Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery including a negative electrode, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. The negative electrode contains a negative electrode active material, the negative electrode active material contains a silicon-based active material, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive, the additive contains a first additive and a second additive, the first additive contains a coumarin-based compound represented by a specific chemical formula, and the second additive contains at least one selected from the group consisting of lithium fluoromalonate(difluoro)borate (LiFMDFB), lithium difluoro(oxalate)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis-(oxalate)phosphate (LiDFOP). The secondary battery according to the present invention can improve high-temperature cycle characteristics and high-temperature storage characteristics by forming a flexible and highly durable film on the negative electrode containing the silicon-based active material.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0103657, filed on August 18, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery. [Background technology]

[0003] Recently, the application areas of lithium secondary batteries have rapidly expanded from supplying power to electronic devices such as electrical, electronic, communication, and computer equipment to storing and supplying power to large-area devices such as automobiles and power storage devices. As a result, there is an increasing need for secondary batteries that have high capacity, high output, and high stability.

[0004] The lithium secondary battery mainly comprises a positive electrode made of a lithium-containing transition metal oxide, a negative electrode capable of storing lithium, an electrolyte serving as a medium for transferring lithium ions, and a separator. The negative electrode may contain a negative electrode active material such as a carbon-based active material or a silicon-based active material.

[0005] In the case of the lithium secondary battery, a coating (SEI coating) is formed on the positive electrode and / or negative electrode during the initial activation process, which protects the positive electrode and negative electrode and prevents electrolyte consumption due to electrolyte side reactions during battery operation. If a strong electrode coating cannot be formed on the positive electrode and / or negative electrode during this initial activation process, problems such as deterioration of capacity and shortened lifespan may occur.

[0006] In particular, silicon-based active materials, among negative electrode active materials, have attracted attention for their high capacity compared to carbon-based active materials, but have the drawback of large volume changes due to lithium insertion and extraction. Such volume expansion of silicon-based active materials can reduce the durability of the SEI film already formed or cause various problems such as continuous electrolyte consumption and an increase in the thickness of the SEI film due to the generation of new negative electrode active material on the surface, which can lead to capacity degradation and shortened lifespan. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a lithium secondary battery in which a flexible and durable coating is formed on a negative electrode containing a silicon-based active material, thereby reducing side reactions in the electrolyte and improving high-temperature cycle life performance and high-temperature storage performance. [Means for solving the problem]

[0008] The present invention provides a lithium secondary battery comprising: a negative electrode; a positive electrode facing the negative electrode; a separator interposed between the negative electrode and the positive electrode; and a non-aqueous electrolyte, wherein the negative electrode comprises a negative electrode active material, the negative electrode active material comprising a silicon-based active material; the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, the additive comprising a first additive and a second additive, the first additive comprising a compound represented by the following Chemical Formula 1, and the second additive comprising at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis(oxalato)phosphate (LiDFOP):

[0009] [ka]

[0010] In Chemical Formula 1, R's each independently represent a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof; and n represents an integer selected from 0 to 6. [Effects of the Invention]

[0011] The lithium secondary battery according to the present invention comprises a negative electrode containing a silicon-based active material and a non-aqueous electrolyte containing, as additives, a first additive containing a coumarin-based compound having a specific structural formula and a second additive containing lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalate)borate (LiDFOB), or the like. When the non-aqueous electrolyte contains both the first and second additives, an SEI coating that is flexible, highly resilient, and highly durable can be formed on the negative electrode. Thus, the lithium secondary battery according to the present invention forms the SEI coating on a silicon-based negative electrode, which is prone to significant volume expansion and therefore potential electrolyte side reactions. This prevents cracking of the SEI coating, prevents electrolyte side reactions, and prevents increased resistance due to an increase in the thickness of the electrode coating. As a result, various performances of the lithium secondary battery, particularly high-temperature cycle life and high-temperature storage performance, can be improved. DETAILED DESCRIPTION OF THE INVENTION

[0012] The terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as having meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best explain their inventions.

[0013] In this specification, the terms "comprises," "includes," "has," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but are not intended to preclude the possible presence or addition of one or more different features, numbers, steps, components, or combinations thereof.

[0014] On the other hand, before describing the present invention, unless otherwise specified, "*" in the present invention means a connecting portion (bonding site) between the same or different atoms or terminal ends of a chemical formula.

[0015] In the present specification, in the description of "number of carbon atoms a to b," "a" and "b" refer to the number of carbon atoms contained in a specific functional group. That is, the functional group can contain "a" to "b" carbon atoms. For example, "an alkyl group having 1 to 5 carbon atoms" refers to an alkyl group containing 1 to 5 carbon atoms, that is, CH3 - , CH3CH2 - , CH3CH2CH2 - , (CH3)2CH-, CH3CH2CH2CH2 - , (CH3)2CHCH2 - , CH3CH2CH2CH2CH2 - , (CH3)2CHCH2CH2 - etc.

[0016] Furthermore, in this specification, both the alkyl group and the aryl group can be substituted or unsubstituted. Unless otherwise specified, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom is replaced with an element other than hydrogen, and includes, for example, an alkyl group having 1 to 20 carbon atoms, an alkenyl group having 2 to 20 carbon atoms, an alkynyl group having 2 to 20 carbon atoms, an alkoxy group having 1 to 20 carbon atoms, a cycloalkyl group having 3 to 12 carbon atoms, a cycloalkenyl group having 3 to 12 carbon atoms, a cycloalkynyl group having 3 to 12 carbon atoms, a heterocycloalkyl group having 3 to 12 carbon atoms, a heterocycloalkenyl group having 3 to 12 carbon atoms, an aryloxy group having 6 to 12 carbon atoms, a halogen atom, a fluoroalkyl group having 1 to 20 carbon atoms, a nitro group, an aryl group having 6 to 20 carbon atoms, a heteroaryl group having 2 to 20 carbon atoms, or a haloaryl group having 6 to 20 carbon atoms.

[0017] The present invention will now be described in more detail.

[0018] [Lithium secondary battery] The present invention relates to a lithium secondary battery.

[0019] Specifically, the lithium secondary battery according to the present invention includes a negative electrode, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte. The negative electrode includes a negative electrode active material, and the negative electrode active material includes a silicon-based active material. The non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive. The additive includes a first additive and a second additive. The first additive includes a compound represented by the following Chemical Formula 1. The second additive includes at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis(oxalato)phosphate (LiDFOP):

[0020] [ka]

[0021] In Chemical Formula 1, R's each independently represent a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof; and n represents an integer selected from 0 to 6.

[0022] The lithium secondary battery according to the present invention comprises a negative electrode containing a silicon-based active material and a non-aqueous electrolyte containing, as additives, a first additive containing a coumarin-based compound having a specific structural formula and a second additive containing lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalate)borate (LiDFOB), or the like. When the non-aqueous electrolyte contains both the first additive and the second additive, a flexible SEI coating with high recovery and excellent durability can be formed on the negative electrode. Thus, the lithium secondary battery according to the present invention forms the SEI coating on a silicon-based negative electrode, which is prone to significant volume expansion and associated electrolyte side reactions. This prevents cracking of the SEI coating, prevents electrolyte side reactions, and prevents increased resistance due to an increase in the thickness of the electrode coating. As a result, various performances of the lithium secondary battery, particularly high-temperature cycle life and high-temperature storage performance, can be improved.

[0023] The lithium secondary battery of the present invention can be manufactured by a conventional method known in the art, for example, by stacking an anode, a cathode, and a separator interposed between the anode and cathode to form an electrode assembly, inserting the electrode assembly into a battery case, and injecting the non-aqueous electrolyte of the present invention into the battery case.

[0024] (1) Negative electrode The negative electrode includes a negative electrode active material.

[0025] The negative electrode active material includes a silicon-based active material.

[0026] Silicon-based active materials have the advantage of having higher capacity and higher energy density than carbon-based active materials such as graphite, but suffer from significant volumetric changes during charging and discharging. Such volumetric expansion and contraction of silicon-based active materials can lead to breakage of conductive connections within the negative electrode, resulting in increased resistance and reduced lifespan. Furthermore, the SEI coating of the negative electrode formed during activation of a lithium secondary battery can crack due to volumetric changes in the silicon-based active material. This can promote electrolyte side reactions, leading to increased SEI coating thickness and problems such as increased resistance and electrolyte depletion, resulting in reduced lifespan and storage characteristics.

[0027] To solve these problems, the present invention is characterized by using a non-aqueous electrolyte containing a first additive and a second additive, which will be described later, together with a negative electrode containing a silicon-based active material. The combined use of the first additive and the second additive improves the flexibility (recovery) and durability of the SEI coating formed on the negative electrode, provides excellent recovery against volumetric changes in the silicon-based active material, prevents cracking of the SEI coating, and significantly prevents electrolyte side reactions and electrolyte depletion, resulting in a lithium secondary battery with excellent life and storage performance.

[0028] The silicon-based active material is SiO x (0≦x<2). In the case of SiO2, since it does not react with lithium ions and therefore cannot store lithium, it is preferable that x is within the above range. Specifically, the silicon-based active material is a compound containing Si and SiO x (0.7≦x≦1.2, specifically, x=1).

[0029] The average particle size (D 50) can be 1 μm to 30 μm, preferably 2 μm to 15 μm, in order to ensure structural stability and reduce side reactions with the electrolyte during charge and discharge.

[0030] The negative electrode active material may further include a carbon-based active material in addition to the silicon-based active material.

[0031] The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and preferably includes at least one selected from the group consisting of artificial graphite and natural graphite.

[0032] The average particle size (D 50 ) can be 10 μm to 30 μm, preferably 15 μm to 25 μm, in order to ensure structural stability and reduce side reactions with the electrolyte during charge and discharge.

[0033] When the negative electrode active material contains a silicon-based active material and a carbon-based active material, the weight ratio of the silicon-based active material to the carbon-based active material can be 1:99 to 50:50, specifically 3:97 to 20:80, and more specifically 3:97 to 10:90.

[0034] In one embodiment, the negative electrode active material may not include a carbon-based active material.

[0035] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material may be included in the negative electrode active material layer.

[0036] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0037] The negative electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0038] The negative electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0039] The negative electrode active material layer may be disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.

[0040] The negative electrode active material may be contained in the negative electrode active material layer in an amount of 60% by weight to 99% by weight, preferably 75% by weight to 95% by weight.

[0041] The negative electrode active material layer may further include a binder and / or a conductive material in addition to the negative electrode active material.

[0042] The binder is used to improve the adhesive strength between the negative electrode active material layer and the negative electrode current collector and thereby improve battery performance. For example, the binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0043] The binder may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 30% by weight, preferably 1% by weight to 15% by weight, and more preferably 5% by weight to 10% by weight.

[0044] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum 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.

[0045] The conductive material may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 30% by weight, preferably 1% by weight to 25% by weight.

[0046] The thickness of the negative electrode active material layer can be 10 μm to 100 μm, and preferably 50 μm to 80 μm.

[0047] The negative electrode may be manufactured by coating at least one surface of a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, and / or a solvent for forming the negative electrode slurry, followed by drying and rolling.

[0048] The solvent for forming the negative electrode slurry may include, for example, at least one selected from the group consisting of distilled water, NMP (N-methyl-2-pyrrolidone), ethanol, methanol, and isopropyl alcohol, preferably distilled water, in order to facilitate dispersion of the negative electrode active material, binder, and / or conductive material.

[0049] (2) Positive electrode The positive electrode faces the negative electrode.

[0050] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector.

[0051] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector may contain at least one material selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, sintered carbon, and an aluminum-cadmium alloy, and preferably aluminum.

[0052] The positive electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0053] The positive electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0054] The positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer can be disposed on one surface or both surfaces of the positive electrode current collector.

[0055] The positive electrode active material layer can contain a positive electrode active material.

[0056] The positive electrode active material can be a compound capable of reversible intercalation and deintercalation of lithium. Specifically, it can include a lithium transition metal composite oxide containing at least one transition metal composed of nickel, cobalt, manganese, and aluminum and lithium, preferably a lithium transition metal composite oxide containing a transition metal containing nickel, cobalt, and manganese and lithium.

[0057] For example, as the lithium transition metal composite oxide, there are lithium-manganese-based oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (e.g., LiCoO2, etc.), lithium-nickel-based oxides (e.g., LiNiO2, etc.), lithium-nickel-manganese-based oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-z Ni z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt-based oxides (e.g., LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (e.g., LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2-z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt-based oxides (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2)O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are the atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), etc.), etc. can be mentioned, and one or two or more of these compounds can be included. Among them, in terms of being able to enhance the capacity characteristics and stability of the battery, the lithium transition metal composite oxide is LiCoO2, LiMnO2, LiNiO2, lithium nickel-manganese-cobalt oxide (e.g., Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc.), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), etc. can be, and considering the significance of the improvement effect by controlling the types and content ratios of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide is Li(Ni 0.6 Mn 0.2 Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2 or Li(Ni 0.8 Mn 0.1 Co 0.1) O2, etc., and any one or a mixture of two or more of these can be used.

[0058] More specifically, the positive electrode active material may contain, as a lithium transition metal composite oxide, 60 mol% or more of nickel relative to the total number of moles of transition metals contained in the lithium transition metal composite oxide. Specifically, the positive electrode active material may be a lithium transition metal composite oxide, and the transition metal may include nickel and at least one selected from manganese, cobalt, and aluminum, and the nickel may be contained in an amount of 60 mol% or more, specifically 60 mol% to 90 mol%, relative to the total number of moles of the transition metals. Such a lithium transition metal composite oxide with a high nickel content is preferable in that it can reduce by-products in gas generated by structural collapse when used together with the above-mentioned nonaqueous electrolyte.

[0059] The positive electrode active material may include a lithium composite transition metal oxide represented by the following chemical formula A:

[0060] [Chemical formula A] Li 1+x (Ni a Co b Mn c M d )O2

[0061] In the chemical formula A, M is one or more selected from 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 1+x, a, b, c, and d are atomic fractions of independent elements, and 0≦x≦0.2, 0.50≦a<1, 0 <b≦0.25、0<c≦0.25、0≦d≦0.1、a+b+c+d=1である。

[0062] Preferably, the a, b, c, and d may be in the ranges 0.70≦a≦0.95, 0.025≦b≦0.20, 0.025≦c≦0.20, and 0≦d≦0.05, respectively.

[0063] Furthermore, the a, b, c, and d may be in the ranges 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.

[0064] Furthermore, the a, b, c, and d may be in the ranges 0.85≦a≦0.90, 0.05≦b≦0.10, 0.05≦c≦0.10, and 0≦d≦0.03, respectively.

[0065] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80% by weight to 99% by weight, preferably 92% by weight to 98.5% by weight, in consideration of sufficient capacity of the positive electrode active material.

[0066] The positive electrode active material layer may further include a binder and / or a conductive material in addition to the positive electrode active material.

[0067] The binder is a component that facilitates binding of the active material and conductive material, etc., and binding to the current collector, and specifically includes at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0068] The binder may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight, in order to ensure sufficient binding strength between components such as the positive electrode active material.

[0069] The conductive material can be used to assist and improve the conductivity of a secondary battery, and is not particularly limited as long as it does not undergo chemical changes and has conductivity. Specifically, the conductive material can include at least one selected from the group consisting of graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives. Preferably, the conductive material can include carbon black in terms of improving conductivity.

[0070] In order to ensure sufficient electrical conductivity, the conductive material may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight.

[0071] The thickness of the positive electrode active material layer may be 30 μm to 400 μm, and preferably 40 μm to 110 μm.

[0072] The positive electrode may be manufactured by coating a positive electrode slurry containing a positive electrode active material and, optionally, a binder, a conductive material, and a solvent for forming the positive electrode slurry on the positive electrode current collector, followed by drying and rolling.

[0073] The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone), etc. The solid content of the positive electrode slurry may be 40 wt% to 90 wt%, specifically 50 wt% to 80 wt%.

[0074] (3) Separator The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homocopolymer, a propylene homocopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminated state, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, etc. Furthermore, to ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may also be used, and may be used in a single-layer or multi-layer structure.

[0075] (4) Nonaqueous electrolyte The non-aqueous electrolyte according to the present invention includes a lithium salt, an organic solvent, and an additive, the additive including a first additive and a second additive, the first additive including a compound represented by the following Chemical Formula 1, and the second additive including at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis-(oxalato)phosphate (LiDFOP):

[0076] [ka]

[0077] In Chemical Formula 1, R's each independently represent a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof; and n represents an integer selected from 0 to 6.

[0078] The nonaqueous electrolyte of the present invention is characterized by including, as additives, a first additive containing a coumarin-based compound having a specific structural formula and a second additive containing lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalate)borate (LiDFOB), or the like. When the nonaqueous electrolyte simultaneously contains the first additive and the second additive, a flexible SEI coating with strong recovery and excellent durability can be formed on the negative electrode. Thus, the lithium secondary battery of the present invention forms the SEI coating on a silicon-based negative electrode, which is prone to significant volume expansion and therefore potential electrolyte side reactions. This prevents cracking of the SEI coating, inhibits electrolyte side reactions, and prevents increased resistance due to an increase in the thickness of the electrode coating. As a result, various performances of the lithium secondary battery, particularly high-temperature cycle life and high-temperature storage performance, can be improved.

[0079] On the other hand, when a silicon-based active material is not used as the negative electrode active material, for example, when the above-mentioned nonaqueous electrolyte is applied to a negative electrode containing only a carbon-based active material, the combined use of the first additive and the second additive may actually cause an increase in resistance, and therefore the effect may not be significantly improved, or may even be reduced, compared to when the first additive or the second additive is used alone.

[0080] 1) Lithium salt The lithium salt used in the present invention may be any of various lithium salts commonly used in non-aqueous electrolytes for lithium secondary batteries. For example, the lithium salt may contain Li as a cation.+ and the anion is F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , B 10 Cl 10 - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CH3SO3 - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - It may contain at least one selected from the group consisting of:

[0081] Specifically, the lithium salts include LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10, LiBOB (LiB(C2O4)2), LiCF3SO3, LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt may include at least one selected from the group consisting of LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), and LiBETI (LiN(SO2CF2CF3)2).

[0082] The lithium salt may be contained in the non-aqueous electrolyte at a concentration of 0.5 M to 5 M, specifically 0.8 M to 4 M, and more specifically 0.8 M to 2.0 M. When the concentration of the lithium salt satisfies the above range, the lithium ion transport number (Li + The transference number and dissociation degree of lithium ions are improved, which can improve the output characteristics of the battery.

[0083] 2) Organic solvents The organic solvent is not particularly limited as long as it is a non-aqueous solvent commonly used in lithium secondary batteries and can minimize decomposition due to oxidation reactions during charging and discharging of the secondary battery.

[0084] Specifically, the organic solvent may include at least one selected from the group consisting of cyclic carbonate organic solvents, linear carbonate organic solvents, linear ester organic solvents, and cyclic ester organic solvents.

[0085] Specifically, the organic solvent may include a cyclic carbonate organic solvent, a linear carbonate organic solvent, or a mixture thereof.

[0086] The cyclic carbonate organic solvent is a high-viscosity organic solvent that has a high dielectric constant and can effectively dissociate lithium salts in the electrolyte. Specifically, the cyclic carbonate organic solvent may include at least one organic solvent selected from the group consisting of ethylene carbonate (EC), fluoroethylene carbonate (FEC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate. More specifically, the cyclic carbonate organic solvent may include at least one organic solvent selected from the group consisting of ethylene carbonate (EC) and fluoroethylene carbonate (FEC). More specifically, the cyclic carbonate organic solvent may include fluoroethylene carbonate (FEC). Fluoroethylene carbonate (FEC) can form an SEI coating with a high content of inorganic components such as LiF on a negative electrode, and can increase the durability of the SEI coating. As a result, when applied to a negative electrode containing a silicon-based active material, improvements in lifespan performance and storage characteristics can be expected.

[0087] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and may include at least one 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. More specifically, it may include at least one selected from the group consisting of ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). Diethyl carbonate is a symmetric linear carbonate that is free from transesterification, and therefore is preferred because it can prevent a decrease in reduction stability due to the generation of by-products caused by transesterification. Diethyl carbonate is particularly preferred when used in a negative electrode containing a silicon-based active material.

[0088] The organic solvent may be a mixture of a cyclic carbonate organic solvent and a linear carbonate organic solvent. Here, the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed at a volume ratio of 5:95 to 40:60, specifically a volume ratio of 7:93 to 30:70, more specifically a volume ratio of 8:92 to 30:70. When the mixing ratio of the cyclic carbonate organic solvent and the linear carbonate organic solvent satisfies this range, high dielectric constant and low viscosity properties are simultaneously achieved, and excellent ionic conductivity properties can be realized.

[0089] In addition, in order to produce an electrolyte having high ionic conductivity, the organic solvent may further include at least one ester organic solvent selected from the group consisting of linear ester organic solvents and cyclic ester organic solvents in addition to the at least one carbonate organic solvent selected from the group consisting of cyclic carbonate organic solvents and linear carbonate organic solvents.

[0090] Specifically, the linear ester organic solvent may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0091] The cyclic ester organic solvent may specifically include at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0092] Meanwhile, the organic solvent may be any organic solvent commonly used in non-aqueous electrolytes, without limitation, if necessary, and may further include at least one organic solvent selected from the group consisting of an ether-based organic solvent, a glyme-based organic solvent, and a nitrile-based organic solvent.

[0093] The ether solvent may be any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL), or a mixture of two or more thereof, but is not limited thereto.

[0094] The glyme-based solvent has a higher dielectric constant and lower surface tension than linear carbonate-based organic solvents, and is less reactive with metals. The glyme-based solvent may include, but is not limited to, at least one selected from the group consisting of dimethoxyethane (DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME).

[0095] The nitrile solvent may be at least one selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but is not limited thereto.

[0096] 3) Additives The additives include a first additive and a second additive.

[0097] The first additive includes a compound represented by the following Chemical Formula 1:

[0098] [ka]

[0099] In Chemical Formula 1, R's each independently represent a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof; and n represents an integer selected from 0 to 6.

[0100] The first additive contains a coumarin-based compound having the structural formula described above. This coumarin-based compound has strong reducibility at the anode, and during initial activation of the lithium secondary battery, its ring structure opens, allowing the formation of a polyethylene oxide-based polymer-type SEI coating. Such a polymer-type SEI coating has the advantages of excellent flexibility and recovery. However, the SEI coating derived from the first additive has poor durability. Therefore, in order to achieve the desired effects of improving high-temperature cycle life performance and high-temperature storage performance, the first additive must be used in combination with a second additive, as described below.

[0101] In Chemical Formula 1, each R may independently include a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof.

[0102] In Chemical Formula 1, specifically, R may each independently include a halogen (the halogen may be selected from F, Cl, Br, and I, and specifically may be F), a nitrile group, a propargyl group, an ester group, an ether group, or a combination of two or more thereof. Such a substituent is preferred in that it has excellent reducibility, is advantageous for forming a polymer-type SEI coating, and also has excellent lithium ion transport performance.

[0103] In the above Chemical Formula 1, n can be an integer selected from 0 to 6, specifically an integer selected from 1 to 6, more specifically n can be 1. In the above Chemical Formula 1, when n is 2 or more, each R may be the same as or different from each other.

[0104] Specifically, the compound represented by Chemical Formula 1 may be at least one selected from the group consisting of compounds represented by Chemical Formula 2 below and compounds represented by Chemical Formula 3 below.

[0105] [ka]

[0106] [ka]

[0107] In Chemical Formula 2 and Chemical Formula 3, R is as defined in Chemical Formula 1.

[0108] The compounds represented by Chemical Formula 2 and Chemical Formula 3 have structures in which substituents exist at positions 3 and 7 (IUPAC nomenclature standard) of the ring structure, respectively, and are preferred in that synthesis at these positions is more advantageous than other substitution positions. In particular, the compound represented by Chemical Formula 2 in which the substituent exists at position 3 is more preferred in that the reaction uniformity is improved when reduced at the anode.

[0109] Specifically, the compound represented by Chemical Formula 1 can include at least one selected from the group consisting of compounds represented by Chemical Formulas 4 to 12 below. The compound represented by Chemical Formula 1 can specifically include at least one selected from the group consisting of compounds represented by Chemical Formulas 4 to 7 below, in terms of being reduced more smoothly on the negative electrode and being more advantageous for forming a polymer-type SEI coating, and more specifically can include at least one selected from the group consisting of Chemical Formulas 4, 5, and 7 below. Even more specifically, the compound represented by Chemical Formula 1 can include a compound represented by Chemical Formula 4 below, in terms of having strong reducing properties and an excellent effect of suppressing the elution of transition metals in addition to the above-mentioned effects.

[0110] [ka]

[0111] [ka]

[0112] [ka]

[0113] [ka]

[0114] [ka]

[0115] [ka]

[0116] [ka]

[0117] [ka]

[0118] [ka]

[0119] The first additive may be included in the non-aqueous electrolyte in an amount of 0.01 wt % to 10 wt %, specifically 0.05 wt % to 7 wt %, more specifically 0.1 wt % to 5 wt %, and even more specifically 0.3 wt % to 2 wt %. When the content of the compound represented by Chemical Formula 1 satisfies this range, it is preferable in that it imparts sufficient flexibility and recovery to the SEI film and prevents an increase in the resistance of the lithium secondary battery due to excessive addition, which in turn prevents a decrease in life performance.

[0120] The second additive includes at least one selected from the group consisting of lithium fluoromalonate(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis-(oxalato)phosphate (LiDFOP).

[0121] The second additive may include a fluorine-containing lithium salt additive, which can form an inorganic SEI film such as LiF during initial activation of the lithium secondary battery. Although an inorganic SEI film such as LiF has excellent adhesion to the negative electrode surface, it is difficult to completely cover the negative electrode surface, making it difficult to sufficiently prevent side reactions in the electrolyte.

[0122] In this regard, the nonaqueous electrolyte is characterized by the combined use of a first additive and a second additive capable of forming a polymer / inorganic composite SEI coating. In the nonaqueous electrolyte according to the present invention, the polymer SEI coating derived from the first additive covers the entire surface of the negative electrode, forming an SEI coating with excellent flexibility and recovery, and the inorganic SEI coating derived from the second additive is distributed within the polymer SEI coating, improving the durability of the SEI coating. Furthermore, the first additive (coumarin-based compound) forms radicals during a reduction reaction, and these radicals promote the release of fluorine contained in the second additive, thereby facilitating the formation of the inorganic SEI coating. Therefore, the nonaqueous electrolyte according to the present invention can form an SEI coating on the surface of the negative electrode that is flexible, has excellent recovery, and is improved in durability and strength. This prevents electrolyte consumption due to electrolyte side reactions and increased resistance due to an increase in the thickness of the electrode coating during battery operation, thereby improving the performance of the lithium secondary battery, particularly its high-temperature cycle life and high-temperature storage performance. In particular, when used with a negative electrode containing a silicon-based active material, the nonaqueous electrolyte according to the present invention is capable of forming a flexible and durable SEI coating on the silicon-based active material, which experiences a large degree of volumetric expansion during charge and discharge. This prevents damage to the SEI coating due to the volumetric expansion of the silicon-based active material, and prevents an increase in the thickness of the SEI coating and electrolyte consumption due to the surface exposure of new silicon-based active material due to the volumetric expansion, which is preferable.

[0123] Specifically, the second additive may include at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB) and lithium difluoro(oxalato)borate (LiDFOB). In this case, it is preferable in that it helps form a polymeric ether component, LiF component, which is advantageous for physical rigidity during reduction at the negative electrode, and that the inclusion of an ester structure in the SEI coating helps improve lithium ion transfer performance. Specifically, the second additive may include lithium fluoromalonato(difluoro)borate (LiFMDFB). Specifically, the second additive may include lithium difluoro(oxalato)borate (LiDFOB).

[0124] The second additive may be included in the non-aqueous electrolyte in an amount of 0.01 wt % to 10 wt %, specifically 0.05 wt % to 7 wt %, more specifically 0.1 wt % to 5 wt %, and even more specifically 0.3 wt % to 2 wt %. When the content of the compound represented by Chemical Formula 1 satisfies this range, it is preferable in that it provides sufficient durability and strength to the SEI film and prevents an increase in the resistance of the lithium secondary battery due to excessive addition, which in turn prevents a decrease in life performance.

[0125] The weight ratio of the first additive to the second additive may be 5:95 to 95:5, specifically 10:90 to 92:8, more specifically 16:84 to 91:9, even more specifically 30:70 to 70:30, and even more specifically 40:60 to 60:40. When the weight ratio is as described above, the flexibility and durability of the SEI film can be simultaneously improved to a desirable level, which is preferable.

[0126] The additive may further include an additional additive or a third additive in addition to the first and second additives. The additional additive may be included in the non-aqueous electrolyte to prevent the non-aqueous electrolyte from decomposing and causing the negative electrode to collapse in a high-power environment, or to improve low-temperature high-rate discharge characteristics, high-temperature stability, prevent overcharging, and suppress battery expansion at high temperatures.

[0127] Specifically, the additional additive may be at least one selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, propane sultone, propene sultone, succinonitrile, adiponitrile, ethylene sulfate, LiBOB (Lithium bis-(oxalato)borate), TMSPa (Tris(trimethylsilyl)phosphate), and TMSPi (Tris(trimethylsilyl)phosphate), and may be vinylene carbonate.

[0128] The additional additive may be included in the non-aqueous electrolyte in an amount of 0.1 wt % to 15 wt %.

[0129] In one embodiment, the non-aqueous electrolyte may be free of a silyl-containing additive.

[0130] The 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.

[0131] The present invention will be described in more detail below with reference to specific examples. However, the following examples are merely illustrative for facilitating understanding of the present invention and are not intended to limit the scope of the present invention. It will be apparent to those skilled in the art that various changes and modifications can be made within the scope and technical concept of the present description, and it goes without saying that such changes and modifications fall within the scope of the appended claims.

[0132] Examples and Comparative Examples Example 1 (Production of non-aqueous electrolyte) As the organic solvent, a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a volume ratio of 10:90 was used.

[0133] A non-aqueous electrolyte was prepared by adding LiPF6 as a lithium salt, a compound represented by the following Formula 4 as a first additive, LiFMDFB as a second additive, and vinylene carbonate (VC) as an additional additive to the organic solvent.

[0134] The LiPF6 was contained in the non-aqueous electrolyte at a concentration of 1.5M.

[0135] The compound represented by Chemical Formula 4 was contained in the non-aqueous electrolyte at 0.5 wt %, the LiFMDFB was contained in the non-aqueous electrolyte at 0.5 wt %, and the vinylene carbonate used as the additional additive was contained in the non-aqueous electrolyte at 0.5 wt %.

[0136] [ka]

[0137] (Lithium secondary battery manufacturing) Cathode active material (LiNi 0.85 Co 0.05 Mn 0.07 Al 0.03 A cathode mixture slurry (solid content 75.5 wt%) was prepared by adding O2: conductive material (carbon nanotubes): binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.74:0.70:1.56. The cathode mixture slurry was applied to one side of a cathode current collector (Al thin film) with a thickness of 15 μm, dried, and roll-pressed to prepare a cathode.

[0138] Anode active material (Si): conductive material (carbon black): binder (styrene-butadiene rubber) was mixed in a weight ratio of 70.0:20.3:9.7 with distilled water as a solvent to prepare anode mixture slurry (solid content 26 wt%). The anode mixture slurry was applied to one side of anode current collector (Cu thin film) with a thickness of 15 μm, dried, and roll pressed to prepare anode.

[0139] A polyethylene porous film separator was interposed between the prepared positive electrode and negative electrode in a dry room, and the prepared non-aqueous electrolyte was then injected to prepare a secondary battery.

[0140] Example 2 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the first additive was added to the non-aqueous electrolyte in an amount of 0.1 wt %.

[0141] Example 3 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the first additive was added to the non-aqueous electrolyte in an amount of 5.0 wt %.

[0142] Example 4 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by the following Chemical Formula 5 was added to the non-aqueous electrolyte at 0.5 wt % as the first additive instead of the compound represented by Chemical Formula 4.

[0143] [ka]

[0144] Example 5 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that, as a first additive, a compound represented by the following Chemical Formula 6 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 4.

[0145] [ka]

[0146] Example 6 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that, as a first additive, a compound represented by the following Chemical Formula 7 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 4.

[0147] [ka]

[0148] Example 7 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that 0.5 wt % of LiDFP was added to the non-aqueous electrolyte instead of LiFMDFB as the second additive.

[0149] Example 8 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that 0.5 wt % of LiDFOB was added to the non-aqueous electrolyte as the second additive instead of LiFMDFB.

[0150] Example 9 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the second additive was added to the non-aqueous electrolyte at 0.1 wt %.

[0151] Example 10 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the second additive was added to the non-aqueous electrolyte at 5 wt %.

[0152] Example 11 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 8, except that the second additive was added to the non-aqueous electrolyte at 0.1 wt %.

[0153] Example 12 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 8, except that the second additive was added to the non-aqueous electrolyte at 5 wt %.

[0154] Comparative Example 1 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the first additive and the second additive were not added.

[0155] Comparative Example 2 A non-aqueous electrolyte lithium secondary battery was produced in the same manner as in Example 1, except that the second additive was not added.

[0156] Comparative Example 3 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the first additive was not added.

[0157] [Table 1A] [Table 1B]

[0158] [Experimental Example] Experimental Example 1: Evaluation of high-temperature cycle capacity retention The lithium secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 3 prepared as above were charged to 4.2 V at 45° C. under CC / CV conditions at 0.33 C, and then discharged to 3 V under CC conditions at 0.33 C using an electrochemical charger / discharger. 300 charge / discharge cycles were performed, and the capacity retention rate was measured.

[0159] The capacity retention rate was calculated using the following formula, and the results are shown in Table 2 below.

[0160] Capacity retention rate (%) = (discharge capacity after 300 cycles / discharge capacity after 1 cycle) x 100

[0161] Experimental Example 2: Evaluation of capacity retention rate after high-temperature storage The lithium secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 3 were initially charged to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and then discharged to 2.5 V at 0.33 C. Subsequently, the secondary batteries were charged to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and then stored at 60° C. After storage, the secondary batteries were charged to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and then discharged to 2.5 V at 0.33 C, and the discharge capacities were measured.

[0162] The capacity retention rate was evaluated using the following formula, and the results are shown in Table 2 below.

[0163] Capacity retention rate (%) = (discharge capacity after N weeks of storage / initial discharge capacity) x 100 (In the above formula, N is an integer of 1 or more)

[0164] [Table 2]

[0165] Referring to Table 2, it can be seen that the lithium secondary batteries of Examples 1 to 12, which used non-aqueous electrolytes containing the first additive and the second additive according to the present invention, had significantly improved high-temperature cycle life performance and high-temperature storage performance compared to the lithium secondary batteries of Comparative Examples 1 to 3.

[0166] [Reference example] Reference example 1 1. Preparation of non-aqueous electrolyte The same non-aqueous electrolyte as used in Example 1 was used.

[0167] 2. Lithium secondary battery manufacturing Cathode active material (Li[Ni 0.85 Co 0.03 Mn 0.07 Al 0.03A cathode mixture slurry (solid content 75.5 wt%) was prepared by adding 02: conductive material (carbon nanotubes): binder (polyvinylidene fluoride) to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97.74:0.70:1.56. The cathode mixture slurry was applied to one side of a cathode current collector (Al thin film) with a thickness of 15 μm, dried, and roll-pressed to prepare a cathode.

[0168] Anode active material (natural graphite), conductive material (carbon black), and binder (styrene-butadiene rubber (SBR)-carboxymethyl cellulose (CMC)) were mixed in a weight ratio of 95.0:1.5:3.5 with distilled water as a solvent to prepare anode mixture slurry (solid content 60 wt%). The anode mixture slurry was applied to one side of anode current collector (Cu thin film) with a thickness of 15 μm, dried, and roll-pressed to prepare anode.

[0169] A polyethylene porous film separator was interposed between the prepared positive electrode and negative electrode in a dry room, and the prepared non-aqueous electrolyte was then injected to prepare a secondary battery.

[0170] Reference example 2 A secondary battery was produced in the same manner as in Reference Example 1, except that the nonaqueous electrolyte used in Comparative Example 2 was used as the nonaqueous electrolyte.

[0171] Reference example 3 A secondary battery was produced in the same manner as in Reference Example 1, except that the nonaqueous electrolyte used in Comparative Example 3 was used as the nonaqueous electrolyte.

[0172] Reference Experiment Example 1 The lithium secondary batteries of Reference Examples 1 to 3 prepared above were charged to 4.2 V at 45° C. under CC / CV conditions at 0.33 C, and then discharged to 3 V under CC conditions at 0.33 C using an electrochemical charger / discharger. 300 charge / discharge cycles were performed, with one cycle being defined as charging to 4.2 V at 45° C. using CC / CV conditions at 0.33 C, and then the capacity retention rate was measured.

[0173] The capacity retention rate was calculated using the following formula, and the results are shown in Table 3 below.

[0174] Capacity retention rate (%) = (discharge capacity after 300 cycles / discharge capacity after 1 cycle) x 100

[0175] Reference Experiment Example 2 The lithium secondary batteries of Reference Examples 1 to 3 prepared above were initially charged to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and then discharged to 2.5 V at 0.33 C, followed by charging to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and storing at 60° C. After storage, the secondary batteries were charged to 4.2 V / 55 mA at room temperature under constant current / constant voltage (CC / CV) conditions of 0.33 C / 4.2 V and then discharged to 2.5 V at 0.33 C, and the discharge capacities were measured.

[0176] The capacity retention rate was evaluated using the following formula, and the results are shown in Table 3 below.

[0177] Capacity retention rate (%) = (discharge capacity after N weeks of storage / initial discharge capacity) x 100 (In the above formula, N is an integer of 1 or more)

[0178] [Table 3]

[0179] Referring to Table 3, it can be seen that when a nonaqueous electrolyte having the characteristics of the present invention is applied to a negative electrode containing only a carbon-based active material (Reference Example 1), the performance is actually worse than Reference Examples 2 and 3, which differ in that only one of the first and second additives is used. Because carbon-based active materials do not expand in volume as much as silicon-based active materials, the polymer-type SEI coating derived from the first additive acts as a resistor, and this problem of increased resistance is thought to be exacerbated when the first and second additives are used simultaneously.

[0180] This clearly shows that the effects of the present invention on improving life performance and storage performance, particularly the effects on improving high-temperature life performance and high-temperature storage performance, are manifested only when a negative electrode containing a silicon-based active material is combined with the above-described nonaqueous electrolyte.

Claims

1. a negative electrode, a positive electrode facing the negative electrode, a separator interposed between the negative electrode and the positive electrode, and a non-aqueous electrolyte; the negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive; the additives include a first additive and a second additive; The first additive includes a compound represented by the following Chemical Formula 1: The second additive comprises at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), lithium difluorophosphate (LiDFP), and lithium difluorobis(oxalate)phosphate (LiDFOP). 【Chemistry 1】 In the above Chemical Formula 1, each R independently represents a halogen, a nitrile group, a propargyl group, an ester group, an ether group, a ketone group, a carboxy group, a substituted or unsubstituted alkyl group, a substituted or unsubstituted alkenyl group, a substituted or unsubstituted alkynyl group, a substituted or unsubstituted alkoxy group, a boron group, a borate group, an isocyanate group, an isothiocyanate group, a silyl group, a siloxane group, a sulfone group, a sulfonate group, a sulfate group, or a combination of two or more thereof; n is an integer selected from 0 to 6.

2. 2. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 includes at least one selected from the group consisting of compounds represented by the following Chemical Formula 2 and compounds represented by the following Chemical Formula 3: 【Chemistry 2】 【Transformation 3】 In Formula 2 and Formula 3, R is as defined in Formula 1.

3. The lithium secondary battery of claim 1, wherein the compound represented by Chemical Formula 1 includes at least one selected from the group consisting of compounds represented by Chemical Formulas 4 to 12: 【Chemistry 4】 【Transformation 5】 【Transformation 6】 【Transformation 7】 【Transformation 8】 【Chemistry 9】 【Chemistry 10】 【Chemistry 11】 【Chemistry 12】

4. 2. The lithium secondary battery according to claim 1, wherein the first additive is contained in the non-aqueous electrolyte in an amount of 0.01% by weight to 10% by weight.

5. 2. The lithium secondary battery according to claim 1, wherein the second additive comprises at least one selected from the group consisting of lithium fluoromalonato(difluoro)borate (LiFMDFB) and lithium difluoro(oxalato)borate (LiDFOB).

6. 2. The lithium secondary battery according to claim 1, wherein the second additive is contained in the non-aqueous electrolyte in an amount of 0.01% by weight to 10% by weight.

7. 2. The lithium secondary battery according to claim 1, wherein a weight ratio of the first additive to the second additive is 10:90 to 90:

10.

8. The lithium salts include LiCl, LiBr, LiI, and LiBF. ４ , LiClO ４ , LiAlO ４ , LiAlCl ４ , LiPF ６ , LiSbF ６ , LiAsF ６ , LiB １０ Cl １０ , LiBOB(LiB(C ２ O ４ ) ２ ), LiCF ３ SO ３ , LiFSI (LiN(SO ２ F) ２ ), LiCH ３ SO ３ , LiCF ３ CO ２ , LiCH ３ CO ２ and LiBETI(LiN(SO ２ CF ２ CF ３ ) ２ 2. The lithium secondary battery according to claim 1, comprising at least one selected from the group consisting of:

9. 2. The lithium secondary battery according to claim 1, wherein the lithium salt is contained in the non-aqueous electrolyte at a molar concentration of 0.5 M to 5.0 M.

10. 2. The lithium secondary battery according to claim 1, wherein the organic solvent includes at least one selected from the group consisting of a cyclic carbonate organic solvent, a linear carbonate organic solvent, a linear ester organic solvent, and a cyclic ester organic solvent.

11. the organic solvent includes a cyclic carbonate organic solvent and a linear carbonate organic solvent, the cyclic carbonate organic solvent contains fluoroethylene carbonate, The lithium secondary battery according to claim 10 , wherein the linear carbonate organic solvent includes diethyl carbonate.

12. 2. The lithium secondary battery of claim 1, wherein the additive further comprises at least one third additive selected from the group consisting of vinylene carbonate, vinyl ethylene carbonate, propane sultone, propene sultone, succinonitrile, adiponitrile, ethylene sulfate, LiBOB (Lithium bis-(oxalato)borate), TMSPa (Tris(trimethylsilyl)phosphate), and TMSPi (Tris(trimethylsilyl)phosphate).

13. The silicon-based active material is SiO ｘ 2. The lithium secondary battery according to claim 1, comprising a compound represented by the formula (0≦x<2).

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