KR20190050651A - Lithium secondary battery - Google Patents

Abstract

The present invention relates to a lithium secondary battery with improved high-temperature storage characteristics, wherein the lithium secondary battery comprises: a positive electrode including a lithium transition metal oxide represented by chemical formula 1, Li(Ni_aCo_bMn_c)O_2; a negative electrode including a carbon-based active material and a silicon-based active material; a non-aqueous electrolyte including a lithium bisfluorosulfonylimide, which is a first lithium salt, a second lithium salt different from the first lithium salt, and a non-aqueous organic solvent; and a separation membrane. According to the present invention, a weight ratio of the carbon-based active material to the silicon-based active material is 99 : 1 to 91 : 9, and a molar ratio of the first lithium salt to the second lithium salt is 1 : 1 to 1 : 4.

Description

LITHIUM SECONDARY BATTERY [0002]

The present invention relates to a lithium secondary battery having improved high-temperature storage characteristics.

As technology development and demand for mobile devices are increasing, the demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries having high energy density and voltage have been commercialized and widely used.

In the lithium secondary battery, charging and discharging proceed while repeating the process of intercalating and deintercalating lithium ions generated in the anode.

For this purpose, a lithium transition metal oxide is used as the positive electrode active material of the lithium secondary battery, and lithium metal, lithium alloy, crystalline or amorphous carbon or carbon composite material is used as the negative electrode active material.

As the lithium transition metal oxide, lithium-containing cobalt oxide (LiCoO ₂ ) has been mainly used up to now. In addition, a lithium-containing manganese oxide such as LiMnO _{2 having a} layered crystal structure and LiMn ₂ O _{4 having a} spinel crystal structure, The use of oxide (LiNiO ₂ ) is also considered.

Of these, LiCoO ₂ has excellent properties such as excellent cycle characteristics and is easy to manufacture. However, since it is low in safety and uses a large amount of expensive cobalt, it is applied as a power source for a field requiring a large amount of cells such as an electric vehicle There are limits to doing so.

In addition, LiNiO ₂ can be reversibly charged and discharged at a cost lower than that of a cobalt-based oxide, but at least 70% of lithium can be reversibly charged and discharged, thus attracting attention as a high-capacity material.

Recently, lithium-nickel-manganese-cobalt-based oxides such as Li (Ni _p Co _q Mn _{r 1} ) O ₂ In (here, 0 <p <1, 0 <q <1, 0 <r1 <1, p + q + r1 = 1) , or Li (Ni _p1 Co _q1 Mn _r2) O ₄ (here, 0 <p1 < 2, 0 <q1 <2, 0 <r2 <2, p1 + q1 + r2 = 2) , etc.), or lithium-nickel-cobalt-transition metal (M) oxide, such as Li (Ni _p2 Co _q2 Mn _r3 M _S2 ) O ₂ (Where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg and Mo, and p2, q2, r3 and s2 are atomic fractions of independent elements, , 0 <q2 <1, 0 <r3 <1, 0 <s2 <1, p2 + q2 + r3 + s2 = 1), and so on.

On the other hand, among such lithium transition metal oxides, a high content (Hi-Ni) nickel transition metal oxide having a nickel content of more than 60% has low structural stability and chemical stability. Therefore, when the secondary battery is exposed to a high temperature of 50 & The decomposition reaction of the electrolytic solution and the dissolution of the dislocation metal component take place on the surface of the cathode active material and the eluted metal grows from the cathode to cause internal short circuit or the like to cause a voltage drop.

Accordingly, there is a high demand for a method capable of improving high-temperature storage characteristics of a secondary battery including a high-content nickel-based transition metal oxide as a cathode active material.

Japanese Patent Application Laid-Open No. 2015-230816

In order to solve the above problems, a technical object of the present invention is to provide a lithium secondary battery having improved high-temperature storage characteristics.

In order to achieve the above object, in one embodiment of the present invention

A positive electrode comprising a lithium transition metal oxide represented by the following formula (1);

A negative electrode comprising a carbon-based active material and a silicon-based active material;

A non-aqueous electrolytic solution comprising lithium bis-fluorosulfonylimide as the first lithium salt, a second lithium salt different from the first lithium salt, and a non-aqueous organic solvent; And

Comprising a separator,

The weight ratio of the carbon-based active material to the silicon-based active material is 99: 1 to 91: 9,

Wherein the molar ratio of the first lithium salt: the second lithium salt is 1: 1 to 1: 4.

[Chemical Formula 1]

Li (Ni _a Co _b Mn _c ) O ₂

In Formula 1,

0.6? A? 0.9, 0.05? B? 0.3, and 0.05? C <0.3.

The lithium transition metal oxide represented by Formula 1 may be Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ , Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ Or Li (Ni _0.8 Mn _0.1 Co _0.1 ) O ₂ Lt; / RTI &gt;

The carbon-based active material may include at least one of natural graphite and artificial graphite.

The silicon-based active material may be SiO _x (0 <x <2), specifically SiO.

Further, as the first lithium salt different from the first lithium salt comprises a Li ⁺ as the cation, the anion is ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 ^- , ClO ₄ ^- , AlO ₄ ^- , AlCl ₄ ^- , PF ₆ ^- , SbF ₆ ^- , AsF ₆ ^- , BF ₂ C ₂ O ₄ ^- , BC ₄ O ₈ ^- , (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 -, C 4 F 9 SO 3 -, CF 3 CF _{^{2 SO 3 -, 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 ^- .

The molar ratio of the first lithium salt to the second lithium salt may be 1: 1.5 to 1: 4.

As described above, according to the present invention, when a lithium secondary battery having a positive electrode containing a high-content nickel-based positive electrode active material having a low temperature stability is provided with a non-aqueous electrolyte and a negative electrode in a specific configuration, A solid SEI film is formed on the surface of the negative electrode at the time of initial charging, and the solubility of the lithium salt is increased to improve the high-temperature storage characteristics.

BRIEF DESCRIPTION OF THE DRAWINGS The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate preferred embodiments of the invention and together with the description of the invention serve to further the understanding of the technical idea of the invention, It is not limited.
1 is a graph showing the results of a voltage drop evaluation test according to Experimental Example 1 of the present invention.

Hereinafter, the present invention will be described in detail in order to facilitate understanding of the present invention. 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.

Specifically, in one embodiment of the present invention

A positive electrode comprising a lithium transition metal oxide represented by the following formula (1);

A negative electrode comprising a carbon-based active material and a silicon-based active material;

A non-aqueous electrolytic solution comprising lithium bis-fluorosulfonylimide as the first lithium salt, a second lithium salt different from the first lithium salt, and a non-aqueous organic solvent; And

Comprising a separator,

The weight ratio of the carbon-based active material to the silicon-based active material is 99: 1 to 91: 9,

Wherein the molar ratio of the first lithium salt: the second lithium salt is 1: 1 to 1: 4.

[Chemical Formula 1]

Li (Ni _a Co _b Mn _c ) O ₂

In Formula 1,

0.6? A? 0.9, 0.05? B? 0.3, and 0.05? C <0.3.

First, in the lithium secondary battery according to an embodiment of the present invention, the positive electrode, the negative electrode, and the separator may be manufactured by a conventional method used in the production of a lithium secondary battery.

At this time, the positive electrode may be manufactured by forming a positive electrode mixture layer on the positive electrode current collector. The positive electrode mixture layer may be formed by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material and a solvent on a positive electrode collector, followed by drying and rolling.

The positive electrode collector is not particularly limited as long as it has electrical conductivity without causing chemical change in the battery. For example, the positive electrode collector may be formed of a metal such as carbon, stainless steel, aluminum, nickel, titanium, sintered carbon, , Nickel, titanium, silver, or the like may be used.

The positive electrode active material is a compound capable of reversibly intercalating and deintercalating lithium. Specifically, the positive active material is a high-rich nickel-based lithium transition metal having a nickel content of more than 50% Oxide. &Lt; / RTI &gt;

Specifically, the lithium transition metal oxide represented by Formula 1 is Li (Ni _0.6 Mn _0.2 Co _0.2 ) O ₂ , Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ Or Li (Ni _0.8 Mn _0.1 Co _0.1 ) O ₂ .

The cathode active material may be contained in an amount of 40% by weight to 90% by weight, specifically 40% by weight to 75% by weight, based on the total weight of the solid content in the cathode slurry.

The binder is a component that assists in bonding of the active material to the conductive material and bonding to the current collector, and is usually added in an amount of 1 to 30 wt% based on the total weight of the solid content in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene (Ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluorine rubber, various copolymers and the like.

The conductive material is not particularly limited as long as it has electrical conductivity without causing chemical changes in the battery. For example, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, Carbon powder; Graphite powder such as natural graphite, artificial graphite, or graphite with a highly developed crystal structure; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

At this time, the average particle diameter (D ₅₀ ) of the conductive material may be 10 μm or less, specifically 0.01 μm to 10 μm, more specifically 0.01 μm to 1 μm. At this time, when the average particle diameter of the conductive material exceeds 10 탆, the dispersibility is poor and the effect of improving the conductivity by the addition of the graphite powder is insignificant.

In the present invention, the average particle diameter (D ₅₀ ) of the conductive material can be defined as a particle diameter corresponding to 50% of the number of accumulated particles in the particle diameter distribution curve of the particles. The average particle diameter (D ₅₀ ) can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of a particle diameter of several millimeters from a submicron region, resulting in high reproducibility and high degradability.

The specific surface area of the conductive material may be 10 m ² / g to 1000 m ² / g. For example, specifically, the specific surface area of the carbon powder is 40 to 80 m 2 / g and the specific surface area of the graphite powder is 10 to 1: 40 m 2 / g, It is superior to carbon powder.

At this time, the larger the specific surface area of the conductive material, the larger the contact area with the cathode active material, and as a result, the conductive path is easily formed between the cathode active material particles. However, when the specific surface area is excessively large, specifically, when the specific surface area exceeds 1000 m ² / g, the energy density of the anode may be deteriorated due to a bulky structural feature. On the other hand, when the specific surface area of the conductive material is excessively small, specifically less than 100 m ² / g, the contact area with the cathode active material may decrease and the conductive material may aggregate. In the present invention, the specific surface area of the conductive material can be defined as a value (BET specific surface area) measured by a nitrogen adsorption method.

The conductive material may typically be added in an amount of 1 to 30% by weight based on the total weight of the solid content in the positive electrode slurry.

Specific examples of such commercially available conductive materials include acetylene black series such as Chevron Chemical Company, Denka Singapore Private Limited and Gulf Oil Company), Ketjenblack, EC (Armak Company), Vulcan XC-72 (Cabot Company), and Super P (Timcal).

In addition, the solvent may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a preferable viscosity when the cathode active material and optionally a binder and a conductive material are included. For example, the solid content in the slurry containing the cathode active material, and optionally the binder and the conductive material may be 10 wt% to 60 wt%, preferably 20 wt% to 50 wt%.

On the other hand, the high-content nickel-based lithium-transition metal oxide represented by Formula 1 has a high content of Ni in the transition metal. Therefore, Li + ions in the layered structure of the cathode active material and Ni The cationic mixing of +2 ions occurs and the structure is collapsed. As a result, the cathode active material causes a side reaction with the electrolyte or elution of the transition metal. This phenomenon is known to occur because Li + ions and Ni +2 ions are similar in size. As a result, this secondary reaction leads to the depletion of the electrolyte and the structural collapse of the cathode active material in the secondary battery, so that the performance of the battery is easily deteriorated and the voltage drop is increased at high temperature storage.

In an embodiment of the present invention, in order to solve such a problem, an anode including a carbon-based active material and a silicon-based active material as a negative electrode active material component at the time of manufacturing a lithium secondary battery, and a lithium bis- A second lithium salt different from the first lithium salt, and a non-aqueous electrolytic solution including a non-aqueous organic solvent.

That is, in the lithium secondary battery according to an embodiment of the present invention, the negative electrode may be a mixture of a carbon-based material capable of reversibly intercalating / deintercalating lithium ions and a silicon-based material as an anode active material .

At this time, the carbon-based material may include at least one of natural graphite and artificial graphite, which are crystalline carbon.

In addition, the silicon-based active material may be SiO _x (0 <x <2), specifically SiO.

The carbon-based active material and the silicon-based active material may be mixed in a weight ratio of 99: 1 to 91: 9.

At this time, when the content ratio of the carbonaceous active material exceeds 99, a thermodynamically metastable substance (ex (CH ₂ OCO ₂ Li) ₂ ) is generated in the SEI film formed on the surface of the active material, The SEI membrane decomposition reaction can be accelerated. When the content of the carbonaceous active material is less than 91, the amount of silicon-based active material having a large volume change before and after reaction with Li ions is high. Therefore, there is a difference from the volume change of the carbonaceous active material, The internal positive electrode active material on which the SEI film is not formed is exposed. As a result, a side reaction between the exposed internal positive electrode active material and the electrolyte occurs, and the SEI film becomes thicker, which leads to rapid degradation of the electrode.

On the other hand, when the carbon-based active material is used alone as the negative electrode active material, the silicon-based active material is used alone, or a material other than the mixture of the carbon-based active material and the silicon-based active material such as lithium and a metal alloy is used, The lithium metal or the mixture of the metal and the metal is exposed to the inside of the separator to cause internal shots.

The negative electrode may be prepared by forming a negative electrode mixture layer on the negative electrode collector. The negative electrode mixture layer may be formed by coating an anode slurry containing an anode active material, a binder, a conductive material, Followed by drying and rolling.

The anode current collector generally has a thickness of 3 to 500 mu m. The negative electrode current collector is not particularly limited as long as it has high conductivity without causing chemical change in the battery. Examples of the negative electrode current collector include copper, stainless steel, aluminum, nickel, titanium, sintered carbon, copper or stainless steel Surface-treated with carbon, nickel, titanium, silver or the like, aluminum-cadmium alloy, or the like can be used. In addition, like the positive electrode collector, fine unevenness can be formed on the surface to enhance the bonding force of the negative electrode active material, and it can be used in various forms such as films, sheets, foils, nets, porous bodies, foams and nonwoven fabrics.

The negative active material may be contained in an amount of 80% by weight to 99% by weight based on the total weight of the solid content in the negative electrode slurry.

The binder is a component that assists in bonding between the conductive material, the active material and the current collector, and is usually added in an amount of 1 to 30% by weight based on the total weight of the solid content in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride (PVDF), polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene Examples thereof include ethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber, fluorine rubber and various copolymers thereof.

The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 1 to 20 wt% based on the total weight of the solid content in the negative electrode slurry. Such a conductive material is not particularly limited as long as it has electrical conductivity without causing a chemical change in the battery, and examples thereof include graphite such as natural graphite or artificial graphite used in the production of a positive electrode; Carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; Conductive fibers such as carbon fiber and metal fiber; Metal powders such as carbon fluoride, aluminum, and nickel powder; Conductive whiskey such as zinc oxide and potassium titanate; Conductive metal oxides such as titanium oxide; Conductive materials such as polyphenylene derivatives and the like can be used.

The solvent may include water or an organic solvent such as NMP, alcohol, etc., and may be used in an amount in which the negative electrode active material and, optionally, a binder, a conductive material, and the like are contained in a desired viscosity. For example, the slurry containing the negative electrode active material and, optionally, the binder and the conductive material may be contained in such a manner that the solid concentration of the slurry is 50% by weight to 75% by weight, preferably 50% by weight to 65% by weight.

Further, in the lithium secondary battery according to an embodiment of the present invention, the non-aqueous electrolytic solution comprises a lithium bisfluorosulfonylimide which is a first lithium salt, a second lithium salt which is different from the first lithium salt, And an organic solvent.

First, the lithium bisfluorosulfonylimide included as the first lithium salt is added to the non-aqueous electrolyte solution to form a solid and thin SEI film on the surface of the negative electrode. Therefore, decomposition of the positive electrode surface And the oxidation reaction of the electrolytic solution can be prevented. Further, since the SEI film formed on the surface of the negative electrode by the first lithium salt is thin, it is possible to more smoothly move the lithium ions in the negative electrode, thereby realizing an effect of further improving the output of the secondary battery .

According to an embodiment of the present invention, the concentration of the lithium bisfluorosulfonylimide in the non-aqueous electrolytic solution is preferably 0.01 M to 2 M, and may be contained at a concentration of 0.1 M to 1 M.

If the concentration of the lithium bisfluorosulfonylimide in the non-aqueous electrolyte solution is more than 3M, side reactions within the electrolyte may occur excessively during charging and discharging of the battery, causing swelling, Corrosion of the positive electrode or negative electrode current collector and the packaging material such as a can or a pouch which surrounds the battery.

The non-aqueous electrolytic solution may be a lithium salt that is a second lithium salt different from the first lithium salt and is typically used in the art. Examples thereof include Li ⁺ as a cation and F ^- , Cl ^{-, Br -, I -,} NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, AlO 4 -, AlCl 4 -, PF 6 -, SbF 6 -, AsF 6 -, BF 2 C _{^{2 O 4 -, BC 4 O}} 8 -, (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}} -, C 4 F 9 SO 3 -, CF 3 CF 2 SO 3 -, 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 ^- number of the at least a lithium salt comprising any one selected from the group consisting of, and may specifically include LiPF _6.

In this case, the molar ratio of the first lithium salt to the second lithium salt may be 1: 1 to 1: 4, specifically 1: 1.5 to 1: 4.

When the mixing ratio of the lithium salt and the lithium bisfluorosulfonylimide is within the range of the above-mentioned molar ratio, side reactions in the electrolyte can be suppressed during charging and discharging of the battery. As a result, a power drop or a swelling phenomenon Can be prevented from occurring.

Specifically, when the molar ratio of the second lithium salt is more than 4, the process of forming the SEI film in the lithium ion battery and the process of inserting the solvated lithium ion between the negative electrode by the carbonate solvent, The effect of improving the low temperature output of the secondary battery and improving the cycle characteristics and the capacity characteristics after high temperature storage may be insufficient due to peeling of the negative electrode surface layer (for example, carbon surface layer) and decomposition of the electrolyte. If the molar ratio of the second lithium salt is less than 1, the content of the lithium bisfluorosulfonylimide in the electrolyte excessively increases and the corrosion of the electrode current collector and the packaging material such as the can surrounding the battery or the pouch Which may affect the stability of the secondary battery.

In addition, the organic solvent is not limited as long as it can minimize decomposition due to an oxidation reaction during charging and discharging of the secondary battery, and can exhibit desired properties together with additives. For example, an ether solvent, an ester solvent or an amide solvent may be used alone or in combination of two or more.

As the ether solvent in the organic solvent, any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether and ethyl propyl ether, or a mixture of two or more thereof may be used , But is not limited thereto.

In addition, the ester solvent may include at least one compound selected from the group consisting of a cyclic carbonate compound, a linear carbonate compound, a linear ester compound, and a cyclic ester compound.

Specific examples of the cyclic carbonate compound include ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate , 2,3-pentylene carbonate, vinylene carbonate, and fluoroethylene carbonate (FEC), or a mixture of two or more thereof.

Specific examples of the linear carbonate compound include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate and ethyl propyl carbonate , Or a mixture of two or more thereof, but the present invention is not limited thereto.

Specific examples of the linear ester compound include any one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, And mixtures thereof, but the present invention is not limited thereto.

Specific examples of the cyclic ester compound include any one selected from the group consisting of? -Butyrolactone,? -Valerolactone,? -Caprolactone,? -Valerolactone and? -Caprolactone, or two or more Mixtures may be used, but are not limited thereto.

The cyclic carbonate-based compound in the ester-based solvent is a highly viscous organic solvent having a high dielectric constant and can dissociate the lithium salt in the electrolyte well. The cyclic carbonate-based compound has a low viscosity such as dimethyl carbonate and diethyl carbonate, The dielectric constant linear carbonate compound and the linear ester compound are mixed in an appropriate ratio, an electrolyte having a high electric conductivity can be prepared, and thus it can be more preferably used.

In addition, the separation membrane blocks the internal short circuit of both electrodes and impregnates the electrolyte. The separation membrane composition is prepared by mixing a polymer resin, a filler and a solvent, and then the separation membrane composition is directly coated on the electrode and dried Or may be formed by casting and drying the separation membrane composition on a support, and then laminating the separation membrane film peeled off from the support on the electrode.

The separator may be a porous polymer film commonly used, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer The polymer film may be used alone or as a laminate thereof, or may be a nonwoven fabric made of a conventional porous nonwoven fabric, for example, glass fiber of high melting point, polyethylene terephthalate fiber or the like, but is not limited thereto.

At this time, the pore diameter of the porous separation membrane is generally 0.01 to 50 μm, and the porosity may be 5 to 95%. The thickness of the porous separator may be generally in the range of 5 to 300 mu m.

Conventionally, in the case of a lithium secondary battery including a cathode active material having a high content of nickel, deterioration of the battery characteristics due to side reactions due to direct contact between the cathode active material and the electrolyte deteriorates high-temperature stability, There was difficulty in doing.

In particular, among such nickel-based cathode active materials, in the case of a high-content nickel-based lithium transition metal oxide having a nickel content of more than 60%, nickel is not only eluted from the cathode active material due to the reaction between the cathode active material and the electrolyte, It is pointed out that the thermal stability is seriously degraded at high temperature.

That is, when an excessive amount of Ni transition metal is contained in the cathode active material, the nickel transition metal having the d orbital in an environment such as high temperature has an octahedron structure at the time of coordination according to the oxidation number change of Ni, The order changes backward or the oxidation number fluctuates (nonuniformization reaction) to form a distorted octahedron. As a result, the crystal structure of the positive electrode active material including the nickel transition metal is deformed, and the probability of nickel metal in the positive electrode active material is increased.

However, as in a lithium secondary battery according to an embodiment of the present invention, a positive electrode comprising a lithium transition metal oxide represented by Formula 1 as a positive electrode active material and a negative electrode comprising a carbonaceous active material and a silicon-based active material as a negative electrode active material, When a non-aqueous electrolyte containing a first lithium salt LiFSI and a second lithium salt in a molar ratio of 1: 1 to 1: 4 is used together, a lithium secondary battery having improved high-temperature stability can be produced, there was.

That is, a protective layer is formed on the surface of the positive electrode due to LiFSI to suppress the cation mixing phenomenon of Li + 1 + ions and Ni +2 ions, and sufficient nickel transition metal The side reaction with the electrolytic solution and the metal elution phenomenon can be effectively suppressed.

As a result, the present inventors have found that the cathode active material containing the oxide according to the above formula (1) exhibits a high output in the combination of LiFSI and LiFSI, and exhibits excellent efficiency in high temperature stability and capacity characteristics.

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

Example  One.

80 g of Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ as a cathode active material, 15 g of carbon black as a conductive agent and 5 g of polyvinylidene fluoride (PVDF) as a binder were dissolved in a solvent N-methyl-2-pyrrolidone NMP) to prepare a positive electrode mixture slurry (solid content 60 wt%). The positive electrode mixture slurry was applied to an aluminum (Al) thin film having a thickness of about 20 탆 and dried to produce a positive electrode, followed by a roll press to prepare a positive electrode.

Subsequently, 96 g of negative electrode active material (natural graphite: SiO 2 = 96: 4), 3 g of PVDF as a binder and 1 g of carbon black as a conductive agent were added to NMP as a solvent to prepare an anode mixture slurry (solid content 75 wt%) Respectively. The negative electrode mixture slurry was applied to a copper (Cu) thin film as an anode current collector having a thickness of 10 mu m and dried to prepare a negative electrode, followed by roll pressing to produce a negative electrode.

Subsequently, a non-aqueous organic solvent having a composition of ethylene carbonate (EC): ethyl methyl carbonate (EMC) = 3: 7 (volume ratio) was added with 0.3M of lithium bisfluorosulfonylimide, which is the first lithium salt, LiPF ₆ 1.1M were mixed to prepare a non-aqueous electrolytic solution.

Then, the positive electrode and the negative electrode thus prepared were fabricated into a polymer type cell by a conventional method together with a separator composed of three layers of polypropylene / polyethylene / polypropylene (PP / PE / PP) Thereby preparing a lithium secondary battery.

Comparative Example  One

In the case of the non-aqueous electrolytic solution prepared in Example 1, the second lithium salt LiPF ₆ A lithium secondary battery was produced in the same manner as in Example 1 except that a non-aqueous electrolytic solution containing only 1.4M was used.

Comparative Example  2

A lithium secondary battery was produced in the same manner as in Example 1, except that natural graphite (100% by weight) was used as the negative electrode active material in preparation of the negative electrode in Example 1 above.

Comparative Example  3

In Comparative Example 2, when the non-aqueous electrolyte was prepared, the second lithium salt LiPF ₆ A lithium secondary battery was produced in the same manner as in Comparative Example 2, except that a non-aqueous electrolytic solution containing only 1.4M was used.

Comparative Example  4

A lithium secondary battery was produced in the same manner as in Example 1, except that natural graphite: SiO 2 was used in an amount of 90:10 by weight as an anode active material in preparation of the negative electrode in Example 1.

Comparative Example  5

In Comparative Example 4, when the non-aqueous electrolyte was prepared, the second lithium salt LiPF ₆ A lithium secondary battery was produced in the same manner as in Comparative Example 4, except that a non-aqueous electrolytic solution containing only 1.4M was used.

Experimental Example

Experimental Example  One. Voltage descent (voltage drop) evaluation experiment

The secondary batteries prepared in Example 1 and Comparative Examples 1 to 5 were charged at 0.2 C up to 4.2 V under a constant current / constant voltage condition at room temperature and then charged at 0.2 C up to 2.5 V under constant current (CC) C &lt; / RTI &gt; The remaining capacity of the secondary batteries after the initial activation was set to 100%, stored at 60 ° C for 60 days, and then the voltage drop was measured by measuring the voltage behavior. At this time, the voltage drop was a unit of mV, and the voltage change amount of each of the five lithium secondary batteries in each of the examples and the comparative examples was measured every day for 60 days. The total amount of the voltage change amount and the five lithium secondary battery deviation values thereof were shown in the following table 1 and Fig.

Cathode active material Anode active material
(weight%) Lithium salt
(M) Average voltage drop
(mV)  Cell voltage deviation (mV) Carbon-based
Active material Silicon-based
Active material LiFSI LiPF ₆ Example 1 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 96 4 0.3 1.1 74 2 Comparative Example 1 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 96 4 - 1.4 366 168 Comparative Example 2 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 100 - 0.3 1.1 153 60 Comparative Example 3 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 100 - - 1.4 229 24 Comparative Example 4 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 90 10 0.3 1.1 291 86 Comparative Example 5 Li (Ni _0.7 Mn _0.15 Co _0.15 ) O ₂ 90 10 - 1.4 354 127

As can be seen from Table 1 and FIG. 1, in the case of the secondary battery of Example 1, the average voltage drop was 74 mV, and the voltage deviation between the five cells was about 2 mV. . Therefore, it was confirmed that the lithium secondary battery of the present invention exhibited a relatively even voltage profile even at high temperature storage.

On the other hand, in the secondary battery of Comparative Example 1 having the non-aqueous electrolyte containing no first lithium salt, the average voltage drop was 366 mV and the voltage deviation between the five cells was 168 mV, which was significantly lower than that of the secondary battery of Example 1 .

Comparative Examples 2 and 3 having a negative electrode containing no silicon-based active material as a negative electrode active material and Comparative Example 4 having a negative electrode containing a carbon-based active material and a silicon-based active material as a negative electrode active material in a 90:10 weight ratio And the voltage drop of the secondary battery of 5 and the voltage deviation of the lithium secondary battery were larger than those of the secondary battery of Example 1. [

Claims (7)

A positive electrode comprising a lithium transition metal oxide represented by the following formula (1);
A negative electrode comprising a carbon-based active material and a silicon-based active material;
A non-aqueous electrolytic solution comprising lithium bis-fluorosulfonylimide as the first lithium salt, a second lithium salt different from the first lithium salt, and a non-aqueous organic solvent; And
Comprising a separator,
The weight ratio of the carbon-based active material to the silicon-based active material is 99: 1 to 91: 9,
Wherein the molar ratio of the first lithium salt: the second lithium salt is 1: 1 to 1: 4.
[Chemical Formula 1]
Li (Ni _a Co _b Mn _c ) O ₂
In Formula 1,
0.6? A? 0.9, 0.05? B? 0.3, and 0.05? C <0.3.
The method according to claim 1,
Lithium transition metal oxide represented by Formula 1 is _{Li (Ni 0.6 Mn 0.2 Co 0.2} ) O 2, Li (Ni 0.7 Mn 0.15 Co 0.15) O 2 or _{Li (Ni 0.8 Mn 0.1 Co 0.1} ) O 2 would lithium Secondary battery.
The method according to claim 1,
Wherein the carbon-based active material is at least one of natural graphite and artificial graphite.
The method according to claim 1,
Wherein the silicon-based active material is SiO _x (0 <x <2).
The method of claim 4,
Wherein the silicon-based active material is SiO.
The method according to claim 1,
In the second lithium salt comprises a Li ⁺ as the cation, the anion is ^{F -, Cl -, Br -} , I -, NO 3 -, N (CN) 2 -, BF 4 -, ClO 4 -, AlO 4 ^{_{-, AlCl 4 -, PF 6}} -, SbF 6 -, AsF 6 -, BF 2 C 2 O 4 -, BC 4 O 8 -, (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 -, C 4 F 9 SO 3 -, CF 3 CF 2 SO 3 -, 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 And at least one selected from the group consisting of CO ₂ ^- , CH ₃ CO ₂ ^- , SCN ^-, and (CF ₃ CF ₂ SO ₂ ) ₂ N ^- .
The method according to claim 1,
Wherein the molar ratio of the first lithium salt: the second lithium salt is 1: 1.5 to 1: 4.

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