KR101277732B1 - Cylindrical lithium secondary battery - Google Patents

Abstract

A cylindrical lithium secondary battery comprising an electrode assembly comprising an anode, a cathode, and a separator interposed between the cathode and the anode, and a nonaqueous electrolyte containing an electrolyte salt and an organic solvent, wherein the organic solvent is A cylindrical lithium secondary battery comprising at least 70% by volume methyl propionate, at most 15% fluoroethylene carbonate, and 5-20% by volume ethylene carbonate, wherein the anode has a loading level of at least 280 mg / 25 cm 2 .

Description

Cylindrical lithium secondary battery

The present invention relates to a cylindrical lithium secondary battery having a non-aqueous electrolyte for lithium secondary batteries containing methyl propionate, fluoroethylene carbonate, and ethylene carbonate and an anode of high rolling level.

Recently, interest in energy storage technology is increasing. As the application fields of cell phones, camcorders, notebook PCs, and electric vehicles further expand, there is a growing demand for higher energy density of batteries used as power sources for such electronic devices. Lithium secondary batteries are the ones that can best meet these demands, and researches on them are actively under way.

Among the secondary batteries currently applied, lithium secondary batteries developed in the early 1990s are nonaqueous materials in which lithium salts are appropriately dissolved in an anode made of carbon material capable of storing and releasing lithium ions, a cathode made of lithium-containing oxide, and a mixed organic solvent. It consists of electrolyte solution.

As the organic solvent, it is preferable to use an organic solvent having high ionic conductivity and low dielectric constant. However, since there is no single organic solvent that simultaneously satisfies these characteristics, a mixed solvent of a high dielectric constant organic solvent and a low viscosity organic solvent is mainly used. Specifically, in a battery using non-graphite carbon as the anode material, a mixed solvent of propylene carbonate, which is a high dielectric constant solvent, and diethyl carbonate or dimethyl carbonate, which is a low viscosity solvent, is used. In a battery using graphite carbon as the anode material, a mixed solvent of ethylene carbonate, which is a high dielectric constant solvent, and diethyl carbonate or dimethyl carbonate, which is a low viscosity solvent, is used.

However, when the organic electrolyte solution containing the mixed solvent is used, the charge and discharge characteristics at room temperature and the lifespan characteristics of the battery are excellent, but the mobility of lithium ions is rapidly decreased at low temperatures (-10 ° C. or lower), so that the inside of the battery As the resistance increases, the capacity characteristic becomes poor. As a result, there is a problem that the battery characteristics at low temperatures are very poor compared to room temperature.

Therefore, there is an urgent need to develop a nonaqueous electrolyte having high ion conductivity and low viscosity characteristics and also improving low-temperature discharge characteristics of a lithium secondary battery and a lithium secondary battery having the same.

The problem to be solved by the present invention is to solve the above-mentioned problems of the prior art, having a high ion conductivity and low viscosity of the electrolyte has improved low temperature characteristics, a high loading level of the anode layer can be stable and cost-effective cylindrical lithium It is to provide a secondary battery.

In order to achieve the above technical problem, an aspect of the present invention

A cylindrical lithium secondary battery comprising an electrode assembly comprising an anode, a cathode, and a separator interposed between the cathode and the anode, and a nonaqueous electrolyte containing an electrolyte salt and an organic solvent.

The organic solvent comprises at least 70% by volume methyl propionate, at most 15% fluoroethylene carbonate, and at least 5-20% ethylene carbonate,

The anode provides a cylindrical lithium secondary battery having a loading level of 280 mg / 25 cm 2 or more.

The electrolyte salt may comprise 1.15 to 1.6M LiPF ₆ .

The non-aqueous electrolyte may further include any one selected from the group consisting of cyclic sulfites, acyclic sulfones, nitrile group-containing compounds, saturated sultones, unsaturated sultones, vinylene carbonates, and vinylethylene carbonates, or mixtures of two or more thereof. Can be.

The anode may include an anode active material layer including a lithium metal, a carbon material, a metal compound, and a mixture thereof.

The metal compound is from the group consisting of Si, Ge, Sn, Pb, P, Sb, Bi, Al, Ga, In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, Mg, Sr, and Ba It may be a compound containing at least one metal element selected or a mixture thereof.

The cathode may have a cathode layer comprising a lithium containing oxide.

The lithium-containing oxide may be a lithium-containing transition metal oxide.

The lithium-containing transition metal oxide is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, 0 <c <1 , a + b + c = 1), LiNi _1-y Co _y O ₂ , LiCo _1-y Mn _y O ₂ , LiNi _1-y Mn _y O ₂ (O ≦ y <1), Li (Ni _a Co _b Mn _c ) O ₄ (0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn _2-z Ni _z O ₄ , LiMn _2-z Co _z O ₄ (0 <z <2), LiCoPO ₄ and LiFePO ₄ , any one selected from the group consisting of, or a mixture of two or more thereof.

Cylindrical lithium secondary battery according to an aspect of the present invention, by having a non-aqueous electrolyte for lithium secondary battery containing methyl propionate, fluoroethylene carbonate, and ethylene carbonate, has a high ion conductivity and low viscosity characteristics of lithium ions The mobility is improved to significantly improve the life characteristics of the battery at low temperatures, to prevent breakage and precipitation of the electrode, and to reduce the interface resistance after long-term storage at high temperatures. In addition, by reducing the content of expensive fluoroethylene carbonate in the non-aqueous electrolyte, increasing the loading level of the anode, and the density of the applied non-aqueous electrolyte is reduced, the amount of the electrolyte is reduced, thereby reducing the cost of battery manufacturing. can do.

1 is a graph showing the results of measuring the ion conductivity of the nonaqueous electrolyte injected into the cylindrical lithium secondary batteries prepared in Example 1 and Comparative Example 1. FIG.
2 is a graph showing the low temperature life characteristics of the cylindrical lithium secondary batteries prepared in Example 1 and Comparative Example 1.
3 is a graph showing the high temperature life characteristics of the cylindrical lithium secondary batteries prepared in Example 1 and Comparative Examples 2 to 4.
Figure 4 is a graph showing the result of measuring the impedance before high temperature storage of the cylindrical lithium secondary battery prepared in Example 1 and Comparative Example 1.
Figure 5 is a graph showing the results of measuring the impedance after high temperature storage (75 ℃, 43 hours) of the cylindrical lithium secondary batteries prepared in Example 1 and Comparative Example 1.

Hereinafter, the present invention will be described in detail. The terms and words used in the present specification and claims should not be construed as limited to ordinary or dictionary terms and the inventor may appropriately define the concept of the term in order to best describe its invention It should be construed as meaning and concept consistent with the technical idea of the present invention.

Cylindrical lithium secondary battery according to an aspect of the present invention is provided with an electrode assembly consisting of an anode, a cathode, and a separator interposed between the cathode and the anode, and a nonaqueous electrolyte containing an electrolyte salt and an organic solvent. Wherein the organic solvent comprises at least 70% by volume methyl propionate, up to 15% by volume fluoroethylene carbonate, and 5-20% by volume ethylene carbonate, wherein the anode has a loading level of at least 280 mg / 25 cm 2.

Since a lithium secondary battery has a high operating voltage, an aqueous solution electrolyte cannot be used. Therefore, an organic solvent must be used. The biggest disadvantage of the lithium secondary battery is that the dielectric constant of the solvent is relatively low.

Therefore, in order to be used as an organic solvent of the electrolyte solution, lithium salts must be well dissolved to have ionic conductivity and have no chemical reactivity with lithium, and therefore, must be non-hydrogen ionic and have high polarity.

In particular, the dielectric constant of an organic solvent has a great influence on ion dissociation and association of lithium salts. The coulomb force between cations and anions constituting the lithium salt is inversely proportional to the dielectric constant of the solvent. Because it is promoted.

In addition, the organic solvent of the non-aqueous electrolyte serves as a medium in which the ions involved in the electrochemical reaction of the battery can move. It is advantageous to use. However, since the organic solvent having a high dielectric constant has a high intramolecular charge deflection, since the solvent has a high viscosity due to the large interaction of solvent molecularization, the organic solvent having a high dielectric constant has a high viscosity and thus cannot be used alone.

Therefore, it is preferable to use two or more mixed solvents composed of a solvent having a high dielectric constant and a high viscosity and a solvent having a low dielectric constant and a low viscosity.

As a result, the organic solvent of the nonaqueous electrolyte solution according to an aspect of the present invention includes a mixture of ethylene carbonate, methyl propionate and fluoroethylene carbonate.

Typically, cyclic carbonate is typically used as an organic solvent of a nonaqueous electrolyte, and examples thereof include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, and 2,3-butylene. Carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and the like.

Among these, ethylene carbonate has a significantly higher dielectric constant than other cyclic carbonates, which greatly promotes dissociation of lithium salts, thereby contributing to the improvement of ion conductivity of the electrolyte. However, ethylene carbonate has a high dielectric constant and can dissociate lithium salts to a high degree, but since it exhibits high viscosity, there is a possibility that the ionic conductivity of the electrolyte prepared by dissolving lithium salts is reduced.

On the other hand, ethylene carbonate exhibits a viscosity of 1.86 cP (measured at 40 ° C.) compared with propylene carbonate having a similar structure, and thus has a relatively low viscosity characteristic, which is smaller than 2.53 cP of propylene carbonate, and is a relative dielectric constant of propylene carbonate. The dielectric constant of 89.6, a much larger value than 64.4, is also excellent in terms of dielectric properties.

Thus, although ethylene carbonate has high viscosity characteristics, it has excellent dielectric properties and is therefore most preferred as a cyclic carbonate used in an organic solvent of a nonaqueous electrolyte.

In addition, according to one aspect of the present invention, methyl propionate is used together to offset the high viscosity properties of ethylene carbonate to improve the ion conductivity of the electrolyte.

Methyl propionate has good miscibility with ethylene carbonate and has a low viscosity of 0.43 cP, which lowers the overall viscosity of the nonaqueous electrolyte so that higher ionic conductivity can be achieved than with ethylene carbonate alone solvent. In addition, methyl propionate has a melting point of -88 ° C, which can greatly contribute to the dissolution of lithium salts as well as the dissolution of lithium salts at low temperatures as well as to maintain the liquid state at low temperatures. By suppressing, the cycle life of a battery can be improved. In addition, because of low viscosity and excellent mobility of lithium ions, the impregnation characteristics in the electrode are superior to other linear esters and linear carbonates that are typically used in nonaqueous electrolytes.

In addition, although fluoroethylene carbonate (FEC) is expensive, when added as an organic solvent of a nonaqueous electrolyte, it imparts flame retardancy of the secondary battery, improves the life characteristics of the secondary battery, and improves the interface between the electrolyte and the anode. It stabilizes and enables high capacity charging per unit time. This is because the fluoroethylene carbonate contains fluorine having a strong electron attraction effect, so that the SEI layer having a high dielectric constant and excellent lithium ion conductivity during battery initial charging can be formed. In fact, when the fluoroethylene carbonate is included in the nonaqueous electrolyte as an additive, it can be seen that the charging characteristics and unit cycle performance of the battery are improved.

However, fluoroethylene carbonate has a high viscosity to reduce the processability, but as described above by increasing the content of methyl propionate and reducing the content of fluoroethylene carbonate, the overall viscosity is lowered to improve the ionic conductivity The pouring of the electrolyte is easy and the processability is improved.

In addition, according to an aspect of the present invention, by increasing the content of high density fluoroethylene carbonate and ethylene carbonate in the composition of the electrolyte, the content of methyl propionate with a low density, thereby reducing the density of the entire electrolyte, and As a result, it is possible to reduce the amount of pouring of the electrolyte solution for the secondary battery, thereby reducing the manufacturing cost of the secondary battery.

The content of the ethylene carbonate in the organic solvent of the nonaqueous electrolyte is 5 to 20% by volume, more preferably 7 to 15% by volume, even more preferably 10 to 13% by volume, based on the total volume of the organic solvent. When the content of the ethylene carbonate is less than 5% by volume, the dielectric constant of the nonaqueous electrolyte decreases, so that the dissociation property of the lithium salt is lowered. When the content of the ethylene carbonate is greater than 20% by volume, the viscosity of the nonaqueous electrolyte is excessively increased so that the ion conductivity is lowered. Injection of electrolyte solution becomes difficult, and mobility of electrolyte solution falls and it is unpreferable.

The content of methyl propionate is at least 70% by volume, more preferably 70 to 80% by volume, even more preferably 70 to 75% by volume relative to the total volume of the organic solvent. When the content of the methyl propionate is less than 70% by volume, the viscosity of the mixed solvent is too high and the mobility of the electrolyte is low, which is not preferable.

The content of the fluoroethylene carbonate is 15% by volume or less, more preferably 5 to 15% by volume, even more preferably 5 to 13% by volume, based on the total volume of the organic solvent. When the content of the fluoroethylene carbonate satisfies 15% by volume or less, it may be maintained without being exhausted even during a prolonged cycle, and the cost of the battery may be increased due to the excessive use of expensive fluoroethylene carbonate. It is possible to control, the resistance of the cathode is excessively increased to prevent the degradation of the battery performance during high rate discharge, and the electrolyte after the injection can evenly penetrate the electrode.

The electrolyte salt contained in the nonaqueous electrolyte solution according to one aspect of the present invention is a lithium salt. The lithium salt can be used without limitation as those conventionally used in an electrolyte for a lithium secondary battery. For example is the above lithium salt anion ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, PF 6 -, (CF 3) 2 PF _{^{4 -, (CF 3) 3}} PF 3 -, (CF 3) 4 PF 2 -, (CF 3) 5 PF -, (CF 3) 6 P -, CF 3 SO 3 -, CF 3 CF 2 SO 3 - _{, (CF 3 SO 2) 2} N -, (FSO 2) 2 N -, CF 3 CF 2 (CF 3) 2 CO -, (CF 3 SO 2) 2 CH -9, (SF 5) 3 C -, _{(CF 3 SO 2) 3 C} -, CF 3 (CF 2) 7 SO 3 -, CF 3 CO 2 -, CH 3 CO 2 -, SCN ^- and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .

Specifically, LiPF ₆ or LiBF ₄ based on fluoride Lewis acid may be used as the electrolyte salt in view of solubility and chemical stability of the salt. The content of such electrolyte salt may be for example 1.15 to 1.6M, or 1.2 to 1.4M. When the content of the electrolyte salt satisfies this range, the electrolyte solution in which the electrolyte salt is dissolved has high ionic conductivity, does not easily cause side reactions in the electrode, prevents dissociation and disconnection of active materials of the electrode, and the content of electrolyte salt When the amount is increased up to a certain amount, the ion conductivity of the electrolyte is increased, thereby suppressing precipitation on the surface of the cathode, thereby reducing disconnection.

The nonaqueous electrolyte of the cylindrical lithium secondary battery according to an aspect of the present invention may further include an additive to improve the performance and safety of the secondary battery.

During the initial charging of a lithium secondary battery, lithium ions derived from a cathode active material such as lithium metal oxide move to an anode active material such as graphite and are inserted between the layers of the anode active material. At this time, since lithium has a high reactivity, the electrolyte and the carbon constituting the anode active material react on the surface of the anode active material such as graphite to generate compounds such as Li ₂ CO ₃ , Li ₂ O, and LiOH. These compounds form a kind of SEI (Solid Electrolyte Interface) layer on the surface of an anode active material such as graphite.

The SEI layer acts as an ion tunnel and passes only lithium ions. The SEI layer is an effect of such an ion tunnel, and prevents the structure of the anode from being destroyed by inserting an organic solvent molecule having a large molecular weight moving with lithium ions in the electrolyte between the layers of the anode active material. Therefore, by preventing contact between the electrolyte solution and the anode active material, decomposition of the electrolyte solution does not occur, and the amount of lithium ions in the electrolyte solution is reversibly maintained to maintain stable charge and discharge.

Therefore, according to one aspect of the present invention, the non-aqueous electrolyte may further include a conventionally known additive for forming the SEI layer in a range not departing from the object of the present invention. As an additive for forming the SEI layer, cyclic sulfite, acyclic sulfone, saturated sultone, unsaturated sultone, vinylene carbonate, vinyl ethylene carbonate and the like may be used alone or in combination of two or more thereof, but is not limited thereto. .

The cyclic sulfites include ethylene sulfite, methyl ethylene sulfite, ethyl ethylene sulfite, 4,5-dimethyl ethylene sulfite, 4,5-diethyl ethylene sulfite, propylene sulfite, 4,5-dimethyl propylene sulfide Pite, 4,5-diethyl propylene sulfite, 4,6-dimethyl propylene sulfite, 4,6-diethyl propylene sulfite, 1,3-butylene glycol sulfite, and the like. And divinyl sulfone, dimethyl sulfone, diethyl sulfone, methylethyl sulfone, methylvinyl sulfone, and the like. Examples of saturated sulfone include 1,3-propane sulfone, 1,4-butane sulfone, and the like. Examples thereof include ethene sultone, 1,3-propene sultone, 1,4-butene sultone, 1-methyl-1,3-propene sultone, and the like.

In addition, according to an aspect of the present invention, it may further include a nitrile group-containing compound as an additive of the non-aqueous electrolyte.

Since the nitrile group-containing compound has a strong polar nitrile device, the nitrile group blocks the active site on the electrode surface, thereby reducing the calorific value generated by side reaction between the electrolyte and the cathode and structural collapse of the cathode, thereby accelerating the combustion of the electrolyte. Thermal runaway occurs to prevent the battery from firing and rupture.

The nitrile group-containing compound may be an aliphatic or aromatic compound, and mononitrile and dinitrile compounds containing 1 to 2 nitrile groups are preferable. In particular, aliphatic dinitrile compounds are more preferred.

The aliphatic dinitrile compound is a linear or branched dinitrile compound having 1 to 12 carbon atoms having one or more substituents, including, but not limited to, succinonitrile, sebaconitrile, glutaronitrile, adipononitrile, 1,5 Dicyanopentane, 1,6-dicyanohexane, 1,7-dicyanoheptane, 1,8-dicyanooctane, 1,9-dicyanononane, 1,10-dicyanodecane, 1,12 Dicyanododecane, tetramethylsuccinonitrile, 2-methylglutaronitrile, 2,4-dimethylglutaronitrile, 2,2,4,4-tetramethylglutaronitrile, 1,4-dicinopentane , 2,5-dimethyl-2,5-hexanedicarbonitrile, 2,6-dicyanoheptane, 2,7-dicyanooctane, 2,8-dicyanononane or 1,6-dicyanodecane have. In particular, succinonitrile or sebaconitrile compounds are preferred.

The additive used in the non-aqueous electrolyte may be included in an appropriate amount according to the specific type of the additive, for example, 0.01 to 10 parts by weight based on 100 parts by weight of the organic solvent.

The nonaqueous electrolyte may be used as an electrolyte of a lithium secondary battery in the form of a gel polymer electrolyte impregnated with a liquid electrolyte or a polymer per se.

In the nonaqueous electrolyte of the cylindrical lithium secondary battery according to an aspect of the present invention, 15% by volume or less of fluoroethylene carbonate and 5 to 20% by volume of ethylene carbonate are mixed with the electrolyte salt, and 70% by volume or more of methyl propionate. It can obtain by adding and dissolving.

At this time, the organic solvent and electrolyte salt to be used can be refine | purified beforehand within the range which does not significantly reduce productivity, and the thing with very few impurities can be used.

By including air or carbon dioxide in the nonaqueous electrolyte, for example, it is possible to further improve battery characteristics such as suppression of gas generation due to decomposition of the electrolyte and long-term cycle characteristics and charge storage characteristics.

From the viewpoint of improving the charge and discharge characteristics at a high temperature, an electrolyte solution in which carbon dioxide is dissolved in a nonaqueous electrolyte solution can be used. The amount of carbon dioxide dissolved may be at least 0.001% by weight, or at least 0.05% by weight, or at least 0.2% by weight relative to the weight of the nonaqueous electrolyte, and may be dissolved in the nonaqueous electrolyte until the carbon dioxide is saturated.

In addition, the cathode, the anode and the separator constituting the electrode assembly of the cylindrical lithium secondary battery according to an aspect of the present invention may be used all that was conventionally used in the manufacture of a lithium secondary battery.

The cathode has a structure in which a cathode layer including a cathode active material, a conductive material and a binder is supported on one or both surfaces of the current collector.

As the cathode active material, a lithium-containing transition metal oxide may be preferably used. For example, Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ <x <1.3, 0 <a <1, 0 <b <1), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x (Ni _a Co _b Mn _c ) O ₂ , 0 <c <1, a + b + c = 1), Li x Ni 1-y Co y O 2 (0.5 <x <1.3, 0 <y <1), Li x Co 1-y Mn y O 2 (0.5 <x <1.3, 0≤y <1), Li x Ni 1-y Mn y O 2 (0.5 <x <1.3, O≤y <1), Li x (Ni a Co b Mn c) O 4 (0.5 <x <1.3, 0 <a <2, 0 <b <2, 0 <c <2, a + b + c = 2), Li _x Mn _{2 -z} Ni _z O ₄ Li _x CoPO ₄ (0.5 <x <1.3) and Li _x FePO ₄ (0.5 <x <1.3, 0 <z <2), 0 <z <2, Li _x Mn _{2 -z} Co _z O ₄ < x < 1.3), or a mixture of two or more thereof.

The lithium-containing transition metal oxide may be coated with a metal or metal oxide such as aluminum (Al). In addition to the lithium-containing transition metal oxide, sulfide, selenide, halide, and the like may also be used.

The conductive material is not particularly limited as long as it is an electron conductive material that does not cause a chemical change in an electrochemical device. In general, carbon black, graphite, carbon fiber, carbon nanotube, metal powder, conductive metal oxide, organic conductive material and the like can be used. Commercially available products as the conductive material include acetylene black series (manufactured by Chevron Chemical Co., (Chevron Chemical Company or Gulf Oil Company products), Ketjen Black EC series (Armak Company), Vulcan XC-72 (Cabot Company) and Super P (MM (MMM)). For example, acetylene black, carbon black and graphite.

The anode has a structure in which the anode layer including the anode active material and the binder is supported on one or both surfaces of the current collector.

As the anode active material, a carbon material, a lithium metal, a metal compound, and a mixture thereof, which may normally occlude and release lithium ions, may be used.

Concretely, as the carbon material, both low-crystalline carbon and highly-crystalline carbon may be used. Soft crystalline carbon and hard carbon are typical low crystalline carbon, and high crystalline carbon is natural graphite, artificial graphite, Kish graphite, pyrolytic carbon, liquid crystal pitch High-temperature calcined carbon such as mesophase pitch based carbon fiber, meso-carbon microbeads, mesophase pitches, and petroleum or coal tar pitch derived cokes are typical.

Examples of the metal compound include metal elements such as Si, Ge, Sn, Pb, P, Sb, Bi, Al, Ga, In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, Mg, , And the like. These metal compounds may be used in any form, such as single, alloys, oxides (TiO ₂ , SnO _2, etc.), nitrides, sulfides, borides, and alloys with lithium. High capacity can be achieved. Among them, it may contain at least one element selected from Si, Ge and Sn, and it may further increase the capacity of the battery including at least one element selected from Si and Sn.

The binders used for the cathode and the anode have a function of retaining the cathode active material and the anode active material in the current collector and connecting the active materials, and a binder commonly used may be used without limitation.

For example, it is possible to use vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, styrene Various types of binder polymers such as styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and the like can be used.

The current collectors used for the cathode and the anode are metals of high conductivity, and metals to which the slurry of the active material can easily adhere can be used as long as they are not reactive in the voltage range of the battery. Specifically, examples of the current collector for a cathode include aluminum, nickel, or a combination thereof. Examples of the current collector for an anode include copper, gold, nickel, or a copper alloy or a combination thereof And the like. In addition, the current collector may be used by laminating the substrates made of the above materials.

The cathode and the anode are kneaded using an active material, a conductive agent, a binder, and a high boiling point solvent to form an electrode mixture, and then the mixture is applied to a copper foil of a current collector, dried, and press-molded, then about 50 ° C to 250 ° C. It may be produced by heat treatment under vacuum at a temperature of about 2 hours.

The thickness of the electrode layer of the cathode may be 30 to 120 占 퐉 or 50 to 100 占 퐉, and the thickness of the electrode layer of the anode may be 1 to 100 占 퐉, or 3 to 70 占 퐉. When the cathode and the anode satisfy this thickness range, the amount of active material in the electrode material layer is sufficiently secured to prevent the battery capacity from decreasing and the cycle characteristics and rate characteristics can be improved.

In addition, the loading level of the electrode is an amount of active material per unit area of the electrode, which is a factor affecting the rate characteristic of the battery, and should be designed in consideration of the diffusion coefficient of lithium ions, the conduction between powders, and the path to the current collector. do.

According to one aspect of the invention, the loading level of the anode is at least 280 mg / 25 cm 2, more preferably 285 to 340 mg / 25 cm 2, even more preferably 290 to 320 mg / 25 cm 2.

When the loading level of the anode is 280 mg / 25 cm 2 or more, the stability of the battery can be improved, resulting in cost reduction in battery manufacturing.

On the other hand, the loading level of the cathode is at least 570 mg / 25 cm 2, more preferably 580 to 640 mg / 25 cm 2, even more preferably 590 to 630 mg / 25 cm 2.

When the loading level of the cathode is 570 mg / 25 cm 2 or more, cost reduction in battery manufacturing can be achieved.

In addition, the separator may be a conventional porous polymer film conventionally used as a separator, such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer and ethylene / methacrylate copolymer. The porous polymer film made of the polyolefin-based polymer may be used alone or by laminating them, or a conventional porous nonwoven fabric, for example, a non-woven fabric made of high melting glass fiber, polyethylene terephthalate fiber, or the like may be used. It doesn't happen.

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 1

(Production of Nonaqueous Electrolyte)

A solution was prepared by mixing 13% by volume fluoroethylene carbonate (FEC), 12% by volume ethylene carbonate (EC), and 75% by volume methyl propionate and dissolving LiPF ₆ to a 1.2M concentration. Thereafter, 3 parts by weight of propylene sulfite (PRS), 2 parts by weight of propane sulfone (PS), and 1 part by weight of succinonitrile (SN) were further added to the obtained solution to prepare a nonaqueous electrolyte. .

(Manufacture of Cylindrical Lithium Secondary Battery)

The cathode was prepared by using LiCoO ₂ as a cathode active material to have a loading level of 590 mg / 25 cm ₂ , and the anode was prepared by using a mixture of natural graphite and artificial graphite as the anode active material to have a loading level of 290 mg / 25 cm 2. After manufacturing, a cylindrical lithium secondary battery was manufactured by a conventional method of injecting the non-aqueous electrolyte prepared above.

Comparative Example 1

(Production of Nonaqueous Electrolyte)

A solution was prepared by mixing 30% by volume of fluoroethylene carbonate (FEC), 5% by volume of propylene carbonate (PC), and 65% by volume of methyl propionate and dissolving LiPF ₆ to a concentration of 1.1 M. Thereafter, 3 parts by weight of propylene sulfite (PRS), 2 parts by weight of propane sulfone (PS), and 1 part by weight of succinonitrile (SN) were further added to the obtained solution to prepare a nonaqueous electrolyte. .

(Manufacture of Cylindrical Lithium Secondary Battery)

The cathode was prepared by using LiCoO ₂ as a cathode active material to have a loading level of 590 mg / 25 cm ₂ , and the anode was prepared by using a mixture of natural graphite and artificial graphite as the anode active material to have a loading level of 290 mg / 25 cm 2. After manufacturing, a cylindrical lithium secondary battery was manufactured by a conventional method of injecting the non-aqueous electrolyte prepared above.

Comparative Example 2

A cylindrical lithium secondary battery was prepared in the same manner as in Comparative Example 1 except that ethyl propionate was used instead of methyl propionate in the nonaqueous electrolyte.

Comparative Example 3

A cylindrical lithium secondary battery was prepared in the same manner as in Comparative Example 1 except that propyl propionate was used instead of methyl propionate in the nonaqueous electrolyte.

Comparative Example 4

A cylindrical lithium secondary battery was prepared in the same manner as in Comparative Example 1 except that dimethyl carbonate was used instead of methyl propionate in the nonaqueous electrolyte.

Characterization of Lithium Secondary Battery

Ionic Conductivity, Viscosity, and Specific Gravity Characteristics of Nonaqueous Electrolyte

The ionic conductivity according to the temperature change of the nonaqueous electrolyte prepared in Example 1 and Comparative Example 1 was measured under conditions of 5 ° C., 20 ° C. and 40 ° C. using an ion conductivity meter device, and the results are shown in FIG. 1.

In addition, the measured values of the ionic conductivity, viscosity, and specific gravity of the prepared nonaqueous electrolyte solutions of Example 1 and Comparative Example 1 are summarized in Table 1 below.

Electrolyte Ion Conductivity (ms / cm) Viscosity (cP) importance 5 ℃ 20 ℃ 40 ℃ 25 ℃ 25 ℃ Example 1 13.3 14.3 14.8 2.18 1.18 Comparative Example 1 11.0 13.9 14.7 2.33 1.22

Low temperature life characteristics

The cylindrical lithium secondary battery (battery capacity 2,800 mAh) prepared in Example 1 and Comparative Example 1 was charged at 5 ° C. with a constant current of 1.0 C until it became 4.3 V, and then charged at a constant voltage of 4.3 V to charge current of 50 mA. Charging was terminated when Thereafter, it was left for 20 minutes and then discharged until it reached 3.0V at a constant current of 0.8C. After 50 cycles of charging and discharging, the discharge capacity of the battery was measured and shown in FIG. 2. Here, C represents the charge / discharge current rate and C-rate of the battery represented by ampere (A) and is usually expressed as a ratio to the battery capacity. That is, 1C of the batteries manufactured previously means a current of 2.8A.

High temperature life characteristics

The cylindrical lithium secondary battery (battery capacity 2,600 mAh) prepared in Examples 1 and Comparative Examples 2 to 4 was charged at 55 ° C. at a constant current of 0.8 C until it became 4.3 V, and then charged at a constant voltage of 4.3 V to charge current. Charging was completed when was 50 mA. After standing for 20 minutes, the battery was discharged until it became 3.0V with a constant current of 0.5C. After 400 cycles of charging and discharging, the discharge capacity of the battery was measured and shown in FIG. 3. Here, C represents the charge / discharge current rate and C-rate of the battery represented by ampere (A) and is usually expressed as a ratio to the battery capacity. That is, 1C of the batteries manufactured above means a current of 2.6A.

Interfacial resistance characteristics before and after high temperature storage

The impedance of the secondary battery before and after storing the cylindrical lithium secondary batteries prepared in Example 1 and Comparative Example 1 in a 75 ° C. oven apparatus for 43 hours at high temperature using a Potentiostat / Galvanostat measuring device was 0.1 to 10,000 Hz. , Amplitude was measured under the condition of 10mV, and the results are shown in FIGS. 4 and 5, respectively.

1 and Table 1, the non-aqueous electrolyte of Example 1 is significantly improved ionic conductivity compared to the non-aqueous electrolyte of Comparative Example 1, the viscosity is lowered, the mobility of the electrolyte is improved, the specific gravity of the electrolyte is reduced It was confirmed that the amount of pouring could be reduced.

Referring to FIG. 2, it was found that the cylindrical lithium secondary battery of Example 1 was remarkably improved in life characteristics at low temperatures. Referring to FIG. 3, the nonaqueous electrolyte of Example 1 contained methyl propionate. In addition, it was confirmed that the cycle sustainability at high temperature cycles was higher than that of ethyl propione, linear ester of propyl propionate or linear carbonate such as dimethyl carbonate.

4 and 5, the cylindrical lithium secondary battery of Example 1 was confirmed that the interface resistance was reduced compared to the secondary battery of Comparative Example 1 before and after the storage for a long time at high temperature.

Claims (8)

A cylindrical lithium secondary battery comprising an electrode assembly comprising an anode, a cathode, and a separator interposed between the cathode and the anode, and a nonaqueous electrolyte containing an electrolyte salt and an organic solvent.
The organic solvent comprises 70 to 80 volume percent methyl propionate, 5 to 15 volume percent fluoroethylene carbonate, and 7 to 15 volume percent ethylene carbonate,
The anode has a cylindrical lithium secondary battery having a loading level of 280 mg / 25 cm 2 or more. The method of claim 1,
Cylindrical lithium secondary battery, characterized in that the electrolyte salt comprises 1.15 to 1.6M LiPF ₆ . The method of claim 1,
The non-aqueous electrolyte further comprises any one selected from the group consisting of cyclic sulfites, acyclic sulfones, nitrile group-containing compounds, saturated sultones, unsaturated sultones, vinylene carbonates, and vinylethylene carbonates or mixtures of two or more thereof. Cylindrical lithium secondary battery, characterized in that. The method of claim 1,
A cylindrical lithium secondary battery, wherein the anode comprises an anode active material layer comprising a lithium metal, a carbon material, a metal compound and a mixture thereof. 5. The method of claim 4,
From the group consisting of Si, Ge, Sn, Pb, P, Sb, Bi, Al, Ga, In, Ti, Mn, Fe, Co, Ni, Cu, Zn, Ag, Mg, Sr, and Ba A cylindrical lithium secondary battery, characterized in that the compound containing one or more selected metal elements or a mixture thereof. The method of claim 1,
Cylindrical lithium secondary battery, characterized in that the cathode comprises a cathode layer containing a lithium-containing oxide. The method according to claim 6,
Cylindrical lithium secondary battery, characterized in that the lithium-containing oxide is a lithium-containing transition metal oxide. The method of claim 7, wherein
The lithium-containing transition metal oxide is LiCoO ₂ , LiNiO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , Li (Ni _a Co _b Mn _c ) O ₂ (0 <a <1, 0 <b <1, 0 <c <1 , a + b + c = 1), LiNi _1-y Co _y O ₂ , LiCo _1-y Mn _y O ₂ , LiNi _1-y Mn _y O ₂ (O ≦ y <1), Li (Ni _a Co _{b Mn c) O 4 (0 <} a <2, 0 <b <2, 0 <c <2, a + b + c = 2), LiMn 2 - z Ni z O 4, LiMn 2-z Co z O 4 Cylindrical lithium secondary battery, characterized in that any one or a mixture of two or more selected from the group consisting of (0 <z <2), LiCoPO ₄ and LiFePO ₄ .

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