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

Abstract

The electrolyte for a lithium secondary battery according to an embodiment of the present invention includes an organic solvent, a lithium salt, and an additive including a cyclic ether sulfate compound. Thereby, the cycle characteristics and high-temperature stability of the lithium secondary battery can be improved.

Description

Technical Field

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same, and more particularly, to an electrolyte for a lithium secondary battery including an organic solvent and a lithium secondary battery including the same. Background Art

[0002] A secondary battery is a battery that can be repeatedly charged and discharged. With the rapid development of the information and communication and display industries, secondary batteries have been widely used as power sources for various portable telecommunication electronic devices (e.g., portable cameras, mobile phones, laptop computers). Recently, battery packs including secondary batteries have also been developed and applied to eco-friendly vehicles such as hybrid vehicles as their power sources.

[0003] Examples of secondary batteries may include lithium secondary batteries, nickel-cadmium batteries, nickel-metal hydride batteries, etc. Among them, lithium secondary batteries have a high operating voltage and a high energy density per unit weight, and are advantageous in terms of charging speed and light weight. In this regard, lithium secondary batteries have been actively developed and used as power sources.

[0004] For example, a lithium secondary battery may include an electrode assembly including a cathode, an anode, and a separator; and an electrolyte that immerses the electrode assembly. The lithium secondary battery may further include, for example, a pouch-type casing in which the electrode assembly and the electrolyte are accommodated.

[0005] Meanwhile, the electrolyte may include a lithium salt, such as LiPF6. The lithium salt may react with moisture to form hydrogen fluoride (HF). Due to hydrogen fluoride, the cycle characteristics and high-temperature storage characteristics of the battery may deteriorate.

[0006] For example, Korean Patent Laid-Open Publication No. 10-2012-0101499 discloses an electrolyte for a lithium secondary battery, but there is always a need to develop an electrolyte for improving the capacity characteristics of the battery.

[0007] [Prior Art Documents]

[0008] [Patent Documents]

[0009] Korean Patent Laid-Open Publication No. 10-2012-0101499 Summary of the Invention

[0010] An object of the present invention is to provide an electrolyte that can enable a lithium secondary battery to operate reliably with excellent chemical stability.

[0011] Another object of the present invention is to provide a lithium secondary battery including the electrolyte, such that the battery can operate reliably with excellent chemical stability.

[0012] To achieve the above object, according to one aspect of the present invention, there is provided an electrolyte for a lithium secondary battery, which includes: an organic solvent; a lithium salt; and an additive containing a cyclic ether sulfate compound.

[0013] In an exemplary embodiment, the cyclic ether sulfate compound may include a cyclic sulfate compound containing an ether group in the ring.

[0014] In an exemplary embodiment, the cyclic ether sulfate compound may be represented by the following Formula 1:

[0015] [Formula 1]

[0016]

[0017] In Formula 1, R 1 and R 2 are each independently a divalent alkyl group having 1 to 4 carbon atoms, and n is an integer of 1 to 8.

[0018] In an exemplary embodiment, the cyclic ether sulfate compound may contain an ethyleneoxy group.

[0019] In an exemplary embodiment, the cyclic ether sulfate compound may contain more than three ether groups.

[0020] In an exemplary embodiment, based on the total weight of the electrolyte, the cyclic ether sulfate compound may be included in an amount of 0.1 to 15% by weight.

[0021] In an exemplary embodiment, the additive may further include a cyclic sulfonate compound.

[0022] In an exemplary embodiment, the organic solvent may include at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC).

[0023] According to another aspect of the present invention, there is provided a lithium secondary battery, which includes: a cathode; an anode; a separator inserted between the cathode and the anode; and an electrolyte for a lithium secondary battery according to the present invention. According to an exemplary embodiment of the present invention, the cyclic ether sulfate compound can be used as an additive for the electrolyte of a lithium secondary battery. The cyclic ether sulfate (CES) compound can also form a solid electrolyte interphase (SEI) with excellent stability on the electrode surface, thereby suppressing the side reaction between the electrolyte and the active material. Therefore, the life, capacity retention rate, and high-temperature stability of the secondary battery can be improved, and the generation of gas and the increase in internal resistance can be suppressed during repeated use and exposure to high temperatures. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] The above and other objects, features, and other advantages of the present invention will be more clearly understood from the following detailed description in conjunction with the accompanying drawings, wherein:

[0025] Figure 1 A schematic cross-sectional view showing a lithium secondary battery according to an exemplary embodiment.

[0026] Figure 2 A graph showing the cycle capacity retention rate of a secondary battery including electrolytes of Examples and Comparative Examples.

[0027] Figure 3 A graph showing the rate of change in thickness during high-temperature storage of the anode of a secondary battery including electrolytes of Examples and Comparative Examples. Detailed Description

[0028] Embodiments of the present invention provide an electrolyte for a lithium secondary battery, which includes an organic solvent, a lithium salt, and a cyclic ether sulfate additive, and a lithium secondary battery including the electrolyte. Accordingly, the cycle characteristics and high-temperature stability of the lithium secondary battery can be improved.

[0029] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. However, since the drawings of the present application are only for showing one of the various preferred embodiments of the present invention to easily understand the technical spirit of the present invention having the above invention, they should not be construed as being limited to the descriptions shown in the drawings.

[0030] <Electrolyte for Lithium Secondary Battery>

[0031] An electrolyte for a lithium secondary battery (hereinafter, may be simply referred to as an electrolyte) according to an embodiment of the present invention may include an organic solvent, and a lithium salt and an additive mixed or dissolved in the organic solvent.

[0032] The organic solvent may include an organic compound that provides sufficient solubility for the lithium salt and the additive and is non-reactive in the lithium secondary battery. In an exemplary embodiment, the organic solvent may include a carbonate solvent, an ester solvent, an ether solvent, a ketone solvent, an alcohol solvent, an aprotic solvent, etc. These solvents may be used alone or in combination of two or more thereof.

[0033] Examples of the carbonate solvent 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, etc.

[0034] Examples of ester solvents may include methyl acetate (MA), ethyl acetate (EA), n-propyl acetate (n-PA), ethyl 1,1-dimethylacetate (DMEA), methyl propionate (MP), ethyl propionate (EP), γ-butyrolactone (GBL), decanolide, valerolactone, mevalonolactone, caprolactone, etc.

[0035] Examples of ether organic solvents may include dibutyl ether, tetraethylene glycol dimethyl ether (TEGDME), diethylene glycol dimethyl ether (DEGDME), dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc.

[0036] Examples of ketone solvents may include cyclohexanone, etc. Examples of alcohol solvents may include ethanol, isopropyl alcohol, etc.

[0037] Examples of aprotic solvents may include nitrile solvents, amide solvents (such as dimethylformamide (DMF), etc.), dioxolane solvents (such as 1,3-dioxolane, etc.), sulfolane solvents, etc.

[0038] In a preferred embodiment, carbonate solvents can be used as organic solvents. For example, the organic solvents may include ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), diethyl carbonate (DEC), or a combination thereof.

[0039] The lithium salt may include, for example, a compound represented by Li + X -

[0040] Non-limiting examples of the anion (X - ) of the lithium salt 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. The anions can be used alone or in combination of two or more of them. Preferably, the lithium salt includes lithium hexafluorophosphate (LiPF6).

[0041] In one embodiment, based on the organic solvent, the lithium salt can be included at a concentration of about 0.1 - 5M, preferably about 0.5 - 2M. Within the above range, lithium ions and / or electrons can move smoothly.

[0042] The additive can include cyclic ether sulfate compounds. The cyclic ether sulfate compounds can refer to cyclic compounds containing an ether group (-O-) and a sulfate group.

[0043] The cyclic ether sulfate compounds can further include an amine group (-NH- or -NR-), an ester group (-COO-), an amide group (-CONR-), a ketone group (-CO-), a carbonate group (-OCOO-), etc.

[0044] In an exemplary embodiment, the cyclic ether sulfate compounds can include a hydrocarbon chain in the ring. The hydrocarbon chain can include single bonds, double bonds or triple bonds. In some embodiments, the carbon in the hydrocarbon chain can be substituted with a hydroxyl group, an amino group, a thiol group, a halogen group, etc.

[0045] The cyclic ether sulfate compounds can include compounds containing an ether group in the ring of the cyclic sulfate compounds.

[0046] In an exemplary embodiment, the cyclic ether sulfate compounds can include compounds in which a sulfate group is substituted in the ring of a cyclic ether compound (such as a crown ether).

[0047] In an exemplary embodiment, the cyclic ether sulfate compounds can be represented by the following formula 1.

[0048] [Formula 1]

[0049]

[0050] In formula 1, R 1and R 2 Each independently represents an alkanediyl group having 1 to 4 carbon atoms, and n can be an integer from 1 to 8. Preferably, n is an integer from 2 to 6.

[0051] For example, R 1 and R 2 can include a methylene group (-CH2-), an ethylene group (-CH2CH2-), a propylene group (-C3H6-), or a butylene group (-C4H8-), and can be branched or linear. Preferably, R 1 and R 2 are each an ethylene group.

[0052] Preferably, R 1 and R 2 can be the same as each other. In this case, its molecular structure can have symmetry, and the two oxygen-bonded groups included in the sulfate group can have uniform reactivity. Therefore, the formation of the SEI film of the cyclic ether sulfate compound can be promoted.

[0053] In an exemplary embodiment, the cyclic ether sulfate compound can include an ethyleneoxy group (-CH2CH2O-). The cyclic ether sulfate compound can include more than three ether groups. In this case, the battery life and high-temperature storage characteristics can be improved.

[0054] In an exemplary embodiment, based on the total weight of the electrolyte, the cyclic ether sulfate compound can be included in an amount of 0.1 to 15 wt.%. If the content of the cyclic ether sulfate compound is less than 0.1 wt.%, the battery cycle characteristics and high-temperature storage characteristics may not be improved. If the content of the cyclic ether sulfate compound exceeds 15 wt.%, the internal resistance of the lithium secondary battery may increase excessively.

[0055] Preferably, based on the total weight of the electrolyte, the cyclic ether sulfate compound is included in an amount of 0.5 to 10 wt.%, and more preferably 0.5 to 5 wt.%.

[0056] For example, depending on its use, a metal (such as a transition metal) can be removed from the cathode of the lithium secondary battery. The removed metal may be electrodeposited on the anode, resulting in a decline in anode performance. Additionally, when the lithium secondary battery is driven at a high voltage, the film on the cathode surface may decompose, causing a side reaction between the cathode surface and the electrolyte.

[0057] Cyclic ether sulfate compounds contain both oxygen and sulfur, and oxygen bonds and sulfur bonds can be formed when the cyclic ether sulfate compounds decompose. The oxygen bonds and / or sulfur bonds can bind to the electrode surface to form a stable SEI film. In this case, the stability of the electrode surface structure can be improved, and side reactions between the electrode and the electrolyte can be suppressed. Therefore, gas generation can be suppressed during repeated charge / discharge or exposure to high temperatures, and battery swelling and resistance increase can be inhibited.

[0058] Cyclic ether sulfate compounds can promote the formation of a stable-structured SEI film to prevent side reactions between the electrolyte and the electrode. In this case, the reduction of lithium ions and the decomposition of the electrolyte and active substances can be suppressed. Therefore, the cycle characteristics and high-temperature stability of the secondary battery can be improved.

[0059] According to an exemplary embodiment, the additive can stabilize the cathode structure. In this case, when using and storing a lithium secondary battery at high temperatures, the elution of metals, gas generation, and volume (thickness) expansion can be suppressed. Therefore, the life and high-temperature storage characteristics of the lithium secondary battery can be improved. Additionally, when driven at a high voltage, an increase in the battery resistance can be suppressed.

[0060] In an exemplary embodiment, the electrolyte may contain additional additives, such as cyclic carbonate compounds containing double bonds, fluorine-substituted cyclic carbonate compounds, sultone compounds, cyclic sulfonate compounds, etc.

[0061] Cyclic carbonate compounds containing double bonds may include vinylene carbonate, ethylene vinylene carbonate, etc.

[0062] Fluorine-substituted cyclic carbonate compounds may include fluoroethylene carbonate.

[0063] Cyclic carbonate compounds containing double bonds and fluorine-substituted cyclic carbonate compounds can improve the thermal and electrical durability of the film formed on the electrode surface.

[0064] For example, based on the total weight of the electrolyte, the contents of cyclic carbonate compounds containing double bonds and fluorine-substituted cyclic carbonate compounds can each be 0.1 - 5 wt%. If their contents are less than 0.1 wt%, the durability of the film may decrease, and if their contents exceed 5 wt%, the thickness of the film may increase excessively. In this case, the battery resistance may increase and the output may decrease.

[0065] Sultone compounds may include 1,3-propane sultone, 1,3-propene sultone, 1,4-butane sultone, etc.

[0066] Cyclic sulfonate compounds may include ethylene sulfite, propylene sulfite, etc.

[0067] Sultone compounds and cyclic sulfonate compounds can form a more stable ion-conductive film on the electrode surface.

[0068] For example, based on the total weight of the electrolyte, the content of sultone compounds and cyclic sulfonate compounds can be 0.1 - 5 wt%. If the content is less than 0.1 wt%, the durability of the film may be reduced, and if the content exceeds 5 wt%, the thickness of the film may increase excessively. In this case, the resistance of the battery may increase and the output may decrease.

[0069] In some embodiments, the electrolyte can be a non-aqueous electrolyte without water.

[0070] <Lithium secondary battery>

[0071] Figure 1 It is a schematic cross-sectional view showing a lithium secondary battery according to an exemplary embodiment.

[0072] Referring to Figure 1 , the lithium secondary battery 100 may include an electrode assembly that includes a cathode 130, an anode 140, and a separator 150 inserted between the cathode and the anode. The electrode assembly may be accommodated in a housing 170 together with the electrolyte according to the above exemplary embodiment to be impregnated.

[0073] The cathode 130 may include a cathode active material layer 115 formed by applying a cathode active material to a cathode current collector 110. The cathode active material may include a compound capable of reversibly intercalating and deintercalating lithium ions.

[0074] In an exemplary embodiment, the cathode active material may include a lithium transition metal oxide. For example, the lithium transition metal oxide includes nickel (Ni), and may further include at least one of cobalt (Co) and manganese (Mn).

[0075] For example, the lithium transition metal oxide may be represented by the following formula 3.

[0076] [Formula 3]

[0077] Li 1+a Ni 1-(x+y) Co x M y O2

[0078] In Formula 3, α, x, and y can be in the ranges of -0.05 ≤ α ≤ 0.15, 0.01 ≤ x ≤ 0.3, and 0.01 ≤ y ≤ 0.3, and M can be at least one element selected from Mn, Mg, Sr, Ba, B, Al, Si, Ti, Zr, and W.

[0079] The slurry can be prepared by mixing the cathode active material with a binder, a conductive material, and / or a dispersant in a solvent and then stirring it. The slurry can be coated on the cathode current collector 110 and then dried and pressed to fabricate the cathode 130.

[0080] The cathode current collector 110 can include, for example, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and preferably, includes aluminum or an aluminum alloy.

[0081] The binder can be selected from, for example, organic binders such as polyvinylidene fluoride - hexafluoropropylene copolymer (PVDF - co - HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, etc., or aqueous binders such as styrene - butadiene rubber (SBR), and the binder can be used together with a thickener (such as carboxymethyl cellulose (CMC)).

[0082] For example, a PVDF - based binder can be used as the binder for the cathode. In this case, the amount of the binder used to form the cathode active material layer can be reduced, and the amount of the cathode active material can be relatively increased, thereby improving the output and capacity of the secondary battery.

[0083] A conductive material can be included to facilitate electron transfer between the active material particles. For example, the conductive material can include carbon - based conductive materials such as graphite, carbon black, graphene, or carbon nanotubes, and / or metal - based conductive materials such as tin, tin oxide, titanium oxide, or perovskite minerals (such as LaSrCoO3 or LaSrMnO3).

[0084] The anode 140 can include an anode current collector 120 and an anode active material layer 125 formed by coating an anode active material on the anode current collector 120.

[0085] According to an exemplary embodiment, a silicon (Si) - based compound can be used as the anode active material. In some embodiments, silicon - carbon particles including silicon carbide (SiC) or a carbon core and a silicon coating can be used as the anode active material.

[0086] For example, the silicon - carbon particles can be formed by depositing a silicon layer on the surface of a graphite core. In one embodiment, the silicon - carbon particles can be formed by coating a silicon layer on commercially available graphite particles using a silicon precursor compound (such as a silane compound) through a chemical vapor deposition (CVD) process.

[0087] In some embodiments, the silicon-carbon particles may have a structure in which multiple carbon coatings and silicon coatings are alternately coated or laminated on a graphite core.

[0088] Generally, carbon-based materials have been mainly used as anode active materials, and in the case of carbon-based materials, the theoretical capacity can be limited to a level of about 370 mAh / g. According to an exemplary embodiment, when a silicon-based compound is used as the anode active material, the output and capacity characteristics of the secondary battery can be significantly improved to exceed the theoretical capacity limit of carbon-based materials.

[0089] The anode current collector 120 may include, for example, gold, stainless steel, nickel, aluminum, titanium, copper, or their alloys, and preferably, includes copper or a copper alloy.

[0090] In some embodiments, a slurry can be prepared by mixing an anode active material with a binder, a conductive material, and / or a dispersant in a solvent and then stirring it. The slurry can be coated on the anode current collector 120 and then dried and pressed to fabricate the anode 140. As the conductive material, materials substantially the same as or similar to the above materials can be used.

[0091] In an exemplary embodiment, styrene-butadiene rubber (SBR) capable of reacting with the reactive additive of the above-described electrolyte can be used as a binder for the anode. In some embodiments, a thickener such as carboxymethyl cellulose (CMC) can be used together with SBR.

[0092] As described above, a silicon-based compound can be used as the anode active material to increase the capacity and output characteristics of the battery. However, the silicon-based compound has a high volume expansion / shrinkage rate, such that the anode active material layer 125 or the anode active material particles may repeatedly expand and contract during repeated charging and discharging.

[0093] In this case, the anode active material particles may decompose or collapse to be exposed to the electrolyte, and electrolyte depletion and an increase in resistance may be caused due to irreversible decomposition of the electrolyte.

[0094] However, according to an exemplary embodiment, by combining SBR as an anode binder with the reactive additive included in the electrolyte, expansion of the anode active material including the silicon-based compound can be suppressed.

[0095] The separator 150 can be inserted between the cathode 130 and the anode 140. The separator 150 can include a porous polymer membrane made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, ethylene / methacrylate copolymer. The separator can include a non-woven fabric made of glass fibers, polyethylene terephthalate fibers, etc. having a high melting point.

[0096] In some embodiments, the area and / or volume of the anode 140 (e.g., the contact area with the separator 150) can be greater than the area and / or volume of the cathode 130. Thus, for example, the lithium ions generated from the cathode 130 can smoothly move to the anode 140 without being precipitated in the middle.

[0097] According to an exemplary embodiment, the electrode unit 160 is defined by the cathode 130, the anode 140, and the separator 150, and a plurality of electrode units 160 are stacked to form, for example, a jelly roll type electrode assembly. For example, the electrode assembly can be formed by winding, laminating, folding, etc. of the separator.

[0098] According to an exemplary embodiment, the electrode assembly can be accommodated in the housing 170 together with the electrolyte to define the lithium secondary battery 100.

[0099] Electrode tabs can be formed respectively from the cathode current collector 110 and the anode current collector 120 belonging to each electrode unit 160, and can extend to one side of the housing 170. The electrode tabs can be fused to one side of the housing 170 to form electrode leads extending to or exposed to the outside of the housing 170.

[0100] The lithium secondary battery 100 can be manufactured, for example, in a cylindrical shape (using a can), a square shape, a pouch shape, or a coin shape.

[0101] Hereinafter, preferred embodiments are presented to more specifically describe the present invention. However, the following examples are only for illustrating the present invention, and those skilled in the relevant art will clearly understand that various changes and modifications can be made within the scope and spirit of the present invention. Such changes and modifications should be included in the appended claims.

[0102] Comparative Example: Preparation of a reference electrolyte solution

[0103] A reference electrolyte solution was prepared as follows: A 1.0 M LiPF6 solution was dissolved in a mixed solvent of EC / EMC (25 / 75; volume ratio), and then, based on the total weight of the mixed solvent, 3 wt% of vinylene carbonate, 1 wt% of LiPO2F2, 0.5 wt% of propane sultone, and 0.5 wt% of 1 - propene - 1,3 - sultone were added.

[0104] Examples 1 and 2

[0105] The electrolytes of Examples 1 and 2 were prepared as follows: Based on the total weight of the mixed solvent, 0.5 wt% and 1 wt% of the compound represented by Formula 2 below were added to the reference electrolyte solution prepared in the comparative example.

[0106] [Formula 2]

[0107]

[0108] Preparation Example: Preparation of a secondary battery

[0109] A slurry was prepared by mixing Li[Ni 0.6 Co 0.2 Mn 0.2 O2 as a cathode active material, carbon black as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 92:5:3. The prepared slurry was uniformly applied onto an aluminum foil with a thickness of 15 μm and vacuum dried at 130 °C to prepare a cathode for a lithium secondary battery.

[0110] An anode slurry was prepared, which included 95 wt% of a silicon graphite composite (SiC) (containing 10 wt% of silicon) as an anode active material, 1 wt% of Super-P as a conductive material, 2 wt% of styrene-butadiene rubber (SBR) as a binder, and 2 wt% of carboxymethyl cellulose (CMC) as a thickener. The prepared anode slurry was uniformly applied onto a copper foil with a thickness of 15 μm, and then dried and pressed to prepare an anode.

[0111] The cathode and anode prepared as described above were each cut into a predetermined size and laminated, and then a separator (polyethylene, thickness: 20 μm) was inserted between the cathode and the anode to fabricate an electrode unit. Subsequently, the tab portions of the cathode and the anode were welded respectively. The welded cathode / separator / anode assembly was placed in a bag, and then three sides of the bag except for the side for injecting the electrolyte were sealed. At this time, the sealed portion included the portion having the electrode tab. After injecting the electrolytes prepared in the comparative example and the examples through the remaining side other than the sealed portion, the remaining side was also sealed, and then impregnated for more than 12 hours to fabricate a lithium secondary battery. The fabricated secondary battery had a capacity of about 2.3 Ah.

[0112] Experimental Example 1: Life evaluation at room temperature

[0113] The discharge capacity was measured by repeatedly charging (CC / CV 1C 4.2V 0.1C cut-off) and discharging (CC 1C 2.7V cut-off) the fabricated secondary battery at about 35 °C, and then the capacity retention rate was calculated as a percentage compared with the initial discharge capacity after performing multiple charge / discharge cycles. The results are as Figure 2 shown in the graph in

[0114] As Figure 2As shown, in the case of the comparative example, it was confirmed that the capacity retention rapidly decreased after about 1000 charge and discharge cycles. However, in the case of the example, it was confirmed that the capacity was maintained at an appropriate level even after up to about 1200 charge and discharge cycles.

[0115] Experimental Example 2: Evaluation of High-Temperature Storage Characteristics

[0116] The fabricated secondary battery was placed in a chamber at 60 °C for 4 weeks and 6 weeks, and then at room temperature for 30 minutes.

[0117] (1) Evaluation of Capacity Retention

[0118] The placed secondary battery was charged (CC / CV 1C 4.2V 0.1C cut-off) and discharged (CC 1C 2.7V cut-off), and the discharge capacity was measured. Then, the capacity retention was calculated as a percentage compared to the initial discharge capacity at room temperature. The results are shown in Table 1 below.

[0119] (2) Evaluation of Anode Thickness Change

[0120] The thickness of the anode after storage at high temperature was measured, and the rate of thickness change was calculated as a percentage relative to the initial thickness of the anode. The results are shown in Table 1 below.

[0121] [Table 1]

[0122]

[0123]

[0124] Referring to Table 1, it can be confirmed that, compared with the comparative example without using the cyclic ether sulfate compound, in the case of the example, the capacity retention was improved and the amount of thickness increase was reduced during high-temperature storage.

[0125] [Description of Reference Numerals]

[0126] 110: Cathode Current Collector

[0127] 115: Cathode Active Material Layer

[0128] 120: Anode Current Collector

[0129] 125: Anode Active Material Layer

[0130] 130: Cathode

[0131] 140: Anode

[0132] 150: Separator

[0133] 160: Electrode Unit

[0134] 170: Case

Claims

1. An electrolyte for a lithium secondary battery, comprising: an organic solvent; a lithium salt; and an additive, the additive comprising a cyclic ether sulfate compound represented by Formula 1: [Formula 1] In Formula 1, R 1 and R 2 each independently represents a divalent alkyl group having 1 to 4 carbon atoms, and n represents an integer from 1 to 8. wherein the cyclic ether sulfate compound is included in an amount of 0.1-15% by weight based on the total weight of the electrolyte.

2. The electrolyte for a lithium secondary battery according to claim 1, wherein, The cyclic ether sulfate compound includes an ethoxy group.

3. The electrolyte for a lithium secondary battery according to claim 1, wherein, The cyclic ether sulfate compound includes three or more ether groups.

4. The electrolyte for a lithium secondary battery according to claim 1, wherein, The additive further comprises a cyclic sulfonate compound.

5. The electrolyte for a lithium secondary battery according to claim 1, wherein, The organic solvent includes at least one of ethylene carbonate (EC), ethyl methyl carbonate (EMC), dimethyl carbonate (DMC), or diethyl carbonate (DEC).

6. A lithium secondary battery, comprising: a cathode; an anode; a separator interposed between the cathode and the anode; and the electrolyte for a lithium secondary battery according to claim 1.

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