Patent Description:
A secondary battery which can be charged and discharged repeatedly has been widely employed as a power source of a mobile electronic device such as a camcorder, a mobile phone, a laptop computer, etc..

A lithium secondary battery is actively developed and applied among various types of secondary batteries due to high operational voltage and energy density per unit weight, a high charging rate, a compact dimension, etc..

For example, the lithium secondary battery may include an electrode assembly including a cathode, an anode and a separation layer, and an electrolyte solution immersing the electrode assembly.

A lithium metal oxide may be used as an active material for a cathode of the lithium secondary battery. Examples of the lithium metal oxide include a nickel-based lithium metal oxide.

Surface damages of the nickel-based lithium metal oxide may occur due to repeated charge/discharge cycles to degrade power and capacity, and a side reaction may occur between the nickel-based lithium metal oxide and the electrolyte.

<CIT> discloses an electrolyte solution comprising e.g. <NUM>-ethyl-<NUM>-butoxymethyl oxetane, LiPF<NUM> and an organic solvent.

According to an aspect of the present disclosures, there is provided an electrolyte solution providing improved high-temperature property.

According to an aspect of the present disclosures, there is provided a lithium secondary battery having improved high-temperature property.

An electrolyte solution for a lithium secondary battery includes an additive including a compound represented by Chemical Formula <NUM>, an organic solvent; and a lithium salt.

In Chemical Formula <NUM>, R<NUM> and R<NUM> are each independently hydrogen or a substituted or unsubstituted C<NUM>-C<NUM> alkyl group, X is O or S, and L is a substituted or unsubstituted C<NUM>-C<NUM> alkylene group.

In some embodiments, R<NUM> and R<NUM> may be each independently an unsubstituted C<NUM>-C<NUM> alkyl group, X is O, and L is an unsubstituted C<NUM>-C<NUM> alkylene group.

In some embodiments, R<NUM> and R<NUM> may be each independently an unsubstituted C<NUM> alkyl group, X is O, and L is an unsubstituted C<NUM> alkylene group.

In some embodiments, the additive may be included in an amount ranging from <NUM> wt% to <NUM> wt% based on a total weight of the electrolyte solution.

In some embodiments, the additive may be included in an amount ranging from <NUM> wt% to <NUM> wt% based on the total weight of the electrolyte solution.

In some embodiments, the organic solvent may include at least one selected from the group consisting of a carbonate-based solvent, an ester-based solvent, a ether-based solvent, a ketone-based solvent, an alcohol-based solvent and an aprotic solvent. In some embodiments, the electrolyte solution may further include an auxiliary additive including at least one selected from the group consisting of a cyclic carbonate-based compound, a fluorine-substituted carbonate-based compound, a sultone-based compound, a cyclic sulfate-based compound and a phosphate-based compound.

In some embodiments, the auxiliary additive may be included in an amount ranging from <NUM> wt% to <NUM> wt% based on a total weight of the electrolyte solution.

In some embodiments, the auxiliary additive may be included in an amount ranging from <NUM> wt% to <NUM> wt% based on the total weight of the electrolyte solution.

A lithium secondary battery includes an electrode assembly in which a plurality of cathodes and a plurality of anodes are repeatedly stacked, a case accommodating the electrode assembly, and the electrolyte solution for a lithium secondary battery according to the above-described embodiments accommodated together with the electrode assembly in the case.

An electrolyte solution including an additive in an electrolyte solution for a lithium secondary battery according to example embodiments may form a robust solid electrolyte interphase (SEI) on an electrode surface.

Accordingly, a lithium secondary battery having improved high-temperature storage properties (e.g., a capacity retention and prevention of resistance/thickness increase of the battery under high-temperature conditions) can be implemented.

The electrolyte solution for a lithium secondary battery according to example embodiments may provide a lithium secondary battery having improved high-temperature stability (e.g., suppression of gas generation in a high-temperature environment).

In the present specification, the term "A-based compound" may refer to a compound including a group A and a derivative of the compound.

In the present specification, the term "Ca-Cb" refers that the number of carbon (C) atoms is from a to b.

An electrolyte solution for a lithium secondary battery according to embodiments of the present disclosures includes a lithium salt, organic solvents and an additive including a compound represented by Chemical Formula <NUM>.

Hereinafter, components of the electrolyte solution for a lithium secondary battery will be described in detail.

The electrolyte solution for a lithium secondary battery according to exemplary embodiments may include an additive including a compound represented by Chemical Formula <NUM>.

In Chemical Formula <NUM>, R<NUM> and R<NUM> may be the same as or different from each other.

For example, R<NUM> may be hydrogen or a substituted or unsubstituted C<NUM>-C<NUM> alkyl group. In one embodiment, R<NUM> may be an unsubstituted C<NUM>-C<NUM> alkyl group. In one embodiment, R<NUM> may be an unsubstituted C<NUM> alkyl group (a methyl group).

For example, R<NUM> may be hydrogen or a substituted or unsubstituted C<NUM>-C<NUM> alkyl group. In one embodiment, R<NUM> may be an unsubstituted C<NUM>-C<NUM> alkyl group, e.g., an unsubstituted C<NUM> alkyl group (a methyl group).

For example, X may be O or S. O represents oxygen and S represents sulfur. In one embodiment, X may be O.

For example, L may be a substituted or unsubstituted C<NUM>-C<NUM> alkylene group. In one embodiment, L may be a substituted or unsubstituted C<NUM>-C<NUM> alkylene group, e.g., an unsubstituted C<NUM> alkylene group (a methylene group).

For example, the alkyl group may mean a portion in a molecule composed of carbon and hydrogen. The alkyl group may mean a partial remaining structure assuming that one hydrogen atom is removed from the alkane (CnH2n+<NUM>). For example, CH<NUM>-CH<NUM>-CH<NUM>- indicates a propyl group.

For example, the alkylene group may refer to a structure in which one hydrogen atom is separated from each of carbon atoms at both terminals of an alkane. For example, -CH<NUM>-CH<NUM>-CH<NUM>- indicates a propylene group.

For example, the term "substituted" used herein refers that a substituent may be further bonded to a carbon atom of the alkyl group or the alkylene group by substituting a hydrogen atom of the alkyl group or the alkylene group with the substituent. For example, the substituent may be at least one of a halogen, a C<NUM>-C<NUM> alkyl group, a C<NUM>-C<NUM> alkenyl group, an amino group, a C<NUM>-C<NUM> alkoxy group, a C<NUM>-C<NUM> cycloalkyl group, and a <NUM> to <NUM>-membered hetero-cycloalkyl group. In some embodiments, the substituent may be a halogen or a C<NUM>-C<NUM> alkyl group.

The additive including the compound represented by Chemical Formula <NUM> may be included in the electrolyte solution for a secondary battery, so that a robust solid electrolyte interphase (SEI) layer may be formed on an electrode through a decomposition reaction of a cyclic ether.

For example, the solid electrolyte interphase layer based on a sulfonate functional group may be formed under a high temperature condition. In example embodiments, the solid electrolyte interphase layer may be formed on an anode. Accordingly, decomposition of an organic solvent (e.g., EC, EMC, etc.) may be effectively prevented, and gas generation and battery thickness increase may be significantly reduced.

In one embodiment, the compound represented by Chemical Formula <NUM> may include <NUM>-(mesyloxymethyl)-<NUM>-methyloxetane. For example, the <NUM>-(mesyloxymethyl)-<NUM>-methyloxetane may be represented by Chemical Formula <NUM>-<NUM>.

<NUM>-(mesyloxymethyl)-<NUM>-methyloxetane may be included as the additive of the electrolyte solution for a secondary battery, so that and a stable SEI layer may be formed on the anode by the sulfonate group. Accordingly, a lithium secondary battery having improved high-temperature storage properties may be implemented. Additionally, decomposition of the electrolyte due to a reaction between the electrolyte solution and the anode may be suppressed to suppress gas generation.

In one embodiment, a content of the additive content may be adjusted to be <NUM> weight percent (wt%) or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more based on a total weight of the electrolyte solution in consideration of sufficient passivation and stable SEI film formation.

In one embodiment, the content of the additive may be adjusted to be <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, or <NUM> wt% or less based on the total weight of the electrolyte solution in consideration of lithium ion mobility and active material activity in the electrolyte solution.

In one embodiment, the content of the additive may be in a range from <NUM> wt% wt% to <NUM> wt%, or from <NUM> wt% to <NUM> wt%. Within the above range, the above-described anode passivation may be sufficiently implemented while preventing an excessive degradation of lithium ion mobility and activity of a cathode active material. Further, increase of a battery resistance and a battery thickness may be prevented under high temperature conditions.

The electrolyte solution for a rechargeable lithium battery according to example embodiments may further include an auxiliary additive together with the above-described additive.

The auxiliary additive may include, e.g., a cyclic carbonate-based compound, a fluorine-substituted carbonate-based compound, a sultone-based compound, a cyclic sulfate-based compound and a phosphate-based compound.

In some embodiments, a content of the auxiliary additive may be adjusted to be <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, 5wt% or less, <NUM> wt% or less, <NUM> wt% or less, <NUM> wt% or less, or <NUM> wt% or less based on the total weight of the electrolyte solution in consideration of an interaction with the additive including the compound represented by Chemical Formula <NUM>.

In one embodiment, the content of the auxiliary additive may be adjusted to be <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, <NUM> wt% or more, or <NUM> wt% or more in consideration of the SEI film stabilization.

In one embodiment, the auxiliary additive may be included in an amount from about <NUM> wt% to <NUM> wt%, from <NUM> wt% to <NUM> wt%, or from <NUM> wt% to <NUM> wt% based on the total weight of the electrolyte solution. Within the above range, durability of the protective film may be enhanced and high-temperature storage properties may be improved without degrading the function of the main additive.

The cyclic carbonate-based compound may include vinylene carbonate (VC), vinyl ethylene carbonate (VEC), etc..

The fluorine-substituted cyclic carbonate-based compound may include fluoroethylene carbonate (FEC).

The sultone-based compound may include <NUM>,<NUM>-propane sultone, <NUM>,<NUM>-propene sultone, <NUM>,<NUM>-butane sultone.

The cyclic sulfate-based compound may include <NUM>,<NUM>-ethylene sulfate, <NUM>,<NUM>-propylene sulfate, etc..

The phosphate-based compound may include an oxalatophosphate-based compound such as lithium bis(oxalato)phosphate.

In one embodiment, the fluorine-substituted cyclic carbonate-based compound, the sultone-based compound, the cyclic sulfate-based compound and the oxalatophosphate-based compound may be used together as the auxiliary additive.

Durability and stability of the electrode may be further improved by the addition of the auxiliary additive. The auxiliary additive may be included in an appropriate amount within a range that does not inhibit the lithium ion mobility in the electrolyte solution.

The organic solvent may include an organic compound that provides sufficient solubility for the lithium salt, the additive and the auxiliary additive and may have no substantial reactivity in the battery.

For example, 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, etc. These may be used alone or in a combination thereof.

In some embodiments, the organic solvent may include a carbonate-based solvent. The carbonate-based solvent may include a linear carbonate-based solvent and a cyclic carbonate-based solvent.

For example, the linear carbonate-based solvent may include at least one of dimethyl carbonate (DMC), ethyl methyl carbonate (EMC), diethyl carbonate (DEC), methyl propyl carbonate, ethyl propyl carbonate and dipropyl carbonate.

For example, the cyclic carbonate-based solvent may include at least one of ethylene carbonate (EC), propylene carbonate (PC) and butylene carbonate.

In some embodiments, in the organic solvent, an amount of the linear carbonate-based solvent may be greater than an amount of the cyclic carbonate-based solvent on a volume basis.

For example, a mixed volume ratio of the linear carbonate-based solvent and the cyclic carbonate-based solvent may be from <NUM>:<NUM> to <NUM>:<NUM>, and may be from <NUM>:<NUM> to <NUM>:<NUM> in an embodiment.

For example, the ester-based solvent may include at least one of methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), <NUM>,<NUM>-dimethylethyl acetate (DMEA), methyl propionate (MP) and ethyl propionate (EP).

For example, the ether-based solvent may include at least one of dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, tetrahydrofuran (THF) and <NUM>-methyltetrahydrofuran.

For example, the ketone-based solvent may include cyclohexanone. The alcohol-based solvent may include, e.g., at least one of ethyl alcohol and isopropyl alcohol.

For example, the aprotic solvent may include at least one of a nitrile-based solvent, an amide-based solvent (e.g., dimethylformamide), a dioxolane-based solvent (e.g., <NUM>,<NUM>-dioxolane), a sulfolane-based solvent, etc. These may be used alone or in a combination thereof.

The electrolyte may include, e.g., a lithium salt. For example, the lithium salt may be expressed as Li+X-. Non-limiting examples of the anion X- may include F-, Cl-, Br-, I-, NO<NUM>-, N(CN)<NUM>-, BF<NUM>-, ClO<NUM>-, PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF<NUM>-, (CF<NUM>)<NUM>PF-, (CF<NUM>)<NUM>P-, CF<NUM>SO<NUM>-, CF<NUM>CF<NUM>SO<NUM>-, (CF<NUM>SO<NUM>)<NUM>N-, (FSO<NUM>)<NUM>N-, CF<NUM>CF<NUM>(CF<NUM>)<NUM>CO-, (CF<NUM>SO<NUM>)<NUM>CH-, (SF<NUM>)<NUM>C-, (CF<NUM>SO<NUM>)<NUM>C-, CF<NUM>(CF<NUM>)<NUM>SO<NUM>-, CF<NUM>CO<NUM>-, CH<NUM>CO<NUM>-, SCN- , (CF<NUM>CF<NUM>SO<NUM>)<NUM>N-, etc..

In some embodiments, the lithium salt may include at least one of LiBF<NUM> and LiPF<NUM>.

In an embodiment, the lithium salt may be included in a concentration from <NUM> to <NUM>, or from <NUM> to <NUM> with respect to the organic solvent. Within the above range, transfer of lithium ions and/or electrons may be promoted during charging and discharging of the lithium secondary battery.

According to example embodiments, a lithium secondary battery having improved high-temperature stability and storage property is provided using the electrolyte solution.

<FIG> are a schematic plan view and a schematic cross-sectional view, respectively, illustrating a lithium secondary battery in accordance with exemplary embodiments. <FIG> is cross-sectional view taken along a line I-I' of <FIG>.

Referring to <FIG>, a lithium secondary battery may include an electrode assembly including a cathode <NUM> and an anode <NUM> facing the cathode <NUM>.

The cathode <NUM> may include a cathode current collector <NUM> and a cathode active material layer <NUM> on the cathode current collector <NUM>. The cathode active material layer <NUM> may include a cathode active material. The cathode active material layer <NUM> may further include a cathode binder and a conductive material.

For example, a cathode slurry may be prepared by mixing and stirring the cathode active material, the cathode binder, the conductive material, a dispersive medium, etc., and then the cathode slurry may be coated on the cathode current collector <NUM>, dried and pressed to form the cathode <NUM>.

For example, the cathode current collector <NUM> may include stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof.

The cathode active material may include lithium metal oxide particles capable of reversibly intercalating and de-intercalating lithium ions. In an embodiment, the cathode active material may include the lithium metal oxide particles containing nickel.

In some embodiments, the lithium metal oxide particle may include <NUM> mol% or more of nickel based on a total number of moles of all elements except lithium and oxygen. In this case, the lithium secondary battery having a high capacity may be implemented.

In some embodiments, the lithium metal oxide particle may include <NUM> mol% or more, <NUM> mol% or more, <NUM> mol% or more, or <NUM> mol% or more of nickel based on the total number of moles of all elements except lithium and oxygen.

In some embodiments, the lithium metal oxide particle may further include at least one of cobalt and manganese.

In some embodiments, the lithium metal oxide particle may further include cobalt and manganese. In this case, the lithium secondary battery having enhanced power and penetration stability may be implemented.

In one embodiment, the lithium metal oxide particle may be represented by Chemical Formula <NUM> below.

[Chemical Formula <NUM>]     LixNi(<NUM>-a-b)CoaMbOy.

For example, in Chemical Formula <NUM>, M may include at least one of Al, Zr, Ti, Cr, B, Mg, Mn, Ba, Si, Y, W and Sr, <NUM>≤x≤<NUM>, <NUM>≤y≤<NUM>, and <NUM>≤a+b≤<NUM>.

In some embodiments, <NUM><a+b≤<NUM>, <NUM><a+b≤<NUM>, <NUM><a+b≤<NUM>, <NUM><a+b≤<NUM>, <NUM><a+b≤<NUM>, <NUM><a+b≤<NUM>, and <NUM><a+b≤<NUM>.

In an embodiment, the lithium metal oxide particles may further include a coating element or a doping element. For example, the coating element or doping element may include Al, Ti, Ba, Zr, Si, B, Mg, P, Sr, W, La, an alloy thereof, or an oxide thereof. In this case, the lithium secondary battery having improved life-span properties may be implemented.

For example, the cathode binder may include an organic based binder such as polyvinylidenefluoride (PVDF), a polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyacrylonitrile, polymethylmethacrylate, etc., or an aqueous based binder such as styrene-butadiene rubber (SBR) that may be used with a thickener such as carboxymethyl cellulose (CMC).

For example, the conductive material may include a carbon-based material such as graphite, carbon black, graphene, carbon nanotube, etc., and/or a metal-based material such as tin, tin oxide, titanium oxide, a perovskite material such as LaSrCoO<NUM> or LaSrMnO<NUM>, etc..

The anode <NUM> may include an anode current collector <NUM> and an anode active material layer <NUM> on the anode current collector <NUM>. The anode active material layer <NUM> may include an anode active material, and may further include an anode binder and a conductive material.

For example, the anode active material may be mixed and stirred together with a binder and conductive material, etc., in a solvent to form an anode slurry. The anode slurry may be coated on the anode current collector <NUM>, dried and pressed to obtain the anode <NUM>.

For example, the anode current collector <NUM> may include gold, stainless steel, nickel, aluminum, titanium, copper, or an alloy thereof. In one embodiment, the anode current collector <NUM> may include copper or a copper alloy.

The anode active material may be a material capable of intercalating and de-intercalating lithium ions. For example, the anode active material may include a lithium alloy, a carbon-based material, a silicon-based material, etc..

For example, the lithium alloy may include a metal element such as aluminum, zinc, bismuth, cadmium, antimony, silicon, lead, tin, gallium, indium, etc..

For example, the carbon-based material may include a crystalline carbon, an amorphous carbon, a carbon composite, a carbon fiber, etc..

For example, the amorphous carbon may include hard carbon, coke, a mesocarbon microbead (MCMB) calcined at <NUM> or less, a mesophase pitch-based carbon fiber (MPCF), etc. The crystalline carbon may include, e.g., natural graphite, graphitized coke, graphitized MCMB, graphitized MPCF, etc..

In one embodiment, the anode active material may include the silicon-based material. For example, the silicon-based material may include Si, SiOx (<NUM><x<<NUM>), Si/C, SiO/C, a Si-Metal, etc. In this case, a lithium secondary battery having a high capacity may be implemented.

For example, when the anode active material includes the silicon-based material, the battery thickness may be increased during repeated charging and discharging. The lithium secondary battery according to embodiments of the present disclosures may include the above-described electrolyte solution, so that the increase of the battery thickness may be reduced or suppressed.

In some embodiments, a content of the silicon-based active material in the anode active material may be in a range from <NUM> wt% to <NUM> wt%, <NUM> wt% to <NUM> wt%, or <NUM> wt% to <NUM> wt%.

The anode binder and the conductive material may include materials substantially the same as or similar the above-described cathode binder and conductive material. For example, the anode binder may include the aqueous binder such as styrene-butadiene rubber (SBR) that may be used together with a thickener such as carboxymethyl cellulose (CMC).

In some embodiments, a separation layer <NUM> may be interposed between the cathode <NUM> and the anode <NUM>.

In some embodiments, an area of the anode <NUM> may be greater than an area of the cathode <NUM>. In this case, lithium ions generated from the cathode <NUM> may be easily transferred to the anode <NUM> without being precipitated.

The separation layer <NUM> may include a porous polymer film prepared from, e.g., a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene/butene copolymer, an ethylene/hexene copolymer, an ethylene/methacrylate copolymer, or the like. The separation layer <NUM> may also include a non-woven fabric formed from a glass fiber with a high melting point, a polyethylene terephthalate fiber, or the like.

In example embodiments, an electrode cell may be defined by the cathode <NUM>, the anode <NUM> and the separation layer <NUM>, and a plurality of the electrode cells may be stacked to form an electrode assembly <NUM>. For example, the electrode assembly <NUM> may be formed by winding, laminating or z-folding of the separation layer <NUM>.

The lithium secondary battery according to example embodiments may include a cathode lead <NUM> connected to the cathode <NUM> to protrude to an outside of a case <NUM>, and an anode lead <NUM> connected to the anode <NUM> to protrude to the outside of the case <NUM>.

For example, the cathode lead <NUM> may be electrically connected to the cathode current collector <NUM>. The anode lead <NUM> may be electrically connected to the anode current collector <NUM>.

The cathode current collector <NUM> may include a protrusion (a cathode tab, not illustrated) at one side thereof. The cathode active material layer <NUM> may not be formed on the cathode tab. The cathode tab <NUM> may be integral with the cathode current collector <NUM> or may be connected to the cathode current collector <NUM> by, e.g., welding. The cathode current collector <NUM> and the cathode lead <NUM> may be electrically connected via the cathode tab.

The anode current collector <NUM> may include a protrusion (an anode tab, not illustrated) at one side thereof. The anode active material layer <NUM> may not be formed on the anode tab. The anode tab <NUM> may be integral with the anode current collector <NUM> or may be connected to the anode current collector <NUM> by, e.g., welding. The anode electrode current collector <NUM> and the anode lead <NUM> may be electrically connected via the anode tab.

The electrode assembly <NUM> may include a plurality of the cathodes and a plurality of the anodes. For example, the cathodes and the anodes may be alternately disposed, and the separation layer may be interposed between the cathode and the anode. Each of the plurality of the cathodes may include the cathode tab. Each of the plurality of the anodes may include the anode tab.

In an embodiment, the cathode tabs (or the anode tabs) may be laminated, pressed and welded to form a cathode tab stack (or an anode tab stack). The cathode tab stack may be electrically connected to the cathode lead <NUM>. The anode tab stack may be electrically connected to the anode lead <NUM>.

The electrode assembly <NUM> may be accommodated together with the electrolyte solution according to the above-described embodiments in a case <NUM> to form the lithium secondary battery.

The lithium secondary battery may be fabricated into a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, etc..

Hereinafter, specific examples and comparative examples are proposed to more concretely describe the present invention. However, the following examples are only given for illustrating the present.

<NUM>-methyl-<NUM>-oxetanemethanol (<NUM>, <NUM> mmol), triethylamine (<NUM>, <NUM> mmol) and <NUM> of dichloromethane were sequentially added to a round-bottom flask to prepare a reaction solution, and then stirred. After cooling to <NUM>, methanesulfonyl chloride (<NUM>, <NUM> mmol) diluted with <NUM> of dichloromethane was slowly added to the reaction solution. Thereafter, the mixture was reacted while being stirred for <NUM> hours and maintaining the temperature.

The obtained product was washed with distilled water and saturated sodium chloride aqueous solution. The solvent was removed from the organic layer under reduced pressure and purified by a silica gel column chromatography to obtain <NUM> of <NUM>-(mesyloxymethyl)-<NUM>-methyloxetane as a yellow liquid sample (yield: <NUM>%).

<NUM>H-NMR (<NUM>, CDCl<NUM>): <NUM> (<NUM>, d), <NUM> (<NUM>, d), <NUM> (<NUM>, s), <NUM> (<NUM>, s), <NUM> (<NUM>, s).

A <NUM> LiPF<NUM> solution (EC/EMC/DEC mixed solvent in a <NUM>:<NUM>:<NUM> volume ratio) was prepared.

In the LiPF<NUM> solution, the additive and auxiliary additives were added and mixed with contents (wt%) shown in Table <NUM> below based on a total weight of an electrolyte solution to form electrolyte solutions of Examples and Comparative Examples.

Li[Ni<NUM>Co<NUM>Mn<NUM>]O<NUM>, carbon black and polyvinylidene fluoride (PVDF) were dispersed in N-methyl pyrrolidone (NMP) in a weight ratio of <NUM>:<NUM>:<NUM> to prepare a cathode slurry.

The cathode slurry was uniformly coated on a region of an aluminum foil having a protrusion (cathode tab) except for the protrusion, and dried and pressed to prepare a cathode.

Anode active material including graphite and SiOx (<NUM><x<<NUM>) in a weight ratio of <NUM>:<NUM>, styrene-butadiene rubber (SBR) and carboxymethyl cellulose (CMC) were dispersed in water by a weight ratio of <NUM>:<NUM>:<NUM> to form an anode slurry.

The anode slurry was uniformly coated on a region of a copper foil having a protrusion (anode tab) except for the protrusion, and dried and pressed to prepare an anode.

An electrode assembly was formed by interposing a polyethylene separator between the cathode and the anode. A cathode lead and an anode lead were connected to the cathode tab and the anode tab, respectively, by welding.

The electrode assembly was accommodated in a pouch (case) so that partial regions of the cathode lead and the anode lead were exposed to an outside, and three sides except for an electrolyte injection side were sealed.

A lithium secondary battery sample was prepared by injecting the electrolyte solution prepared in the above (<NUM>), sealing the electrolyte injection side, and then impregnating for <NUM> hours.

The components listed in Table <NUM> are as follows.

The lithium secondary batteries of Examples and Comparative Examples were subjected to <NUM>. 5C CC/CV charging (<NUM>. 05C CUT-OFF) and <NUM>. 5C CC discharging (<NUM>. 7V CUT-OFF) at <NUM> by three cycles, and a discharge capacity C<NUM> at the 3rd cycle was measured.

After storing the charged lithium secondary battery at <NUM>° C for <NUM> weeks, the batteries were additionally maintained at room temperature for <NUM> minutes, and a discharge capacity C2 was measured by <NUM>. 5C CC discharging (<NUM>. 75V CUT-OFF). A capacity retention was calculated as follows. The results are shown in Table <NUM> below and <FIG>.

The lithium secondary battery of each Examples and Comparative Examples was <NUM>. 5C CC/CV charged (<NUM>. 05C CUT-OFF) at <NUM>, and then <NUM>. 5C CC discharged until a SOC <NUM>%. At the SOC <NUM>% point, DCIR R1 was measured by discharging for <NUM> seconds and complementary charging while changing the C-rate to <NUM>. 5C, 1C, <NUM>. 5C, 2C and <NUM>.

The charged lithium secondary battery was exposed to air at <NUM> for <NUM> weeks. The battery was further left at room temperature for <NUM> minutes, and then DCIR R2 was measured by the same method as described above. An internal resistance increase ratio was calculated as follows. The results are shown in Table <NUM> below and <FIG>.

After charging the lithium secondary batteries of Examples and Comparative Examples at <NUM> under conditions of <NUM>. 5C CC/CV (<NUM>. 05C CUT-OFF), a battery thickness T1 was measured.

After exposing the charged lithium secondary batteries to air at <NUM> for <NUM> weeks (using a thermostat), a battery thickness T2 was measured. The battery thickness was measured using a plate thickness measuring device (Mitutoyo, <NUM>-490B). A battery thickness increase ratio was calculated as follows. The results are shown in Table <NUM> below and <FIG>.

Referring to Table <NUM> and <FIG>, the high-temperature storage performance of the lithium secondary battery of Example <NUM> was improved.

Specifically, in the lithium secondary battery of Example <NUM>, the resistance increase was suppressed, and gas generation was suppressed thereby reducing the thickness increase ratio.

Claim 1:
An electrolyte solution for a lithium secondary battery, comprising:
an additive including a compound represented by Chemical Formula <NUM>;
an organic solvent; and
a lithium salt:
<CHM>
wherein, in Chemical Formula <NUM>, R<NUM> and R<NUM> are each independently hydrogen or a substituted or unsubstituted C<NUM>-C<NUM> alkyl group, X is O or S, and L is a substituted or unsubstituted C<NUM>-C<NUM> alkylene group.