Nonaqueous electrolyte for lithium secondary battery and lithium secondary battery containing same

Abstract

The present invention relates to a non-aqueous electrolyte for a lithium secondary battery, the non-aqueous electrolyte comprising an organic solvent, a lithium salt, a first additive which is a compound represented by Chemical Formula 1, a second additive which is lithium tetrafluoroborate, and a third additive which contains a saturated nitrile compound and an unsaturated nitrile compound, the lithium salt being different from the second additive, and a lithium secondary battery comprising the non-aqueous electrolyte.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0114193, filed with the Korean Intellectual Property Office on August 27, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. [Background technology]

[0003] Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode including a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode including a negative electrode active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting a non-aqueous electrolyte solution that serves as a medium for transferring lithium ions, and then sealing the battery case.

[0004] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and therefore are applied in various fields such as mobile devices, electronic products, electric vehicles, etc. As the application fields of lithium secondary batteries become more diverse, the required physical properties are also becoming more stringent. In particular, there is a demand for the development of lithium secondary batteries that can be stably operated even under high voltage and high temperature conditions.

[0005] On the other hand, when a lithium secondary battery is operated under high voltage and high temperature conditions, lithium salts such as LiPF6 contained in the electrolyte are converted to PF6 - The thermal decomposition of anions can generate Lewis acids such as PF5, which react with water to generate HF. These decomposition products, such as PF5 and HF, can destroy the coating on the electrode surface and can also cause decomposition reactions of the organic solvent. Furthermore, they can react with the decomposition products of the positive electrode active material to leach transition metal ions, which can then be electrodeposited on the negative electrode and destroy the coating on the negative electrode.

[0006] If the electrolyte decomposition reaction continues on the destroyed coating, battery performance will further deteriorate, so there is a need to develop secondary batteries that can maintain excellent performance even under high voltage and high temperature conditions. In particular, it is known that oxidation of the electrolyte accelerates in the high voltage range of 4.25 V or higher, and the decomposed electrolyte undergoes side reactions at the positive and negative electrode interfaces, forming unstable structures that reduce life characteristics and high-temperature storage characteristics. Summary of the Invention [Problem to be solved by the invention]

[0007] The present invention has been made to solve the above-mentioned problems, and an object of the present invention is to provide a nonaqueous electrolyte that can effectively solve not only the problem of metal ions eluting from the positive electrode in a high voltage region of 4.25 V or more, but also the problem of severe decomposition of the SEI film of a negative electrode containing a silicon-based negative electrode active material, and a lithium secondary battery containing the same. [Means for solving the problem]

[0008] According to one embodiment, the present invention comprises: an organic solvent; A lithium salt, a first additive which is a compound represented by the following chemical formula 1; a second additive which is lithium tetrafluoroborate; a third additive including a saturated nitrile compound and an unsaturated nitrile compound; The nonaqueous electrolyte for a lithium secondary battery is provided, wherein the lithium salt is different from the second additive.

[0009] [ka]

[0010] In the above Chemical Formula 1, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, or R1 and R2 are bonded to each other to form a ring having 2 to 10 carbon atoms and containing a sulfate group.

[0011] According to another embodiment, the present invention provides a lithium secondary battery including: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte solution for the lithium secondary battery. [Effects of the Invention]

[0012] The present invention can provide a lithium secondary battery that contains the above-mentioned nonaqueous electrolyte solution for a lithium secondary battery and that has excellent life and resistance characteristics even under high voltage and high temperature conditions. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 2 is a graph showing the discharge energy retention rates measured after high-voltage charging and high-temperature storage of lithium secondary batteries manufactured in Examples and Comparative Examples. DETAILED DESCRIPTION OF THE INVENTION

[0014] The present invention will now be described in more detail.

[0015] Generally, anions such as LiPF6, a lithium salt widely used in lithium secondary batteries, will form decomposition products such as hydrogen fluoride (HF) and PF5 due to thermal decomposition or moisture. Such decomposition products have acidic properties and deteriorate the coating or electrode surface in the battery.

[0016] Specifically, the decomposition products tend to dissolve transition metals constituting the positive electrode into the electrolyte, and the dissolved transition metal ions migrate to the negative electrode through the electrolyte and are then electrodeposited on a solid electrolyte interphase (SEI) film formed on the negative electrode, causing further electrolyte decomposition reactions.

[0017] This series of reactions not only reduces the amount of available lithium ions in the battery, resulting in a decrease in battery capacity, but also leads to an increase in resistance due to the accompanying further electrolyte decomposition reactions.

[0018] Furthermore, if metal impurities are present in the positive electrode during initial charging, they will leach out of the positive electrode, migrate to the negative electrode, and be electrodeposited as metal ions on the surface of the negative electrode. The electrodeposited metal ions will grow in a dendritic pattern, causing an internal short circuit in the battery and becoming a major cause of low-voltage defects.

[0019] In the present invention, the eluted metal ions that cause such deterioration and poor behavior are removed from inside the battery, preventing them from being electrodeposited on the surface of the electrodes, and by forming a strong coating on the surface of the positive and negative electrodes, it is possible to suppress the elution of transition metals and control the electrodeposition reaction at the negative electrode.By controlling the electrochemical decomposition reaction of the electrolyte, it is possible to control the gas phase by-products caused by the decomposition of the electrolyte, thereby improving the durability of the battery.

[0020] Specifically, the inventors have confirmed that by using a compound represented by the following chemical formula 1, LiBF4, and a nitrile-based compound as additives to a non-aqueous electrolyte, decomposition products generated from lithium salts can be effectively removed and a coating can be formed on the positive electrode / negative electrode, thereby preventing a continuous decomposition reaction between the positive electrode and an organic solvent.

[0021] The compound represented by the formula 1 forms a strong SEI film on the initial negative electrode surface, creating active sites on the negative electrode surface, and - It was confirmed that inducing stable production of ions has the effect of reducing the resistance of the negative electrode.

[0022] Furthermore, it was confirmed that LiBF4 forms a protective film on the surface of the positive electrode due to the strong bonding of BF, which prevents metal elution at high temperatures and improves electrical conductivity.

[0023] Therefore, when the compound represented by Chemical Formula 1 is used together with LiBF4, the surfaces of the positive and negative electrodes are protected even in a high voltage region of 4.25 V or more where the battery degradation is accelerated due to the oxidation of the electrolyte and the resulting side reactions, thereby improving the high-temperature storage characteristics.

[0024] Furthermore, it was confirmed that the nonaqueous electrolyte solution of the present invention, which contains a nitrile compound, can form a stable SEI film on the surface of the negative electrode during initial charge and discharge, and that the nitrile compound has the property of having strong binding energy with metal ions in the positive electrode active material, and thus forms a coating with strong binding force on the surface of the positive electrode active material through stable and strong bonding with the metal ions, thereby achieving a synergistic effect with the additive.

[0025] In particular, since the composition contains both a saturated nitrile compound and an unsaturated nitrile compound, a synergistic effect can be achieved. Specifically, the unsaturated nitrile compound can form a stable coating because it can form a stronger binding bond with the positive electrode active material than the saturated nitrile compound, and the saturated nitrile compound can prevent the transition metal remaining in the electrolyte from migrating to the negative electrode and being deposited thereon due to a chelate reaction with the transition metal eluted from the positive electrode.

[0026] non-aqueous electrolyte (1) Additives The non-aqueous electrolyte of the present invention contains a first additive, which is a compound represented by the following chemical formula 1:

[0027] [ka]

[0028] In the above Chemical Formula 1, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, or R1 and R2 are bonded to each other to form a ring having 2 to 10 carbon atoms and containing a sulfate group.

[0029] In one embodiment of the present invention, the first additive may be represented by the following chemical formula 1-1.

[0030] [ka]

[0031] In the above chemical formula 1-1, m is an integer of 0 to 6, and n is an integer of 1 or 2.

[0032] In one embodiment of the present invention, m in Formula 1-1 may be 0 and n may be 1. That is, the first additive may be ethylene sulfate.

[0033] In one embodiment of the present invention, the content of the first additive may be 0.1 wt % to 2 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.2 wt % to 0.5 wt %, based on the total weight of the non-aqueous electrolyte solution.

[0034] When the content of the first additive is 0.1 wt % or more, a strong SEI film is formed on the surface of the negative electrode and the collapse of the SEI film due to the decomposition of the electrolyte can be prevented, and when the content is 2 wt % or less, an SEI film of appropriate thickness can be formed on the surface of the negative electrode. If the SEI film thickness is excessively large, it acts as a resistance that hinders lithium migration, which can cause a problem of reduced capacity due to reversible lithium loss.

[0035] The non-aqueous electrolyte of the present invention contains lithium tetrafluoroborate (LiBF4) as a second additive.

[0036] In one embodiment of the present invention, the content of the second additive may be 0.1 wt % to 1 wt %, preferably 0.1 wt % to 0.5 wt %, and more preferably 0.1 wt % to 0.3 wt %, based on the total weight of the non-aqueous electrolyte solution.

[0037] When the content of the second additive is 0.1 wt % or more, a protective coating is formed on the surface of the positive electrode, which prevents metal elution from the positive electrode and improves electrical conductivity, and when the content is 1 wt % or less, a protective coating of appropriate thickness is formed. If the thickness of the protective coating formed on the surface of the positive electrode is excessively increased, it may act as resistance, resulting in a problem of reduced capacity due to loss of soluble lithium.

[0038] In the nonaqueous electrolyte of the present invention, the weight ratio of the first additive to the second additive may be 1:1 or more and 3:1 or less, preferably more than 1:1 and 3:1 or less, and more preferably 2:1. When the first additive and the second additive are contained within this weight ratio, a coating of an appropriate thickness is formed on the surfaces of the positive electrode and the negative electrode, thereby suppressing metal elution at high voltage and high temperature, improving electrical conductivity, and preventing loss of soluble lithium.

[0039] The non-aqueous electrolyte of the present invention contains a third additive containing a saturated nitrile compound and an unsaturated nitrile compound.

[0040] The saturated nitrile compound may be any one or more selected from the group consisting of succinonitrile (SN), adiponitrile (ADN), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), and 1,2,3-tris(2-cyanoethyl)propane (TCEP), preferably any one or more selected from the group consisting of succinonitrile, adiponitrile, and 1,3,6-hexanetricarbonitrile, more preferably succinonitrile.

[0041] The unsaturated nitrile compounds include 1,4-dicyano-2-butene (DCB), 1,4-dicyano-2-methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene, 1,6-dicyano-2-methyl-5-methyl-3-hexene, 2-fluorobenzonitrile, 4 ... The fluorophenyl acetonitrile may be any one or more selected from the group consisting of 1,4-dicyano-2-butene, 1,4-dicyano-2-methyl-2-butene, 1,4-dicyano-2-butene, 1,4-dicyano-2-butene, 1,4-dicyano-2-butene, or 1,4-dicyano-2-butene.

[0042] In one embodiment of the present invention, the third additive may be succinonitrile and 1,4-dicyano-2-butene, which not only forms a stable coating on the positive electrode and negative electrode but also prevents the deposition of transition metals, as described above.

[0043] In one embodiment of the present invention, the content of the third additive may be 0.1 wt % to 2 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.2 wt % to 0.8 wt %, based on the total weight of the non-aqueous electrolyte solution.

[0044] When the third additive is 0.1 wt % or more in the non-aqueous electrolyte solution, the effect of the nitrile compound described above is significantly obtained, and when it is 2 wt % or less, a coating of an appropriate thickness is formed, which is preferable in that an increase in resistance can be prevented and battery performance can be maintained.

[0045] The non-aqueous electrolyte of the present invention may optionally further contain a fourth additive, as needed, to prevent the non-aqueous electrolyte from decomposing in a high-voltage environment, thereby causing electrode collapse, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and the effects of suppressing battery expansion at high temperatures.

[0046] The fourth additive may be at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, amine-based compounds, silane-based compounds, benzene-based compounds, and lithium salt-based compounds.

[0047] The cyclic carbonate compound may be vinylene carbonate (VC), vinyl ethylene carbonate (VEC), or a mixture thereof, and specifically may be vinylene carbonate.

[0048] The halogen-substituted carbonate compound may be fluoroethylene carbonate (FEC).

[0049] The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be any one or more compounds selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone, and specifically may be 1,3-propane sultone (PS).

[0050] The phosphate-based or phosphite-based compound may be any one or more selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0051] The borate-based compound may be lithium tetraphenylborate.

[0052] The amine-based compound may be triethanolamine, ethylenediamine, or a mixture thereof, and the silane-based compound may be tetravinylsilane.

[0053] The benzene-based compound may be any one or more selected from the group consisting of monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

[0054] The lithium salt-based compound is a compound different from the lithium salt and the second additive contained in the non-aqueous electrolyte, and may be any one or more compounds selected from the group consisting of lithium difluorophosphate (LiDFP; LiPOF), lithium bis(oxalato)borate (LiBOB; LiB(C0)), and lithium difluoro(bis(oxalato)phosphate) (LiDFOP).

[0055] Preferably, the additive for the non-aqueous electrolyte according to one embodiment of the present invention may further include, as a fourth additive, vinylene carbonate (VC), 1,3-propane sultone (PS), or a mixture thereof, and more preferably may include both vinylene carbonate (VC) and 1,3-propane sultone (PS).

[0056] The content of the fourth additive may be 0.1 wt % to 3 wt %, preferably 0.1 wt % to 2 wt %, and more preferably 0.3 wt % to 2 wt %, based on the total weight of the non-aqueous electrolyte solution. When the content of the fourth additive is within the above range, it is effective in suppressing gas generation and surface structure collapse.

[0057] (2) Organic solvent The non-aqueous electrolyte of the present invention contains an organic solvent.

[0058] The organic solvent may be any of various organic solvents commonly used in lithium electrolytes, without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a linear carbonate solvent, a linear ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof, and preferably may contain a cyclic carbonate solvent and a linear carbonate solvent. In this case, the volume ratio of the cyclic carbonate solvent to the linear carbonate solvent may be 3:7 to 1:9.

[0059] The cyclic carbonate solvent is a highly viscous organic solvent having a high dielectric constant, and therefore can effectively dissociate the lithium salt in the electrolyte. The cyclic carbonate solvent may be at least one selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and specifically may be ethylene carbonate (EC).

[0060] The linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be any one or more selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate, and specifically may be dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC).

[0061] The organic solvent is preferably a mixture of a cyclic carbonate solvent and a linear carbonate solvent in order to produce an electrolyte solution having high ionic conductivity.

[0062] The linear ester solvent may be any one or more selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0063] The cyclic ester solvent may be any one or more selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0064] Unless otherwise specified, the remainder of the total weight of the non-aqueous electrolyte solution excluding other components other than the organic solvent, for example, the additive and the lithium salt, is the organic solvent.

[0065] (3) Lithium salt The non-aqueous electrolyte of the present invention contains a lithium salt different from the second additive.

[0066] The lithium salt may be any one that is commonly used in electrolytes for lithium secondary batteries, and may be any one of those containing Li as a cation. + and as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO4 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N- , BF2C2O4 - , BC4O8 - , BF2C2O4CHF - , PF4C2O4 - , PF2C4O8 - , PO2F2 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - , and SCN - It may include any one or more selected from the group consisting of:

[0067] Specifically, the lithium salts include LiPF6, LiN(FSO2)2 (LiFSI), LiClO4, LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO3CF3, LiPO2F2, lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFBP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiTFOP). The lithium salt may be any one or more selected from the group consisting of LiF6, LiN(FSO2)2 (LiFSI), or a mixture thereof. When both LiPF6 and LiFSI are used as the lithium salt, PF6 - This has the advantage of being able to suppress the above-mentioned effects and improve high-temperature characteristics.

[0068] In one embodiment of the present invention, the concentration of the lithium salt in the non-aqueous organic solution containing the lithium salt and the organic solvent may be 0.3 M to 3.0 M, specifically 1.0 M to 2.0 M, more specifically 1.4 M to 2.0 M. When the concentration of the lithium salt is within the above range, the effects of improving low-temperature output and cycle characteristics are sufficiently ensured, and excessive increases in viscosity and surface tension are prevented, resulting in appropriate electrolyte impregnation.

[0069] Lithium secondary battery Next, the lithium secondary battery according to the present invention will be described.

[0070] The lithium secondary battery according to the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte solution, wherein the non-aqueous electrolyte solution is the non-aqueous electrolyte solution according to the present invention. Since the non-aqueous electrolyte solution is as described above, a description thereof will be omitted and the other components will be described below.

[0071] (1) Positive electrode The positive electrode according to the present invention includes a positive electrode active material and can be manufactured by coating a positive electrode slurry containing the positive electrode active material, a binder, a conductive material, a solvent, etc., on a positive electrode current collector, followed by drying and rolling.

[0072] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like may be used.

[0073] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and includes LiNiO (LiCoO2), LNO (LiNiO2, Li2NiO2), LMO (LiMnO2), LiMn2O4, LiCoPO4, LFP (LiFePO4), and LiNiMnCoO2. 1-x-y-z Co x M 1 y M 2 z O2(M 1 and M 2 are each independently any one selected from the group consisting of Al, Ni, Co, Fe, Mn, V, Cr, Ti, W, Ta, Mg, and Mo, and x, y, and z are each independently any one or more selected from the group consisting of atomic fractions of oxide composition elements, where 0≦x<0.5, 0≦y<0.5, 0≦z<0.5, and x+y+z=1.

[0074] In one embodiment of the present invention, the positive electrode active material may be a lithium transition metal composite oxide in which the content of nickel in all metals excluding lithium is 70 mol % or more, preferably 80 mol % or more, and more preferably 85 mol % or more.

[0075] In one embodiment of the present invention, the positive electrode active material may be represented by the following chemical formula 2:

[0076] [Chemical formula 2] Li 1+e (Ni a Co b Mn c M d )O2

[0077] In the above Chemical Formula 2, M is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; a, b, c, and d are the atomic fractions of each independent element; 0≦e≦0.5, 0.8≦a<1, 0 <b≦0.15、0<c≦0.15、0≦d≦0.1、a+b+c+d=1である。

[0078] Preferably, the a, b, c, and d in the chemical formula 2 are 0.85≦a<1, 0 <b≦0.1、0<c≦0.1、0≦d≦0.05、a+b+c+d=1である。

[0079] More preferably, the a, b, c, and d in the chemical formula 2 are in the range of 0.88≦a<1, 0 <b≦0.09、0<c≦0.05、0≦d≦0.03であってもよい。

[0080] The positive electrode active material of the present invention can achieve high energy density due to its high Ni content, but has the disadvantage of deteriorating stability at high voltages due to the occurrence of a phase transition from a surface layered structure to a rock salt structure, which causes elution of Ni, Co, and Mn metals. However, the nonaqueous electrolyte according to one embodiment of the present invention has the effect of strengthening the electrode coating and preventing performance degradation at high voltages and high temperatures, thereby improving the performance of lithium secondary batteries containing high-nickel positive electrode active materials.

[0081] In one embodiment of the present invention, the positive electrode may further include a compound represented by the following Chemical Formula 3 containing an excess amount of lithium:

[0082] [Chemical formula 3] Li2M a 1-k M b k O2

[0083] In the above Chemical Formula 3, M a is one or more elements selected from the group consisting of Ni and Cu, M b is one or more elements selected from the group consisting of Mn, Fe, Co, Zn, Mg, and Cd; k is 0≦k<1.

[0084] In one embodiment of the present invention, M of Formula 3 a is Ni.

[0085] In one embodiment of the present invention, the compound represented by Formula 3 is Li2NiO2.

[0086] To realize a battery with a high energy density, it is preferable to use a silicon-based material with a high capacity as the anode active material. However, since silicon-based anode active materials have a high lithium ion loss rate due to irreversible reactions during the initial charge / discharge process, when they are used together with a cathode active material with high initial charge / discharge efficiency, problems such as a decrease in charge capacity and lithium precipitation may occur.

[0087] According to one embodiment of the present invention, when the compound represented by Chemical Formula 3 is used together with a cathode active material, it acts as a sacrificial cathode material that compensates for the high initial irreversible capacity of a lithium secondary battery containing a silicon-based material as an anode active material, and is therefore preferred in terms of being applicable to batteries having high energy density.

[0088] In one embodiment of the present invention, the initial efficiency of the positive electrode may be 85% to 89%, which can reduce the loss of lithium ions in a lithium secondary battery that does not contain the compound represented by Chemical Formula 3, which is a sacrificial positive electrode material, and that contains a silicon-based negative electrode active material.

[0089] Here, the initial efficiency refers to the ratio of the initial discharge capacity to the initial charge capacity, measured after charging and discharging a coin cell-type half cell manufactured using a positive electrode containing the lithium composite transition metal oxide represented by Chemical Formula 2 as a positive electrode active material, a lithium metal counter electrode, and an electrolyte prepared by dissolving 1.0 M LiPF in a mixed solvent of ethylene carbonate and dimethyl carbonate (volume ratio: 3:7) once at a rate of 0.2 C in the range of 2.5 V to 4.25 V.

[0090] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry. In this case, if the content of the positive electrode active material is less than 80 wt%, the energy density may be reduced, resulting in a decrease in capacity.

[0091] The binder is a component that aids in binding the active material and conductive material, etc., and in binding to the current collector, and may typically be added in an amount of 1 wt % to 30 wt % based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0092] The conductive material of the positive electrode slurry is a substance that imparts conductivity to the battery without causing any chemical change to the battery, and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the positive electrode slurry.

[0093] The conductive material of the positive electrode slurry may be selected from, but is not limited to, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powder such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0094] The solvent for the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, binder, conductive material, etc. are contained. For example, the positive electrode slurry containing the positive electrode active material, binder, and conductive material may be contained so that the solids concentration in the positive electrode slurry is 40 wt % to 90 wt %, preferably 50 wt % to 80 wt %.

[0095] (2) Negative electrode The negative electrode according to the present invention contains a negative electrode active material, and can be manufactured by coating a negative electrode slurry containing a negative electrode active material, a binder, a conductive material, a solvent, etc. on a negative electrode current collector, and then drying and rolling.

[0096] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity. For example, copper; stainless steel; aluminum; nickel; titanium; fired carbon; a surface-treated one with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel; or an aluminum-cadmium alloy, etc. may be used. Also, similar to the positive electrode current collector, the bonding force of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and it may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0097] In one embodiment of the present invention, the negative electrode active material may include a silicon-based material, a carbon-based material, or a mixture thereof, and preferably may include a carbon-based material and a silicon-based material.

[0098] The silicon-based material is Si, SiO x (0 < x < 2) and Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Si), and is any one or more selected from the group, and preferably SiO.

[0099] Since the silicon-based negative electrode active material has a capacity nearly about 10 times higher than that of graphite, the mass loading (mg·cm -2) and improve the fast charging performance of the battery. Silicon-based negative electrode active materials have problems such as a high lithium ion loss rate due to irreversible reactions and large volume changes, which can adversely affect the battery life. However, the use of the aforementioned nonaqueous electrolyte can solve these problems. Specifically, negative electrodes containing silicon-based negative electrode active materials contain more oxygen-rich (O-rich) components in the SEI film than negative electrodes made of 100% graphite. SEI films containing such oxygen-rich components tend to be more susceptible to decomposition in the presence of Lewis acids such as HF or PF5 in the electrolyte. Therefore, maintaining a stable SEI film is important for negative electrodes containing silicon-based negative electrode active materials. The nonaqueous electrolyte according to the present invention, which includes the aforementioned nonaqueous electrolyte, can effectively solve the SEI film decomposition problem that occurs during the use of negative electrodes containing silicon-based active materials.

[0100] In one embodiment of the present invention, the silicon-based material may be present in an amount of 1 wt % to 20 wt %, preferably 3 wt % to 15 wt %, based on the total weight of the negative electrode active material. When the silicon-based material is present in the above range, it has the effect of increasing the negative electrode capacity and improving the fast charging performance.

[0101] In one embodiment of the present invention, the carbon-based material may be any material commonly used in lithium-ion secondary batteries without any particular limitation, and representative examples thereof include at least one selected from the group consisting of crystalline carbon such as natural graphite and artificial graphite; and amorphous carbon such as soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke.

[0102] In addition to the silicon-based material and carbon-based material, the negative electrode active material of the present invention may further include metals or alloys of these metals with lithium; metal composite oxides; materials capable of doping and dedoping lithium; lithium metal; and transition metal oxides.

[0103] As the metal or an alloy of these metals and lithium, a metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn or an alloy of these metals and lithium can be used.

[0104] As the metal composite oxide, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1), and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements of Group 1, Group 2, Group 3 of the periodic table, halogen; 0<x≦1; 1≦y≦3; 1≦z≦8) Any one or more selected from the group consisting of can be used.

[0105] As the substance capable of doping and undoping lithium, Si, SiO x (0<x<2), Si-Y alloy (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Si), Sn, SnO2, Sn-Y (where Y is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a transition metal, a rare earth element, and a combination thereof, and is not Sn), etc. can be mentioned, and at least one of these and SiO2 may be mixed and used. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (Dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and a combination thereof.

[0106] Examples of the transition metal oxide include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide.

[0107] The negative electrode active material may be contained in an amount of 80 wt % to 99 wt % based on the total weight of the solid content in the negative electrode slurry.

[0108] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and may be added in an amount of 1 to 30% by weight based on the total weight of the solid content in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0109] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the negative electrode slurry. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and may be selected from, for example, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powder such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fiber and metal fiber; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0110] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a suitable viscosity when containing the negative electrode active material, binder, conductive material, etc. For example, the solvent may be included so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 30 wt % to 80 wt %, preferably 40 wt % to 70 wt %.

[0111] (3) Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

[0112] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator that is commonly used as a separator in a lithium secondary battery can be used without any particular limitation. In particular, a separator that has low resistance to the movement of ions in the electrolyte, excellent electrolyte impregnation ability, and safety is preferred.

[0113] Specifically, the separator may be a porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer; or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymeric substance may also be used, and may be used in a single-layer or multi-layer structure.

[0114] The lithium secondary battery according to the present invention can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs).

[0115] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0116] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0117] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0118] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in a medium- to large-sized battery module containing a large number of battery cells.

[0119] The present invention will be specifically described below with reference to specific examples.

[0120] <Examples and Comparative Examples: Manufacture of Lithium Secondary Battery> Comparative Example 1 (Production of non-aqueous electrolyte) Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed in a volume ratio of 20:10:70, and then LiPF6 was dissolved in the mixture to a concentration of 1.4 M to prepare a non-aqueous organic solution. 2 wt% vinylene carbonate (VC), 1 wt% 1,3-propane sultone (PS), 0.3 wt% succinonitrile (SN), 0.3 wt% 1,4-dicyano-2-butene (DCB), and the remainder of the non-aqueous organic solution were mixed to prepare a 100 wt% non-aqueous electrolyte.

[0121] (Lithium secondary battery manufacturing) N-methyl-2-pyrrolidone (NMP) was used as the positive electrode active material, and Li[Ni 0.88 Co 0.07 Mn 0.04 Al 0.01 A cathode slurry (solid content: 70 wt%) was prepared by mixing ]O2(NCMA) and Li2NiO2(LNO) (NCMA:LNO = 95:5 weight ratio), a conductive material (carbon black), and a binder (polyvinylidene fluoride) in a weight ratio of 97.5:1:1.5. The cathode slurry was applied to a 15 μm-thick aluminum (Al) thin film cathode current collector, dried, and then roll-pressed to prepare a cathode.

[0122] Additionally, a negative electrode mixture slurry was prepared by mixing a 95:5 blend of natural graphite and silicon as the negative electrode active material, styrene-butadiene rubber (SBR) as the binder, sodium carboxymethyl cellulose (CMC) as the thickener, and carbon black as the conductive material in a 97:1:1:1 weight ratio, and then adding the mixture to NMP as the solvent. The negative electrode mixture slurry was then applied to a 10 μm-thick copper (Cu) thin film as the negative electrode current collector, dried, and then roll pressed to prepare the negative electrode.

[0123] In a dry room, a porous polypropylene separator was interposed between the positive electrode and the negative electrode, and the non-aqueous electrolyte solution prepared above was then injected to prepare a coin-shaped half-cell lithium secondary battery.

[0124] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 0.2 wt % of ethylene sulfate (Esa) was further added during the preparation of the non-aqueous electrolyte.

[0125] Comparative Example 3. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 40.2 wt % of LiBF was further added during the preparation of the non-aqueous electrolyte solution.

[0126] Comparative Example 4. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that succinonitrile and 1,4-dicyano-2-butene were not added, and 0.4 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added during the preparation of the non-aqueous electrolyte.

[0127] Comparative Example 5. A lithium secondary battery was produced in the same manner as in Comparative Example 1, except that 1,4-dicyano-2-butene was not added during the production of the non-aqueous electrolyte solution.

[0128] Comparative Example 6. A lithium secondary battery was produced in the same manner as in Comparative Example 1, except that succinonitrile was not added during the production of the non-aqueous electrolyte solution.

[0129] Comparative Example 7. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 1,4-dicyano-2-butene was not added during the preparation of the non-aqueous electrolyte, and 0.4 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added.

[0130] Comparative Example 8. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that succinonitrile was not added during the preparation of the non-aqueous electrolyte, and 0.4 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added.

[0131] Example 1 A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 0.4 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added during the preparation of the non-aqueous electrolyte solution.

[0132] Example 2. A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 0.2 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added during the preparation of the non-aqueous electrolyte solution.

[0133] Example 3 A lithium secondary battery was manufactured in the same manner as in Comparative Example 1, except that 0.8 wt % of ethylene sulfate and 40.2 wt % of LiBF were further added during the preparation of the non-aqueous electrolyte solution.

[0134] Example 4. A lithium secondary battery was manufactured in the same manner as in Example 1, except that a positive electrode having an initial efficiency of 87% was used instead of the positive electrode of Example 1.

[0135] Specifically, N-methyl-2-pyrrolidone (NMP) was mixed with LiNi as a positive electrode active material. 0.93 Co 0.02 Mn 0.03 Al 0.02 A cathode slurry (solid content: 70 wt%) was prepared by adding O2, a conductive material (carbon black), and a binder (polyvinylidene fluoride) in a weight ratio of 97.5:1:1.5. The cathode slurry was applied to a 15 μm-thick aluminum (Al) thin film cathode current collector, dried, and then roll-pressed to fabricate a cathode with an initial efficiency of 87%. Here, the initial efficiency refers to the ratio of the initial discharge capacity to the initial charge capacity, measured after a coin-cell-type half cell was fabricated using the fabricated cathode, a lithium metal counter electrode, and an electrolyte of 1.0 M LiPF6 dissolved in a mixed solvent of ethylene carbonate and dimethyl carbonate (3:7 volume ratio). The half cell was charged and discharged once between 2.5 V and 4.25 V at a rate of 0.2 C.

[0136] <Experimental example: Performance evaluation of lithium secondary batteries> Experimental Example 1: Evaluation of high temperature (55°C) storage characteristics Each of the lithium secondary batteries manufactured in the Examples and Comparative Examples was activated at 0.1 C / C and then degassed.

[0137] Next, the battery was charged at 0.33 C CC to 4.25 V under constant current-constant voltage (CC-CV) charging conditions at 25°C, and then the current was cut by 0.05 C, followed by discharging at 0.33 C to 2.5 V under CC conditions.

[0138] Next, the battery was recharged at 0.33C CC until SOC reached 100%, and then stored at a high temperature of 55°C for 8 weeks.

[0139] After CC-CV charging and discharging at a rate of 0.33C every 4 weeks, the discharge energy was measured using a PNE-0506 charger / discharger (manufacturer: PNE Solution Co., Ltd., 5V, 6A). The discharge energy was then substituted into equation (1) to calculate the discharge energy retention. The results are shown in Table 1 below and FIG. 1.

[0140] In addition, DC-iR (Direct Current Internal Resistance) was calculated from the voltage drop observed when a discharge pulse of 2.5 C was applied for 10 seconds at an SOC of 50%, and this was substituted into equation (2) to calculate the resistance increase rate (%), which is shown in Table 1 and Figure 1. The voltage drop was measured using a PNE-0506 charger / discharger (manufacturer: PNE Solution Co., Ltd., 5 V, 6 A).

[0141] Equation (1): Discharge energy retention rate (%) = (discharge energy after high-temperature storage / discharge energy before high-temperature storage) × 100

[0142] Equation (2): Resistance increase rate (%) = {(resistance value after high-temperature storage - initial resistance value) / initial resistance value} × 100

[0143] [Table 1]

[0144] From the results in Table 1 and the attached FIG. 1, it can be seen that the lithium secondary batteries of Examples 1 to 4, which employ non-aqueous electrolyte solutions containing all of the first to third additives of the present invention, have superior performance after high-temperature storage compared to the lithium secondary batteries of Comparative Examples 1 to 3, 5, and 6, which employ non-aqueous electrolyte solutions not containing the first and / or second additive, the lithium secondary battery of Comparative Example 4, which employs non-aqueous electrolyte solutions not containing the third additive, and the lithium secondary batteries of Comparative Examples 7 and 8, which employ only one nitrile-based additive as the third additive.

[0145] Specifically, the lithium secondary batteries manufactured in Examples 1 to 4 were confirmed to have excellent energy retention and resistance characteristics even under adverse conditions such as high-voltage charging and high-temperature storage, despite containing high-nickel positive electrode active materials and silicon-based negative electrode active materials.

[0146] On the other hand, it can be confirmed that the lithium secondary battery of Comparative Example 7, which contained both the first and second additives but used an electrolyte solution containing only SN, a saturated nitrile-based compound, as the third additive, and the lithium secondary battery of Comparative Example 8, which used an electrolyte solution containing only DCB, an unsaturated nitrile-based compound, both had lower energy retention rates after high-voltage charging and high-temperature storage, and lower resistance characteristics, compared to the lithium secondary battery of Example 1.

[0147] Furthermore, even when comparing the lithium secondary battery of Comparative Example 1 with the lithium secondary batteries of Comparative Examples 5 and 6, among the lithium secondary batteries using nonaqueous electrolyte solutions that do not contain the first or second additive, it can be confirmed that those containing both a saturated nitrile compound and an unsaturated nitrile compound are advantageous in improving the energy retention rate and resistance characteristics after high-voltage charging and high-temperature storage.

[0148] Furthermore, it can be confirmed that among Examples 1 to 4, Examples 1 and 4 in which the weight ratio of the first additive to the second additive was 2:1 exhibited the greatest effect of improving performance.

Claims

1. an organic solvent; A lithium salt, a first additive which is a compound represented by the following chemical formula 1; a second additive which is lithium tetrafluoroborate; a third additive including a saturated nitrile compound and an unsaturated nitrile compound; the lithium salt is different from the second additive; The saturated nitrile compound is at least one selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, ethylene glycol bis(2-cyanoethyl)ether, 1,3,6-hexanetricarbonitrile, and 1,2,3-tris(2-cyanoethyl)propane; A non-aqueous electrolyte for a lithium secondary battery, wherein the content of the third additive is 0.1 wt % to 2 wt % based on the total weight of the non-aqueous electrolyte: 【Chemical 1】 (In the above Chemical Formula 1, R1 and R2 are each independently an alkyl group having 1 to 5 carbon atoms, or R1 and R2 are bonded to each other to form a ring having 2 to 10 carbon atoms containing a sulfate group.

2. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the first additive is represented by the following chemical formula 1-1: 【Chemistry 2】 (In the above chemical formula 1-1, m is an integer from 0 to 6; n is an integer of 1 or 2.

3. 3. The nonaqueous electrolyte for a lithium secondary battery according to claim 2, wherein m is 0 and n is 1 in Chemical Formula 1-1.

4. 2. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the content of the first additive is 0.1 wt % to 2 wt % based on the total weight of the non-aqueous electrolyte solution.

5. 2. The non-aqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the content of the second additive is 0.1 wt % to 1 wt % based on the total weight of the non-aqueous electrolyte solution.

6. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein a weight ratio of the first additive to the second additive is 1:1 or more and 3:1 or less.

7. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, wherein the unsaturated nitrile compound is at least one selected from the group consisting of 1,4-dicyano-2-butene, 1,4-dicyano-2-methyl-2-butene, 1,4-dicyano-2-ethyl-2-butene, 1,4-dicyano-2,3-dimethyl-2-butene, 1,4-dicyano-2,3-diethyl-2-butene, 1,6-dicyano-3-hexene, 1,6-dicyano-2-methyl-3-hexene, 1,6-dicyano-2-methyl-5-methyl-3-hexene, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

8. 2. The nonaqueous electrolyte solution for a lithium secondary battery according to claim 1, further comprising a fourth additive selected from the group consisting of vinylene carbonate, 1,3-propane sultone, and a mixture thereof.

9. a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode; A lithium secondary battery comprising the nonaqueous electrolyte solution according to claim 1 .

10. 10. The lithium secondary battery of claim 9, wherein the positive electrode active material comprises a lithium composite transition metal oxide represented by the following Chemical Formula 2: [Chemical formula 2] Li １＋ｅ (N ａ Co ｂ Mn ｃ M ｄ )O ２ (In the above chemical formula 2, M is one or more elements selected from the group consisting of W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, and Mo; a, b, c, and d are atomic fractions of each independent element; 0≦e≦0.5, 0.8≦a<1, 0<b≦0.15, 0<c≦0.15, 0≦d≦0.1, a+b+c+d=1.)

11. 10. The lithium secondary battery of claim 9, wherein the positive electrode further comprises a compound represented by the following Chemical Formula 3: [Chemical formula 3] Li ２ M ａ １－ｋ M ｂ ｋ O ２ (In the above chemical formula 3, M ａ is one or more elements selected from the group consisting of Ni and Cu, M ｂ is one or more elements selected from the group consisting of Mn, Fe, Co, Zn, Mg, and Cd; k is 0≦k<1.

12. 10. The lithium secondary battery according to claim 9, wherein the initial efficiency of the positive electrode is 85% to 89%.

13. The lithium secondary battery of claim 9 , wherein the negative electrode active material comprises a silicon-based material, a carbon-based material, or a mixture thereof.

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