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 the contents disclosed in the document of the Korean Patent Application are incorporated herein by reference in their entirety.

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

Background Art

[0003] Recently, as the application fields of lithium secondary batteries have rapidly expanded not only to the power supply of electronic devices such as electricity, electronics, communication, and computers but also to the power storage and supply of large-area devices such as automobiles and power storage devices, the need for secondary batteries with high capacity, high output, and high stability has been increasing.

[0004] The lithium secondary battery mainly includes a positive electrode made of a transition metal oxide containing lithium, a negative electrode capable of storing lithium, an electrolyte serving as a medium for transmitting lithium ions, and a separator. Here, the negative electrode may include 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 film (SEI film) is formed on the positive electrode and / or the negative electrode in the initial activation process, thereby protecting the positive electrode and the negative electrode and preventing the consumption of the electrolyte due to side reactions of the electrolyte during battery operation. If a strong electrode film cannot be formed on the positive electrode and / or the negative electrode in such an initial activation process, problems such as capacity degradation and lifespan reduction may occur.

[0006] In particular, among the negative electrode active materials, silicon-based active materials have attracted attention for their high capacity compared to carbon-based active materials, but they have the drawback of a large volume change due to the insertion and desorption of lithium. Such volume expansion of silicon-based active materials causes various problems such as reducing the durability of the already formed SEI film or causing continuous electrolyte consumption and an increase in the thickness of the SEI film due to the generation on the surface of the new negative electrode active material, which leads to problems such as capacity degradation and reduced life.

Summary of the Invention

Problems to be Solved by the Invention

[0007] One problem of the present invention is to provide a lithium secondary battery that can form a flexible and highly durable film on a negative electrode containing a silicon-based active material, reduce side reactions of the electrolyte, and improve high-temperature cycle life performance and high-temperature storage performance.

Means for Solving the Problems

[0008] 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 includes a negative electrode active material, 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, and the second additive includes 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).

[0009]

Chemical

[0010] In the formula (1), each R is independently 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 is an integer selected from 0 to 6.

Advantages of the Invention

[0011] The lithium secondary battery according to the present invention includes a negative electrode containing a silicon-based active material, and as additives, a first additive containing a coumarin-based compound having a specific structural formula and a second additive containing lithium fluoro(malonato)(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), etc. in a non-aqueous electrolyte. When the non-aqueous electrolyte contains the first additive and the second additive at the same time, an SEI film that is flexible, highly recoverable, and excellent in durability can be formed on the negative electrode. Thereby, the lithium secondary battery according to the present invention forms the above-mentioned SEI film on a silicon-based negative electrode with a large degree of volume expansion and thus a risk of side reactions of the electrolyte, thereby preventing cracks in the SEI film, preventing side reactions of the electrolyte, and preventing an increase in resistance due to an increase in the thickness of the electrode film. As a result, various performances of the lithium secondary battery, particularly high-temperature cycle life performance and high-temperature storage performance, can be improved.

Embodiments for Carrying Out the Invention

[0012] Terms and words used in this specification and the claims should not be construed as being limited to their ordinary or dictionary meanings. The inventors should interpret them in accordance with the meaning and concept that conform to the technical idea of the present invention, in accordance with the principle that they can appropriately define the concept of the terms in order to explain their invention in the best way.

[0013] In this specification, terms such as "comprising", "including" or "having" are intended to specify that there are implemented features, numbers, steps, components, or combinations thereof, and it should be understood that they do not preclude the presence or addition possibility of one or more different features, numbers, steps, components, or combinations thereof in advance.

[0014] On the other hand, before explaining the present invention, unless otherwise specified in the present invention, "*" means a connected part (bonding site) between the ends of the same or different atoms or chemical formulas.

[0015] Also, in this specification, in the description of "alkyl group having a carbon number of a to b", "a" and "b" mean 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, "alkyl group having a carbon number of 1 to 5" means an alkyl group containing carbon atoms having a carbon number of 1 to 5, that is, CH3 - , CH3CH2 - , CH3CH2CH2 - , (CH3)2CH-, CH3CH2CH2CH2 - , (CH3)2CHCH2 - , CH3CH2CH2CH2CH2 - , (CH3)2CHCH2CH2 - and so on.

[0016] In addition, in this specification, both the alkyl group and the aryl group can be either substituted or unsubstituted. The term "substituted" means that, unless otherwise specified, at least one or more hydrogens bonded to carbon are substituted with elements other than hydrogen. For example, it means being substituted with 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, a haloaryl group having 6 to 20 carbon atoms, etc.

[0017] Hereinafter, the present invention will 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 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 compound represented by the following Chemical Formula 1, and the second additive is characterized by containing 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).

[0020]

Chemical formula

[0021] In the above Chemical Formula 1, each R is independently 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 is an integer selected from 0 to 6.

[0022] The lithium secondary battery according to the present invention includes a negative electrode containing a silicon-based active material, and as additives, a first additive containing a coumarin-based compound having a specific structural formula and a second additive containing lithium fluoro(malonato)(difluoro)borate (LiFMDFB), lithium difluoro(oxalato)borate (LiDFOB), etc. in a non-aqueous electrolyte. When the non-aqueous electrolyte contains the first additive and the second additive at the same time, an SEI film that is flexible, has strong recoverability, and is excellent in durability can be formed on the negative electrode. Thereby, the lithium secondary battery according to the present invention forms the above-mentioned SEI film on a silicon-based negative electrode that has a large degree of volume expansion and may cause side reactions of the electrolyte, thereby preventing cracks in the SEI film, preventing side reactions of the electrolyte, and preventing an increase in resistance due to an increase in the thickness of the electrode film. As a result, various performances of the lithium secondary battery, particularly high-temperature cycle life performance and high-temperature storage performance, can be improved.

[0023] Here, the lithium secondary battery of the present invention can be manufactured according to a conventional method well-known in the art. For example, after a negative electrode, a positive electrode, and a separator interposed between the negative electrode and the positive electrode are laminated in order to form an electrode assembly, the electrode assembly is inserted into the interior of a battery case, and the non-aqueous electrolyte according to the present invention is injected for manufacturing.

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

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

[0026] The silicon-based active material has a higher capacity than carbon-based active materials such as graphite and has the advantage of a high energy density. However, during charge and discharge, there is a drawback that the volume change of the active material is large. Such volume expansion and contraction of the silicon-based active material result in the breakage of the conductive connection in the negative electrode, causing an increase in resistance and a decrease in life performance. In addition, the SEI film on the negative electrode formed during the activation process of the lithium secondary battery may be cracked due to the volume change of the silicon-based active material, which promotes side reactions of the electrolyte and causes problems such as an increase in resistance due to an increase in the thickness of the SEI film and depletion of the electrolyte. As a result, the performance may deteriorate in terms of life performance and storage characteristics.

[0027] To solve such problems, the present invention is characterized in that a non-aqueous electrolyte using a first additive and a second additive described later in combination as an additive is used together with a negative electrode containing a silicon-based active material. By using the first additive and the second additive in combination, the SEI film formed on the negative electrode can simultaneously improve its flexibility (recovery property) and durability, has excellent recovery property against the volume change of the silicon-based active material, prevents the cracking phenomenon of the SEI film, and can prevent the problems of side reactions of the electrolyte and depletion of the electrolyte at a remarkable level, enabling the realization of a lithium secondary battery having excellent life performance and storage performance.

[0028] The silicon-based active material can contain a compound represented by SiO x (0 ≦ x < 2). In the case of SiO2, since it does not react with lithium ions and cannot store lithium, x is preferably within the above range. Specifically, the silicon-based active material can be at least one selected from the group consisting of 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 terms of achieving structural stability during charge and discharge and reducing side reactions with the electrolyte.

[0030] In addition to the silicon-based active material, the negative electrode active material can further contain a carbon-based active material.

[0031] The carbon-based active material can contain 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 can contain 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 terms of achieving structural stability during charge and discharge and reducing side reactions with the electrolyte.

[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 and 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 contain a carbon-based active material.

[0035] The negative electrode can 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. Here, the negative electrode active material can 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 can be copper, stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface treatment of copper or stainless steel with carbon, nickel, titanium, silver, etc., and an aluminum-cadmium alloy can be used.

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

[0038] The negative electrode current collector can also form fine irregularities on its surface to strengthen the binding force of the negative electrode active material. For example, the negative electrode current collector can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric body, etc.

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

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

[0041] The negative electrode active material layer can further contain a binder and / or a conductive material together with the negative electrode active material.

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

[0043] The binder can 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, 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. For example, graphite such as natural graphite and artificial graphite; carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, 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; conductive materials such as polyphenylene derivatives can be used.

[0045] The conductive material can 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, preferably 50 μm to 80 μm.

[0047] The negative electrode can be manufactured by coating at least one surface of a negative electrode current collector with a negative electrode slurry containing 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 can 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 including distilled water, in terms of facilitating the dispersion of the negative electrode active material, the binder, and / or the conductive material.

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

[0050] The positive electrode can 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 a chemical change in the battery and has high conductivity. Specifically, the positive electrode current collector can be at least one selected from the group consisting of copper, stainless steel, aluminum, nickel, titanium, fired carbon, and an aluminum-cadmium alloy, preferably including aluminum.

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

[0053] The positive electrode current collector can also form fine irregularities on its surface to strengthen the binding force of the positive electrode active material. For example, the positive electrode current collector can be used in various forms such as a film, a sheet, a foil, a net, a porous body, a foam, and a non-woven fabric body.

[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 is 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 selected from 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 (such as LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (such as LiCoO2, etc.), lithium-nickel-based oxides (such as LiNiO2, etc.), lithium-nickel-manganese-based oxides (such as 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 (such as LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (such as 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 (such as 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, and 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 the respective independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), etc.), and one 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.), and considering the remarkable 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)It can be O2 or the like, and any one or a mixture of two or more of these can be used.

[0058] More specifically, the positive electrode active material can be a lithium transition metal composite oxide and can contain 60 mol% or more of nickel based on the total number of moles of transition metals contained in the lithium transition metal composite oxide. Specifically, the positive electrode active material is a lithium transition metal composite oxide, the transition metal contains nickel and at least one selected from manganese, cobalt, and aluminum, and the nickel can be contained in an amount of 60 mol% or more, specifically 60 mol% to 90 mol%, based on the total number of moles of the transition metals. When using such a lithium transition metal composite oxide containing a high content of nickel together with the above-described non-aqueous electrolyte, it is preferable in terms of reducing by-products in the gas generated by the structural collapse.

[0059] Further, the positive electrode active material can contain 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 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, respectively, where 0 ≦ x ≦ 0.2, 0.50 ≦ a < 1, 0 < b ≦ 0.25, 0 < c ≦ 0.25, 0 ≦ d ≦ 0.1, and a + b + c + d = 1.

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

[0063] Also, a, b, c, and d can be 0.80 ≦ a ≦ 0.95, 0.025 ≦ b ≦ 0.15, 0.025 ≦ c ≦ 0.15, and 0 ≦ d ≦ 0.05, respectively.

[0064] Also, a, b, c, and d can be 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 can be contained in the positive electrode active material layer in an amount of 80% to 99% by weight, preferably 92% to 98.5% by weight, in consideration of sufficient capacity demonstration of the positive electrode active material and the like.

[0066] The positive electrode active material layer can further contain a binder and / or a conductive material together with the above-described positive electrode active material.

[0067] The binder is a component that facilitates binding of the active material, the conductive material, etc., and binding to the current collector. Specifically, it can be 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 including polyvinylidene fluoride.

[0068] The binder can 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 terms of sufficiently ensuring the binding force between components such as the positive electrode active material.

[0069] The conductive material can be used to assist and improve conductivity in a secondary battery, and is not particularly limited as long as it does not cause a chemical change 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, in terms of improving conductivity, it can include carbon black.

[0070] In terms of sufficiently ensuring electrical conductivity, the conductive material can 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 can be 30 μm to 400 μm, preferably 40 μm to 110 μm.

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

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

[0074] (3) Separator In addition, as the separator, conventionally, a normal porous polymer film used as a separator, for example, a porous polymer film made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer can be used alone or by laminating these, or a normal porous non-woven fabric, for example, a non-woven fabric made of high melting point glass fibers, polyethylene terephthalate fibers, etc. can be used, but it is not limited thereto. Further, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer substance can also be used, and optionally, it can be used in a single-layer or multi-layer structure.

[0075] (4) Non-aqueous electrolyte The non-aqueous electrolyte according to the present invention 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 compound represented by the following Chemical Formula 1, 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).

[0076]

Chemical formula

[0077] In the formula (1), each R is independently 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 is an integer selected from 0 to 6.

[0078] The non-aqueous electrolyte of the present invention contains, as additives, a first additive containing a coumarin compound having a specific structural formula and a second additive containing lithium fluoromalonate (difluoro) borate (LiFMDFB), lithium difluoro (oxalate) borate (LiDFOB), etc. When the non-aqueous electrolyte contains the first additive and the second additive at the same time, an SEI film that is flexible, has strong recoverability, and is excellent in durability can be formed on the negative electrode. As a result, the lithium secondary battery according to the present invention forms the above-described SEI film on a silicon-based negative electrode that has a large degree of volume expansion and thus may cause side reactions of the electrolyte, thereby preventing cracks in the SEI film, preventing side reactions of the electrolyte, and preventing an increase in resistance due to an increase in the thickness of the electrode film. As a result, various performances of the lithium secondary battery, particularly, high-temperature cycle life performance 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-described non-aqueous 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 rather cause an increase in resistance. Therefore, compared with the case where the first additive or the second additive is used alone, the improvement in effect is not much, and rather, it may decrease.

[0080] 1) Lithium salt As the lithium salt used in the present invention, various lithium salts usually used in non-aqueous electrolytes for lithium secondary batteries can be used without limitation. For example, the lithium salt has Li as a cation+ including, as anions, 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 - and may include at least any one selected from the group consisting of.

[0081] Specifically, the lithium salt is LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10It can contain at least one selected from the group consisting of LiBOB (LiB(C2O4)2), LiCF3SO3, LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt can contain 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 can 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 transference number (Li + transference number) and the dissociation degree of lithium ions can be improved, and the output characteristics of the battery can be improved.

[0083] 2) Organic solvent The organic solvent is a non-aqueous solvent commonly used in lithium secondary batteries, and is not particularly limited as long as decomposition due to oxidation reaction or the like can be minimized during the charge and discharge process of the secondary battery.

[0084] Specifically, the organic solvent can contain at least one selected from the group consisting of cyclic carbonate-based organic solvents, linear carbonate-based organic solvents, linear ester-based organic solvents, and cyclic ester-based organic solvents.

[0085] Specifically, the organic solvent can contain a cyclic carbonate-based organic solvent, a linear carbonate-based organic solvent, or a mixture thereof.

[0086] The cyclic carbonate-based organic solvent is a high-viscosity organic solvent with a high dielectric constant that can well dissociate lithium salts in the electrolyte. Specifically, it can contain 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, it can contain at least one selected from the group consisting of ethylene carbonate (EC) and fluoroethylene carbonate (FEC). Even more specifically, it can contain fluoroethylene carbonate (FEC). The fluoroethylene carbonate (FEC) can form an SEI film with a high content of inorganic components such as LiF on the negative electrode, and can enhance the durability of the SEI film. Thereby, especially when applied to a negative electrode containing a silicon-based active material, an improvement in life performance and storage characteristics can be expected.

[0087] In addition, the linear carbonate-based organic solvent is an organic solvent with low viscosity and low dielectric constant. Specifically, it can contain 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 can contain at least one selected from the group consisting of ethyl methyl carbonate (EMC) and diethyl carbonate (DEC). Even more specifically, it can contain diethyl carbonate (DEC). The diethyl carbonate is a symmetrical linear carbonate. Since there is no risk of transesterification, it can prevent a decrease in reduction stability due to the generation of by-products by transesterification, which is preferable. The diethyl carbonate is even more preferable especially when applied to a negative electrode containing a silicon-based active material.

[0088] The organic solvent can 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 can be mixed in a volume ratio of 5:95 to 40:60, specifically a volume ratio of 7:93 to 30:70, and 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 the above range, it can simultaneously satisfy the characteristics of high dielectric constant and low viscosity, and realize excellent ionic conductivity characteristics.

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

[0090] Specifically, the linear ester organic solvent can contain 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] In addition, specifically, the cyclic ester organic solvent can contain at least one selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0092] On the other hand, the organic solvent can be added and used without limitation to the organic solvents usually used in non-aqueous electrolytes as needed. For example, it can further contain at least one of an ether organic solvent, a glyme solvent, and a nitrile organic solvent.

[0093] As the ether solvent, 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 can be used, but it is not limited thereto.

[0094] The glyme solvent has a high dielectric constant and a low surface tension compared to a linear carbonate organic solvent, and is a solvent with low reactivity with metals, and can contain at least one selected from the group consisting of dimethoxyethane (Gleim, DME), diethoxyethane, diglyme, triglyme, and tetra-glyme (TEGDME), but is not limited thereto.

[0095] The nitrile solvent can be one or more 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) Additive The additive includes a first additive and a second additive.

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

[0098]

Chemical Formula

[0099] 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, and n is an integer selected from 0 to 6.

[0100] The first additive contains a coumarin compound having the above structural formula. Such a coumarin compound has strong reducibility at the negative electrode, and when the lithium secondary battery is initially activated, the ring structure is opened to form a polyethylene oxide-based polymer type SEI film. Such a polymer type SEI film has the advantages of excellent flexibility and recoverability. However, in the case of the SEI film derived from the first additive, since the durability is not good, in order to achieve the improvement effect of the high-temperature cycle life performance and the high-temperature storage performance aimed at by the present invention, it is necessary to use the second additive described below in combination with the first additive.

[0101] In the above Chemical Formula 1, each R independently can contain 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 the above Chemical Formula 1, specifically, each R independently can contain a halogen (the halogen can be selected from F, Cl, Br, and I, and specifically can be F), a nitrile group, a propargyl group, an ester group, an ether group, or a combination of two or more thereof. Such substituents are excellent in reducibility, are advantageous for the formation of a polymer type SEI film, and are preferable in terms of also being excellent in lithium ion transfer performance.

[0103] In the above Chemical Formula 1, n can be an integer selected from 0 to 6, specifically, it can be an integer selected from 1 to 6, and 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 the above Chemical Formula 1 can be at least one selected from the group consisting of the compound represented by the following Chemical Formula 2 and the compound represented by the following Chemical Formula 3.

[0105]

Chemical Formula

[0106]

Chemical Formula

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

[0108] The compounds represented by the above Chemical Formulas 2 and 3 each have a structure in which substituents are present at the 3rd and 7th positions (based on the IUPAC nomenclature) of the ring structure. In this case, compared with other substitution positions, it is preferable in terms of being advantageous for synthesis at the above positions. In particular, in the case of the compound represented by the above Chemical Formula 2 in which the substituent is present at the 3rd position, it is more preferable in terms of improving the uniformity of the reaction when it is reduced at the negative electrode.

[0109] Specifically, the compound represented by the chemical formula 1 can include at least one selected from the group consisting of the compounds represented by the following chemical formulas 4 to 12. The compound represented by the chemical formula 1 can specifically include at least one selected from the group consisting of the compounds represented by the following chemical formulas 4 to 7 in terms of being more smoothly reduced at the negative electrode and being more advantageous for the formation of the polymer-type SEI film. More specifically, it can include at least one selected from the group consisting of the following chemical formula 4, the following chemical formula 5, and the following chemical formula 7. Even more specifically, in terms of having strong reducibility and excellent suppression effect on the elution of transition metals together with the above-mentioned effects, the compound represented by the chemical formula 1 can include the compound represented by the following chemical formula 4.

[0110]

Chem.

[0111]

Chem.

[0112]

Chem.

[0113]

Chem.

[0114]

Chem.

[0115]

Chem.

[0116]

Chem.

[0117]

Chem.

[0118]

Chem.

[0119] The first additive can be contained in the non-aqueous electrolyte in an amount of 0.01% by weight to 10% by weight, specifically 0.05% by weight to 7% by weight, more specifically 0.1% by weight to 5% by weight, and even more specifically 0.3% by weight to 2% by weight. When the content of the compound represented by Chemical Formula 1 satisfies the above range, it is preferable in terms of sufficiently imparting flexibility and recoverability to the SEI film and preventing an increase in the resistance of the lithium secondary battery due to excessive addition and a consequent decrease in the life performance.

[0120] 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).

[0121] The second additive can contain a lithium salt additive containing fluorine. The above-mentioned lithium salt additive can form an inorganic-type SEI film such as LiF during the initial activation of the lithium secondary battery. An inorganic-type SEI film such as LiF has excellent adhesion to the surface of the negative electrode, but it is difficult to entirely cover the surface of the negative electrode, so it is difficult to sufficiently prevent side reactions of the electrolyte.

[0122] In this regard, the non-aqueous electrolyte is characterized by using a first additive and a second additive that can form a polymer-type / inorganic-type composite SEI film in combination. According to the non-aqueous electrolyte of the present invention, the polymer-type SEI film derived from the first additive entirely covers the surface of the negative electrode, forming an SEI film with excellent flexibility and recoverability. The inorganic-type SEI film derived from the second additive is distributed in the polymer-type SEI film, and the durability of the SEI film can be improved. Further, the first additive (coumarin-based compound) forms radicals during the reduction reaction, and these radicals promote the detachment of fluorine contained in the second additive, so that the formation of the inorganic-type SEI film can be carried out more smoothly. Therefore, the non-aqueous electrolyte according to the present invention can form an SEI film with excellent flexibility, recoverability, durability, and strength on the surface of the negative electrode, preventing the consumption of the electrolyte due to side reactions of the electrolyte and the increase in resistance due to the increase in the thickness of the electrode film during battery operation, and improving the performance of the lithium secondary battery, particularly the high-temperature cycle life performance and high-temperature storage performance. In particular, when the non-aqueous electrolyte according to the present invention is used together with a negative electrode containing a silicon-based active material, it can form a flexible and durable SEI film on the silicon-based active material with a large volume expansion degree during charge and discharge, preventing damage to the SEI film due to the volume expansion of the silicon-based active material, and preventing an increase in the thickness of the SEI film and electrolyte consumption due to the surface exposure of the new silicon-based active material caused by the volume expansion, which is preferable.

[0123] Specifically, the second additive can include at least one selected from the group consisting of lithium fluoromalonate (difluoro) borate (LiFMDFB) and lithium difluoro (oxalate) borate (LiDFOB). In this case, it is advantageous for the physical rigidity during reduction at the negative electrode in terms of forming a polymeric ether component and a LiF component, and is preferable in that it can contribute to improving the lithium ion transfer performance by including an ester structure in the SEI film. Specifically, the second additive can include lithium fluoromalonate (difluoro) borate (LiFMDFB). Specifically, the second additive can include lithium difluoro (oxalate) borate (LiDFOB).

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

[0125] The weight ratio of the first additive to the second additive can 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 preferable level, which is preferable.

[0126] The additive can further include an additional additive or a third additive together with the first additive and the second additive. The additional additive can be included in the non-aqueous electrolyte to prevent the non-aqueous electrolyte from decomposing in a high-power environment and causing the collapse of the negative electrode, or for purposes such as low-temperature high-rate discharge characteristics, high-temperature stability, prevention of overcharging, and suppression of battery swelling at high temperatures.

[0127] Specifically, the additional additive can 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)Phosphite). Specifically, it can be vinylene carbonate.

[0128] The additional additive can be contained in the non-aqueous electrolyte in an amount of 0.1% by weight to 15% by weight.

[0129] In one embodiment, the non-aqueous electrolyte may not contain a silyl group-containing additive.

[0130] The outer shape of the lithium secondary battery of the present invention is not particularly limited, and can be, for example, a cylindrical shape, a rectangular shape, a pouch shape, or a coin shape using a can.

[0131] Hereinafter, the present invention will be described more specifically with reference to specific examples. However, the following examples are illustrative for facilitating the understanding of the present invention and do not limit the scope of the present invention. It is obvious to those skilled in the art that various changes and modifications can be made within the scope of the described category and technical idea, and it goes without saying that such modifications and variations belong to the scope of the appended claims.

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

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

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

[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]

Chemical Formula

[0137] (Manufacture of Lithium Secondary Battery) Cathode active material (LiNi 0.85 Co 0.05 Mn 0.07 Al 0.03 O2): Conductive material (carbon nanotube): Binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) as a solvent at a weight ratio of 97.74:0.70:1.56 to produce a cathode mixture slurry (solid content 75.5 wt%). The cathode mixture slurry was applied to one side of a cathode current collector (Al thin film) with a thickness of 15 μm, and drying and roll press were carried out to produce a cathode.

[0138] The negative electrode active material (Si), conductive material (carbon black), and binder (styrene-butadiene rubber) were added to distilled water, which is a solvent, at a weight ratio of 70.0:20.3:9.7 to produce a negative electrode mixture slurry (solid content: 26% by weight). The negative electrode mixture slurry was applied to one side of a negative electrode current collector (Cu thin film) with a thickness of 15 μm, followed by drying and roll pressing to produce a negative electrode.

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

[0140] Example 2 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the first additive was added to the non-aqueous electrolyte at 0.1% by weight.

[0141] Example 3 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the first additive was added to the non-aqueous electrolyte at 5.0% by weight.

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

[0143]

Chem.

[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 the first additive, a compound represented by the following Chemical Formula 6 was included in the non-aqueous electrolyte at 0.5% by weight instead of the compound represented by Chemical Formula 4.

[0145]

Chem.

[0146] Example 6 As the first additive, except that 0.5% by weight of the compound represented by the following Chemical Formula 7 was contained in the non-aqueous electrolyte instead of the compound represented by the above Chemical Formula 4, a non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1.

[0147] [Chem.]

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

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

[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 contained in the non-aqueous electrolyte at 0.1% by weight.

[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 contained in the non-aqueous electrolyte at 5% by weight.

[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 contained in the non-aqueous electrolyte at 0.1% by weight.

[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 contained in the non-aqueous electrolyte at 5% by weight.

[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 and a lithium secondary battery were 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 produced above were charged to 4.2 V at 45 °C under CC / CV and 0.33C conditions using an electrochemical charger / discharger, and then discharged to 3 V under CC and 0.33C conditions. One cycle was defined as such, and 300 cycles of charge and discharge were performed to measure the capacity retention.

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

[0160] Capacity retention (%) = (Discharge capacity after 300 cycles / Discharge capacity after 1 cycle) × 100

[0161] Experimental Example 2: Evaluation of Capacity Retention after High-Temperature Storage The lithium secondary batteries of Examples 1 to 12 and Comparative Examples 1 to 3 manufactured as described above were charged at room temperature under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V up to 4.2V / 55mA, and discharged at 0.33C down to 2.5V to perform initial charge and discharge. Then, after charging at room temperature under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V up to 4.2V / 55mA, they were stored at 60°C. After storage, the secondary batteries were charged at room temperature under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V up to 4.2V / 55mA, and discharged at 0.33C down to 2.5V to measure the discharge capacity during discharge.

[0162] The capacity retention rate was evaluated according to 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) × 100 (In the above formula, N is an integer of 1 or more)

[0164]

Table 2

[0165] Referring to Table 2, it can be confirmed that the lithium secondary batteries of Examples 1 to 12 using the non-aqueous electrolyte containing the first additive and the second additive according to the present invention showed 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. Manufacture of non-aqueous electrolyte The same non-aqueous electrolyte as that used in Example 1 was used.

[0167] 2. Manufacture of lithium secondary battery Positive electrode active material (Li[Ni 0.85 Co 0.03 Mn 0.07 Al 0.03(O2): Conductive material (carbon nanotube): Binder (polyvinylidene fluoride) was added to N-methyl-2-pyrrolidone (NMP), which is a solvent, at a weight ratio of 97.74:0.70:1.56 to produce a positive electrode mixture slurry (solid content 75.5 wt%). The positive electrode mixture slurry was applied to one side of a positive electrode current collector (Al thin film) with a thickness of 15 μm, and drying and roll press were carried out to produce a positive electrode.

[0168] Negative electrode active material (natural graphite): Conductive material (carbon black): Binder (styrene-butadiene rubber (SBR)-carboxymethyl cellulose (CMC)) was added to distilled water, which is a solvent, at a weight ratio of 95.0:1.5:3.5 to produce a negative electrode mixture slurry (solid content 60 wt%). The negative electrode mixture slurry was applied to one side of 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.

[0169] In a dry room, after interposing a polyethylene porous film separator between the produced positive electrode and negative electrode, the produced non-aqueous electrolyte was injected to produce a secondary battery.

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

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

[0172] Reference Experimental Example 1 Using an electrochemical charger, the lithium secondary batteries of Reference Examples 1 to 3 produced above were charged to 4.2 V at 45 °C under the conditions of CC / CV and 0.33C, and then discharged to 3 V under the conditions of CC and 0.33C. One cycle was defined as such, and 300 cycles of charge and discharge were carried out to measure the capacity retention rate.

[0173] The capacity retention rate was calculated by 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) × 100

[0175] Reference Experimental Example 2 The lithium secondary batteries of Reference Examples 1 to 3 manufactured above were charged at room temperature under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V up to 4.2V / 55mA, and discharged at 0.33C down to 2.5V to perform initial charge and discharge. Then, after charging up to 4.2V / 55mA under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V at room temperature, they were stored at 60°C. After storage, the secondary batteries were charged at room temperature under the conditions of constant current / constant voltage (CC / CV) of 0.33C / 4.2V up to 4.2V / 55mA, and discharged at 0.33C down to 2.5V to measure the capacity during discharge.

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

[0177] Capacity retention rate (%) = (Discharge capacity after N-week storage / Initial discharge capacity) × 100 (In the above formula, N is an integer of 1 or more)

[0178]

Table 3

[0179] Referring to Table 3 above, when the non-aqueous electrolyte having the characteristics according to the present invention is applied to the negative electrode containing only the carbon-based active material (Reference Example 1), it can be confirmed that the performance deteriorates rather than in Reference Examples 2 and 3 where only one of the first additive and the second additive was used. Since the carbon-based active material has little or no volume expansion compared to the silicon-based active material, the polymer-type SEI film derived from the first additive acts as a resistance instead. Such a problem of resistance increase is considered to be more severe when the first additive and the second additive are used simultaneously.

[0180] Thus, it can be clearly understood that the improvement effects of the life performance and storage performance of the present invention, particularly the improvement effects of the 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 non-aqueous electrolyte.

Claims

1. 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 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 compound represented by the following Chemical Formula 1, 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). 【Chemical 1】 In Chemical Formula 1, R is independently of each other halogen, nitrile group, propargyl group, ester group, ether group, ketone group, carboxy group, substituted or unsubstituted alkyl group, substituted or unsubstituted alkenyl group, substituted or unsubstituted alkynyl group, substituted or unsubstituted alkoxy group, boron group, borate group, isocyanate group, isothiocyanate group, silyl group, siloxane group, sulfone group, sulfonate group, sulfate group, or a combination of two or more thereof, and n is an integer selected from 0 to 6.

2. The lithium secondary battery according to Claim 1, wherein the compound represented by Chemical Formula 1 contains at least one selected from the group consisting of a compound represented by the following Chemical Formula 2 and a compound represented by the following Chemical Formula 3. 【Chemical 2】 [Chemical Formula 3] In Chemical Formulas 2 and 3, R is as defined in Chemical Formula 1.

3. The lithium secondary battery according to Claim 1, wherein the compound represented by Chemical Formula 1 contains at least one selected from the group consisting of the compounds represented by the following Chemical Formulas 4 to 12. 【Chemical 4】 【Chemical Formula 5】 [Chemical Formula 6] 【Chemical Formula 7】 [Chemical Formula 8] 【Chemical Formula 9】 【Chemical 10】 【Chemical Formula 11】 【Chemical 12】

4. 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. The lithium secondary battery according to Claim 1, wherein the second additive contains at least one selected from the group consisting of lithium fluoromalonate (difluoro) borate (LiFMDFB) and lithium difluoro (oxalate) borate (LiDFOB).

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

10.

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

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

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

12. The lithium secondary battery according to claim 1, wherein the additive further contains 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)Phosphite).

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

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