Electrolyte for lithium secondary battery and lithium secondary battery comprising same

Abstract

An electrolyte for a lithium secondary battery according to an exemplary embodiment of the present invention includes an organic solvent, a lithium salt, a first additive represented by a predetermined chemical formula, and a second additive represented by a predetermined chemical formula. A protective film is formed from the additive to suppress expansion of the lithium secondary battery and improve storage performance at high temperatures.

Description

[0001] This application is a divisional application of the Chinese patent application with an application date of March 30, 2021, Chinese patent application number 202110340177.6 and an invention name of “Electrolyte for lithium secondary batteries and lithium secondary batteries containing the same”, and this application claims priority to the Korean application with application number 10-2020-0040945. Technical Field

[0002] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same. More particularly, the present invention relates to an electrolyte for a lithium secondary battery comprising an organic solvent and a lithium secondary battery comprising the same. Background Art

[0003] With the development of information technology and display technology, secondary batteries that can be repeatedly charged and discharged have been widely used as power sources for portable electronic devices such as camcorders, mobile phones, notebook computers, etc. Recently, battery packs including secondary batteries are being developed and used as environmentally friendly power sources for electric vehicles such as hybrid vehicles.

[0004] Secondary batteries include, for example, lithium secondary batteries, nickel-cadmium batteries, nickel-hydrogen batteries, etc. Lithium secondary batteries have attracted attention due to high operating voltage and high energy density per unit weight, high charging rate, compact size, and the like.

[0005] For example, a lithium secondary battery may include an electrode assembly including a positive electrode, a negative electrode, and a separator layer (separator); and an electrolyte that soaks the electrode assembly. The lithium secondary battery may further include a case having, for example, a pouch.

[0006] Recently, as the application of lithium secondary batteries has expanded, lithium secondary batteries with higher capacity and power have been developed. For example, materials for positive and negative electrodes that can provide higher capacity are being studied.

[0007] For example, research is underway to develop materials that can replace conventional carbon-based materials. However, when the materials used for the positive and negative electrodes are changed, the electrolytes that contact or react with the positive and negative electrodes also need to be changed or redesigned.

[0008] For example, Korean Patent Application Publication No. 10-2012-0101499 discloses a high-voltage electrolyte for a lithium secondary battery. Summary of the Invention

[0009] According to one aspect of the present invention, there is provided an electrolyte for a lithium secondary battery that offers improved chemical stability and operational reliability.

[0010] According to one aspect of the present invention, there is provided a lithium secondary battery including an electrolyte and providing improved chemical stability and operational reliability.

[0011] According to an exemplary embodiment of the present invention, an electrolyte for a lithium secondary battery includes an organic solvent, a lithium salt, a first additive represented by Chemical Formula 1, and a second additive represented by Chemical Formula 2. The respective amounts of the first additive and the second additive are independently in the range of 0.01 wt % to 5 wt % based on the total weight of the electrolyte:

[0012] [Chemical Formula 1]

[0013]

[0014] In Chemical Formula 1, R 1 is a saturated hydrocarbon skeleton structure having 3 to 10 carbon atoms, and n is an integer of 1 to 3.

[0015] [Chemical Formula 2]

[0016]

[0017] In Chemical Formula 2, R 11 to R 13 are each independently hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, and A is a substituent represented by Chemical Formula 3 or Chemical Formula 4.

[0018] [Chemical Formula 3]

[0019]

[0020] [Chemical Formula 4]

[0021]

[0022] In Chemical Formula 3 and Chemical Formula 4, R 14 and R 15 are each independently a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or -OR 16 .

[0023] R 16 It is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or an alkynyl group having 2 to 6 carbon atoms.

[0024] In some embodiments, in Chemical Formula 1, R 1 The saturated hydrocarbon skeleton structure may be linear.

[0025] In some embodiments, the first additive may include at least one of the compounds represented by Chemical Formula 1-1 to Chemical Formula 1-3:

[0026] [Chemical Formula 1-1]

[0027]

[0028] [Chemical formula 1-2]

[0029]

[0030] [Chemical formula 1-3]

[0031]

[0032] In some embodiments, the second additive may include a compound represented by Chemical Formula 4-1.

[0033] [Chemical Formula 4-1]

[0034]

[0035] In some embodiments, the amount of the first additive may be 0.1 wt % to 3 wt % based on the total weight of the electrolyte.

[0036] In some embodiments, the amount of the first additive may be 0.1 wt % to 2 wt % based on the total weight of the electrolyte.

[0037] In some embodiments, the amount of the second additive may be 0.1 wt % to 3 wt % based on the total weight of the electrolyte.

[0038] In some embodiments, the weight ratio of the first additive to the second additive may be 1:0.25 to 1:3.

[0039] In some embodiments, the organic solvent may include at least one selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0040] In some embodiments, the lithium salt may include at least one of lithium hexafluorophosphate (LiPF6) and lithium difluorophosphate (LiPO2F2).

[0041] In some embodiments, the electrolyte may further include a cyclic carbonate-based compound containing a double bond or a fluorine-substituted cyclic carbonate-based compound.

[0042] In some embodiments, the electrolyte may further include a sultone-based compound.

[0043] In some embodiments, the electrolyte may further include a cyclic sulfonate-based compound.

[0044] According to an exemplary embodiment, the lithium secondary battery according to the above embodiment includes a positive electrode, a negative electrode, a separator layer interposed between the positive electrode and the negative electrode, and an electrolyte for a lithium secondary battery.

[0045] In the electrolyte for a lithium secondary battery according to an embodiment of the present invention, the first additive and the second additive can suppress the side reaction between the electrode and the electrolyte. For example, the first additive can passivate the metal of the positive electrode, and the second additive can form an electrically and thermally stable protective film on the surface of the negative electrode. In this case, the side reaction between the electrode and the electrolyte can be suppressed, thereby preventing the consumption of the electrolyte. In addition, battery expansion caused by gas generation can be prevented.

[0046] Therefore, the lifespan and capacity retention rate of the lithium secondary battery can be improved, and reliability and storage performance at high temperatures can also be improved. BRIEF DESCRIPTION OF THE DRAWINGS

[0047] Figure 1 is a schematic cross-sectional view illustrating a lithium secondary battery according to an exemplary embodiment. Specific implementation plan

[0048] According to an exemplary embodiment of the present invention, an electrolyte for a lithium secondary battery is provided, the electrolyte comprising an organic solvent, a lithium salt, and different first and second additives. The additives can form a protective film to inhibit expansion of the lithium secondary battery and improve high-temperature storage performance. According to an exemplary embodiment of the present invention, a lithium secondary battery including the electrolyte is provided.

[0049] Hereinafter, the present invention will be described in detail with reference to the accompanying drawings. However, those skilled in the art will appreciate that these embodiments described with reference to the accompanying drawings are provided for further understanding of the spirit of the present invention and do not limit the subject matter to be protected as disclosed in the detailed description and the appended claims.

[0050] Electrolyte for lithium secondary batteries

[0051] The electrolyte for a lithium secondary battery according to an embodiment of the present invention (hereinafter, may be abbreviated as electrolyte) may include an organic solvent, a lithium salt mixed or dissolved in the organic solvent, and two or more types of additives. For example, the electrolyte may be used as a non-aqueous electrolyte for a lithium secondary battery.

[0052] The organic solvent may include an organic compound that provides sufficient solubility for the lithium salt and additives and is non-reactive with the lithium secondary battery. In an exemplary embodiment, the organic solvent may include a carbonate-based solvent, an ester-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, an aprotic solvent, and the like. These solvents may be used alone or in combination.

[0053] Examples of carbonate-based solvents may include dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), methyl propyl carbonate, ethyl propyl carbonate, diethyl carbonate (DEC), dipropyl carbonate, propylene carbonate (PC), ethylene carbonate (EC), fluoroethylene carbonate (FEC), butylene carbonate, and the like.

[0054] Examples of ester-based solvents may include methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), 1,1-dimethylethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), γ-butyrolactone (GBL), decanolide, valerolactone, mevalonolactone, caprolactone, and the like.

[0055] Examples of the ether-based organic solvent may include dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, and the like.

[0056] Cyclohexanone can be used as the ketone-based solvent. Examples of the alcohol-based solvent may include ethanol, isopropyl alcohol, and the like.

[0057] The aprotic solvent may include a nitrile-based solvent, an amide-based solvent (eg, dimethylformamide (DMF)), a dioxolane-based solvent (eg, 1,3-dioxolane), a sulfolane-based solvent, and the like.

[0058] In a preferred embodiment, a carbonate-based solvent can be used as the organic solvent. For example, the organic solvent can include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or a combination thereof. Preferably, a mixture of EC and EMC can be used as the organic solvent.

[0059] The lithium salt may include, for example, Li + X - Represents the compound.

[0060] The anion of lithium salt (X - Non-limiting examples of ) may include F - 、Cl - Br - , I - 、NO3 - 、N(CN)2- 、BF4 - 、ClO4 - PF6 - 、SbF6 - 、AsF6 - 、(CF3)2PF4 - 、(CF3)3PF3 - 、(CF3)4PF2 - 、(CF3)5PF - 、(CF3)6P - CF3SO3 - CF3CF2SO3 - 、(CF3SO2)2N - 、(FSO2)2N - CF3CF2(CF3)2CO - 、(CF3SO2)2CH - 、(SF5)3C - 、(CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - 、CH3CO2 - 、SCN - 、(CF3CF2SO2)2N - PO2F2 - etc. These anions may be used alone or in combination.

[0061] Preferably, a mixture of lithium hexafluorophosphate (LiPF6) and lithium difluorophosphate (LiPO2F2) can be used as the lithium salt. For example, LiPO2F2 can form a film with improved thermal stability on the electrode surface. LiPF6 and LiPO2F2 can be mixed in a weight ratio of 1:0.5 to 1:2. Within this weight ratio range, the electrolyte can have enhanced ionic conductivity and electrode protection performance.

[0062] In one embodiment, the lithium salt may be included at a concentration of about 0.01 M to about 5 M, preferably about 0.01 M to 2 M, based on the organic solvent. Within the above range, transfer of lithium ions and / or electrons may be promoted during charge and discharge of the lithium secondary battery, thereby providing improved capacity.

[0063] The first additive can be represented by the following chemical formula (1).

[0064] [Chemical Formula 1]

[0065]

[0066] In the above chemical formula 1, R 1is a saturated hydrocarbon skeleton structure having 3 to 10 carbon atoms, and n is an integer of 1 to 3.

[0067] As used herein, the term "saturated hydrocarbon backbone structure" may refer to a substituent group derived from a hydrocarbon that does not contain an unsaturated bond. For example, a saturated hydrocarbon backbone structure may refer to a group in which one or more hydrogen atoms are removed from a saturated hydrocarbon, and may include alkyl and alkylene groups. A saturated hydrocarbon backbone structure may be linear, branched, or cyclic.

[0068] Preferably, the saturated hydrocarbon skeleton structure may be linear so that a coordination bond with a metal element contained in the positive electrode or a chelate with the metal element may be effectively formed.

[0069] For example, when using a lithium secondary battery, metals (such as transition metals) may be released from the positive electrode. The released metal may be electrodeposited on the negative electrode, thereby degrading the performance of the negative electrode. In addition, when the lithium secondary battery is driven at a high voltage, the coating on the positive electrode surface may decompose, thereby causing side reactions between the positive electrode surface and the electrolyte.

[0070] The first additive can stabilize the structure of the positive electrode by coordinating with the metal of the positive electrode. In this case, when the lithium secondary battery is used and stored at high temperatures, the release of metal, the generation of gas, and the expansion of volume (thickness) can be suppressed. Therefore, the life span and high temperature storage performance of the lithium secondary battery can be improved. In addition, when driven at high voltage, the increase in the resistance of the battery can be suppressed.

[0071] In an exemplary embodiment, in Chemical Formula 1, when n is 1, R 1 It may be an alkyl group having 3 to 10 carbon atoms. In this case, the first additive may include a compound represented by the following Chemical Formula 1-1.

[0072] [Chemical Formula 1-1]

[0073]

[0074] In an exemplary embodiment, when n is 2, R 1 The first additive may include a compound represented by the following Chemical Formula 1-2.

[0075] [Chemical formula 1-2]

[0076]

[0077] In an exemplary embodiment, when n is 3, R 1It can be an alkylene group having 3 to 10 carbon atoms. In the alkylene group, the terminal of the hydrocarbon skeleton structure can be substituted by -OPF2. In addition, at least one -OPF2 group can be bonded to the middle carbon atom of the hydrocarbon skeleton structure. For example, when R 1 In the case of a butylene group in which a secondary carbon atom is substituted with -OPF2, the first additive may include a compound represented by the following Chemical Formulas 1-3.

[0078] [Chemical formula 1-3]

[0079]

[0080] The first additive may be included in an amount of 0.01-5 wt % based on the total weight of the electrolyte. If the amount of the first additive is less than 0.01 wt %, the stabilization effect of the positive electrode structure may be insufficient. If the amount of the first additive exceeds 5 wt %, the internal resistance of the lithium secondary battery may be excessively increased, or the capacity of the lithium secondary battery may be reduced.

[0081] Preferably, the content of the first additive may be 0.1 wt % to 2 wt %, more preferably 0.3 wt % to 2 wt %, or 0.5 wt % to 2 wt %.

[0082] The second additive may be represented by Chemical Formula 2 below.

[0083] [Chemical Formula 2]

[0084]

[0085] In the above chemical formula 2, R 11 to R 13 Each is independently hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms.

[0086] A is a substituent represented by the following Chemical Formula 3 or Chemical Formula 4.

[0087] [Chemical Formula 3]

[0088]

[0089] [Chemical Formula 4]

[0090]

[0091] In Chemical Formula 3 and Chemical Formula 4, R 14 and R 15 are each independently a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or -OR 16 .

[0092] R 16It is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or an alkynyl group having 2 to 6 carbon atoms.

[0093] For example, the second additive may include a compound represented by the following Chemical Formula 4-1.

[0094] [Chemical Formula 4-1]

[0095]

[0096] The second additive can form a coating or film with excellent thermal stability on the electrode. For example, when operating a lithium secondary battery, the second additive can form a complex with the metal removed. Therefore, the metal component removed can be removed from the electrolyte, thereby suppressing the side reaction caused by the metal component (for example, the formation of dendrites of the metal removed). In addition, the complex can be provided as a coating or film to improve the stability of the electrode.

[0097] The second additive may be included in an amount of 0.01% to 5% by weight based on the total weight of the electrolyte. If the amount of the second additive is less than 0.01% by weight, the stabilization effect of the negative electrode structure may be insufficient. If the amount of the second additive exceeds 5% by weight, the internal resistance of the lithium secondary battery may be excessively increased or the capacity may be reduced.

[0098] Preferably, the content of the second additive may be 0.1 wt % to 3 wt %, more preferably 0.3 wt % to 2 wt %, or 0.5 wt % to 2 wt %.

[0099] In an exemplary embodiment, the total amount of the first additive and the second additive may be 0.5 wt % to 10 wt % based on the total weight of the electrolyte. Within the above range, the positive and negative electrodes of the battery can be effectively protected. Preferably, the total amount of the first additive and the second additive may be 1 wt % to 10 wt %, or 1 wt % to 5 wt %, based on the total weight of the electrolyte.

[0100] In an exemplary embodiment, the first additive and the second additive may be included in a weight ratio of 1:0.25 to 1:3. Within this weight ratio range, the positive electrode protection due to the first additive and the negative electrode protection due to the second additive can be balanced. If the amount of the first additive is less than the above range, the degradation of the positive electrode relative to the negative electrode may be accelerated. If the amount of the first additive exceeds the above range, the degradation of the negative electrode relative to the positive electrode may be accelerated.

[0101] Preferably, the weight ratio of the first additive to the second additive may be 1:0.5 to 1:2, more preferably, 1:0.5 to 1:1.

[0102] In an exemplary embodiment, the electrolyte may include other additives such as a cyclic carbonate-based compound including a double bond, a fluorine-substituted cyclic carbonate-based compound, a sultone-based compound, a cyclic sulfonate-based compound, and the like.

[0103] The cyclic carbonate-based compound including a double bond may include vinylene carbonate, vinylethylene carbonate, and the like.

[0104] The fluorine-substituted cyclic carbonate-based compound may include fluoroethylene carbonate.

[0105] The cyclic carbonate-based compound containing a double bond and the fluorine-substituted cyclic carbonate-based compound can improve thermal and electrical durability of a film formed on an electrode surface.

[0106] For example, based on the total weight of the electrolyte, the content of the cyclic carbonate-based compound including a double bond and the fluorine-substituted cyclic carbonate-based compound can be 0.1% to 5% by weight respectively. If the content is less than 0.1% by weight, the durability of the film may decrease, and if the content exceeds 5% by weight, the thickness of the film may increase excessively. In this case, the resistance of the battery may increase, thereby reducing the power of the battery.

[0107] The sultone-based compound may include 1,3-propane sultone, 1,3-propylene sultone, 1,4-butane sultone, and the like.

[0108] The cyclic sulfonate-based compound may include 1,2-ethylene sulfate, 1,2-propylene sulfate, and the like.

[0109] The sultone-based compound and the cyclic sulfonate-based compound can form a more stable ion conductive film on the electrode surface. For example, the sultone-based compound and the cyclic sulfonate-based compound can form a structurally stable protective film on the electrode surface by reacting with the second additive.

[0110] For example, the content of the sultone-based compound and the cyclic sulfonate-based compound can be 0.1% to 5% by weight based on the total weight of the electrolyte. If the content is less than 0.1% by weight, the durability of the membrane may decrease, and if the content exceeds 5% by weight, the membrane thickness may increase excessively. In this case, the resistance of the battery may increase, thereby reducing the battery power.

[0111] lithium secondary batteries

[0112] Figure 1 is a schematic cross-sectional view illustrating a lithium secondary battery according to an exemplary embodiment.

[0113] Reference Figure 1The lithium secondary battery 100 may include an electrode assembly including a positive electrode 130, a negative electrode 140, and a separator layer 150 interposed therebetween. The electrode assembly may be housed in a case 170 together with the electrolyte according to the exemplary embodiment to be impregnated therein.

[0114] The positive electrode 130 may include a positive active material layer 115 formed by applying a positive active material to the positive current collector 110. The positive active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0115] In an exemplary embodiment, the positive active material may include a lithium-transition metal oxide. For example, the lithium-transition metal oxide may include nickel (Ni) and may further include at least one of cobalt (Co) and manganese (Mn).

[0116] For example, the lithium-transition metal oxide may be represented by the following Chemical Formula 5.

[0117] [Chemical Formula 5]

[0118] Li 1+a Ni 1-(x+y) Co x M y O2

[0119] In the above Chemical Formula 5, -0.05≤a≤0.15, 0.01≤x≤0.3, 0.01≤y≤0.3, and M may be at least one element selected from Mn, Mg, Sr, Ba, B, Al, Si, Ti, Zr, and W.

[0120] The slurry may be prepared by mixing and stirring the positive active material with a binder, a conductive agent, and / or a dispersant in a solvent. The slurry may be coated on the positive electrode collector 110 and then dried and pressed to form the positive electrode 130.

[0121] The positive electrode current collector 110 may include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or an alloy thereof may be used.

[0122] The adhesive may include an organic-based adhesive such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or a water-based adhesive such as styrene-butadiene rubber (SBR), which may be used with a thickener such as carboxymethyl cellulose (CMC).

[0123] For example, a PVDF-based binder can be used as a positive electrode binder. In this case, the amount of binder used to form the positive electrode active material layer can be reduced, and the amount of positive electrode active material can be relatively increased. Therefore, the capacity and power of the lithium secondary battery can be further improved.

[0124] Conductive agents may be added to promote electron migration between active material particles. For example, the conductive agent may include carbon-based materials such as graphite, carbon black, graphene, carbon nanotubes, and / or metal-based materials such as tin, tin oxide, titanium oxide, perovskite materials (e.g., LaSrCoO3 or LaSrMnO3), and the like.

[0125] The negative electrode 140 may include a negative electrode current collector 120 and a negative electrode active material layer 125 formed by coating a negative electrode active material on the negative electrode current collector 120 .

[0126] In an exemplary embodiment, a silicon (Si)-based compound may be used as the negative electrode active material. In some embodiments, silicon carbide (SiC) or silicon-carbon particles including a carbon core and a silicon coating may be used as the negative electrode active material.

[0127] Silicon-carbon particles can be formed, for example, by depositing a silicon layer on the surface of a graphite core. In one embodiment, silicon-carbon particles can be formed by coating a silicon layer on commercially available graphite particles using a silicon precursor compound (e.g., a silane-based compound) through a chemical vapor deposition (CVD) process.

[0128] In some embodiments, the silicon-carbon particles may have a structure in which a plurality of carbon coating layers and silicon coating layers are alternately coated or stacked on a graphite core.

[0129] The negative electrode current collector 120 may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof, and preferably, may include copper or a copper alloy.

[0130] In some embodiments, the negative electrode active material may be mixed with a binder, a conductive agent, and / or a dispersing additive in a solvent and stirred to form a slurry. The slurry may be coated on the negative electrode current collector 120, dried, and pressed to obtain the negative electrode 140. A conductive agent substantially the same as or similar to that described above may be used.

[0131] In some embodiments, the binder for the negative electrode may include styrene-butadiene rubber (SBR) that can react with the additives of the electrolyte as described above. In some embodiments, a thickener (eg, carboxymethyl cellulose (CMC)) may be used together with the SBR.

[0132] The separator layer 150 may be interposed between the positive electrode 130 and the negative electrode 140. The separator layer 150 may include 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, or an ethylene / methacrylate copolymer. The separator layer may also include a non-woven fabric formed of glass fibers having a high melting point, polyethylene terephthalate fibers, or the like.

[0133] In some embodiments, the area and / or volume of the negative electrode 140 (e.g., the contact area with the separator layer 150) may be greater than the area and / or volume of the positive electrode 130. Therefore, lithium ions generated from the positive electrode 130 can be easily transferred to the negative electrode 140 without being lost due to, for example, precipitation or deposition.

[0134] In an exemplary embodiment, the electrode unit 160 may be defined by the positive electrode 130, the negative electrode 140, and the separator layer 150, and a plurality of electrode units 160 may be stacked to form an electrode assembly having, for example, a jelly roll shape. For example, the electrode assembly may be formed by winding, laminating, or folding separator layers.

[0135] The electrode assembly may be housed in the case 170 together with the electrolyte according to the exemplary embodiment to form a lithium secondary battery.

[0136] An electrode tab may be formed from each of the positive and negative current collectors 110 and 120 to extend to one end of the case 170. The electrode tab may be welded to one end of the case 170 to form an electrode lead exposed outside the case 170.

[0137] Lithium secondary batteries can be manufactured in cylindrical (using cans), square, pouch, or coin shapes.

[0138] Hereinafter, preferred embodiments are set forth to more particularly describe the present invention. However, the following examples are intended to illustrate the present invention only, and those skilled in the art will clearly understand that various changes and modifications may be made within the scope and spirit of the present invention. Such changes and modifications should be included in the appended claims.

[0139] Examples and Comparative Examples

[0140] (1) By mixing Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2, carbon black as a conductive material and polyvinylidene fluoride (PVDF) as a binder to prepare a slurry. The slurry was evenly applied to an aluminum foil with a thickness of 15 μm and vacuum dried at 130°C. The dried slurry was pressed to prepare a density of 3.667 g / cm3 Positive electrode for lithium secondary battery.

[0141] (2) The following slurry was prepared, which included 97 wt% of a negative electrode active material (artificial graphite and natural graphite were mixed at a weight ratio of 7:3), 1 wt% of styrene-butadiene rubber (SBR) as a binder, and 2 wt% of carboxymethyl cellulose (CMC) as a thickener. The negative electrode slurry was uniformly coated on a copper foil having a thickness of 15 μm, dried, and pressed to prepare a copper foil having a density of 1.684 g / cm 3 of the negative electrode.

[0142] (3) After dissolving 1 M LiPF6 in a mixed solvent of EC / EMC (1:3; volume ratio), 0.5 wt% of vinylene carbonate, 1 wt% of fluoroethylene carbonate, 1 wt% of LiPO2F2, 0.5 wt% of 1,3-propane sultone, 1 wt% of 1,3-propylene sultone, and 0.5 wt% of 1,2-ethylenesulfonate were mixed to prepare a base electrolyte.

[0143] The electrolyte solutions of Examples and Comparative Examples were prepared by adding the additives shown in Table 1 to the basic electrolyte solution.

[0144] (4) The positive electrode and the negative electrode obtained as described above are notched and stacked in an appropriate size, and a separator (polyethylene, thickness: 20 μm) is inserted between the positive electrode and the negative electrode to form an electrode unit. The tab portions of each of the positive electrode and the negative electrode are welded. The welded positive electrode / separator / negative electrode assembly is inserted into a soft package, and the three sides of the soft package except the side where the electrolyte is injected are sealed. The tab portion is also included in the sealing portion. The electrolyte according to each embodiment and comparative example is injected through the electrolyte injection side, and then the electrolyte injection side is also sealed. Subsequently, the above structure is immersed for more than 12 hours to prepare a lithium secondary battery with a capacity rating of about 2 Ah.

[0145] [Table 1]

[0146]

[0147] [Chemical Formula 1-1]

[0148]

[0149] [Chemical formula 1-2]

[0150]

[0151] [Chemical formula 1-3]

[0152]

[0153] [Chemical Formula 4-1]

[0154]

[0155] Experimental Example 1: Evaluation of Initial Capacity and Resistance

[0156] The electrolyte-injected secondary batteries of Examples and Comparative Examples were charged (CC / CV 1 / 3C 4.2V 0.05C cutoff) and discharged (CC 1 / 3C 2.5V cutoff), and the initial discharge capacity of each battery was measured.

[0157] At a point where the battery's state of charge (SOC) is set to 60%, the C-rate is sequentially increased or decreased to 0.2C, 0.5C, 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C. When charging and discharging at the corresponding C-rates are performed for 10 seconds, the voltage endpoints are set to a linear equation. The slope of the linear equation is used as the DCIR (direct current internal resistance).

[0158] Experimental Example 2: Evaluation of gas generation after 2 weeks of storage at high temperature

[0159] The secondary battery injected with the electrolyte of the embodiment and the comparative example was placed at 60 ° C for 2 weeks and then placed at room temperature for 30 minutes. The battery was placed in a chamber for measuring the amount of gas generated. After forming a vacuum in the chamber, nitrogen was filled to form atmospheric pressure, and the nitrogen volume (V0) and the indoor pressure (P0) were measured. After forming a vacuum in the chamber again, a hole was punched on the battery and the pressure (P1) in the chamber was measured. The amount of gas generated was calculated according to the following formula.

[0160] Gas production (mL) = (V0 / P0)*P1

[0161] Experimental Example 3: Evaluation of Capacity and Resistance after 8 Weeks of High-Temperature Storage

[0162] The secondary batteries injected with the electrolytic solutions of Examples and Comparative Examples were stored in a room at 60° C. for 8 weeks and then left at room temperature for 30 minutes.

[0163] Thereafter, the DCIR of each battery was measured again by the method described in Experimental Example 1.

[0164] After the DCIR measurement, the capacity measured when performing 1C rate CC discharge (2.7V cut-off) was divided by the initial discharge capacity and expressed as a percentage.

[0165] The evaluation results are shown in Table 2 below.

[0166] [Table 2]

[0167]

[0168] Referring to Table 2, in the battery of the example using the first additive and the second additive, the capacity retention rate was increased while suppressing the volume expansion and the increase in the internal resistance, as compared with the battery of the comparative example.

Claims

1. An electrolyte for a lithium secondary battery, comprising: organic solvents; lithium salts; a first additive represented by Chemical Formula 1; and The second additive is represented by Chemical Formula 2: [Chemical Formula 1] In Chemical Formula 1, R 1 is a saturated hydrocarbon skeleton structure having 3 to 10 carbon atoms, and n is an integer of 1 to 3, [Chemical Formula 2] In Chemical Formula 2, R 11 to R 13 are each independently hydrogen or a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, and A is a substituent represented by Chemical Formula 3 or Chemical Formula 4. [Chemical Formula 3] [Chemical Formula 4] In Chemical Formula 3 and Chemical Formula 4, R 14 and R 15 are each independently a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or -OR 16 ,and R 16 It is a substituted or unsubstituted alkyl group having 1 to 6 carbon atoms, a substituted or unsubstituted aryl group having 6 to 12 carbon atoms, or an alkynyl group having 2 to 6 carbon atoms.

2. The electrolyte for a lithium secondary battery according to claim 1, wherein In Chemical Formula 1, R 1 The saturated hydrocarbon skeleton structure is linear.

3. The electrolyte for a lithium secondary battery according to claim 1, wherein the first additive comprises at least one of the compounds represented by Chemical Formula 1-1 to Chemical Formula 1-3: [Chemical Formula 1-1] [Chemical formula 1-2] [Chemical formula 1-3] 4. The electrolyte for a lithium secondary battery according to claim 1, wherein the second additive comprises a compound represented by Chemical Formula 4-1: [Chemical Formula 4-1] 5. The electrolyte for a lithium secondary battery according to claim 1, wherein The amount of the first additive is 0.1 wt % to 3 wt % based on the total weight of the electrolyte.

6. The electrolyte for a lithium secondary battery according to claim 1, wherein The amount of the first additive is 0.1 wt % to 2 wt % based on the total weight of the electrolyte.

7. The electrolyte for a lithium secondary battery according to claim 1, wherein The amount of the second additive is 0.1 wt % to 3 wt % based on the total weight of the electrolyte. 8 . The electrolyte for a lithium secondary battery according to claim 1 , wherein a weight ratio of the first additive to the second additive is 1:0.25 to 1:

3.

9. The electrolyte for a lithium secondary battery according to claim 1, wherein The organic solvent includes at least one selected from the group consisting of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC) and diethyl carbonate (DEC).

10. The electrolyte for a lithium secondary battery according to claim 1, wherein The lithium salt includes at least one of lithium hexafluorophosphate (LiPF6) and lithium difluorophosphate (LiPO2F2). 11 . The electrolyte for a lithium secondary battery according to claim 1 , further comprising a cyclic carbonate-based compound containing a double bond or a fluorine-substituted cyclic carbonate-based compound. 12 . The electrolyte for a lithium secondary battery according to claim 1 , further comprising a sultone-based compound. 13 . The electrolyte for a lithium secondary battery according to claim 1 , further comprising a cyclic sulfonate-based compound.

14. A lithium secondary battery comprising: positive electrode; negative electrode; a separator layer inserted between the positive electrode and the negative electrode; and The electrolyte for a lithium secondary battery according to claim 1.

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