Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery including a non-aqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by Chemical Formula 1, and a second additive represented by Chemical Formula 2; a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material made of silicon; and a separator interposed between the positive electrode and the negative electrode.

Description

Technical Field

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0087179 filed on July 14, 2022 and Korean Patent Application No. 10-2023-0087210 filed on July 5, 2023, and all of its contents are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery including an electrolyte containing a combination of specific additives and a pure silicon (Pure Si) negative electrode material.

Background Art

[0003] A lithium secondary battery is generally manufactured by forming an electrode assembly with a separator interposed between a positive electrode including a positive electrode active material made of a transition metal oxide containing lithium and a negative electrode including a negative electrode active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting an electrolyte serving as a medium for transmitting lithium ions, and then sealing it.

[0004] Lithium secondary batteries can be miniaturized, have high energy density and operating voltage, and are applied to various fields such as mobile devices, electronic products, and electric vehicles. As the application fields of lithium secondary batteries become more diverse, the required physical property conditions are gradually increasing. In particular, the development of lithium secondary batteries that can be stably driven even under high-temperature conditions is required.

[0005] On the other hand, when a lithium secondary battery is driven under high-temperature conditions, PF6 - anions such as LiPF6 contained in the electrolyte may be thermally decomposed to generate Lewis acids such as PF5, which react with moisture to produce HF. Such decomposition products such as PF5 and HF not only destroy the film formed on the electrode surface, but may also cause a decomposition reaction of the organic solvent, react with the decomposition products of the positive electrode active material to elute transition metal ions, and the eluted transition metal ions may be electrodeposited on the negative electrode to destroy the film formed on the negative electrode surface.

[0006] When the electrolyte decomposition reaction continues on the film thus damaged, the performance of the battery further deteriorates. Therefore, there is a demand for the development of a secondary battery that can maintain excellent performance even under high-temperature conditions.

Summary of the Invention

Problems to be Solved by the Invention

[0007] The present invention is for solving the above problems, and aims to improve the high-temperature performance of a lithium secondary battery containing a pure silicon negative electrode active material by introducing a non-aqueous electrolyte containing a combination of two specific additives.

Means for Solving the Problems

[0008] According to one embodiment, the present invention provides a non-aqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2; a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material made of silicon; and a lithium secondary battery including a separator interposed between the positive electrode and the negative electrode.

[0009]

Chemical Formula

[0010] In the above Chemical Formula 1, A is a heterocyclic ring having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R1 is an alkylene group having 1 to 3 carbon atoms,

[0011]

Chemical Formula

[0012] In the above Chemical Formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorines.

Advantages of the Invention

[0013] The lithium secondary battery according to the present invention can improve the high-temperature life and the amount of gas generation of a lithium secondary battery containing a silicon negative electrode active material by including a non-aqueous electrolyte containing a specific combination of additives.

Modes for Carrying Out the Invention

[0014] Hereinafter, the present invention will be described in more detail.

[0015] Generally, anions contained in lithium salts such as LiPF6 widely used in electrolytes for lithium secondary batteries form decomposition products such as hydrogen fluoride (HF) and PF5 due to thermal decomposition or moisture, etc., and such phenomena are deepened when driving under high-temperature conditions. Since the decomposition products have the property of an acid, they deteriorate the characteristics of the electrode surface in the battery.

[0016] Due to the decomposition products of the electrolyte and the structural change of the positive electrode caused by repeated charge and discharge, transition metals in the positive electrode are easily eluted into the electrolyte, and the eluted transition metals are re-deposited on the positive electrode again, increasing the resistance of the positive electrode.

[0017] Moreover, when the eluted transition metal moves to the negative electrode through the electrolyte, it electrodeposits on the negative electrode, causing the destruction of the SEI (solid electrolyte interphase) film and further decomposition reaction of the electrolyte, thereby causing problems such as consumption of lithium ions and increase in resistance.

[0018] In particular, when silicon (Si) is used as the negative electrode active material, due to the large volume change in the charge and discharge process, the SEI film is easily damaged and the reaction of re-generation continuously occurs, resulting in the problem that the battery durability deteriorates and gas generation deepens.

[0019] To solve such problems, the inventors have found that by including a first additive represented by the following Chemical Formula 1 and a second additive represented by the following Chemical Formula 2 in a non-aqueous electrolyte, a strong SEI film can be formed on the negative electrode through this, and thus the performance of the battery, especially the amount of gas generated at high temperatures, can be improved.

[0020] Hereinafter, each component constituting the present invention will be described in more detail.

[0021] Non-aqueous electrolyte The lithium secondary battery according to the present invention includes a non-aqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2.

[0022] Hereinafter, each component of the non-aqueous electrolyte will be specifically described.

[0023] (1) First additive and second additive The non-aqueous electrolyte of the present invention contains a first additive represented by the following Chemical Formula 1.

[0024]

Chemical formula

[0025] In the above Chemical Formula 1, A is a heterocycle having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R1 is an alkylene group having 1 to 3 carbon atoms.

[0026] Since the first additive represented by Chemical Formula 1 contains a propargyl functional group, when it is included in the electrolytic solution, an SEI film of a PEO (poly(ethylene oxide))-based polymer component can be formed on the surface of the negative electrode while being reductively decomposed through a radical reaction. Through this, not only can the high-temperature durability of the negative electrode itself be improved, but there is also an effect of preventing the electrodeposition of transition metals on the surface of the negative electrode. Further, since the propargyl group has the property of adsorbing to metal ions, it can perform functions of adsorbing to the surface of metallic impurities contained in the positive electrode to prevent the elution of impurities, and suppressing internal short circuits by preventing the deposited metal ions from being deposited on the negative electrode. Moreover, the first additive can combine with PF5, which is a decomposition product of the electrolytic solution, to suppress the generation of HF, thereby preventing the destruction of the CEI (cathode electrolyte interphase) film formed on the surface of the positive electrode and further suppressing the decomposition of the electrolytic solution.

[0027] In one embodiment of the present invention, A in Chemical Formula 1 may be a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms, and preferably may be an imidazole group.

[0028] As a preferred example, the first additive may be represented by the following Chemical Formula 1-1.

[0029]

Chemical Formula

[0030] In Chemical Formula 1-1, R1 is as defined in Chemical Formula 1.

[0031] In one embodiment of the present invention, R1 in Chemical Formula 1 may be a linear or branched alkylene group having 1 to 3 carbon atoms, preferably a linear alkylene group having 1 to 3 carbon atoms, and more preferably a methylene group.

[0032] As a preferred example, the first additive may be represented by the following Chemical Formula 1A.

[0033] [Chemical Formula]

[0034] In one embodiment of the present invention, the content of the first additive may be from 0.05% by weight to 10% by weight, preferably from 0.1% by weight to 1% by weight, and more preferably from 0.1% by weight to 0.5% by weight, based on the total weight of the non-aqueous electrolyte. When the content of the first additive is excessive, considering that there may be a problem that it excessively participates in the decomposition reaction at the interface between the electrode and the electrolyte and the film resistance becomes too large, increasing the resistance of the battery, the content of the first additive is preferably 10% by weight or less.

[0035] Further, the non-aqueous electrolyte of the present invention contains a second additive represented by the following Chemical Formula 2.

[0036] [Chemical Formula]

[0037] In Chemical Formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorines.

[0038] Since the second additive represented by Chemical Formula 2 contains a cyclic phosphate structure, when this is included in the electrolyte, a SEI film of a polyphosphoester (PPE) component can be formed on the negative electrode surface through a ring-opening reaction, which can impart elasticity to the silicon negative electrode. That is, since the durability against the volume change of silicon is enhanced, deterioration of the negative electrode in a cycle and a high-temperature environment can be prevented. Not only that, since the second additive contains a fluoroalkyl group, an inorganic SEI film based on LiF can be formed on the positive and negative electrodes through a reaction with lithium ions. LiF has physically strong properties and lithiated silicon (Lix Since it has strong binding properties with Si), there is an effect of suppressing the deterioration of the battery due to the reaction at the electrolyte-electrode interface.

[0039] In one embodiment of the present invention, R2 in the chemical formula 2 is -(CR2) n is CF3, R is hydrogen or fluorine, and n may be an integer from 0 to 9. Preferably, R2 in the chemical formula 2 is -(CH2) n is CF3, n may be an integer from 0 to 5, and more preferably, n may be an integer from 1 to 3.

[0040] As a preferred example, the second additive may be represented by the following chemical formula 2A.

[0041]

Chemical formula

[0042] In one embodiment of the present invention, the content of the second additive may be from 0.1% by weight to 15% by weight based on the total weight of the non-aqueous electrolyte.

[0043] Specifically, the content of the second additive may be 0.5% by weight or more, preferably 2% by weight or more, more preferably 5% by weight or more, and most preferably 10% by weight or more based on the total weight of the non-aqueous electrolyte. However, considering that when the second additive is added excessively, the resistance of the battery may increase due to their decomposition, the content of the second additive is preferably 15% by weight or less.

[0044] In the non-aqueous electrolyte according to the present invention, when both the first additive and the second additive are used, the A substituent of Chemical Formula 1, specifically, the electron lone-pair present in imidazole further promotes the ring-opening reaction of Chemical Formula 2 of the second additive, and the reduction reaction can occur smoothly even with less energy. As a result, when forming the SEI layer on the silicon negative electrode, the polymerized film components can be further increased, and PF5 can be chelated to suppress the generation of HF. In particular, in the case of the Si negative electrode material, since the volume change is large, the phenomenon of film damage and durability reduction due to HF is deepened in a high-temperature environment. Therefore, the effect of improving durability and reducing the gas generation amount can be shown more dramatically through the combination of these additives.

[0045] In one embodiment of the present invention, the weight ratio of the second additive to the first additive, that is, the ratio (W2 / W1) of the weight (W2) of the second additive to the weight (W1) of the first additive in the non-aqueous electrolyte may be 1.5 or more, preferably 6 or more, more preferably 16 or more, and most preferably 30 or more. The higher the relative ratio of the second additive, not only is the formation of the polymerized film due to the ring-opening reaction of the cyclic phosphate structure advantageous, but also LiF formed by the binding of -CF3 and lithium ions can play a role in strengthening the durability of the film, which is preferable for improving the high-temperature performance of a battery using Pure Si as the negative electrode material. However, considering that when the film of the polymer component is formed excessively thick due to the decomposition of the excessive additive, the battery resistance increases and side reactions and by-products occur during the decomposition process, the weight ratio of the second additive to the first additive is preferably 40 or less.

[0046] (2) The third additive The non-aqueous electrolyte may contain any one or more selected from the group consisting of a cyclic carbonate-based compound, a sultone-based compound, a sulfate-based compound, a phosphorus-based compound, a nitrile-based compound, an amine-based compound, a silane-based compound, a benzene-based compound, and a lithium salt-based compound as the third additive.

[0047] The cyclic carbonate compound may be any one or more selected from the group consisting of vinylene carbonate (VC), vinyl ethylene carbonate (VEC), and fluoroethylene carbonate (FEC), and specifically, it may be vinylene carbonate.

[0048] The sultone compound is a substance capable of forming a stable SEI film by a reduction reaction on the surface of the negative electrode, and may be any one or more 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, it may be 1,3 - propane sultone (PS).

[0049] The sulfate compound is a substance capable of being electrically decomposed on the surface of the negative electrode to form a stable SEI film without cracks even during high - temperature storage, and may be any one or more selected from the group consisting of ethylene sulfate (Ethylene Sulfate; Esa), trimethylene sulfate (Trimethylene sulfate; TMS), and methyl trimethylene sulfate (Methyl trimethylene sulfate; MTMS).

[0050] The phosphorus compound may be a phosphate - based or phosphite - based compound, and specifically, it may be any one or more selected from the group consisting of tris(trimethylsilyl) phosphate, tris(trimethylsilyl) phosphite, tris(2,2,2 - trifluoroethyl) phosphate, and tris(trifluoroethyl) phosphite.

[0051] The 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, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, ethylene glycol bis(2-cyanoethyl) ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), and 1,2,3-tris(2-cyanoethyl)propane (TCEP).

[0052] The amine compound may be any one or more selected from the group consisting of triethanolamine and ethylenediamine, and the silane compound may be tetravinylsilane.

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

[0054] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and may be any one or more selected from the group consisting of lithium difluorophosphate (LiDFP; LiPO2F2), lithium bisoxalatoborate (LiBOB; LiB(C2O4)2), lithium tetrafluoroborate (LiBF4), lithium tetraphenylborate, lithium difluoro(oxalate)borate (LiDFOB), and lithium difluoro(bisoxalate)phosphate (LiDFOP).

[0055] Preferably, the non-aqueous electrolyte according to an embodiment of the present invention may further contain any one or more third additives selected from the group consisting of vinylene carbonate (VC), 1,3-propane sultone (PS), 1,3-propene sultone (PRS), and lithium difluorophosphate (LiDFP). In this case, since a more robust SEI film can be formed on the negative electrode surface during the initial activation process of the secondary battery, gas generation due to electrolyte decomposition at high temperatures can be suppressed, and the resistance increase rate due to cycle increase during battery operation can be reduced.

[0056] In one embodiment of the present invention, the content of the third additive may be from 0.05% by weight to 5% by weight, preferably from 0.1% by weight to 3% by weight, based on the total weight of the non-aqueous electrolyte. It is preferable that the content of the third additive is 5% by weight or less in terms of reducing the initial resistance.

[0057] (3) Organic solvent The non-aqueous electrolyte of the present invention contains an organic solvent.

[0058] As the organic solvent, various organic solvents commonly used in lithium electrolytes may be used without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a chain carbonate solvent, a chain ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof, preferably may contain a mixture of a cyclic carbonate solvent and a chain carbonate solvent, and more preferably may be a mixture of a fluorinated cyclic carbonate solvent and a chain carbonate solvent.

[0059] The cyclic carbonate-based solvent is a high-viscosity organic solvent with a high dielectric constant, which can well dissociate the lithium salt in the electrolyte. It may be any one or more selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), fluoroethylene carbonate (FEC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate. Preferably, it can contain fluoroethylene carbonate (FEC). When using a fluorinated cyclic carbonate-based solvent such as FEC, an inorganic SEI film based on LiF can be formed. Therefore, there is an advantage that a hybrid film can be formed together with the organic film of the polymer component formed by the additive.

[0060] Also, the chain carbonate-based solvent is an organic solvent having a low viscosity and a 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. Preferably, it can contain diethyl carbonate (DEC), ethyl methyl carbonate (EMC), or a mixture thereof. More preferably, it can contain diethyl carbonate and ethyl methyl carbonate. DMC is an organic solvent effective for increasing ionic conductivity and improving the room temperature resistance thereby. However, when DMC is used alone as the chain carbonate-based solvent, it is unstable at high temperatures and gas formation due to reduction reaction increases, and there is a disadvantage that the low-temperature performance is greatly reduced due to a high freezing point. Therefore, it is preferable to use DEC, EMC, or a combination thereof to reduce the amount of gas generated and strengthen the durability of the film formed on the electrode, thereby improving the low-temperature performance of the battery. In particular, EMC is advantageous for improving ionic conductivity because it has a lower viscosity than DEC.

[0061] The chain ester-based 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, and preferably may be methyl propionate, ethyl propionate, or propyl propionate.

[0062] The cyclic ester-based solvent may be any one or more selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0063] The nitrile-based solvent may be any one or more selected from the group consisting of succinonitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, and preferably may be succinonitrile.

[0064] Among the total weight of the non-aqueous electrolyte, other constituent components excluding the organic solvent, for example, the remainder excluding the contents of the first to third additives and the lithium salt may all be organic solvents unless otherwise specified.

[0065] (4) Lithium salt The non-aqueous electrolyte of the present invention contains a lithium salt.

[0066] The lithium salt may be used without limitation such as those commonly used in electrolytes for lithium secondary batteries. Specifically, the lithium salt contains 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 - may contain any one or more selected from among them.

[0067] Specifically, the lithium salt may be any one or more selected from the group consisting of LiPF6, LiClO4, LiBF4, LiN(FSO2)2 (LiFSI), LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO3CF3, LiPO2F2, lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiFMDFB), and preferably, it may be LiPF6.

[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, that is, the mixed solution of the lithium salt and the organic solvent, may be 0.5 M or more, specifically, 1.0 M or more, and more specifically, 1.5 M or more. When the concentration of the lithium salt is within the above range, the effects of improving low-temperature output and cycle characteristics can be sufficiently ensured. In particular, when the concentration of the lithium salt is a high concentration of 1.5 M or more, the participation degree of anions in the lithium ion solvation shell on the electrolyte can be increased. As a result, anions in the lithium salt, for example, PF6 -By adding an inorganic component such as LiF to the negative electrode SEI film, the durability against the volume change of the silicon negative electrode can be enhanced. On the other hand, in order to prevent the viscosity and surface tension from becoming excessively high and obtain appropriate electrolyte impregnation properties, the concentration of the lithium salt is preferably 4.0 M or less, specifically 3.0 M or less, and more specifically 2.0 M or less.

[0069] Positive electrode The positive electrode according to the present invention contains a positive electrode active material, and can be manufactured by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, a solvent, etc. on a positive electrode current collector, followed by drying and rolling.

[0070] The positive electrode current collector is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, stainless steel; aluminum; nickel; titanium; fired carbon; or a surface-treated product of the surface of aluminum or stainless steel with carbon, nickel, titanium, silver, etc. may be used.

[0071] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, and may be any one or more selected from the group consisting of LCO (LiCoO2); LNO (LiNiO2); LMO (LiMnO2); LiMn2O4, LiCoPO4; LFP (LiFePO4); and lithium composite transition metal oxides containing nickel (Ni), cobalt (Co), and manganese (Mn).

[0072] On the other hand, the positive electrode active material may be a lithium composite transition metal oxide in which the molar ratio of nickel among the transition metals is 70 mol% or more, preferably 80 mol% or more, and more preferably 85 mol% or more.

[0073] In the case of a high-nickel (High-Ni) cathode active material in which the nickel content is 70 mol% or more of the total transition metal content, since it is structurally unstable, the cathode active material may disintegrate and crack at high temperatures. In the case of such an active material that disintegrates in this way, the surface area increases and the sites where side reactions with the electrolyte can occur increase. As a result, when exposed to high temperatures, a large amount of gas is generated. In this way, when gas is continuously generated, the internal pressure of the battery increases and venting occurs early, and the operation stops before the battery life ends. Therefore, it is important to improve such side reactions and gas generation, but this can be improved by strengthening film formation through the non-aqueous electrolyte containing the first additive and the second additive of the present invention.

[0074] In one embodiment of the present invention, the lithium composite transition metal oxide may be a compound represented by the following Chemical Formula 3. That is, the cathode active material according to one embodiment of the present invention may include a lithium composite transition metal oxide represented by the following Chemical Formula 3.

[0075] [Chemical Formula 3] Li 1+x (Ni a Co b Mn c M d )O2

[0076] In Chemical Formula 3, M is any one or more 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, 1 + x, a, b, c, and d are atomic fractions of independent elements, respectively, -0.2 ≦ x ≦ 0.2, 0.60 ≦ a < 1, 0 < b ≦ 0.30, 0 < c ≦ 0.30, 0 ≦ d ≦ 0.10, and a + b + c + d = 1.

[0077] Since the above 1 + x indicates the molar ratio of lithium in the lithium composite transition metal oxide, it may be -0.1 ≦ x ≦ 0.2, or 0 ≦ x ≦ 0.2. When the molar ratio of lithium satisfies the above range, the crystal structure of the lithium composite transition metal oxide can be stably formed.

[0078] Since the above a indicates the molar ratio of nickel among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0.70 ≦ a < 1, 0.80 ≦ a < 1, or 0.85 ≦ a < 1. When the molar ratio of nickel satisfies the above range, it shows a high energy density and high capacity can be realized.

[0079] Since the above b indicates the molar ratio of cobalt among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0 < b ≦ 0.20, 0 < b ≦ 0.15, or 0 < b ≦ 0.10. When the molar ratio of cobalt satisfies the above range, good resistance characteristics and output characteristics can be realized.

[0080] Since the above c indicates the molar ratio of manganese among all the metals excluding lithium in the lithium composite transition metal oxide, it may be 0 < c ≦ 0.20, 0 < c ≦ 0.15, or 0 < c ≦ 0.10. When the molar ratio of manganese satisfies the above range, the structural stability of the positive electrode active material is excellently shown.

[0081] In one embodiment of the present invention, the lithium composite transition metal oxide may contain any one or more doping 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, and preferably may contain Al as the doping element. In other words, the above d indicating the molar ratio of the doping element among all the metals excluding lithium in the lithium composite transition metal oxide may be 0 < d ≦ 0.10, 0 < d ≦ 0.08, or 0 < d ≦ 0.05.

[0082] Preferably, a, b, c, and d in Chemical Formula 3 may be 0.70 ≦ a < 1, 0 < b ≦ 0.20, 0 < c ≦ 0.20, and 0 ≦ d ≦ 0.10, respectively.

[0083] The positive electrode active material may be contained in an amount of 80% by weight to 99% by weight, specifically 90% by weight to 99% by weight, based on the total weight of the solid content in the positive electrode slurry. At this time, when the content of the positive electrode active material is 80% by weight or less, the energy density may be low and the capacity may decrease.

[0084] The binder is a component that assists in binding the active material and the conductive material and binding to the current collector, and may usually be added in an amount of 1% by weight to 30% by weight based on the total weight of the solid content in the positive electrode slurry. Examples of such binders may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer, sulfonated ethylene-propylene-diene polymer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0085] In addition, the conductive material is a substance that imparts conductivity without inducing a chemical change in the battery, and may be added in an amount of 0.5% by weight to 20% by weight based on the total weight of the solid content in the positive electrode slurry.

[0086] The conductive material may be selected from, for example, carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders 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.

[0087] Further, the solvent of the positive electrode slurry may contain an organic solvent such as NMP (N-methyl-2-pyrrolidone), and may be used so as to have a preferable viscosity when including the positive electrode active material, binder, conductive material, etc. For example, the solid content concentration in the positive electrode slurry including the positive electrode active material, binder, and conductive material may be included so as to be from 40% by weight to 90% by weight, preferably from 50% by weight to 80% by weight.

[0088] Negative electrode The lithium secondary battery according to the present invention includes a negative electrode containing a negative electrode active material, and the negative electrode can be manufactured by coating a negative electrode slurry containing a negative electrode active material, binder, conductive material, solvent, etc. on a negative electrode current collector, followed by drying and rolling.

[0089] The negative electrode current collector generally has a thickness of from 3 μm to 500 μm. Such a negative electrode current collector is not particularly limited as long as it has high conductivity without inducing a chemical change in the battery. For example, copper; stainless steel; aluminum; nickel; titanium; fired carbon; a surface-treated one of copper or stainless steel with carbon, nickel, titanium, silver, etc.; or an aluminum-cadmium alloy, etc. may be used. Also, similar to the positive electrode current collector, fine irregularities can be formed on the surface to strengthen the bonding force of the negative electrode active material, and it may be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

[0090] In the present invention, the negative electrode active material is made of silicon (Si). As the negative electrode active material, silicon not only enables the realization of a high-capacity cell because its theoretical capacity compared to graphite is nearly about 10 times higher, but also mass loading (mg·cm -2Lowering ([0]) can improve the rapid charging performance of the battery. However, there is a problem that the lithium ion loss rate due to the irreversible reaction is high, the volume change is large, and it may have an adverse effect on the lifespan. In particular, in the case of a 100% Si negative electrode, the phenomenon in which the reaction of the SEI film being easily cracked and regenerated due to the large volume change during the charge and discharge process continuously occurs deepens. However, when applying the non-aqueous electrolyte according to the present invention, as described above, the SEI film can be strengthened, so such problems can be effectively solved.

[0091] The negative electrode active material may be contained in an amount of 60% by weight to 99% by weight based on the total weight of the solid content in the negative electrode slurry.

[0092] The binder is a component that assists in the bonding between the conductive material, the active material, and the current collector, and may usually be added in an amount of 1% by weight 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 polymer, sulfonated ethylene-propylene-diene polymer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0093] 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% by weight to 20% by weight based on the total weight of the solid content in the negative electrode slurry. Such a conductive material is not particularly limited as long as it has conductivity without inducing a chemical change in the battery. For example, carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black or thermal black; graphite powder such as natural graphite, artificial graphite, carbon nanotube or graphite with a highly developed crystal structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as carbon fluoride powder, aluminum powder or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive materials such as polyphenylene derivatives; or a combination thereof may be used.

[0094] The solvent of the negative electrode slurry may contain water; or an organic solvent such as NMP and alcohol, and may be used so as to have a preferable viscosity when containing the negative electrode active material, binder, conductive material and the like. For example, the solid content concentration in the slurry containing the negative electrode active material, binder and conductive material may be included so as to be 20% by weight to 80% by weight, preferably 20% by weight to 40% by weight.

[0095] Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

[0096] The separator separates the negative electrode and the positive electrode and provides a migration path for lithium ions. Usually, it can be used without particular limitation as long as it is used as a separator in a lithium secondary battery. In particular, those having low resistance to ion migration of the electrolyte solution, excellent electrolyte solution moisture absorption ability and excellent safety are preferable.

[0097] Specifically, as the separation membrane, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as a homopolymer of ethylene, a homopolymer of propylene, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer; or a laminate structure of two or more layers thereof may be used. Further, a normal porous nonwoven fabric, for example, a nonwoven fabric made of high-melting glass fibers, polyethylene terephthalate fibers, etc. may be used. Further, in order to ensure heat resistance or mechanical strength, a coated separation membrane containing a ceramic component or a polymer substance may be used, and it may be used in a single-layer or multilayer structure.

[0098] The lithium secondary battery according to the present invention as described above may be usefully used in portable devices such as mobile phones, notebook personal computers, digital cameras; and electric vehicle fields such as hybrid electric vehicles (HEVs).

[0099] Accordingly, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the same are provided.

[0100] The battery module or the battery pack can be used as a power source for any one or more medium and large-sized devices including power tools; electric vehicles including electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0101] There is no particular limitation on the outer shape of the lithium secondary battery of the present invention, and it may be a cylindrical shape, a rectangular shape, a pouch type, a coin type, etc. using a can.

[0102] The lithium secondary battery according to the present invention can be preferably used not only as a battery cell for powering small devices, but also as a unit cell in medium and large-sized battery modules containing a large number of battery cells.

[0103] Hereinafter, the present invention will be specifically described through specific examples.

[0104] <Example> Example 1. (Manufacture of non-aqueous electrolyte) After mixing fluoroethylene carbonate (FEC): diethyl carbonate (DEC): ethyl methyl carbonate (EMC) in a volume ratio of 10:45:45, a non-aqueous organic solution was prepared by dissolving LiPF6 to a concentration of 1.5 M. 0.3 wt% of the compound represented by Chemical Formula 1A, 0.5 wt% of the compound represented by Chemical Formula 2A, and the balance of the non-aqueous organic solution were mixed to produce 100 wt% of the non-aqueous electrolyte.

[0105] (Manufacture of lithium secondary battery) As the positive electrode active material, Li(Ni 0.85 Co 0.05 Mn 0.08 Al 0.02 )O2, carbon black as the conductive material, and polyvinylidene fluoride as the binder were added to N-methyl-2-pyrrolidone (NMP), which is a solvent, in a weight ratio of 97.74:0.7:1.56 to produce a positive electrode slurry (solid content: 75.5 wt%). The positive electrode slurry was applied to a positive electrode current collector (Al thin film) with a thickness of 15 μm, and drying and roll press were performed to manufacture the positive electrode.

[0106] Si particles (Wacker) with an average particle size of 5 μm as the negative electrode active material, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as the binder, and carbon black as the conductive material were mixed in a weight ratio of 70:9.7:20.3 in distilled water as the solvent to produce a negative electrode active material slurry with a solid content of 26.0 wt%. The negative electrode active material slurry was applied to a negative electrode current collector (Cu thin film) with a thickness of 15 μm, and drying and roll press were carried out to produce a negative electrode.

[0107] The positive electrode, the polyolefin-based porous separator coated with inorganic particles (Al2O3), and the negative electrode were sequentially laminated to produce an electrode assembly.

[0108] The assembled electrode assembly was housed in a pouch-type battery case, and the manufactured non-aqueous electrolyte was injected to produce a lithium secondary battery.

[0109] Example 2. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 2A was changed to 2 wt% during the production of the non-aqueous electrolyte.

[0110] Example 3. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 2A was changed to 5 wt% during the production of the non-aqueous electrolyte.

[0111] Example 4. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 2A was changed to 10 wt% during the production of the non-aqueous electrolyte.

[0112] Example 5. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 1A was changed to 10 wt% during the production of the non-aqueous electrolyte.

[0113] Example 6. A lithium secondary battery was manufactured in the same manner as in Example 1, except that 2.5 wt% of vinylene carbonate (VC) was further added during the production of the non-aqueous electrolyte.

[0114] Comparative Example 1. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 1A and the compound represented by Chemical Formula 2A were not added during the production of the non-aqueous electrolyte.

[0115] Comparative Example 2. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 2A was not added during the production of the non-aqueous electrolyte.

[0116] Comparative Example 3. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 1A was not added during the production of the non-aqueous electrolyte.

[0117] Comparative Example 4. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula Z1 (5-Ethynyl-1-methyl-1H-imidazole) was used instead of the compound represented by Chemical Formula 1A during the production of the non-aqueous electrolyte.

[0118]

Chemical Formula

[0119] Comparative Example 5. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula Z2 (2-Ethoxy-1,3,2-dioxaphospholane 2-Oxide) was used instead of the compound represented by Chemical Formula 2A during the production of the non-aqueous electrolyte.

[0120]

Chem.

[0121] Comparative Example 6-1. Instead of the slurry described in Example 1 as the negative electrode active material slurry, a mixture of SiO and artificial graphite as the negative electrode active material blended at a weight ratio of 2:8 in distilled water as the solvent, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as the binder, carbon black as the conductive material, and sodium carboxymethyl cellulose (CMC) as the thickener were mixed at a weight ratio of 96.0:2.3:0.7:1. A lithium secondary battery was manufactured in the same manner as in Example 1 except for using this mixture.

[0122] Comparative Example 6-2. A lithium secondary battery was manufactured in the same manner as in Comparative Example 6-1 except that the compound represented by Chemical Formula 2A was not added during the production of the non-aqueous electrolyte.

[0123] Comparative Example 6-3. A lithium secondary battery was manufactured in the same manner as in Comparative Example 6-1 except that the compound represented by Chemical Formula 1A was not added during the production of the non-aqueous electrolyte.

[0124] <Experimental Example> Experimental Example 1: Evaluation of Gas Generation Amount after High Temperature (45°C) Cycling After performing the activation (formation) process on the lithium secondary batteries manufactured in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3, constant current / constant voltage (CC / CV) charging up to 4.2V at a rate of 1C at 45°C (0.05C cut off) was performed, and constant current (CC) discharging up to 3.0V at a rate of 0.5C was performed.

[0125] After transferring the battery charged after 200 cycles to a charge / discharge machine at room temperature (25°C), the gas collected in the pouch was analyzed using GC-TCD (gas chromatography-thermal conductivity detector). When the gas generation amount measured in Comparative Example 1 was set to 100%, the relative gas generation amounts of each battery were calculated and listed in Table 1 below.

[0126] Experimental Example 2: Evaluation of Gas Generation Amount after Storage at High Temperature (60°C) After performing the activation (formation) process on the lithium secondary batteries manufactured in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3, charging was carried out under constant current / constant voltage conditions up to 4.2V at a rate of 0.33C at 25°C (0.05C cut off), and full charge was achieved up to SOC 100%. After storing the fully charged battery at 60°C for 8 weeks, it was transferred to a charge / discharge machine at room temperature (25°C), and the gas collected in the pouch was analyzed using GC-TCD (gas chromatography-thermal conductivity detector). When the gas generation amount measured in Comparative Example 1 was set to 100%, the relative gas generation amounts of each battery were calculated and listed in Table 1 below.

[0127] [Table 1]

[0128] Through the results in Table 1 above, it can be confirmed that when the first additive and the second additive of the present application are simultaneously included as electrolyte additives, it is effective in reducing the gas generation amount after high-temperature cycling and after high-temperature storage of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0129] Specifically, when only the first additive or the second additive is used alone (Comparative Examples 2 and 3), it can be confirmed that the effect of reducing the gas generation amount is very slight compared to the case where neither the first additive nor the second additive is included (Comparative Example 1). Further, the compound of Chemical Formula Z1 used in Comparative Example 4 is a propargyl-substituted imidazole compound, but when used in place of the first additive of the present application, it can be confirmed that the effect of reducing the gas generation amount is significantly reduced. Furthermore, the compound of Chemical Formula Z2 used in Comparative Example 5 is a cyclic phosphate compound like the second additive of the present application, but since a methyl group is located at the terminal (R2 in Chemical Formula 2) instead of a fluoroalkyl group, it can be confirmed that, on the contrary, the increase in film resistance deepens and the gas generation amount increases more compared to Comparative Example 1 where no additive is used. This is a phenomenon that occurs because in the case of fluoroalkyl, it contributes to the inclusion of components excellent in durability and ion conductivity such as LiF in the SEI film, while the methyl group only generates a polyolefin component with no conductivity and reduced durability. That is, a part of the SEI film cannot withstand the volume expansion of the Si negative electrode and is lost, and other films are continuously generated by side reactions of the electrolyte in the part lost during cycle progress, so the increase in film resistance deepens.

[0130] On the other hand, in the case of Comparative Example 6-1 where a mixture of SiO and graphite is used instead of Si as the negative electrode active material, not only is the gas generation amount larger than that in Example 1, but it can also be confirmed that the effect of reducing the gas generation amount is at a slight level compared to Comparative Examples 6-2 and 6-3 where the first additive and the second additive are used alone, respectively. Through Comparative Examples 6-1 to 6-3, it can be understood that the combination of the first additive and the second additive of the present invention shows a particularly remarkable effect on improving the high-temperature gas generation amount of a battery containing a pure silicon (Pure Si) negative electrode active material.

[0131] Also, among Examples 1 to 6, it can be confirmed that the effect of reducing the gas generation amount is most excellent in Examples 3 and 4 where the weight ratio (W2 / W1) of the second additive to the first additive is 15 or more.

[0132] Experimental Example 3: Evaluation of Capacity Retention Rate after High Temperature (45°C) Cycle After performing the activation process on the lithium secondary batteries manufactured in Examples 1 to 6 and Comparative Examples 1 to 5, constant current / constant voltage (CC / CV) charging (0.05C cut off) was performed at 45°C up to 4.2V at a rate of 1C, and constant current (CC) discharge was performed at a rate of 0.5C down to 3.0V.

[0133] Performing the above charge / discharge once each was defined as 1 cycle, and the same charge / discharge was repeated 200 times. The capacity retention rate after 200 cycles was measured with respect to the initial discharge capacity after 1 cycle, and the results are shown in Table 2 below.

[0134]

Table 2

[0135] From the results in Table 2 above, it can be confirmed that when the first additive and the second additive of the present application are simultaneously included as electrolyte additives, it is effective in improving the high temperature life of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0136] Specifically, it can be confirmed that the capacity retention rate was greatly improved not only when neither the first additive nor the second additive was included (Comparative Example 1), but also when only the first additive or the second additive was used alone (Comparative Examples 2 and 3). In addition, it can be confirmed that the compound of chemical formula Z1 used in Comparative Example 4 and the compound of chemical formula Z2 used in Comparative Example 5 have structures similar to the first additive and the second additive of the present invention, respectively, but conversely deteriorate the life characteristics.

[0137] On the other hand, among Examples 1 to 6, it can be confirmed that the effect of improving the capacity retention rate was most excellent in Examples 3 and 4 where the weight ratio (W2 / W1) of the second additive to the first additive was 15 or more.

[0138] Experimental Example 4: Evaluation of Rapid Charging Performance For each of the lithium secondary batteries manufactured in Examples 1 to 6, Comparative Examples 1 to 5, and Comparative Examples 6-1 to 6-3, after performing the activation (formation) process, constant current / constant voltage (CC / CV) charging (0.05C cut off) was carried out at 25°C at a rate of 1C up to 4.2V, and constant current (CC) discharge was carried out at a rate of 0.5C down to 3.0V, and the initial discharge capacity, initial resistance, and initial volume were measured. At this time, the volume was measured by the buoyancy method.

[0139] Next, after adjusting the lithium secondary battery to a state of 15% SOC (State Of Charge), charging was carried out at 25°C while changing the C-rate according to the SOC state as described in Table 3 below, the voltage value was confirmed at 1-second intervals for each charging section, and the voltage profile was measured.

[0140] [Table 3]

[0141] Thereafter, the end condition was set for each section having the voltage value obtained in each section, and the charge amount when charging in the CC mode was recorded. Then, discharge was carried out again in the CC mode at 0.33C down to 15% SOC.

[0142] Performing the above charging and discharging was regarded as 1 cycle, and after 100 cycles were carried out, the capacity retention rate, resistance increase rate, and volume increase rate were calculated according to the following Formulas 1 to 3 and described in Table 4 below.

[0143] [Formula 1] Capacity retention rate (%) = (Discharge capacity after 100 cycles / Initial discharge capacity) × 100 [Formula 2] Resistance increase rate (%) = { (Resistance after 100 cycles - Initial resistance) / Initial resistance} × 100 [Formula 3] Volume increase rate (%) = { (Volume after 100 cycles - Initial volume) / Initial volume} × 100

[0144]

Table 4

[0145] Through the results in Table 4 above, it can be confirmed that when the first additive and the second additive of the present application are simultaneously included as electrolyte additives, it is effective in improving the performance after rapid charging of a lithium secondary battery using 100 wt% Si as the negative electrode active material.

[0146] Specifically, it can be confirmed that the battery according to the present invention shows significantly improved results in terms of capacity, resistance, and volume after rapid charging, not only when neither the first additive nor the second additive is included (Comparative Example 1), but also when only the first additive or the second additive is used alone (Comparative Examples 2 and 3). Also, the compound of Chemical Formula Z1 used in Comparative Example 4 has a slight effect of improving the rapid charging performance compared to the first additive of the present invention, and it can be confirmed that the compound of Chemical Formula Z2 used in Comparative Example 5 deteriorates the rapid charging performance instead. Further, it can be confirmed that Comparative Example 6-1, which uses a mixture of SiO and graphite instead of Si as the negative electrode active material, also has a low capacity retention rate after rapid charging, a high resistance increase rate, and a high volume increase rate when compared with Example 1.

[0147] On the other hand, among Examples 1 to 6, it can be confirmed that Examples 3 and 4, in which the weight ratio (W2 / W1) of the second additive to the first additive is 15 or more, have the best performance after rapid charging.

Claims

1. A non-aqueous electrolyte containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2; A positive electrode containing a positive electrode active material; A negative electrode containing a negative electrode active material made of silicon; and A lithium secondary battery including a separator interposed between the positive electrode and the negative electrode: 【Chemical 1】 In the above chemical formula 1, A is a heterocycle having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R1 is an alkylene group having 1 to 3 carbon atoms, [Chemical Formula 2] In the above chemical formula 2, R2 is an alkyl group having 1 to 10 carbon atoms substituted with one or more fluorines.

2. The lithium secondary battery according to claim 1, wherein A in the chemical formula 1 is a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms.

3. R2 in Formula 2 is -(CR ２ ) ｎ CF ３ and R is hydrogen or fluorine, n is an integer from 0 to 9, and the lithium secondary battery according to claim 1.

4. The content of the first additive is 0.05% by weight to 10% by weight based on the total weight of the non-aqueous electrolyte, and the lithium secondary battery according to claim 1.

5. The content of the second additive is 0.1% by weight to 15% by weight based on the total weight of the non-aqueous electrolyte, and the lithium secondary battery according to claim 1.

6. The weight ratio of the second additive to the first additive is 1.5 or more, and the lithium secondary battery according to any one of claims 1 to 5.

7. The weight ratio of the second additive to the first additive is 6 or more, and the lithium secondary battery according to any one of claims 1 to 5.

8. The non-aqueous electrolyte further contains any one or more third additives selected from the group consisting of vinylene carbonate, 1,3 - propane sultone, 1,3 - propene sultone, and lithium difluorophosphate, and the lithium secondary battery according to claim 1.

9. The organic solvent contains a mixture of a fluorinated cyclic carbonate solvent and a chain carbonate solvent, and the lithium secondary battery according to claim 1.

10. The chain carbonate solvent contains diethyl carbonate, ethyl methyl carbonate, or a mixture thereof, and the lithium secondary battery according to claim 9.

11. The concentration of the lithium salt in the mixed solution of the lithium salt and the organic solvent is 1.5 M or more, and the lithium secondary battery according to claim 1.

12. The lithium secondary battery according to claim 1, wherein the positive electrode active material contains a lithium composite transition metal oxide represented by the following Chemical Formula 3: [Chemical Formula 3] Li １＋ｘ (Ni ａ Co ｂ Mn ｃ M ｄ )O ２ In the Chemical Formula 3, M is any one or more 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; 1 + x, a, b, c, and d are atomic fractions of independent elements, -0.2 ≤ x ≤ 0.2, 0.60 ≤ a < 1, 0 < b ≤ 0.30, 0 < c ≤ 0.30, 0 ≤ d ≤ 0.10, and a + b + c + d = 1.

13. The lithium secondary battery according to claim 1, wherein the positive electrode active material contains a lithium composite transition metal oxide in which the molar ratio of nickel among transition metals is 70 mol% or more.

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