KR101573380B1 - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery. More specifically, the lithium secondary battery according to the present invention comprises a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte solution, wherein the nonaqueous electrolyte solution contains (a) 1 to 10 parts by weight of a vinyl group-containing compound; And (b) from 0.1 to less than 10 parts by weight of a nitrile compound having an alkoxyalkyl group having from 2 to 12 carbon atoms, wherein the separation membrane comprises a porous substrate; And a coating layer comprising a mixture of inorganic particles and a binder polymer coated on at least one side of the porous substrate.
The lithium secondary battery of the present invention greatly suppresses battery swelling at high temperature storage, has high safety at high temperature, and has a reduced internal resistance of the battery, thereby exhibiting excellent electrochemical characteristics.

Description

LITHIUM SECONDARY BATTERY [0002]

The present invention relates to a lithium secondary battery which is excellent in safety by using a separator having a two-component additive and an inorganic coating layer in combination and suppresses swelling at high temperatures.

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 currently applied secondary batteries, the lithium secondary batteries developed in the early 1990s include a cathode made of a carbon material capable of absorbing and desorbing lithium ions, a cathode made of a lithium-containing oxide, and a mixed organic solvent, And a non-aqueous electrolytic solution.

Organic solvents widely used in non-aqueous electrolytic solutions at present include ethylene carbonate, propylene carbonate, dimethoxyethane, gamma-butyrolactone, N, N-dimethylformamide, tetrahydrofuran or acetonitrile. However, such an organic solvent generally has a problem in that when the battery is stored at a high temperature for a long time, the stable structure of the battery is deformed due to the generation of gas due to the oxidation of the electrolyte, or the battery is ignited or exploded due to internal short- . ≪ / RTI >

In order to solve these problems in recent years, it has been recently proposed to use (1) a porous polyolefin separator having a melting point of melting which does not easily dissolve in a high temperature environment, (2) a flame retardant solvent or an additive to the electrolyte, Attempts are being made.

However, in the case of a polyolefin-based separator, in order to realize the melting point of the polyolefin-based separator, the thickness of the film must be increased, which leads to a reduction in the capacity of the battery by lowering the loading amount of the anode and cathode. In addition, due to the characteristics of polyolefin film composed of PE and PP, since the melting point is before and after 150 ° C., if a rapid internal heat generated by the oxidation of the electrolyte occurs during overcharging, extreme heat shrinkage occurs and battery ignition and explosion Is difficult to avoid.

In order to solve the problems of the polyolefin-based membrane, a separator using an inorganic material has been introduced. U.S. Patent No. 6,432,586 discloses a composite membrane in which calcium carbonate, silica, and the like are coated on a polyolefin-based membrane. Korean Unexamined Patent Publication No. 2006-0072065 discloses a polyolefin-based separator using an inorganic or organic composite material having inorganic particles having piezoelectricity due to high dielectric constant and / or inorganic particles having lithium ion-transferring ability as main components, Discloses a separator. These organic and inorganic composite membranes are effective for the mechanical stability of conventional polyolefin separators and for preventing high temperature heat shrinkage.

However, in the composite separator, trace moisture and organic substances generated during the production process remain in the separator. They cause generation of gas during high-temperature storage of the battery, and the storage characteristics of the separator are somewhat weak compared with general polyolefin separators.

Accordingly, a first object of the present invention is to provide a lithium secondary battery having a separator having excellent mechanical stability and high-temperature heat shrinkage prevention performance.

A second problem to be solved by the present invention is to provide a lithium secondary battery having improved safety by preventing swelling of the battery during high-temperature storage.

A third problem to be solved by the present invention is to provide a lithium secondary battery having excellent battery chemical characteristics.

In order to solve the above problems, a lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a nonaqueous electrolyte solution according to the present invention is characterized in that the nonaqueous electrolyte solution contains (a) 1 to 10 parts by weight of a compound containing a phenol group or a vinyl group; And (b) from 0.1 to less than 10 parts by weight of a nitrile compound having an alkoxyalkyl group having from 2 to 12 carbon atoms, wherein the separation membrane comprises a porous substrate; And a coating layer comprising a mixture of inorganic particles and a binder polymer coated on at least one side of the porous substrate.

In the present invention, "vinylene group" is defined as -CH = CH- and "vinyl group" is defined as CH ₂ = CH-.

In the present invention, the vinylene or vinyl group-containing compound may be a vinylene carbonate-based compound, an acrylate-based compound containing a vinyl group, a sulfonate-based compound containing a vinyl group, an ethylene carbonate- They may be used alone or in combination of two or more. More specifically, typical examples of the vinylene carbonate-based compound include vinylene carbonate, and the vinyl-containing compound may be represented by the following Formula 1:

In Formula 1,

R ¹ to R ⁴ at least one of which contains a vinyl group, and the remains are each independently hydrogen, halogen, optionally substituted by halogen or unsubstituted C ₁ ~ C ₆ alkyl group, C ₆ ~ C ₁₂ aryl group, C ₂ ~ of C ₆ alkenyl group, or sulfonate group.

In the present invention, the nitrile compound having an alkoxyalkyl group may be represented by the following general formula (2).

In Formula 2,

n is an integer of 1 to 6, and m is an integer of 1 to 6.

In the present invention, the porous substrate of the separator may be a polyolefin porous substrate; Or a group consisting of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfite, polyethylene naphthalene , Or a mixture of two or more thereof.

In the present invention, the inorganic particles may be selected from the group consisting of inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion-transporting ability, and mixtures thereof.

INDUSTRIAL APPLICABILITY The lithium secondary battery of the present invention has high safety even in an internal short circuit using an organic / inorganic composite separator having excellent mechanical safety and high temperature heat shrinkage prevention performance.

Furthermore, the phenomenon of swelling at a high temperature due to a slight amount of moisture present in the composite separator can be effectively suppressed by using a nonaqueous electrolytic solution containing a compound containing a vinylene group or a vinyl group and an alkoxyalkylnitrile compound.

In addition, the nonaqueous electrolyte solution containing the two compounds can suppress the heat generation and prevent internal short-circuit due to the rapid temperature rise of the battery, and the electrochemical characteristics such as low viscosity and no decrease in conductivity, great.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate exemplary embodiments of the invention and, together with the description of the invention, It should not be construed as limited.
BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a graph illustrating impedance of a secondary battery manufactured according to Example 2 and Comparative Example 2. FIG.

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.

As described above, the polyolefin-based separator exhibits extreme heat shrinkage behavior when the internal temperature of the battery rises during overcharging, thereby causing ignition and explosion of the battery due to an internal short circuit.

Accordingly, the lithium secondary battery of the present invention comprises a porous substrate; And a coating layer comprising a mixture of inorganic particles coated on at least one surface of the porous substrate and a binder polymer. The separation membrane according to the present invention greatly improves the thermal and mechanical stability due to the inorganic particle / binder coating layer. Especially, high temperature heat shrinkage can be suppressed due to heat resistance of inorganic particles.

However, the inventors of the present invention have recently found that the use of the lithium secondary battery is still unsatisfactory in order to secure sufficient safety with the separator according to the present invention under harsh environmental conditions. Particularly, at the time of high-temperature storage, the organic and inorganic composite separator of the present invention is excessively swollen by the fine water content contained therein, thereby posing a problem of safety of the battery. Accordingly, the inventors of the present invention have further improved the safety of a lithium secondary battery, in particular, the safety at the time of high temperature storage, by using a two-component additive in the nonaqueous electrolyte solution.

The nonaqueous electrolyte solution according to the present invention comprises an ionizable lithium salt and an organic solvent, wherein as an additive, (a) 1 to 10 parts by weight of a compound containing a vinylene group or a vinyl group per 100 parts by weight of the nonaqueous electrolyte; And (b) from 0.1 to less than 10 parts by weight of a nitrile compound having an alkoxyalkyl group having from 2 to 12 carbon atoms.

Since the vinylene or vinyl group-containing compound used in the present invention can form a passivation film called SEI film on the surface of the negative electrode when the battery is charged, the micro- or macro thermal short occurring in the battery can be delayed, It is possible to prevent or delay the ignition of the battery by heat inside or outside the battery, compared with ethylene carbonate which is a widely used organic solvent.

However, when a compound containing a vinylene group or a vinyl group according to the present invention is used in a carbonate electrolyte, it has a disadvantage in that it is very fragile thermally and is easily decomposed at a high temperature. The decomposition gas generated at this time causes the pouch- Short circuit may occur and the battery may ignite or explode in severe cases.

When only the above-mentioned compound containing a vinylene group or a vinyl group is used, safety can not be sufficiently secured in the event of an internal short circuit of the battery. Therefore, the present invention solves the problem by further mixing the alkoxyalkylnitrile compound. When an alkoxyalkylnitrile compound is used in combination, a complex is formed on the surface of a positive electrode composed of a lithium-transition metal oxide, thereby suppressing the oxidation reaction between the electrolyte and the positive electrode to suppress heat generation and prevent internal short- . In addition, at the time of high-temperature storage, it has the effect of suppressing decomposition of the above-mentioned compounds containing vinylene group or vinyl group and electrolytic solution to suppress cell swelling phenomenon.

Further, when alkoxyalkylnitrile compounds are used, other properties of the battery are also improved. The alkoxyalkylnitrile has higher permittivity and higher viscosity than the conventional linear carbonate and aliphatic alkylmononitrile and can increase the conductivity of the electrolyte and improve the interfacial resistance and initial capacity of the assembled battery. .

Therefore, the lithium secondary battery of the present invention significantly improves the safety of a battery, particularly, the safety at the time of high-temperature storage, by using physical protection means by a separator having a specific structure according to the present invention and chemical protection means using the two- , The electrochemical characteristics of the battery can be simultaneously improved.

Specific examples of the vinylene group or vinyl group-containing compound according to the present invention include vinylene carbonate-based compounds, acrylate-based compounds containing vinyl groups, sulfonate-based compounds containing vinyl groups, ethylene carbonate- And the like.

More specifically, the vinylene carbonate-based compound may be vinylene carbonate, and the vinyl-containing compound may be a compound represented by the following formula (1): < EMI ID =

[Chemical Formula 1]

In Formula 1,

R ¹ to R ⁴ at least one of which contains a vinyl group, and the remains are each independently hydrogen, halogen, optionally substituted by halogen or unsubstituted C ₁ ~ C ₆ alkyl group, C ₆ ~ C ₁₂ aryl group, C ₂ ~ of C ₆ alkenyl group, or sulfonate group.

The vinylene or vinyl group-containing compound according to the present invention is used in an amount of 1 part by weight to 10 parts by weight, preferably 1 part by weight to 8 parts by weight, more preferably 1 part by weight And 7 parts by weight or less. If the content is less than 1 part by weight, it is difficult to form a dense SEI film, and the charge / discharge performance of the battery may be deteriorated. If the content is more than 10 parts by weight, the dense SEI film becomes excessively thick, Can be degraded.

 In the present invention, the nitrile compound having an alkoxyalkyl group preferably has a total carbon number of 2 to 12 in the alkoxyalkyl group. For example, the nitrile compound having an alkoxyalkyl group according to the present invention may be represented by the following formula (2)

(2)

In Formula 2,

n is an integer of 1 to 6, and m is an integer of 1 to 6.

More specific examples of the alkoxyalkylnitrile compound according to the present invention include methoxyacetonitrile, methoxypropionitrile, methoxybutyronitrile, methoxyvaleronitrile, ethoxyacetonitrile, ethoxypropionitrile, and Ethoxy valeronitrile, etc. may be used alone or in admixture of two or more, but the present invention is not limited thereto.

The alkoxyalkylnitrile compound according to the present invention is used in an amount of 0.1 to 10 parts by weight, preferably 0.1 to 7 parts by weight, more preferably 1 to 7 parts by weight, relative to 100 parts by weight of the non- ≪ / RTI > If the content is less than 0.1 part by weight, the effect of suppressing cell swelling is insignificant. If the content is more than 10 parts by weight, the long-term charge-discharge life at high temperature is reduced.

In addition, the present invention may further include a compound capable of forming a passivation film on the surface of the anode in addition to the above-mentioned cyclic carbonate and vinyl group-containing compound substituted with halogen within the scope of the present invention, if necessary . Non-limiting examples of the compound include S-based compounds such as propanesultone, ethylene sulfite and 1,3-propane sultone, and lactam-based compounds such as N-acetyllactam.

The lithium salt contained as an electrolyte in a non-aqueous electrolyte solution of the present invention described above can be used without limitation, those which are commonly used in a lithium secondary battery electrolyte, such as the lithium salt, the anion is 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 -, (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 ^- .

Examples of the organic solvent included in the non-aqueous electrolyte of the present invention include propylene carbonate (PC), ethylene carbonate (EC), and ethylene carbonate (EC) (DEC), dimethyl carbonate (DMC), ethylmethyl carbonate (EMC), methylpropyl carbonate, dipropyl carbonate, dimethyl sulfoxide, acetonitrile, dimethoxyethane, diethoxyethane, vinyl And a mixture of two or more of them selected from the group consisting of propylene sulfide and tetrahydrofuran, and the like can be used as typical examples. In particular, ethylene carbonate and propylene carbonate, which are cyclic carbonates in the carbonate-based organic solvent, can be preferably used because they have high permittivity as a high viscosity organic solvent and dissociate the lithium salt in the electrolyte well. To such a cyclic carbonate, dimethyl carbonate and diethyl When a low viscosity, low dielectric constant linear carbonate such as carbonate is mixed in an appropriate ratio, an electrolyte having a high electric conductivity can be prepared, and thus it can be more preferably used.

Further, in the separation membrane according to the present invention, the porous substrate may be a polyolefin-based porous substrate; Or a group consisting of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyether sulfone, polyphenylene oxide, polyphenylene sulfite, polyethylene naphthalene , Or a mixture of two or more thereof. The polyolefin-based porous substrate may be any polyolefin-based porous substrate that is commonly used. More specifically, a film formed of a polymer in which a polyolefin-based polymer such as high density polyethylene, linear low density polyethylene, low density polyethylene, ultra high molecular weight polyethylene, polyethylene, polypropylene, polybutylene or polypentene, ) Or nonwoven fabric.

In the separation membrane according to the present invention, the inorganic particles serve as a kind of spacer that can maintain the physical form of the porous coating layer, thereby suppressing heat shrinkage of the porous substrate when the electrochemical device is overheated, . In addition, interstitial volumes exist between the inorganic particles to form micropores.

Such inorganic particles are not particularly limited as long as they are electrochemically stable. That is, the inorganic particles usable in the present invention are not particularly limited as long as oxidation and / or reduction reaction does not occur in the operating voltage range of the applied electrochemical device (for example, 0 to 5 V based on Li / Li ⁺ ). Particularly, when inorganic particles having a high dielectric constant are used as the inorganic particles, the dissociation of the electrolyte salt, for example, the lithium salt in the liquid electrolyte, can be increased, and the ion conductivity of the electrolyte can be improved.

For the reasons stated above, it is preferable that the inorganic particles include high-permittivity inorganic particles having a dielectric constant of 5 or more, preferably 10 or more. Non-limiting examples of the inorganic particles is less than a dielectric constant of 5 is _{BaTiO 3, Pb (Zr, Ti} ) O 3 (PZT), Pb 1 - x La x Zr 1 -y Ti y O 3 (PLZT, 0 <x <1 , 0 <y <1), Pb (Mg 1/3 Nb 2/3) O 3 -PbTiO 3 (PMN-PT), hafnia _{(HfO 2), SrTiO 3,} SnO 2, CeO 2, MgO, NiO, and the like _{CaO, ZnO, ZrO 2, SiO} 2, Y 2 O 3, Al 2 O 3, SiC , and TiO ₂ or mixtures thereof.

As the inorganic particles, inorganic particles having a lithium ion transferring ability, that is, inorganic particles containing a lithium element but having a function of transferring lithium ions without storing lithium can be used. Non-limiting examples of inorganic particles having lithium ion transferring ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y < (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), 14Li ₂ O-9Al ₂ O ₃ -38TiO ₂ -39P ₂ O ₅ (0 <x <4, 0 <y <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), Li (LiAlTiP) _x O _y series glass _3.25 Ge _0.25 P _0.75 S ₄ lithium germanium thiophosphate _{(Li x Ge y P z S} w, 0 <x <4, 0 <y <1, 0 <z <1, 0 <w <5) such as, Li ₃ (Li _x N _y , 0 <x <4, 0 <y <2), Li ₃ PO ₄ -Li ₂ S-SiS ₂ SiS ₂ family, such as _{glass (Li x Si y S z} , 0 <x <3, 0 <y <2, 0 <z <4), LiI-Li 2 SP 2 S 5 There is P ₂ S ₅ based _{glass (Li x P y S z} , 0 <x <3, 0 <y <3, 0 <z <7) or a mixture thereof as such.

The binder polymer is preferably a polymer having a glass transition temperature (T _g ) of -200 to 200 ° C because it can improve the mechanical properties such as flexibility and elasticity of the coating layer finally formed .

In addition, the binder polymer does not necessarily have ion conductivity, but the performance of the electrochemical device can be further improved by using a polymer having ion conductivity. Therefore, it is preferable that the binder polymer has a high permittivity constant. In fact, since the dissociation degree of the salt in the electrolyte depends on the permittivity constant of the electrolyte solvent, the higher the permittivity constant of the binder polymer, the better the salt dissociation in the electrolyte. The dielectric constant of such a binder polymer may be in the range of 1.0 to 100 (measurement frequency = 1 kHz), preferably 10 or more.

In addition to the functions described above, the binder polymer may be characterized by being capable of exhibiting a high degree of swelling of the electrolyte due to gelation upon impregnation with a liquid electrolyte. Accordingly, it is preferable to use a solubility parameter of 15 to 45 MPa ^1/2 of a polymer, and the more preferred solubility parameter of 15 to 25 MPa ^1/2, and 30 to 45 MPa ^1/2 range. Therefore, it is preferable to use hydrophilic polymers having many polar groups, rather than hydrophobic polymers such as polyolefins. If the solubility is more than 15 MPa ^1/2 and less than 45 MPa ^1/2, it is difficult to be impregnated with (swelling) by conventional liquid electrolyte batteries.

Non-limiting examples of such a binder polymer include polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, But are not limited to, polymethylmethacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyvinyl alchol, ethylene vinyl acetate copolymer polyethylene-co-vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, Cyanoethylpullulan, Cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan, carboxyl methyl cellulose, and the like can be given. .

The weight ratio of the inorganic particles to the binder polymer is preferably in the range of, for example, 50:50 to 99: 1, more preferably 70:30 to 95: 5. When the content ratio of the inorganic particles to the binder polymer is less than 50:50, the content of the polymer is increased and the pore size and porosity of the formed coating layer can be reduced. If the content of the inorganic particles exceeds 99 parts by weight, the fillerability of the formed coating layer may be weakened because the content of the binder polymer is small.

As the solvent of the binder polymer, it is preferable that the solubility index is similar to that of the binder polymer to be used, and the boiling point is low. This is to facilitate uniform mixing and subsequent solvent removal. Non-limiting examples of usable solvents include acetone, tetrahydrofuran, methylene chloride, chloroform, dimethylformamide, N-methyl-2-pyrrolidone ( N-methyl-2-pyrrolidone, NMP), cyclohexane, water or a mixture thereof.

The separation membrane of the present invention can be obtained by dissolving the binder polymer in the solvent, dispersing the inorganic particles, applying the dispersion to the porous substrate by a conventional method in the art, and drying the dispersion. The pore size and porosity of the separator of the present invention are preferably 0.001 to 10 탆 and 5 to 95%, respectively. The thickness of the separator of the present invention is not particularly limited and may be adjusted in consideration of battery performance, and is preferably 1 to 100 μm, more preferably 1 to 30 μm.

The nonaqueous electrolyte solution for a lithium secondary battery of the present invention described above is manufactured as a lithium secondary battery by injecting the positive electrode, the negative electrode, and the electrode structure composed of the separator of the present invention interposed between the positive electrode and the negative electrode. The positive electrode and the negative electrode constituting the electrode structure may be all those conventionally used in the production of a lithium secondary battery.

As a specific example, lithium-containing transition metal oxides can be preferably used as the cathode active material. Examples thereof include Li _x CoO ₂ (0.5 <x <1.3), Li _x NiO ₂ (0.5 <x <1.3), Li _x MnO ₂ 0.5 <x <1.3, 0 <a <1, 0 <b <0.5), Li _x Mn ₂ O ₄ (0.5 <x <1.3), Li _x (Ni _a Co _b Mn _c ) O ₂ 1, 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 (0.5 <x < 1.3, 0 <z <2), Li _x Mn _{2 -z} Co _z O ₄ (0.5 <x <1.3, 0 <z <2), Li _x CoPO ₄ (0.5 <x <1.3) and Li _x FePO ₄ 0.5 <x <1.3), or a mixture of two or more thereof. The lithium-containing transition metal oxide may be coated with a metal such as aluminum (Al) or a metal oxide. In addition to the lithium-containing transition metal oxide, sulfide, selenide and halide may also be used.

As the anode active material, a carbon material, lithium metal, silicon or tin, which lithium ions can be occluded and released, can be used, and metal oxides such as TiO ₂ and SnO ₂ having a potential with respect to lithium of less than ₂ V are also possible. Preferably, carbon materials can be used, and carbon materials such as low-crystalline carbon and highly-crystalline carbon can be used. 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. 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 may be used.

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  One

<Preparation of separation membrane>

Polvinylidene fluoride-chlorotrifluoroethylene copolymer (PVdF-CTFE) was added to acetone in an amount of about 5% by weight and dissolved at 50 ° C for about 12 hours to prepare a polymer solution. The BaTiO ₃ powder was added to the polymer solution so as to have a BaTiO ₃ / PVdF-CTFE = 90/10 (weight% ratio), and the BaTiO ₃ powder was broken through ball milling for 12 hours or longer to prepare a slurry. The BaTiO ₃ particle size of the slurry can be controlled according to the bead size (particle size) used in the ball mill method or the process time, but in the first embodiment, the slurry was pulverized to about 400 nm. The slurry thus prepared was coated on a polyethylene separator (porosity of 45%) having a thickness of about 18 탆 by a dip coating method and the coating thickness was adjusted to about 3 mm. As a result of porosimeter measurement, the pore size and porosity were 0.5 mm and 58%, respectively.

&Lt; Preparation of electrolytic solution &

1M LiPF ₆ solution having a composition of EC: PC: DEC = 3: 2: 5 (weight ratio) was used as an electrolytic solution and 3 parts by weight of vinylene carbonate (VC) and 3 parts by weight of methoxypropionitrile By weight.

&Lt; Preparation of lithium secondary battery &

The cathode and the anode were formed on both sides of the separator using the artificial graphite as the anode active material and LiCoO ₂ as the cathode active material. Thereafter, a lithium polymer battery was manufactured by a conventional method, and the prepared electrolyte was injected into a battery assembled with an aluminum laminate packing material.

Example  2

A lithium polymer battery was produced in the same manner as in Example 1 except that the content of methoxypropionitrile was 5 parts by weight.

Example  3

A lithium polymer battery was produced in the same manner as in Example 1, except that the content of methoxypropionitrile was 7 parts by weight.

Comparative Example  One

A lithium polymer battery was produced in the same manner as in Example 1, except that a polyethylene (PE) separator was used as the separator.

Comparative Example  2

A lithium polymer battery was produced in the same manner as in Example 1, except that methoxypropionitrile was not added to the electrolytic solution.

Comparative Example  3

A lithium polymer battery was produced in the same manner as in Comparative Example 1, except that methoxypropionitrile was not added to the electrolytic solution.

Comparative Example  4

A lithium polymer battery was produced in the same manner as in Example 1, except that 10 parts by weight of methoxypropionitrile was added to the electrolytic solution.

Test Example  1: Safety evaluation of battery

In order to evaluate the safety of the secondary batteries manufactured according to the above-described Examples and Comparative Examples, the following tests were conducted.

<Safety test for internal short circuit>

A local crush test was carried out to evaluate the safety of the batteries manufactured in Example 1 and Comparative Example 1 in the event of an internal short circuit.

In the local crush test, a coin with a diameter of 1 cm was placed on a battery and compressed at a constant speed, so that an anode and a cathode were artificially adhered to each other to generate an internal short circuit, and then the battery was observed for explosion. Are shown in Table 1 below.

Membrane Whether ignited Example 1 Composite membrane No ignition Comparative Example 1 PE Ignition

As shown in Table 1, in Comparative Example 1 using the conventional polyolefin separator, when the separator ruptures due to an external impact, the inside of the battery is short-circuited to cause an explosion. In Example 1, however, Thereby exhibiting excellent safety.

<High Temperature Storage Safety Test>

The initial thickness and the thickness after storage were measured while storing the cells prepared in Examples 1 and 2 and Comparative Examples 2 and 3 at 90 ° C for 4 hours under the condition of full charge of 4.2 V. The results are shown in Table 2 Respectively. The change in thickness (? T) was expressed as a relative value with the increase in thickness of Comparative Example 2 being 100%.

Example Membrane Presence or absence of nitrile ? T (%) Example 1 Composite membrane ○ 69 Example 2 Composite membrane ○ 15 Comparative Example 2 Composite membrane X 100 Comparative Example 3 PE X 88

As shown in Table 2 above, it can be seen that the comparison of Comparative Example 2 and Comparative Example 3 shows that the thickness increases more when the composite membrane is used than the PE membrane.

However, when the nonaqueous electrolyte solution according to the present invention was used in combination with the composite membrane (Example 1 and Example 2), it was confirmed that the thickness increase (swelling phenomenon) was further remarkably reduced at a higher temperature than that of Comparative Example 3 using the PE membrane have.

Test Example  2: Evaluation of electrochemical performance of cell

<Impedance Measurement of Battery>

The cells of Example 2 and Comparative Example 2 were measured for impedance using Potentistat and the results are shown in Fig. Impedance measurements were performed by applying a small voltage (10 mV) from a frequency of 3 kHz to 100 mHz after charging the battery to 4.2 V and measuring the current response accordingly.

As shown in Fig. 1, it can be seen that the battery of Example 2 has a lower impedance than the battery of Comparative Example 2. From this, it can be confirmed that the electrolyte containing the alkoxyalkylnitrile compound of the present invention has reduced battery resistance through improvement in viscosity and conductivity.

< Charging and discharging  Cycle test>

Each battery was charged at a constant current of 1 C = 980 mA in a temperature environment of 45 ° C. After the voltage of the battery reached 4.2 V, the first charge was performed until the charge current value reached 50 mA at a constant voltage of 4.2 V Respectively. The battery which had been charged for the first time was discharged at a constant current of 1 C until the battery voltage reached 3 V to obtain the discharge capacity at the first cycle. Subsequently, charging and discharging were repeated 350 times at 45 캜 to obtain a high-temperature cycle test result as a discharge capacity retention ratio (%) according to the following formula (1).

Charge-discharging cycle characteristics of Example 2, Example 3 and Comparative Example 4 are summarized in Table 3.

Membrane Nitride content (wt%) Capacity retention rate (%) Example 2 Composite membrane 5 87.1 Example 3 Composite membrane 7 82.3 Comparative Example 4 Composite membrane 10 67.4

As shown in Table 3, it can be seen that the capacity retention rate of Comparative Example 4 in which methoxypropionitrile was added in an excessive amount (10% by weight) was drastically lowered than that of Example 2 and Example 3. [

Claims (12)

A lithium secondary battery comprising a positive electrode, a negative electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte,
The nonaqueous electrolyte solution contains, relative to 100 parts by weight of the nonaqueous electrolyte solution,
(a) 1 to 10 parts by weight of a compound containing a vinylene group or a vinyl group; And
(b) from 0.1 to less than 10 parts by weight of a nitrile compound having an alkoxyalkyl group having from 2 to 12 carbon atoms,
The separation membrane comprises a porous substrate; And a coating layer comprising a mixture of inorganic particles and a binder polymer coated on at least one side of the porous substrate. The method according to claim 1,
The vinylene or vinyl group-containing compound may be any one selected from the group consisting of a vinylene carbonate-based compound, an acrylate-based compound containing a vinyl group, a sulfonate-based compound containing a vinyl group, and an ethylene carbonate- Or a mixture of two or more thereof. The method according to claim 1,
Wherein the vinyl group-containing compound is a compound represented by the following formula (1): &lt; EMI ID =
[Chemical Formula 1]

In Formula 1,
R ¹ to R ⁴ at least one of which contains a vinyl group, and the remains are each independently hydrogen, halogen, optionally substituted by halogen or unsubstituted C ₁ ~ C ₆ alkyl group, C ₆ ~ C ₁₂ aryl group, C ₂ ~ of C ₆ alkenyl group, or sulfonate group. The method according to claim 1,
Wherein the nitrile compound having an alkoxyalkyl group is represented by the following formula (2): &lt; EMI ID =
(2)

In Formula 2,
n is an integer of 1 to 6, and m is an integer of 1 to 6. The method according to claim 1,
The nitrile compound having an alkoxyalkyl group may be at least one selected from the group consisting of methoxyacetonitrile, methoxypropionitrile, methoxybutyronitrile, methoxyvaleronitrile, ethoxyacetonitrile, ethoxypropionitrile, ethoxybutyronitrile and ethoxy Valeronitrile, and a mixture of two or more thereof. The method according to claim 1,
The porous substrate includes a polyolefin-based porous substrate; Or a group consisting of polyethylene terephthalate, polybutylene terephthalate, polyester, polyacetal, polyamide, polycarbonate, polyimide, polyether ether ketone, polyethersulfone, polyphenylene oxide, polyphenylene sulfite, polyethylene naphthalene , Or a mixture of two or more thereof. The method according to claim 1,
Wherein the inorganic particles are inorganic particles selected from the group consisting of inorganic particles having a dielectric constant of 5 or more, inorganic particles having lithium ion transporting ability, and mixtures thereof. 8. The method of claim 7,
The inorganic particles having a dielectric constant of 5 or more include BaTiO ₃ , Pb (Zr, Ti) O ₃ (PZT), Pb _{1 -x} La _x Zr _{1 -y} Ti _y O ₃ (PLZT, 0 <x <_{<1), Pb (Mg 1} /3 Nb 2/3) O 3 -PbTiO 3 (PMN-PT), hafnia _{(HfO 2), SrTiO 3,} SnO 2, CeO 2, MgO, NiO, CaO, ZnO, ZrO ₂ , SiO ₂ , Y ₂ O ₃ , Al ₂ O ₃ , SiC, and TiO ₂ , or a mixture of two or more thereof. 8. The method of claim 7,
The inorganic particles having lithium ion transferring ability include lithium phosphate (Li ₃ PO ₄ ), lithium titanium phosphate (Li _x Ti _y (PO ₄ ) ₃ , 0 <x <2, 0 <y <3), lithium aluminum titanium phosphate (Li _x Al _y Ti _z (PO ₄ ) ₃ , 0 <x <2, 0 <y <1, 0 <z <3), (LiAlTiP) _x O _y series glass <13), lithium lanthanum titanate (Li _x La _y TiO ₃ , 0 <x <2, 0 <y <3), lithium germanium thiophosphate (Li _x Ge _y P _z S _w , , 0 <y <1, 0 <z <1, 0 <w <5), lithium nitrides _{(Li x N y, 0 <} x <4, 0 <y <2), SiS 2 (Li x Si y S _{z, 0 <x <3,} 0 <y <2, 0 <z <4) based glass, and _{P 2 S 5 (Li x P} y S z, 0 <x <3, 0 <y <3, 0 <z &Lt; 7) series glass, or a mixture of two or more thereof. The method according to claim 1,
Wherein the binder polymer has a solubility index of 15 to 45 MPa & ^{lt ; 1/2 &} gt ;. The method according to claim 1,
Wherein the binder polymer has a dielectric constant of 1.0 to 100 (measured frequency = 1 kHz). The method according to claim 1,
The binder polymer may be at least one selected from the group consisting of polyvinylidene fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichlorethylene, polymethylmethacrylate, But are not limited to, polybutyl acrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyvinyl alchol, polyethylene-co- polyvinyl acetate, vinyl acetate, polyethylene oxide, polyarylate, cellulose acetate, cellulose acetate butyrate, cellulose acetate propionate, Cyanoethylpullulan, cyanoethylpolyvinyl, Or one or more selected from the group consisting of cyanoethylpolyvinylalcohol, cyanoethylcellulose, cyanoethylsucrose, pullulan and carboxyl methyl cellulose, Wherein the polymer is a binder polymer.

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