 1,
The lithium salt is selected from the group consisting of LiPF ₆ , LiBF ₄ , LiSbF ₆ , LiAsF ₆ , LiN (SO ₂ CF ₃ ) ₂ , LiN (SO ₃ C ₂ F ₅ ) ₂ , LiC ₄ F ₉ SO ₃ , LiClO ₄ , LiAlO ₂ , _{4, LiN (C x F 2x} +1 SO 2) (C y F 2y +1 SO 2) (x and y are natural numbers), LiCl, LiI, and LiB (C ₂ O ₄₎ the group consisting of ₂ (LiBOB) Wherein the electrolyte is at least one selected from the group consisting of lithium, lithium, and lithium.
A lithium secondary battery comprising an electrode assembly comprising a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, and an electrolyte injected into the electrode assembly, wherein the electrolyte is a lithium battery according to any one of claims 1 to 9 Lithium secondary battery which is an electrolyte for secondary battery.
The method of claim 10,
Wherein the positive electrode comprises a positive electrode active material layer containing lithium metal, a carbon material, a metal compound, and a mixture thereof.
The method of claim 11,
Wherein the metal compound is selected from the group consisting of Si, Ge, Sn, Pb, P, Sb, Bi, Al, Ga, In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, A compound containing at least one selected metal element, or a mixture thereof.
The method of claim 10,
Wherein the cathode comprises a cathode layer containing a lithium-containing oxide.
14. The method of claim 13,
Wherein the lithium-containing oxide is a lithium-containing transition metal oxide.

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