Electrolyte for lithium secondary battery and lithium secondary battery comprising the same

Abstract

An electrolyte for a lithium secondary battery includes an organic solvent, a lithium salt, an additive including at least one of isothiocyanate-based compounds represented by Chemical Formula 1 or Chemical Formula 2, and a co-additive including at least one of fluorine-containing carbonate-based compounds, lithium phosphate-based compounds, sultone-based compounds, or sulfate-based compounds. The present invention also provides a lithium secondary battery including the electrolyte.

Description

[0001] Cross-reference and priority claims of related applications

[0002] This application claims priority to Korean Patent Application No. 10-2020-0164547, filed with the Korean Intellectual Property Office (KIPO) on November 30, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] This invention relates to an electrolyte for lithium secondary batteries and a lithium secondary battery comprising the electrolyte. More specifically, this invention relates to an electrolyte for lithium secondary batteries comprising an organic solvent, a lithium salt, and additives, and a lithium secondary battery comprising the electrolyte. Background Technology

[0004] Rechargeable and dischargeable secondary batteries have been widely used as power sources for mobile electronic devices such as portable cameras, mobile phones, and laptops.

[0005] Lithium-ion batteries have gained attention and development among various types of secondary batteries due to their high operating voltage, high energy density per unit weight, high charge rate, and compact size.

[0006] For example, a lithium secondary battery may include an electrode assembly comprising a positive electrode, a negative electrode, a separator layer, and an electrolyte immersing the electrode assembly.

[0007] Lithium secondary batteries may further include a housing having, for example, a pouch shape for containing electrode components and electrolyte.

[0008] For example, the positive electrode of a lithium secondary battery can be manufactured by coating a positive electrode slurry, including positive electrode active material, binder, conductive material, etc., onto a positive electrode current collector, drying and pressing it.

[0009] The positive electrode active material can be a material that enables reversible insertion and extraction of lithium ions. For example, the positive electrode active material can include lithium metal oxides, which include metal elements such as nickel (Ni), cobalt (Co), or manganese (Mn).

[0010] Recently, with the expanding applications of lithium-ion batteries, there is a need to improve their lifespan, capacity, and operational stability. Therefore, there is a need to develop lithium-ion batteries that can provide uniform power and capacity even during repeated charging and discharging.

[0011] However, during repeated charging and discharging, power and capacity may decrease due to surface damage to nickel-based lithium metal oxides, and side reactions between nickel-based lithium metal oxides and the electrolyte may occur.

[0012] For example, in Korean Patent Application Publication No. 10-2019-0119615, an additive is included in an electrolyte for a lithium secondary battery to enhance the performance of the lithium secondary battery. SUMMARY

[0013] According to an aspect of the present application, there is provided an electrolyte for a lithium secondary battery that provides improved chemical stability in a lithium secondary battery.

[0014] According to an aspect of the present application, there is provided a lithium secondary battery having improved chemical stability.

[0015] The electrolyte for a lithium secondary battery according to exemplary embodiments includes an organic solvent, a lithium salt, an additive including at least one of isothiocyanate-based compounds represented by Chemical Formula 1 or Chemical Formula 2, and an auxiliary additive including at least one selected from the group consisting of fluorine-containing carbonate-based compounds, lithium phosphate-based compounds, sultone-based compounds, and sulfate-based compounds.

[0016] [Chemical Formula 1]

[0017] R1-N=C=S

[0018] [Chemical Formula 2]

[0019]

[0020] In Chemical Formula 1, R1 is a substituted or unsubstituted C1-C6 linear or branched alkyl, or a substituted or unsubstituted C3-C6 cycloalkyl.

[0021] In Chemical Formula 2, R2, R3, R4, and R5 are each independently hydrogen, a substituted or unsubstituted C1-C6 linear or branched alkyl, or a substituted or unsubstituted C3-C6 cycloalkyl, and at least two of R2 to R5 can be connected to each other to form a ring.

[0022] In some embodiments, the additive can include both isothiocyanate-based compounds represented by Chemical Formula 1 and isothiocyanate-based compounds represented by Chemical Formula 2.

[0023] In some embodiments, R1 can be an unsubstituted C1-C6 linear or branched alkyl.

[0024] In some embodiments, R2 can be hydrogen.

[0025] In some embodiments, R1may be methyl or ethyl, and R2, R3, R4, and R5may be hydrogen.

[0026] In some embodiments, the fluorine-containing carbonate-based compound can have a ring structure.

[0027] In some embodiments, the lithium phosphate-based compound can include a fluorine-containing lithium phosphate-based compound.

[0028] In some embodiments, the sultone-based compound can include an alkyl sultone-based compound and an alkenyl sultone-based compound.

[0029] In some embodiments, the fluorine-containing carbonate-based compound can include fluorinated ethylene carbonate. The lithium phosphate-based compound can include lithium difluorophosphate. The sultone-based compound can include 1,3-propane sultone, 1,4-butane sultone, ethylene sultone, 1,3-propene sultone, 1,4-butene sultone, and / or 1-methyl-1,3-propene sultone. The sulfate-based compound can include ethylene sulfate, trimethylene sulfate, and / or methyl trimethylene sulfate.

[0030] In some embodiments, the content of the additive can be 0.5 to 2% by weight, based on the total weight of the electrolyte.

[0031] In some embodiments, the content of the auxiliary additive can be 1 to 5% by weight, based on the total weight of the electrolyte.

[0032] In some embodiments, the weight ratio of the auxiliary additive to the additive in the electrolyte can be 1 to 5.

[0033] A lithium secondary battery according to exemplary embodiments includes an electrode assembly including a negative electrode and a positive electrode opposite the negative electrode, and an electrolyte for a lithium secondary battery according to the above-described embodiments impregnating the electrode assembly.

[0034] According to exemplary embodiments, by using the electrolyte for a lithium secondary battery, a swelling phenomenon caused during charging and discharging can be prevented.

[0035] A lithium secondary battery according to exemplary embodiments can include an electrolyte for a lithium secondary battery. Accordingly, the lithium secondary battery can have an improved capacity retention rate and lifespan while suppressing an increase in thickness in a high-temperature environment. BRIEF DESCRIPTION OF DRAWINGS

[0036] Figure 1 FIG. 1 illustrates a schematic top view of a lithium secondary battery according to exemplary embodiments.

[0037] Figure 2FIG. 1 illustrates a schematic cross-sectional view of a lithium secondary battery according to an exemplary embodiment. DETAILED DESCRIPTION

[0038] According to an exemplary embodiment of the present application, an electrolyte for a lithium secondary battery including an additive of a predetermined chemical structure is provided. According to an exemplary embodiment of the present application, a lithium secondary battery including the electrolyte is also provided.

[0039] Hereinafter, the present application will be described in detail with reference to Examples and the accompanying drawings. However, those skilled in the art will appreciate that these embodiments described with reference to the Examples and the accompanying drawings are provided to further understand the spirit of the present application, and are not intended to limit the subject matter to be protected disclosed in the detailed description and the appended claims.

[0040] The term "A-based compound" used herein can mean a compound including a moiety represented by "A" as a backbone or a substituent. For example, an "isothiocyanate-based compound" can be a compound in which an isothiocyanate moiety is included in a backbone or an isothiocyanate group is bonded to a backbone as a substituent.

[0041] The term "Ca-Cb" used herein can mean a carbon number of a to b.

[0042]

[0043] The electrolyte for a lithium secondary battery according to an exemplary embodiment (hereinafter, which can be abbreviated as an electrolyte) can include an organic solvent, a lithium salt, and an additive.

[0044] In an exemplary embodiment, the additive can include at least one of isothiocyanate-based compounds represented by the following Chemical Formula 1 or Chemical Formula 2.

[0045] [Chemical Formula 1]

[0046] R1— N=C=S

[0047] [Chemical Formula 2]

[0048]

[0049] In Chemical Formula 1, R1may be a substituted or unsubstituted C1-C6 linear or branched alkyl, or a substituted or unsubstituted C3-C6 cycloalkyl.

[0050] ​In Chemical Formula 2, R2, R3, R4, and R5 can each independently be hydrogen, a substituted or unsubstituted C1-C6 linear or branched alkyl, or a substituted or unsubstituted C3-C6 cycloalkyl. In one embodiment, at least two of R2 to R5 can be connected to each other to form a ring.

[0051] For example, the branched alkyl can be a C3-C6 alkyl.

[0052] In some embodiments, R1 to R5 can include a substituent. For example, the substituent can include at least one of a halogen, a C1-C6 alkyl, a C3-C6 cycloalkyl, a C1-C6 alkoxy, a 3- to 7-membered heterocycloalkyl, a hydroxyl (-OH), -NR6R7 (R6 and R7 each independently are hydrogen or a C1-C6 alkyl), a nitro (-NO2), a cyano (-CN), a thiocyanate (-SCN), and an isothiocyanate (-NCS).

[0053] The above-described additive can be added to the electrolyte such that the high-temperature storage performance can be improved. For example, during high-temperature storage, an improved capacity retention and a capacity recovery can be achieved while suppressing an increase in the thickness and internal resistance of the battery.

[0054] In one embodiment, the additive can include both an isothiocyanate compound represented by Chemical Formula 1 and an isothiocyanate compound represented by Chemical Formula 2. In this case, the high-temperature storage performance of the lithium secondary battery can be further improved.

[0055] In one embodiment, in Chemical Formula 1, R1 can be an unsubstituted C1-C6 linear or branched alkyl.

[0056] In one embodiment, in Chemical Formula 2, R2 can be hydrogen.

[0057] In one embodiment, in Chemical Formula 1, R1 can be a methyl or an ethyl, and in Chemical Formula 2, R2, R3, R4, and R5 can be hydrogen.

[0058] In one embodiment, the content of the additive can be 0.5 to 2% by weight, based on the total weight of the electrolyte. Within the above range, the high-temperature storage performance of the lithium secondary battery can be further improved.

[0059] In one embodiment, the electrolyte can further include an auxiliary additive, which can include a fluorine-containing carbonate-based compound, a lithium phosphate-based compound, a sultone-based compound, and a sulfate-based compound. The additive and the auxiliary additive can be used in combination, which can effectively provide a lithium secondary battery having improved high-temperature storage performance.

[0060] For example, the fluorine-containing carbonate-based compound can include a fluorine atom or a fluorine-combined substituent (e.g., a fluorine-substituted alkyl such as -CF3) combined with at least one carbon atom in the carbonate-based compound.

[0061] In some embodiments, the fluorine-containing carbonate-based compound can include a fluorine-containing cyclic carbonate-based compound having a ring structure. For example, the fluorine-containing cyclic carbonate-based compound can have a 5- to 7-membered ring structure.

[0062] For example, the fluorine-containing cyclic carbonate-based compound can include fluoroethylenecarbonate (FEC).

[0063] In one embodiment, the lithium phosphate-based compound can include a fluorine-containing lithium phosphate-based compound.

[0064] For example, the fluorine-containing lithium phosphate-based compound can include a fluorine atom or a fluorine-combined substituent (e.g., a fluorine-substituted alkyl such as -CF3) combined with a phosphorus atom of the lithium phosphate-based compound.

[0065] In some embodiments, the fluorine-containing lithium phosphate-based compound can include at least one of lithium difluorophosphate (LiPO2F2) and lithium difluoro(bisoxalato)phosphate.

[0066] In some embodiments, the sultone-based compound can include at least one of an alkyl sultone-based compound and an alkenyl sultone-based compound.

[0067] In some embodiments, the sultone-based compound can include both an alkyl sultone-based compound and an alkenyl sultone-based compound.

[0068] For example, the alkyl sultone-based compound can include at least one of 1,3-propane sultone (PS) and 1,4-butane sultone.

[0069] For example, the alkenyl sultone-based compound can include at least one of ethylene sultone, 1,3-propylene sultone (PRS), 1,4-butylene sultone, and 1-methyl-1,3-propylene sultone.

[0070] In some embodiments, the sulfate-based compound can include a cyclic sulfate-based compound including a ring structure. The cyclic sulfate-based compound can have a 5- to 7-membered ring structure.

[0071] For example, the cyclic sulfate-based compound can include at least one of vinyl sulfate (ESA), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS).

[0072] In one embodiment, the content of the auxiliary additive can be 1 to 5% by weight, based on the total weight of the electrolyte. Within the above range, the high-temperature storage performance of the lithium secondary battery can be further improved.

[0073] In one embodiment, the weight ratio of the auxiliary additive with respect to the additive in the electrolyte can be 1 to 5, preferably 1.75 to 4.75, and more preferably 2 to 4.5. Within the above range, the high-temperature storage performance of the lithium secondary battery can be further improved.

[0074] In one embodiment, the auxiliary additive can further include at least one of a borate-based compound, a nitrile-based compound, an amine-based compound, a silane-based compound, and a phenyl-based compound.

[0075] For example, the borate-based compound can include at least one of lithium tetraphenylborate and lithium difluoro(oxalato)borate (LiODFB).

[0076] For example, the nitrile-based compound can include succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, octanenitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenyl nitrile, and 4-fluorophenyl nitrile.

[0077] For example, the organic solvent can include, for example, an organic compound that can provide sufficient solubility to the lithium salt, and an additive and an auxiliary additive that can not have reactivity in the battery.

[0078] In one embodiment, the organic solvent can include a carbonate-based solvent, a carboxylate-based solvent, an ether-based solvent, a ketone-based solvent, an alcohol-based solvent, an aprotic solvent, etc. These organic solvents can be used alone or in combination thereof.

[0079] The carbonate-based solvent can include, for example, dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate, dipropyl carbonate, ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate, etc. These carbonate-based solvents can be used alone or in combination thereof.

[0080] The carboxylate-based solvent can include, for example, methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), ethyl 1,1-dimethyl acetate (DMEA), methyl propionate (MP), ethyl propionate (EP), gamma-butyrolactone (GBL), decanolactone, valerolactone, mevalonolactone, hexalactone, or the like. These carboxylate-based solvents can be used alone or in combination thereof.

[0081] The ether-based solvent can include, for example, dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF), 2-methyltetrahydrofuran, or the like. These ether-based solvents can be used alone or in combination thereof.

[0082] The ketone-based solvent can include, for example, cyclohexanone.

[0083] The alcohol-based solvent can include, for example, ethanol, isopropyl alcohol, or the like.

[0084] The aprotic solvent can include, for example, a nitrile-based solvent, an amide-based solvent (e.g., dimethylformamide), a dioxolane-based solvent (e.g., 1,3-dioxolane), a sulfolane, or the like. These aprotic solvents can be used alone or in combination thereof.

[0085] In some embodiments, the organic solvent can include a carbonate-based solvent. For example, the organic solvent can include at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), and diethyl carbonate (DEC).

[0086] The electrolyte can include a lithium salt, and the lithium salt can be represented by Li + X - .

[0087] The anion (X - ) of the lithium salt can include, for example, 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 - , etc. These anions can be used alone or in combination thereof.

[0088] In some embodiments, the lithium salt can include at least one of LiBF4and LiPF6.

[0089] In one embodiment, the lithium salt can be included at a concentration of about 0.01 M to about 5 M, preferably about 0.01 M to 2 M, with respect to the organic solvent. Within the above range, transfer of lithium ions and / or electrons can be facilitated during charging and discharging of the lithium secondary battery.

[0090] < Lithium secondary battery >

[0091] Figure 1 and Figure 2 are a schematic plan view and a schematic cross-sectional view, respectively, showing a lithium secondary battery according to an exemplary embodiment. Specifically, Figure 2 is a cross-sectional view taken along the I-I' line of Figure 1 .

[0092] Referring to Figure 1 and Figure 2 , the lithium secondary battery can include an electrode assembly 150 including a cathode 100, an anode 130, and a separator layer 140 interposed between the cathode and the anode. The electrode assembly 150 can be accommodated in a case 160 together with an electrolyte according to the above exemplary embodiment impregnating the electrode assembly.

[0093] The cathode 100 can include a cathode current collector 105 and a cathode active material layer 110 formed on the cathode current collector 105.

[0094] For example, the cathode active material layer 110 can include a cathode active material layer and a binder, and can further include a conductive material.

[0095] For example, the positive electrode slurry can be prepared by mixing and stirring the positive electrode active material with a binder, a conductive material, a dispersant, and the like in a solvent. The slurry can be coated on the positive electrode current collector 105, and then dried and pressed to form the positive electrode 100.

[0096] The positive electrode current collector 105 can include stainless steel, nickel, aluminum, titanium, copper, or alloys thereof. Preferably, aluminum or an aluminum alloy can be used.

[0097] The positive electrode active material can be a material capable of reversibly intercalating and deintercalating lithium ions. The positive electrode active material can include, for example, a lithium metal oxide including, for example, metal elements such as nickel, cobalt, manganese, aluminum, and the like.

[0098] For example, the lithium metal oxide can be represented by the following Chemical Formula 3.

[0099] [Chemical Formula 3]

[0100] Li x Ni a Co b M c O y

[0101] In Chemical Formula 3, M is at least one of Al, Zr, Ti, Cr, B, Mg, Mn, Ba, Si, Y, W, and Sr, and 0.9≤x≤1.2, 1.9≤y≤2.1, 0.5≤a≤1, 0≤c / (a+b)≤0.13.

[0102] For example, in Chemical Formula 3, 0≤c≤0.11.

[0103] In some embodiments, the content of nickel among the elements other than lithium and oxygen in the lithium metal oxide can be 60 mol% or more, preferably 70 mol% or more, more preferably 80 mol% or more, 83 mol% or more, or 85 mol% or more.

[0104] For example, in Chemical Formula 3, 0.6≤a≤1, more preferably, 0.8≤a≤1.

[0105] In some embodiments, the above-described positive electrode active material or lithium metal oxide can further include a coating element or a doping element. For example, the coating element or the doping element can include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, alloys thereof, or oxides thereof. These elements can be used alone or in combination thereof. The positive electrode active material can be passivated by the coating element or the doping element, and thus the stability and the lifespan with respect to penetration of an external object can be further improved.

[0106] As the content of nickel in the lithium metal oxide increases, the relative chemical stability, for example, high-temperature storage performance can deteriorate. However, in the case of a battery including an electrolyte according to exemplary embodiments, improved high-temperature storage performance can be achieved even when including a high-nickel lithium metal oxide (for example, 80 mol% of nickel).

[0107] The binder for the cathode can include an organic-based binder such as polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc. Or a water-based binder such as styrene butadiene rubber (SBR) that can be used together with a thickening agent such as carboxymethyl cellulose (CMC).

[0108] The conductive material can include a carbon-based substance such as graphite, carbon black, graphene, carbon nanotube, etc., and / or a metal-based material such as tin, tin oxide, titanium oxide, a perovskite substance such as LaSrCoO3 or LaSrMnO3, etc.

[0109] The anode 130 can include an anode current collector 125 and an anode active material layer 120 on the anode current collector 125.

[0110] The anode active material layer 120 can include an anode active material and an anode binder, and can further include a conductive material.

[0111] For example, the anode active material can be mixed and stirred together with the anode binder, the conductive material in a solvent to form an anode slurry. The anode slurry can be coated on the anode current collector 125, dried and pressed to obtain the anode 130.

[0112] For example, the anode current collector 125 can include gold, stainless steel, nickel, aluminum, titanium, copper, or alloys thereof, and preferably, can include copper or a copper alloy.

[0113] The anode active material can include a substance capable of adsorbing and ejecting lithium ions. For example, a carbon-based substance such as crystalline carbon, amorphous carbon, a carbon complex, or carbon fiber; a lithium alloy, a silicon-based substance, etc. can be used.

[0114] The amorphous carbon can include hard carbon, coke, mesocarbon microbeads (MCMB) fired at a temperature of 1500℃ or less, mesophase pitch-based carbon fiber (MPCF), etc. The crystalline carbon can include artificial graphite, natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc.

[0115] The silicon-based substance can include, for example, Si, SiO x(0 < x < 2), Si / C, SiO / C, Si-metal, etc.

[0116] The lithium alloy can further include aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc.

[0117] The binder and the conductive material substantially the same as or similar to the above can also be used for the negative electrode. In some embodiments, the binder used to form the negative electrode can include, for example, an aqueous binder such as styrene butadiene rubber (SBR) to be compatible with a carbon-based active material, and can be used with a thickening agent such as carboxymethyl cellulose (CMC).

[0118] The separator layer 140 can be interposed between the positive electrode 100 and the negative electrode 130. In some embodiments, the area and / or volume of the negative electrode 130 (e.g., the contact area with the separator layer 140) can be greater than that of the positive electrode 100. Thus, lithium ions generated from the positive electrode 100 can be easily transferred to the negative electrode 130 without being lost due to, for example, precipitation or sedimentation.

[0119] The separator layer 140 can include a porous polymer film prepared from, for example, a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, an ethylene / methacrylate copolymer, etc.

[0120] The separator layer 140 can further include a non-woven fabric formed of high-melting point glass fibers, polyethylene terephthalate fibers, etc.

[0121] The electrode unit can be defined by the positive electrode 100, the negative electrode 130, and the separator layer 140, and a plurality of electrode units can be stacked to form an electrode assembly 150 which can have, for example, a jelly roll shape. For example, the electrode assembly 150 can be formed by winding, layering, or z-folding of the separator layer 140.

[0122] According to an exemplary embodiment, the electrode assembly 150 can be accommodated in a case 160 together with an electrolyte to define a lithium secondary battery.

[0123] As shown in FIG. 1, the electrode assembly 150 can include a plurality of electrode units each including the positive electrode 100 and the negative electrode 130. Figure 1 The tab (positive electrode tab or negative electrode tab) can protrude from each of the respective positive electrode current collector 105 and the negative electrode current collector 125 included in each electrode unit, and can extend to one side of the case 160. The tabs can be welded together to be connected with an electrode lead (negative electrode lead 107 or positive electrode lead 127) extending outside the case 160.

[0124] The lithium secondary battery can be manufactured in a cylindrical shape (using a can), a prismatic shape, a pouch shape, a coin shape, etc.

[0125] Hereinafter, preferred embodiments are proposed to more specifically describe the present application. However, the following examples are given only for illustrating the present application, and it will be apparent to those skilled in the art that various changes and modifications can be made within the scope and spirit of the present application. Such changes and modifications are properly included in the appended claims.

[0126] Examples and Comparative Examples

[0127] Example 1

[0128] (1) Preparation of electrolyte

[0129] A 1.0 M LiPF6solution (a mixed solvent of EC / EMC, a volume ratio of 25:75) was prepared.

[0130] 1 wt% of allyl isothiocyanate (AITC) was added as an additive based on the total weight of the electrolyte.

[0131] 1 wt% of LiPO2F2(W3), 1 wt% of fluoroethylene carbonate (FEC), 0.5 wt% of 1,3-propane sultone (PS), 0.5 wt% of 1,3-propene sultone (PRS), and 0.5 wt% of 1,2-ethylene sulfate (ESA) were added as auxiliary additives, and mixed to prepare the electrolyte of Example 1.

[0132] (2) Preparation of lithium secondary battery sample

[0133] A cathode active material including Li[Ni 0.6 Co 0.2 Mn 0.2 ]O2and Li[Ni 0.8 Co 0.1 Mn 0.1 ]O2in a weight ratio of 6:4, a carbon black conductive material, and a polyvinylidene fluoride (PVDF) binder were mixed in a weight ratio of 92:5:3 to prepare a cathode slurry.

[0134] The cathode slurry was uniformly coated on an area of an aluminum foil (thickness: 15 µm) having a protrusion (a cathode tab portion) on one side, except for the protrusion, and then dried and pressed to form a cathode.

[0135] A negative electrode slurry was prepared by mixing a negative active material including artificial graphite and natural graphite at a weight ratio of 7:3, a styrene butadiene rubber (SBR) binder, and a carboxymethyl cellulose (CMC) thickener in a weight ratio of 97:1:2 in distilled water.

[0136] The negative electrode slurry was uniformly coated on an area of a copper foil (thickness: 15 μm) having a protrusion (a negative electrode tab portion) on one side, except for the protrusion, and then dried and pressed to form a negative electrode.

[0137] An electrode assembly was formed by interposing a polyethylene separator (thickness: 20 μm) between the positive electrode and the negative electrode. The positive electrode lead and the negative electrode lead were respectively welded and connected to the positive electrode tab and the negative electrode tab.

[0138] The electrode assembly was accommodated in a soft pack (a case) so that a portion of the positive electrode lead and a portion of the negative electrode lead were exposed to the outside, and three sides except for an electrolyte injection side were sealed.

[0139] The electrolyte prepared in the above (1) was injected, and the electrolyte injection side was also sealed, and immersed for 12 hours to manufacture a lithium secondary battery sample.

[0140] Example 2

[0141] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1% by weight of ethyl isothiocyanate (EITC) was added as an additive in the preparation of the electrolyte.

[0142] Example 3

[0143] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 0.5% by weight of allyl isothiocyanate and 0.5% by weight of ethyl isothiocyanate were added as additives in the preparation of the electrolyte.

[0144] Example 4

[0145] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 0.5% by weight of LiPO2F2, 0.5% by weight of fluoroethylene carbonate, 0.25% by weight of 1,3-propane sultone, 0.25% by weight of 1,3-propene sultone, and 0.25% by weight of 1,2-ethylene sulfate were added as auxiliary additives in the preparation of the electrolyte.

[0146] Example 5

[0147] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1.25 wt% of LiPO2F2, 1.25 wt% of fluoroethylene carbonate, 0.75 wt% of 1,3-propane sultone, 0.75 wt% of 1,3-propene sultone, and 0.75 wt% of 1,2-ethylene sulfate were added as auxiliary additives in the preparation of the electrolyte solution.

[0148] Example 6

[0149] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1.25 wt% of LiPO2F2, 1.25 wt% of fluoroethylene carbonate, 0.75 wt% of 1,3-propane sultone, 1 wt% of 1,3-propene sultone, and 1 wt% of 1,2-ethylene sulfate were added as auxiliary additives in the preparation of the electrolyte solution.

[0150] Comparative Example 1

[0151] A lithium secondary battery sample was prepared by the same method as in Example 1, except that no additives were added in the preparation of the electrolyte solution.

[0152] Comparative Example 2

[0153] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1 wt% of trimethylsilyl isothiocyanate (TMS-ITC) was added as an additive in the preparation of the electrolyte solution.

[0154] Comparative Example 3

[0155] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1 wt% of 4-fluorophenylisothiocyanate (4-FP-ITC) was added as an additive in the preparation of the electrolyte solution.

[0156] Comparative Example 4

[0157] A lithium secondary battery sample was prepared by the same method as in Example 1, except that no auxiliary additives were added in the preparation of the electrolyte solution.

[0158] Comparative Example 5

[0159] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1 wt% of LiPO2F2, 1 wt% of fluoroethylene carbonate, 0.5 wt% of 1,3-propane sultone, 0.5 wt% of 1,3-propene sultone, and 1 wt% of LiBOB were added as auxiliary additives in the preparation of the electrolyte solution.

[0160] Comparative Example 6

[0161] A lithium secondary battery sample was prepared by the same method as in Example 1, except that 1 wt% of LiPO2F2, 1 wt% of fluoroethylene carbonate, 0.5 wt% of 1,2-ethylene sulfate, and 1 wt% of LiBOB were added as auxiliary additives in the preparation of the electrolyte solution.

[0162] The components and contents of the additives and auxiliary additives used in the examples and comparative examples are shown in Table 1 below.

[0163] [Table 1]

[0164]

[0165]

[0166] Experimental Example 1: Evaluation of initial performance (room temperature, 25°C)

[0167] (1) Evaluation of initial capacity

[0168] The secondary batteries of the examples and comparative examples were subjected to three times of charging (0.5C-rate CC / CV, 4.2 V; 0.05C cutoff) and discharging (0.5C-rate CC; 2.7 V cutoff). The discharge capacity of the third cycle was measured as the initial capacity C1 of the battery.

[0169] (2) Evaluation of initial thickness of battery

[0170] The batteries of the examples and comparative examples were charged at room temperature (0.5C-rate CC / CV, 4.2 V; 0.05C cutoff), and then the initial thickness T1 of the battery was measured using a plate thickness measuring device (Mitutoyo Corporation, Model 543-490B).

[0171] (3) Evaluation of internal resistance (direct current internal resistance (DCIR))

[0172] At the point where SOC is 60%, the C-rate is changed to 0.2C, 0.5C, 1.0C, 1.5C, 2.0C, 2.5C, and 3.0C, and charging and discharging are performed at the respective C-rates for 10 seconds. The voltage termination points during the charging and discharging are included in a linear equation, and the slope of the equation is measured as DCIR.

[0173] Experimental Example 2: Evaluation of storage performance at high temperature

[0174] The batteries of the examples and the comparative examples were exposed to air at 60°C for 3 weeks using a thermostat, and further left at room temperature for 30 minutes. After that, the performance after high-temperature storage was evaluated as follows.

[0175] (1) Evaluation of thickness of battery after storage at high temperature

[0176] After charging the batteries of the examples and the comparative examples, the thickness T2 of the batteries was measured using a thickness measuring device (Mitutoyo Corporation, Model 543-490B).

[0177] The increase ratio of the thickness after high-temperature storage was calculated as follows.

[0178] Thickness increase ratio = (T2 - T1) / T1 x 100 (%)

[0179] (2) Evaluation of capacity retention rate after storage at high temperature

[0180] The batteries of the examples and the comparative examples after charging were stored at high temperature, and 0.5C-rate CC discharge (2.7V cutoff) was performed to measure the discharge capacity C2.

[0181] The capacity retention rate was calculated as follows.

[0182] Capacity retention rate after high-temperature storage (%) = C2 / C1 x 100 (%)

[0183] (3) Evaluation of capacity recovery rate after storage at high temperature

[0184] After measuring the capacity retention rate of the batteries of the examples and the comparative examples according to the above (2), 0.5C-rate CC / CV charging (4.2V, 0.05C cutoff) and 0.5C-rate CC discharge (2.7V cutoff) were performed to measure the discharge capacity C3.

[0185] The capacity recovery rate was calculated as follows.

[0186] Capacity recovery rate after high-temperature storage (%) = C3 / C1 x 100 (%)

[0187] (4) Evaluation of internal resistance (DCIR) after storage at high temperature

[0188] DCIR was measured after high-temperature storage as described in (3) of Experimental Example 1.

[0189] The measurement results are shown in Tables 2 and 3 below.

[0190] [Table 2]

[0191]

[0192]

[0193] [Table 3]

[0194]

[0195] Referring to Tables 1 and 2, the secondary batteries of the examples provide improved high-temperature storage performance (e.g., an increase in capacity retention rate and capacity recovery rate, and suppression of thickness increase and resistance increase) compared to the secondary batteries of Comparative Examples 1 to 3.

[0196] In Comparative Examples 4 to 6, which do not include an auxiliary additive or use an auxiliary additive that is different from the combination of Example 1, the high-temperature storage performance is reduced.

[0197] Referring to Example 3, a combination of allyl isothiocyanate and ethyl isothiocyanate is used as an additive to further improve the high-temperature storage performance.

[0198] Referring to Examples 1, 4, 5, and 6, better high-temperature storage performance is provided when the weight ratio of the auxiliary additive to the additive in the electrolyte is within a specific numerical range.

Claims

1.An electrolyte for a lithium secondary battery, comprising: an organic solvent; a lithium salt; an additive, the additive including at least one of isothiocyanate-based compounds represented by Chemical Formula 1 or Chemical Formula 2; and a co-additive, the co-additive including fluorine-containing carbonate-based compounds, lithium phosphate-based compounds, sultone-based compounds, and sulfate-based compounds: [Chemical Formula 1] R 1-N=C=S [Chemical Formula 2] wherein, in Chemical Formula 1, R 1 is a substituted or unsubstituted C 1-C 6 straight chain or branched alkyl, or a substituted or unsubstituted C 3-C 6 cycloalkyl, wherein, in Chemical Formula 2, R 2, R 3, R 4, and R 5 are each independently hydrogen, a substituted or unsubstituted C 1-C 6 straight chain or branched alkyl, or a substituted or unsubstituted C 3-C 6 cycloalkyl, and at least two of R 2 to R 5 can be connected to each other to form a ring. 2.The electrolyte for a lithium secondary battery according to claim 1, the additive including both isothiocyanate-based compounds represented by Chemical Formula 1 and Chemical Formula 2. 3.The electrolyte for a lithium secondary battery according to claim 1, wherein R 1 is an unsubstituted C 1-C 6 straight chain or branched alkyl. 4.The electrolyte for a lithium secondary battery according to claim 1, wherein R 2 is hydrogen. 5.The electrolyte for a lithium secondary battery according to claim 1, wherein R 1 is methyl or ethyl, and R 2, R 3, R 4, and R 5 are hydrogen. 6.The electrolyte for a lithium secondary battery according to claim 1, wherein the fluorine-containing carbonate-based compounds have a ring structure. 7.The electrolyte for a lithium secondary battery according to claim 1, wherein the lithium phosphate-based compounds include fluorine-containing lithium phosphate-based compounds. 8.The electrolyte for a lithium secondary battery according to claim 1, wherein the sultone-based compounds include alkyl sultone-based compounds and alkenyl sultone-based compounds. 9.The electrolyte for a lithium secondary battery according to claim 1, wherein the fluorine-containing carbonate-based compounds include fluorinated ethylene carbonate, the lithium phosphate-based compounds include lithium difluorophosphate, the sultone-based compounds include at least one selected from the group consisting of 1,3-propane sultone, 1,4-butane sultone, ethylene sultone, 1,3-propene sultone, 1,4-butene sultone, and 1-methyl-1,3-propene sultone, the sulfate-based compounds include at least one selected from the group consisting of ethylene sulfate, trimethylene sulfate, and methyl trimethylene sulfate. 10.The electrolyte for a lithium secondary battery according to claim 1, wherein the additive is contained in an amount of 0.5 to 2% by weight based on the total weight of the electrolyte. 11.The electrolyte for a lithium secondary battery according to claim 1, wherein the co-additive is contained in an amount of 1 to 5% by weight based on the total weight of the electrolyte. 12.The electrolyte for a lithium secondary battery according to claim 1, wherein the weight ratio of the co-additive to the additive in the electrolyte is 1 to 5. ​ ​ ​ 13. A lithium secondary battery comprising: an electrode assembly including a negative electrode and a positive electrode opposite to the negative electrode; and an electrolyte for a lithium secondary battery according to claim 1 impregnated in the electrode assembly. ​

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