KR100693287B1 - Electrolyte for lithium secondary battery and lithium secondary battery comprising the same - Google Patents

Abstract

Provided are an electrolyte solution which forms a very stable film on the surface of a carbon material used as a negative electrode active material, and a lithium secondary battery containing the electrolyte solution which is improved in initial charge/discharge efficiency and lifetime characteristics. The electrolyte solution comprises a lithium salt; an organic solvent; and 0.5-8 wt% of a fluorinated vinyl carbonate-based compound selected from the group consisting of the compound represented by the formula 1 and the compound represented by the formula 2. Preferably, the organic solvent comprises a cyclic carbonate-based organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, and their mixture.

Description

ELECTROLYTE FOR LITHIUM SECONDARY BATTERY AND LITHIUM SECONDARY BATTERY COMPRISING THE SAME

1 is a graph showing the initial charge and discharge characteristics of the lithium secondary battery prepared according to Example 1.

Figure 2 is a graph showing the initial charge and discharge life characteristics of the lithium secondary battery prepared according to Examples 1-3 and Comparative Examples 1,2.

3 to 7 are graphs showing the results of measuring the ignition of lithium secondary batteries according to Examples 1-3 and Comparative Examples 1,2.

The present invention relates to a lithium secondary battery electrolyte and a lithium secondary battery employing the same, and more particularly, has an improved initial charge and discharge efficiency and life characteristics, electrolyte electrolyte for lithium secondary batteries that can suppress the ignition of the battery due to overcharge misuse And it relates to a lithium secondary battery employing the same.

As electronic devices are miniaturized, reduced in weight, thinned, and portable in recent years with the development of the information and communication industry, demand for high energy density of batteries used as power sources of such electronic devices is increasing. Lithium secondary batteries are the batteries that can best meet these demands, and research on these is being actively conducted.

A lithium secondary battery is composed of a cathode, an anode, and an electrolyte and a separator that provide a migration path of lithium ions between the cathode and the anode, and are used for oxidation and reduction reactions when lithium ions are inserted / deinserted from the cathode and the anode. Thereby generating electrical energy.

The first form of such a lithium secondary battery was a lithium metal having a high energy density as a negative electrode, and a liquid solvent as an electrolyte. The lithium secondary battery had a disadvantage of shortening its life due to a dendrite phenomenon. In order to improve this disadvantage, a lithium secondary battery has been developed in which a carbon material capable of absorbing a large amount of lithium ions is used as a negative electrode instead of lithium metal, and an organic liquid or a solid polymer is composed of an electrolyte.

However, a lithium secondary battery using a carbon material as a negative electrode active material has a low boiling point, high ion conductivity as an electrolyte, and has a low temperature characteristic and an electrochemically stable propylene carbonate. Have This is because propylene carbonate promotes decomposition of the carbon material. In order to improve these disadvantages, instead of propylene carbonate, ethylene carbonate is mixed with dimethyl carbonate or diethyl carbonate, which is a linear carbonate organic solvent, or vinyl is mixed with an organic solvent. Although acetate, ethyl sulfite, and vinyl carbonate have been used as additives, there are still limitations in sufficiently improving initial charge and discharge efficiency and life characteristics. It did not solve the problem of ignition. Therefore, in order to commercialize a lithium secondary battery, research for improving an initial charge and discharge efficiency and solving a ignition problem of a battery is required.

Therefore, the technical problem to be achieved by the present invention is to provide an electrolyte solution for a lithium secondary battery that can improve the charge and discharge efficiency and life characteristics of the lithium secondary battery, and can suppress the ignition of the battery during overcharge.

Another object of the present invention is to provide a lithium secondary battery employing the electrolyte as described above.

In order to achieve the above technical problem, the present invention is a lithium salt; And in the lithium secondary battery electrolyte comprising an organic solvent, a compound represented by the following formula (1), a compound represented by the following formula (2), and mixtures thereof are further selected from the group consisting of fluorinated vinyl carbonate (fluorinated vinyl carbonate) It provides a lithium secondary battery electrolyte characterized in that it comprises.

The lithium secondary battery electrolyte prevents the carbonaceous material used as the negative electrode active material from being decomposed, thereby improving charge and discharge efficiency and lifespan characteristics of the lithium secondary battery, and suppressing ignition during overcharge misuse, thereby improving stability.

In order to achieve the above another technical problem, the present invention is a cathode comprising a lithium-containing metal oxide; An anode comprising a carbon material; A separator interposed between the cathode and the anode; And in a lithium secondary battery comprising an electrolyte, the electrolyte provides a lithium secondary battery comprising the electrolyte as described above.

The lithium secondary battery exhibits excellent charge and discharge efficiency, lifespan characteristics, and stability by employing the above electrolyte solution.

The lithium secondary battery is a lithium ion battery, the electrolyte may be a liquid electrolyte.

In addition, the lithium secondary battery is a lithium polymer battery, the electrolyte may be a gel-type solid electrolyte further comprising a polymer resin.

Hereinafter, the lithium secondary battery electrolyte according to the present invention will be described in detail.

Since a lithium secondary battery generates electricity by a complex reaction in a cathode, an anode, and an electrolyte, the use of an appropriate electrolyte is one of important factors for improving the performance of a lithium secondary battery. The present invention relates to such a lithium secondary battery electrolyte, the electrolyte of the present invention comprises a lithium salt, an organic solvent and fluorinated vinyl carbonate (fluorinated vinyl carbonate). The electrolyte solution of the present invention can be prepared by mixing a lithium salt, an organic solvent and fluorinated vinyl carbonate.

The fluorinated vinyl carbonate is added to the electrolyte for improving charge and discharge efficiency and stability, and is selected from the group consisting of a compound represented by the following formula (1), a compound represented by the following formula (2), and a mixture thereof. Fluorinated vinyl carbonate (fluorinated vinyl carbonate) is a part of the chemical structure of the vinyl carbonate (vinyl carbonate) is replaced by fluorine, has a carbonate structure, is easily mixed with the carbonate-based organic solvent, and does not reduce the ion conductivity of the electrolyte.

<Formula 1>

 <Formula 2>

In particular, fluorinated vinyl carbonate has a vinyl double bond therein to form a very stable film on the surface of the carbon material used as the negative electrode active material through polymerization, thereby preventing the carbon material from being decomposed by the electrolyte solution. The charging and discharging efficiency and lifespan characteristics of the lithium secondary battery are improved, and the flame retardance inherent in fluorine suppresses ignition when the battery is overcharged, thereby improving stability.

The addition amount of the fluorinated vinyl carbonate is not particularly limited, but considering the charge and discharge characteristics and stability, it is preferably added in 0.5-8% by weight based on the total weight of the electrolyte.

As the lithium salt, those conventionally used in an electrolyte for a lithium secondary battery may be used without limitation. Representative examples of the lithium salt include LiPF _6, LiBF _4, LiSbF _6, LiAsF _6, LiClO ₄ , and LiN (C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ , CF ₃ SO ₃ Li and LiC (CF ₃ SO ₂ ) ₃ . The lithium salt concentration in the electrolyte is preferably 0.8 to 1.5 M in consideration of the ionic conductivity of the electrolyte.

In addition, the organic solvent may also be used without limitation those commonly used in lithium secondary batteries, typically ethylene carbonate (ethylene carbonate), propylene carbonate (propylene carbonate), dimethyl carbonate (dimethyl carbonate), diethyl carbonate (diethyl carbonate) ), Methylpropyl carbonate, ethylpropyl carbonate, methylethyl carbonate, methylene carbonate, butylene carbonate, dimethylsulfoxide, acetonitrile, dimethoxyethane (dimethoxyethane) and diethoxyethane (diethoxyethane) and the like can be used. In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates among the carbonate-based organic solvents, are highly viscous organic solvents, and thus have high dielectric constant, which may dissociate lithium salts in the electrolyte, and thus may be preferably used. When a low viscosity, low dielectric constant linear carbonate such as dimethyl carbonate and diethyl carbonate is mixed in an appropriate ratio, it can be used more preferably because an electrolyte having high electrical conductivity can be made. .

Hereinafter, a lithium secondary battery employing the lithium secondary battery electrolyte of the present invention will be described.

Lithium secondary batteries are largely classified into lithium ion batteries using liquid electrolytes and lithium polymer batteries using solid electrolytes. The electrolyte solution for lithium secondary batteries of the present invention is not only a liquid electrolyte for lithium ion batteries, but also a solid for lithium polymer batteries. It can also be used for electrolytes. Lithium polymer batteries are classified into fully solid forms containing no electrolyte in the electrolyte and gels containing electrolyte in the electrolyte. The electrolyte of the present invention is used in a gel type lithium polymer battery.

The lithium ion battery uses the above-described electrolyte solution of the present invention as an electrolyte, and the method of manufacturing the lithium ion battery will be described below.

First, an electrode active material layer is formed on a current collector using an electrode active material composition containing an electrode active material, a binder, a conductive agent, and a solvent. At this time, the method of forming the electrode active material layer is a method of directly coating the electrode active material composition on the current collector, or by coating the electrode active material composition on a separate support and dried, and peeling from the support, the film obtained on the current collector There is a way to laminate. Herein, the support may be used as long as it can support the active material layer, and specific examples thereof include a mylar film and a polyethylene terephthalate (PET) film.

As the electrode active material, the binder, the conductive agent, and the solvent, all of those conventionally used in manufacturing a lithium secondary battery may be used. As a specific example, as the electrode active material of the cathode, lithium-containing metal oxides such as LiCoO ₂ , LiNiO ₂ and LiMn ₂ O ₄ and LiNi _1-x Co _x O ₂ prepared by adding Co, Ni or Mn to the lithium-containing metal oxides Lithium-containing metal oxides such as may be used, in addition to the oxide (sulfide), sulfide (sulfide), selenide (selenide) and halide (halide) may be used. Carbon material is used as the electrode active material of the anode, and both low crystalline carbon and high crystalline carbon may be used. The low crystalline carbon includes soft carbon and hard carbon, and high crystalline. Carbon includes natural graphite, Kish graphite, pyrolytic carbon, liquid phase pitch based carbon fiber, meso-carbon microbeads, liquid phase pitches and High-temperature calcined carbon such as petroleum or coal tar pitch derived cokes is typical. The binder may be vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, and the like. Mixtures can be used. As the conductive agent, carbon black or acetylene block is typical, and acetone and N-methylpyrrolidone are typical as the solvent.

When the electrode is manufactured according to the method as described above, the separator is inserted between the cathode electrode plate and the anode electrode plate, and the electrode assembly of the electrode assembly or bicell structure formed by winding it in a jelly roll manner is made. Subsequently, the prepared electrode assembly is placed in a case, and the lithium ion battery of the present invention is completed by injecting the electrolyte solution for a lithium secondary battery of the present invention as described above.

Unlike a lithium ion battery, a gel lithium polymer battery uses a gel formed from a composition containing a polymer resin and the electrolyte solution of the present invention as an electrolyte, and a method of manufacturing such a gel lithium polymer battery is as follows.

First, a cathode, an anode, and a separator are manufactured in the same manner as in the case of a lithium ion battery to form an electrode assembly, and the electrode assembly is placed in a case. Thereafter, the electrolyte-forming composition comprising the polymer resin and the above-described electrolyte solution of the present invention is injected into the case and warmed, and when left at room temperature, a gel electrolyte is formed, thereby forming the gel lithium polymer battery of the present invention. Is completed.

As the polymer resin, all of those conventionally used in the conventional lithium polymer battery may be used. Typically, polyvinylidene fluoride, vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP) , Polyurethane, polyethyleneoxide, polyacrylonitrile, polymethylmethacrylate, polyacrylamide and polyacetate.

Hereinafter, the present invention will be described in detail with reference to Examples. However, embodiments according to the present invention can be modified in many different forms, the scope of the present invention should not be construed as limited to the embodiments described below. Embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

Example 1

LiPF ₆ was added to an organic solvent in which ethylene carbonate and propylene carbonate were mixed in a weight ratio of 1: 1 to 1M concentration, and then, to the prepared solution, fluorinated vinyl carbonate of Chemical Formula 1 carbonate) was added to 2 wt% based on the total weight of the electrolyte to prepare an electrolyte solution.

A thin lithium ion battery sealed in a plastic pack was prepared using an electrolyte solution prepared as described above as an electrolyte, LiCoO ₂ as a cathode active material, and natural graphite as an anode active material.

Example 2

Example 1 except that the fluorinated vinyl carbonate of Formula 2 was added to 2% by weight based on the total weight of the electrolyte instead of the fluorinated vinyl carbonate of Formula 1 when preparing the electrolyte In the same manner, a lithium ion battery was prepared.

Comparative Example 1

A lithium ion battery was manufactured in the same manner as in Example 1, except that fluorinated vinyl carbonate was not added when the electrolyte was prepared.

Example 3

LiPF ₆ was added to a concentration of 1 M in an organic solvent in which ethylene carbonate and propylene carbonate were mixed at a weight ratio of 1: 1, and then the fluorinated vinyl carbonate of Chemical Formula 2 was added as an additive to the prepared solution. fluorinated vinyl carbonate) was added to 2 wt% based on the total weight of the electrolyte to prepare an electrolyte solution.

A thin lithium polymer battery sealed in a plastic pack was prepared by using the prepared electrolyte and a polyethylene oxide (polyethyleneoxide) as an electrolyte, LiCoO ₂ as a cathode active material, and natural graphite as an anode active material.

Comparative Example 2

A lithium polymer battery was manufactured in the same manner as in Example 3, except that fluorinated vinyl carbonate was not added when the electrolyte was prepared.

Initial charge and discharge efficiency evaluation experiment

The initial charge and discharge characteristics of the lithium secondary battery according to Example 1 were evaluated at a 1C charge rate. 1 is a graph showing the initial charge and discharge characteristics of the lithium secondary battery prepared according to Example 1. It can be seen from the graph that the initial charge and discharge efficiency is greater than 93.5%. In the same manner, the initial charge and discharge efficiency of the lithium secondary batteries according to Example 2, Example 3, and Comparative Examples 1 and 2 was measured, and the initial stages of the lithium secondary batteries according to Examples 1-3 and Comparative Examples 1 and 2 were measured. Charge and discharge efficiency is shown in Table 1 below.

division Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Initial charge and discharge efficiency (%) 93.5% 93.2% 92.3% 87.5% 86.5%

As a result, it was found that the lithium secondary battery added with fluorinated vinyl carbonate showed much better initial charge and discharge efficiency.

Life characteristic evaluation experiment

Life characteristics were evaluated at 60 ° C. while repeating charging and discharging for the lithium secondary batteries according to Examples 1-3 and Comparative Examples 1,2. Figure 2 is a graph showing the initial charge and discharge life characteristics of the lithium secondary battery prepared according to Examples 1-3 and Comparative Examples 1,2. Through the graph of FIG. 2, the lithium secondary battery prepared according to Examples 1-3 hardly exhibits a decrease in capacity even over 100 cycles. In contrast, the lithium secondary battery according to Comparative Examples 1 and 2 exhibits a rapid decrease in capacity. Could. Capacity retention rates after 100 cycles of the lithium secondary batteries according to Examples 1-3 and Comparative Examples 1 and 2 are shown in Table 2 below.

division Example 1 Example 2 Example 3 Comparative Example 1 Comparative Example 2 Capacity maintenance rate (%) 95.4% 93.5% 93.2% 68.3% 55.3%

As a result, it was found that the lithium secondary battery to which fluorinated vinyl carbonate was added showed a much better capacity retention rate.

Ignition test

For lithium secondary batteries prepared according to Examples 1-3 and Comparative Examples 1 and 2, it was observed whether ignition occurred upon overcharging up to 12V. 3 to 7 are graphs showing the results of measuring the ignition of lithium secondary batteries according to Examples 1-3 and Comparative Examples 1,2. Although the ignition did not occur in the lithium secondary battery of Example 1-3 to which fluorinated vinyl carbonate was added through the measurement results shown in FIGS. 3 to 7, the comparison without adding fluorinated vinyl carbonate was performed. It was confirmed that ignition occurred in the lithium secondary batteries of Examples 1 and 2.

The lithium secondary battery electrolyte according to the present invention forms a very stable film on the surface of the carbon material used as the negative electrode active material, improves the initial charge and discharge efficiency and life characteristics of the lithium secondary battery employing it, Ignition can be suppressed.

Claims (7)

Lithium salts; And In the electrolyte for lithium secondary batteries containing an organic solvent, An electrolyte solution for a lithium secondary battery, further comprising a fluorinated vinyl carbonate compound selected from the group consisting of a compound represented by Formula 1, a compound represented by Formula 2, and a mixture thereof. <Formula 1>

 <Formula 2>

The electrolyte of claim 1, wherein the fluorinated vinyl carbonate compound is added in an amount of 0.5-8% by weight based on the total weight of the electrolyte. The electrolyte of claim 1, wherein the organic solvent comprises a cyclic carbonate-based organic solvent selected from the group consisting of ethylene carbonate, propylene carbonate, and mixtures thereof. . The lithium secondary battery of claim 3, wherein the organic solvent further comprises a linear carbonate-based organic solvent selected from the group consisting of dimethyl carbonate, diethyl carbonate, and mixtures thereof. Electrolyte solution. A cathode comprising a lithium-containing metal oxide; An anode comprising a carbon material; A separator interposed between the cathode and the anode; And In a lithium secondary battery comprising an electrolyte, The electrolyte is a lithium secondary battery comprising the electrolyte according to claim 1 or 4. The lithium secondary battery of claim 5, wherein the electrolyte is a liquid electrolyte. The lithium secondary battery according to claim 5, wherein the electrolyte is a gel solid electrolyte.

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