KR20170061018A - Electrolyte and lithium secondary battery with the same - Google Patents

Abstract

The present invention relates to an electrolyte and a lithium secondary battery comprising the electrolyte, wherein the electrolyte comprises a first electrolyte system including a cyclic carbonate-based solvent and LiPF ₆ , and a second electrolyte system including a chain carbonate solvent, lithium bis (fluorosulfonyl) imide And a second electrolyte system comprising lithium difluoro (bisoxalate) phosphate.
The electrolyte has improved lifetime and output characteristics, and does not lower the lithium ion transfer number (Li ⁺ transference number) relative to the commercial electrolyte.

Description

ELECTROLYTE AND LITHIUM SECONDARY BATTERY WITH THE SAME BACKGROUND OF THE INVENTION 1. Field of the Invention [0001]

The present invention relates to an electrolyte and a lithium secondary battery including the same, and more particularly, to an electrolyte having improved lifetime and output characteristics and having a low lithium ion transfer number (Li ⁺ transference number) relative to a commercial electrolyte and lithium The present invention relates to a secondary battery.

Lithium secondary batteries can be used not only as portable power sources such as mobile phones, notebook computers, digital cameras and camcorders but also as power tools, electric bicycles, hybrid electric vehicles (HEVs), plug-in hybrid electric vehicles HEV, PHEV), and the like. As the application field is expanded and demand is increased, the external shape and size of the battery are variously changed, and performance and stability are demanded more than the characteristics required in the conventional small batteries. In order to meet such a demand, the performance of the battery should be stabilized in a condition where battery components are flowing in a large current.

The lithium secondary battery is manufactured by using a material capable of inserting and desorbing lithium ions as a cathode and an anode, providing a porous separator between the two electrodes, and injecting a liquid electrolyte. The insertion and extraction of lithium ions in the cathode and the anode, Electricity is generated or consumed by the redox reaction due to desorption.

Various non-aqueous solvents and additives have been studied as electrolyte-containing components in order to improve battery characteristics such as output characteristics, cycle characteristics, and storage characteristics of lithium ion batteries. In addition, even when a specific compound is added to an electrolyte as an additive for improving battery performance, performance of some items of most battery performance can be expected to be improved, but the performance of other items is rather reduced.

An object of the present invention is to provide an electrolyte which has improved lifetime and output characteristics but does not lower the lithium ion transfer number (Li ⁺ transference number) relative to a commercial electrolyte.

Another object of the present invention is to provide a lithium secondary battery comprising the electrolyte.

In order to achieve the above object, an electrolyte according to an embodiment of the present invention includes a first electrolyte system including a cyclic carbonate-based solvent and LiPF ₆ , and a first electrolyte system including a chain carbonate solvent, lithium bis (fluorosulfonyl) And a second electrolyte system comprising lithium difluoro (bis oxalate) phosphate.

The second electrolyte system may be included in an amount of 5 to 50 parts by weight based on 100 parts by weight of the first electrolyte system.

The first electrolyte system may include 6 to 30% by weight of the LiPF ₆ based on the entire first electrolyte system.

The second electrolyte system may include 0.1 to 10% by weight of the lithium bis (fluorosulfonyl) imide based on the entire second electrolyte system.

The second electrolyte system may include the lithium difluoro (bis oxalate) phosphate in an amount of 0.1 to 10% by weight based on the entire second electrolyte system.

The electron density value of the first electrolyte system may be 2.32 to 2.64 [phi] * [Phi].

The upper limit value and the lower limit value of the electron density value of the second electrolyte system may be within ± 10% of the upper limit value and the lower limit value of the electron density value of the first electrolyte system, respectively.

The electron density value of the second electrolyte system may be 2.28 to 2.64 [Phi] * [Phi].

The electron density value of the electrolyte may be 2.36 to 2.79 Φ * Φ.

The cyclic carbonate-based solvent may include any one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, and mixtures thereof.

The chain carbonate-based solvent may include any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl ethyl carbonate, ethyl methyl carbonate, and mixtures thereof .

A lithium secondary battery according to another embodiment of the present invention includes a cathode including a cathode active material, a cathode disposed opposite to the cathode, a cathode including the anode active material, and an electrolyte according to the present invention.

The electrolyte according to the present invention has improved lifetime and output characteristics and does not lower the lithium ion transfer number (Li ⁺ transference number) relative to the commercial electrolyte.

1 is an exploded perspective view of a lithium secondary battery according to an embodiment of the present invention.

The present invention is capable of various modifications and various embodiments and is intended to illustrate and describe the specific embodiments in detail. It is to be understood, however, that the invention is not to be limited to the specific embodiments, but includes all modifications, equivalents, and alternatives falling within the spirit and scope of the invention.

The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention. The singular expressions include plural expressions unless the context clearly dictates otherwise. In the present invention, terms such as comprise, having, or the like are intended to designate the presence of stated features, integers, steps, operations, elements, parts or combinations thereof, and may include one or more other features, , But do not preclude the presence or addition of one or more other features, elements, components, components, or combinations thereof.

The electrolyte according to an embodiment of the present invention includes a first electrolyte system including a cyclic carbonate-based solvent and LiPF ₆ , and a second electrolyte system including a chain carbonate solvent, lithium bis (fluorosulfonyl) imide, and lithium difluoro Oxalate < / RTI > phosphate).

In the first electrolyte system, the cyclic carbonate-based solvent includes any one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, fluoroethylene carbonate (FEC) And may preferably contain ethylene carbonate which is superior in terms of the characteristics of the cell.

In the first electrolyte system, the LiPF ₆ is dissolved in the carbonate-based solvent to function as a source of lithium ions in the lithium secondary battery and accelerate the movement of lithium ions between the positive electrode and the negative electrode. The LiPF ₆ may be contained in an amount of 6 to 30% by weight, preferably 8 to 20% by weight based on the total weight of the first electrolyte system. When the content of LiPF ₆ is less than 6% by weight, the conductivity of the electrolyte may be lowered to deteriorate the performance of the electrolyte. When the content of LiPF ₆ is more than 30% by weight, the viscosity of the electrolyte may increase and the mobility of lithium ions may be lowered.

The second electrolyte system may include at least one of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, methyl propyl carbonate, ethyl propyl carbonate, methyl ethyl carbonate, ethyl methyl carbonate (EMC) And mixtures thereof.

In the second electrolyte system, the lithium bis (fluorosulfonyl) imide may be contained in an amount of 0.1 to 10% by weight, preferably 0.3 to 5% by weight based on the total weight of the second electrolyte system. If the content of the lithium bis (fluorosulfonyl) imide is less than 0.1 wt%, the coating may not be formed, and if it exceeds 10 wt%, excessive coating may be formed and the resistance may be increased.

The first electrolyte system is a commonly used electrolyte system. However, when the lithium bis (fluorosulfonyl) imide is added directly to the first electrolyte system in order to improve the lifetime and output characteristics of the first electrolyte system, the lithium ion transition of the first electrolyte system There is a problem in that the number of sheets is reduced.

The inventors of the present invention have found that the lithium bis (fluorosulfonyl) imide is not directly added to the first electrolyte system, but the lithium bis (fluorosulfonyl) imide (Fluorosulfonyl) imide is added when the first electrolyte system and the second electrolyte system are mixed to produce an electrolyte, the life and output characteristics of the lithium bis (fluorosulfonyl) And the present invention has been completed.

In order to prevent the lithium ion transfer rate of the electrolyte from lowering even when the second electrolyte system including the lithium bis (fluorosulfonyl) imide is mixed with the first electrolyte system, It is presumed that the electron density values of the second electrolyte system should be adjusted to be substantially similar.

In this case, when the organic solvent of the second electrolyte system is the chain carbonate-based solvent, the second electrolyte system together with the lithium bis (fluorosulfonyl) imide and the lithium difluoro (bisoxalate) phosphate The electron density value of the second electrolyte system can be adjusted to be substantially similar to the electron density value of the first electrolyte system.

In the second electrolyte system, the lithium difluoro (bis oxalate) phosphate may be contained in an amount of 0.1 to 10% by weight, preferably 0.5 to 5% by weight based on the total weight of the second electrolyte system. If the content of lithium difluoro (bisoxalate) phosphate is less than 0.1% by weight, the coating may not be formed, and if it exceeds 10% by weight, excessive coating may be formed and the resistance may be increased.

The electron density value of the electrolyte system can be measured by a Langmuir probe and the electron density value of the electrolyte system can be measured by a Langmuir probe of Hiden Analytical ESPION . The Langmuir probe is a device for measuring plasma characteristics such as ion and electron density values or electron temperature or potential from an I-V curve obtained by changing the probe voltage. The probe can be a single probe rod with a diameter of 1.2 mm and a length of 6 mm.

The electron density value can be measured by gasifying the electrolyte system in a magnetic field, converting the gas into a plasma state, and measuring the relationship between current and voltage using the Langmuir probe. The relationship between the current and the voltage is determined by setting a copper tube antenna which is wound four times in a cylindrical shape so that the high frequency power is effectively absorbed into the plasma region outside the discharge tube and the RF frequency is 13.56 MHz and the power of the frequency generator is 20 W can do. For reference, when the RF power is 10 W or less, generation of plasma is weak and accurate discharge may not be exhibited.

The sheath potential ( VV _p ) and the current measured by the above method are substituted into the following equation (1), and the electron temperature ( T _e ) corresponding to the slope is obtained. Then, the saturation current ( I _e ) measured using the above method can be substituted into the following equation (2) to calculate the electron density value.

Where e , k, and T _e are the electron charge, the Boltzmann constant, and the electron temperature, respectively, and n _e , I _e , A _p And m _e are the electron density, the saturation current, the surface area of the probe, and the mass of the electron, respectively.

The electron density value of the first electrolyte system is about 2.32 to 2.64 Φ * Φ. On the other hand, when the lithium bis (fluorosulfonyl) imide is directly added to the first electrolyte system, the electron density value of the electrolyte is about 1.98 to 2.21? *? A difference of more than -10% of the lower limit value of the density value is produced, and the upper limit value thereof is more than -10% of the upper limit value of the electron density value of the first electrolyte system. In this case, the improvement of the life and output characteristics of the electrolyte is also insignificant, and the lithium ion transition number is remarkably lowered as compared with the first electrolyte system.

On the other hand, the electron density value of the second electrolyte system comprising the lithium difluoro (bis oxalate) phosphate together with the lithium bis (fluorosulfonyl) imide in the chain carbonate solvent is about 2.27 to 2.59 Φ * 陸, the lower limit value thereof is within ± 10% of the lower limit value of the electron density value of the first electrolyte system, and the upper limit value is within ± 10% of the upper limit value of the electron density value of the first electrolyte system. In this case, the electrolyte in which the first electrolyte system and the second electrolyte system are mixed not only improves the life and output characteristics but also does not lower the lithium ion transition water.

That is, the upper limit value and the lower limit value of the electron density value of the second electrolyte system are respectively within ± 10%, preferably within ± 5% of the upper limit value and the lower limit value of the electron density value of the first electrolyte system, As the electron density value of the second electrolyte system is similar to the electron density value of the first electrolyte system, deterioration of the lithium ion transfer number can be prevented when these two electrolyte systems are mixed.

Therefore, the electron density value of the second electrolyte system is preferably 2.28 to 2.64? *?, More preferably 2.27 to 2.59? *?. In addition, the electron density value of the electrolyte in which the first electrolyte system and the second electrolyte system are mixed may be 2.36 to 2.79 Φ * Φ, and preferably 2.48 to 2.74 Φ * Φ.

As described above, when the first electrolyte system and the second electrolyte system have similar electron density values, even when the first electrolyte system and the second electrolyte system are mixed, the life and output characteristics of the electrolyte deteriorate Rather, the life and output characteristics of the electrolyte are superior to the life and output characteristics of the first electrolyte system.

The second electrolyte system may be included in an amount of 5 to 50 parts by weight, and may be 10 to 30 parts by weight based on 100 parts by weight of the first electrolyte system. If the content of the second electrolyte system is less than 5 parts by weight, the life and output characteristics may not be improved. If the content of the second electrolyte system exceeds 30 parts by weight, swelling may occur.

In addition, the electrolyte may further include an organic solvent and a lithium salt which are conventionally used.

The organic solvent may be any organic solvent that can act as a medium through which ions involved in an electrochemical reaction of a battery can move. Specifically, examples of the organic solvent include an ester solvent, an ether solvent, a ketone solvent, an aromatic hydrocarbon solvent, and an alkoxyalkane solvent. These solvents may be used alone or in combination of two or more.

Specific examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, n-butyl acetate, dimethyl acetate, But are not limited to, methyl propionate, ethyl propionate, propyl propionate, butyl propionate,? -Butyrolactone, - fluoro? -Butyrolactone, 1,3-dioxolane, decanolide,? -Valerolactone, mevalonolactone, ),? -caprolactone,? -valerolactone,? -caprolactone, and the like.

Specific examples of the ether solvent include dibutyl ether, tetraglyme, 2-methyltetrahydrofuran, tetrahydrofuran, and the like.

Specific examples of the ketone solvent include cyclohexanone and the like. Specific examples of the aromatic hydrocarbon organic solvent include organic solvents such as benzene, fluorobenzene, chlorobenzene, iodobenzene, toluene, fluorotoluene, xylene). Examples of the alkoxyalkane solvent include dimethoxy ethane and diethoxy ethane.

The lithium salt can be used without particular limitation as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. Specifically, LiClO _4, as to the lithium salt _{LiAsF 6, LiBF 4, LiSbF 6} , LiAl0 4, LiAlCl 4, LiCF 3 SO 3, LiC 4 F 9 SO 3, LiN (C 2 F 5 SO 3) 2, LiN ( C ₂ F ₅ SO ₂ ) ₂ , LiN (CF ₃ SO ₂ ) ₂ . _{LiN (C a F 2a + 1} SO 2) (C b F 2b + 1 SO 2) ( However, a and b is a natural number, preferably 1≤a≤20, 1≤b≤20 and Im), LiCl, LiI, LiB (C ₂ O ₄ ) _2, and mixtures thereof.

When the lithium salt is dissolved in the electrolyte, the lithium salt functions as a source of lithium ions in the lithium secondary battery, and can promote the movement of lithium ions between the cathode and the anode. Accordingly, the lithium salt is preferably contained in the electrolyte at a concentration of about 0.6 mol% to 2 mol%. If the concentration of the lithium salt is less than 0.6 mol%, the conductivity of the electrolyte may be lowered and the performance of the electrolyte may be deteriorated. If the concentration exceeds 2 mol%, the viscosity of the electrolyte may increase and the lithium ion mobility may be decreased. Considering the conductivity of the electrolyte and the mobility of lithium ions, it is more preferable that the lithium salt is controlled to be approximately 0.7 mol% to 1.6 mol% in the electrolyte.

In addition to the above electrolyte components, the electrolyte may further include an additive (hereinafter, referred to as another additive) which can be generally used for an electrolyte for the purpose of improving lifetime characteristics of the battery, suppressing reduction of battery capacity, have.

Examples of the other additives include vinylene carbonate (vinylenecarbonate, VC), metal fluoride (metal fluoride, for example, LiF, RbF, TiF, AgF , AgF 2, BaF 2, CaF 2, CdF 2, FeF 2, HF ₂ , Hg ₂ F ₂ , MnF ₂ , NiF ₂ , PbF ₂ , SnF ₂ , SrF ₂ , XeF ₂ , ZnF ₂ , AlF ₃ , BF ₃ , BiF ₃ , CeF ₃ , CrF ₃ , DyF ₃ , EuF ₃ , _{GaF 3, GdF 3, FeF 3} , HoF 3, InF 3, LaF 3, LuF 3, MnF 3, NdF 3, PrF 3, SbF 3, ScF 3, SmF 3, TbF 3, TiF 3, TmF 3, YF 3 _{, YbF 3, TIF 3, CeF} 4, GeF 4, HfF 4, SiF 4, SnF 4, TiF 4, VF 4, ZrF4 4, NbF 5, SbF 5, TaF 5, BiF 5, MoF 6, ReF 6, SF ₆ , WF ₆ , CoF ₂ , CoF ₃ , CrF ₂ , CsF, ErF ₃ , PF ₃ , PbF ₃ , PbF ₄ , ThF ₄ , TaF ₅ and SeF ₆ ), glutaronitrile Succinonitrile (SN), adiponitrile (AN), 4-tolunitrile, 1,3,6-hexanetricarbonitrile (1,3,6-hexanetricarbonitrile) , Propylene sulfide (PS), 3,3'- (3,3'-thiodipropionitrile, TPN), vinylethylene carbonate (VEC), fluoroethylene carbonate (FEC), difluoroethylenecarbonate, fluorodimethyl carbonate fluoromethylcarbonate, fluorodimethylcarbonate, fluoroethylmethylcarbonate, lithium bis (trifluoromethylsulfonyl) imide, LiTFSI, lithium tetrafluoroborate (LiBF ₄ ), lithium tetrafluoroborate Lithium difluoro (oxalate) borate, LiDFOB), lithium (malonato oxalato) borate, LiMOB), LiPF ₂ C ₄ O ₈ , LiSO ₃ CF _{3, LiPF 4 (C 2 O} 4), LiP (C 2 O 4) 3, LiC (SO 2 CF 3) 3, LiBF 3 (CF 3 CF 2), LiPF 3 (CF 3 CF 2) 3, Li 2 B ₁₂ F ₁₂ , 1,3-propane sultone, 1,3-propene sultone, biphenayl, cyclohexyl benzene, 4-fluorotoluene, succinic anhydride, ethylene sulfate anhydride, tris (methylsilyl) borate, and the like. , Or a mixture of two or more of them.

The other additives may be included in an amount of 0.1 to 20% by weight, preferably 0.2 to 5% by weight based on the total weight of the electrolyte.

According to another embodiment of the present invention, there is provided a lithium secondary battery including the electrolyte. The lithium secondary battery according to an embodiment of the present invention can be classified into a lithium ion battery, a lithium ion polymer battery, and a lithium polymer battery depending on the type of the separator and the electrolyte used. The lithium secondary battery can be classified into a cylindrical shape, a square shape, Etc., and can be divided into a bulk type and a thin film type depending on the size. The electrolyte according to the embodiment of the present invention may be particularly excellent for application to a lithium ion battery, an aluminum laminated battery and a lithium polymer battery.

Specifically, the lithium secondary battery includes a cathode including a cathode active material disposed opposite to each other, a cathode including a cathode active material, and the electrolyte interposed between the anode and the cathode.

1 is an exploded perspective view of a lithium secondary battery 1 according to an embodiment of the present invention. 1, a lithium secondary battery 1 according to another embodiment of the present invention includes a separator 7 between a cathode 3, an anode 5, the cathode 3, and an anode 5, (5) and the separator (7) are impregnated into the electrolyte (15), and the electrolyte is injected to impregnate the negative electrode (3), the positive electrode (5) and the separator .

Conductive lead members 10 and 13 for collecting a current generated during a battery operation can be attached to the cathode 3 and the anode 5 respectively and the lead members 10 and 13 are respectively connected to the anode 5, And the current generated in the cathode (3) can be led to the positive electrode and the negative electrode terminal.

The anode 5 is prepared by mixing a cathode active material, a conductive agent and a binder to prepare a composition for forming a cathode active material layer, applying the composition for forming a cathode active material layer to a cathode current collector such as aluminum foil, can do.

As the cathode active material, a compound capable of reversibly intercalating and deintercalating lithium (a lithiated intercalation compound) can be used. Specifically, an olivine-type lithium metal compound represented by the following formula (3) can be used.

(3)

Li _x M _y M ' _z XO _4-w Y _w

M and M 'are independently Fe, Ni, Co, Mn, Cr, Zr, Nb, Cu, V, Mo, Ti, Zn, Al, Ga, Mg, B, And X is an element selected from the group consisting of P, As, Bi, Sb, Mo, and combinations thereof, and Y is selected from the group consisting of F, S, and combinations thereof 0 <x? 1, 0 <y? 1, 0 <z? 1, 0 <x + y + z? 2, and 0? W?

Among these compounds, LiCoO ₂ , LiMnO ₂ , LiMn ₂ O ₄ , LiNiO ₂ , LiNi _x Mn _(1-x) O ₂ (where 0 <x <1) and LiM _lx M _2y O ₂ (However, 0≤x≤1, 0≤y≤1, 0≤x + y≤1 , M 1 and M ₂ is one selected from the group consisting of Al, Sr, Mg and La are each independently One), and a mixture thereof.

The negative electrode 3 is prepared by mixing a negative electrode active material, a binder and an optional conductive agent in the same manner as the positive electrode 5 to prepare a composition for forming a negative electrode active material layer and then applying the composition to a negative electrode current collector such as a copper foil .

As the negative electrode active material, a compound capable of reversible intercalation and deintercalation of lithium may be used. Specific examples of the negative electrode active material include carbonaceous materials such as artificial graphite, natural graphite, graphitized carbon fiber and amorphous carbon. Further, in addition to the carbonaceous material, a compound including a metallic compound capable of alloying with lithium or a metallic compound and a carbonaceous material may be used as the negative electrode active material.

At least one of Si, Al, Sn, Pb, Zn, Bi, In, Mg, Ga, Cd, Si alloys, Sn alloys and Al alloys may be used as the metal capable of being alloyed with lithium. Further, a metal lithium thin film may be used as the negative electrode active material.

As the negative electrode active material, any one selected from the group consisting of crystalline carbon, amorphous carbon, carbon composite, lithium metal, lithium-containing alloy, and mixtures thereof may be used in view of high stability.

On the other hand, since the electrolyte is the same as described above with respect to the electrolyte, the description thereof will be omitted. Since the lithium secondary battery can be manufactured by a conventional method, a detailed description thereof will be omitted herein. Although the pouch type lithium secondary battery is described as an example in the present embodiment, the present invention is not limited to the pouch type lithium secondary battery, and any shape can be used as long as it can operate as a battery.

As described above, the lithium secondary battery including the electrolyte according to the embodiment of the present invention can exhibit low DC-IR characteristics, high-temperature storage characteristics, and improved output characteristics, It can be useful for portable devices such as computers, digital cameras, camcorders, electric vehicles such as hybrid electric vehicles (HEV), plug-in hybrid electric vehicles (HEV, PHEV) have.

Hereinafter, embodiments of the present invention will be described in detail so that those skilled in the art can easily carry out the present invention. The present invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein.

[Preparation Example 1: Preparation of electrolyte and lithium secondary battery]

Each of the electrolytes according to Examples and Comparative Examples was prepared with the composition shown in Table 1 below.

On the other hand, LiCoO ₂ as a cathode active material, carbon black as a conductive agent, polyvinylidene fluoride (PVDF) as a binder, n-methyl-2-pyrrolidone as a solvent, 2-pyrrolidone, NMP) was coated on an aluminum (Al) substrate. As the negative electrode, a slurry prepared by mixing MCFC (mesocarbon microbead) and carbon black, PVDF as a binder, and NMP as a solvent was coated on a copper (Cu) substrate, .

An Al-pouch type lithium secondary battery was fabricated using the electrolyte prepared above, the positive electrode and the negative electrode.

The first electrolyte system The second electrolyte system The mixing ratio (weight ratio) of the first and second electrolyte systems Organic solvent (wt%) additive
(wt%) Sum
(wt%) Organic solvent (wt%) Additive 1
(wt%) Additive 2
(wt%) Sum
(wt%) Comparative Example 1 EC 84, LiPF ₆ (16) 100 - - - - - Comparative Example 2 EC 84, LiPF ₆ (16) 100 - LiFSI (100) - 100 95: 5 Comparative Example 3 - - - DEC (85) LiFSI (14) Add.1 (1) 100 - Example 1 EC 84, LiPF ₆ (16) 100 DEC (85) LiFSI (14) Add.1 (1) 100 80:20

1) EC: ethylene carbonate

2) DEC: Diethyl carbonate

3) LiFSI: Lithium bis (fluorosulfonyl) imide

4) Add.1: Lithium difluoro (bisoxalate) phosphate

[Experimental Example 1: Characteristic evaluation of the produced electrolyte]

The properties of the electrolytes prepared in the Examples and Comparative Examples were evaluated and the results are shown in Table 2 below.

In Table 2, the respective characteristics were measured by the following methods.

1) Electron density (Φ * Φ): Measured using Langmuir probe.

2) Lithium-ion transfer coefficient: Measured by electrochemical impedance spectroscopy (EIS) measurement method. Specifically, the lithium ion transfer coefficient can be calculated by the following equation (3). However, I _o R _o and I _ss R _ss are negligible and can be calculated through I _ss / I _o . At this time, DC polarization induction was applied at 10 mV until the current value stabilized.

&Quot; (3) &quot;

t ₊ = I _SS (? VI _o R _o ) / I _o (? VI _SS R _SS )

t ₊ = lithium ion transference number, I _o = I polarization current, Iss = after polarization current

3) Normal temperature service life (%): After charging 4.2V, 2.7V discharging was repeatedly performed, and 300 cycle discharge capacity was measured at room temperature (25 ° C).

4) Output resistance increase rate (%): The resistance value was obtained after charging 4.2 V by using an electrochemical impedance spectroscopy (EIS) measurement method.

5) Presence or absence of swelling: After charging at 4.2 V and storing at 60 캜 for 4 weeks, swelling of the battery was measured.

6) High temperature service life (%): 4.2V After charging, 2.7V discharge was repeatedly performed, and 300 cycle discharge capacity was measured under high temperature (45 ° C) condition.

7) High Temperature Recovery Capacity (%): After charging at 4.2V and storing at 4O &lt; 0 &gt; C for 4 weeks, the remaining discharge capacity of the battery was measured.

Electron density value Lithium ion transition number Normal temperature service life Output resistance increase rate Swelling
Occurrence High temperature life High temperature recovery capacity Comparative Example 1 2.32 ~ 2.64 0.21 to 0.24 74 232 U 65 44 Comparative Example 2 1.98 ~ 2.21 0.19-0.22 76 184 U 68 51 Comparative Example 3 2.27 to 2.59 - - - - - - Example 1 2.48 ~ 2.74 0.38 to 0.43 81 168 radish 72 67

Referring to Table 2, Comparative Example 2 is a case where the lithium bis (fluorosulfonyl) imide is added directly to the first electrolyte system, and its electron density value is about 1.98 to 2.21? *? The lower limit value thereof is different by more than -10% of the lower limit value of the electron density value of the first electrolyte system and the upper limit value thereof is more than -10% of the upper limit value of the electron density value of the first electrolyte system. In this case, the improvement of the lifetime and the output characteristics of the electrolyte is also insignificant, and the lithium ion transition number is remarkably lowered compared to the first electrolyte system.

On the other hand, the electron density value of the electrolyte of Comparative Example 3 including the lithium bis (fluorosulfonyl) imide and the lithium difluoro (bis oxalate) phosphate in the chain carbonate solvent was about 2.27 to 2.59 Φ * Φ, wherein the lower limit value is within ± 10% of the lower limit value of the electron density value of the first electrolyte system, and the upper limit value is within ± 10% of the upper limit value of the electron density value of the first electrolyte system . In this case, the electrolyte of Example 1 in which the electrolyte of Comparative Example 1 is mixed with the electrolyte of Comparative Example 3 not only improves the life and output characteristics, but also does not lower the lithium ion transition water.

That is, the upper limit value and the lower limit value of the electron density value of the second electrolyte system are respectively within ± 10%, preferably within ± 5% of the upper limit value and the lower limit value of the electron density value of the first electrolyte system, It can be seen that as the electron density value of the second electrolyte system is similar to the electron density value of the first electrolyte system, the lowering of the lithium ion transition number can be prevented when these two electrolyte systems are mixed.

While the present invention has been particularly shown and described with reference to exemplary embodiments thereof, it is to be understood that the invention is not limited to the disclosed exemplary embodiments, Of the right.

1: Lithium secondary battery
3: cathode 5: anode
7: separator 9: electrode assembly
10, 13: lead member 15: case

Claims (12)

A first electrolyte system comprising a cyclic carbonate-based solvent and LiPF ₆ , and
A second electrolyte system comprising a chain carbonate-based solvent, lithium bis (fluorosulfonyl) imide and lithium difluoro (bisoxalato) phosphate,
&Lt; / RTI &gt; The method according to claim 1,
Wherein the second electrolyte system is included in an amount of 5 to 50 parts by weight based on 100 parts by weight of the first electrolyte system. The method according to claim 1,
Wherein the first electrolyte system comprises 6 to 30% by weight of the LiPF ₆ based on the entire first electrolyte system. The method according to claim 1,
Wherein the second electrolyte system comprises 0.1 to 10% by weight of the lithium bis (fluorosulfonyl) imide based on the total of the second electrolyte system. The method according to claim 1,
Wherein the second electrolyte system comprises the lithium difluoro (bis oxalate) phosphate in an amount of 0.1 to 10 wt% based on the entire second electrolyte system. The method according to claim 1,
Wherein the first electrolyte system has an electron density value of 2.32 to 2.64? *?. The method according to claim 6,
Wherein an upper limit value and a lower limit value of an electron density value of the second electrolyte system are within ± 10% of an upper limit value and a lower limit value of an electron density value of the first electrolyte system, respectively. 8. The method of claim 7,
Wherein the second electrolyte system has an electron density value of 2.28 to 2.64? *?. The method according to claim 1,
Wherein the electrolyte has an electron density value of 2.36 to 2.79? *?. The method according to claim 1,
Wherein the cyclic carbonate-based solvent comprises any one selected from the group consisting of ethylene carbonate, propylene carbonate, butylene carbonate, fluoroethylene carbonate, and mixtures thereof. The method according to claim 1,
Wherein the chain carbonate-based solvent includes any one selected from the group consisting of dimethyl carbonate, diethyl carbonate, dipropyl carbonate, methylpropyl carbonate, ethylpropyl carbonate, methyl ethyl carbonate, ethyl methyl carbonate, Electrolyte. An anode including a cathode active material,
A negative electrode disposed opposite to the positive electrode and including a negative electrode active material, and
A lithium secondary battery comprising the electrolyte according to claim 1.

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