Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein the negative electrode comprises a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by a specific chemical formula.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0146436, filed November 4, 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, and more specifically to a lithium secondary battery that includes a nonaqueous electrolyte containing an additive capable of forming a strong SEI film on a positive electrode / negative electrode, thereby suppressing an increase in initial resistance and improving output characteristics and life performance. [Background technology]

[0003] With the development of the information society, personal IT devices and computer networks have become more prevalent, and as a result, society as a whole has become more dependent on electrical energy, which has created a demand for the development of technologies to efficiently store and utilize electrical energy.

[0004] Of all the technologies developed, secondary batteries are the most suitable for a variety of applications. Among these secondary batteries, interest is growing in lithium secondary batteries, which not only can be miniaturized to a degree that makes them suitable for personal IT devices, but also have the highest energy density.

[0005] Generally, a lithium secondary battery is manufactured by injecting or impregnating a non-aqueous electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a porous separator.

[0006] Possible positive electrode active materials for such lithium secondary batteries include lithium-containing cobalt oxide, LiMnO2 with a layered crystal structure, LiMn2O4 with a spinel crystal structure, lithium-containing nickel oxide (LiNiO2), and lithium nickel-cobalt-manganese transition metal oxide.

[0007] On the other hand, carbon-based active materials such as graphite have been used as negative electrode active materials, but recently, silicon-based active materials have been considered for use because they have higher capacity than carbon-based active materials.

[0008] While silicon-based active materials have the advantage of high capacity, they suffer from the problem of significant volume expansion / contraction during charge / discharge. This significant volume expansion / contraction significantly reduces the conductivity of the negative electrode, resulting in reduced lifespan. Furthermore, during initial activation, a solid electrolyte interface layer (SEI) is formed on the surface of the negative electrode. Silicon-based active materials experience significant volume expansion, which can lead to cracking of the SEI layer and the continuous formation of new negative electrode surfaces. This can accelerate electrolyte side reactions due to the continuous SEI layer formation reaction, leading to increased SEI layer thickness and increased resistance. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 2017-0012308 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to solve the above-mentioned problems, and to provide a lithium secondary battery that contains a silicon-based active material as a negative electrode active material, in which a strong and low-resistance SEI film is formed on the negative electrode, and in which the output characteristics and life characteristics are simultaneously improved. [Means for solving the problem]

[0011] The present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein the negative electrode comprises a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1: [ka] In the above chemical formula 1, R1 and R2 are each independently an alkylene group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms. [Effects of the Invention]

[0012] The present invention relates to a lithium secondary battery in which a silicon-based active material is included in the negative electrode and the compound represented by Chemical Formula 1 is included as an additive in a non-aqueous electrolyte. The compound represented by Chemical Formula 1 contains within its structure a propargyl group (—C≡C—) and a fluorocarbon functional group substituted with one or more fluorine elements, and is therefore reduced before an organic solvent, allowing the formation of a low-resistance SEI film containing a fluorocarbon component on the surface of the negative electrode. The SEI film formed on the negative electrode by the compound represented by Chemical Formula 1 is strong and has low resistance, preventing cracking of the SEI film due to volumetric expansion of the silicon-based active material. The low resistance simultaneously improves the life and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0013] First, before describing the present invention, 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 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 describe their inventions.

[0014] Meanwhile, the terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0015] 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.

[0016] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0017] Before describing the present invention, in the description of "number of carbon atoms a to b" in the specification, "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.

[0018] Furthermore, in this specification, unless otherwise defined, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom has been substituted with an element other than hydrogen, for example, with an alkyl group having 1 to 5 carbon atoms or a fluorine atom.

[0019] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0020] The present invention will now be described in further detail.

[0021] [Lithium secondary battery] The present invention provides a lithium secondary battery comprising a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein the negative electrode comprises a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1:

[0022] [ka]

[0023] In the above chemical formula 1, R1 and R2 are each independently an alkylene group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms.

[0024] The present invention relates to a lithium secondary battery in which a silicon-based active material is included in the negative electrode and the compound represented by Chemical Formula 1 is included as an additive in a non-aqueous electrolyte. The compound represented by Chemical Formula 1 contains within its structure a propargyl group (—C≡C—) and a fluorocarbon functional group substituted with one or more fluorine elements, and is therefore reduced before the organic solvent, allowing the formation of a low-resistance SEI film containing a fluorocarbon component on the surface of the negative electrode. The SEI film formed on the negative electrode by the compound represented by Chemical Formula 1 is strong and has low resistance, preventing cracking of the SEI film due to volume expansion of the silicon-based active material. The low resistance simultaneously improves the life and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery.

[0025] The lithium secondary battery includes a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte. Specifically, the lithium secondary battery 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 lithium secondary battery can be manufactured by housing an electrode assembly including the negative electrode, a positive electrode facing the negative electrode, and a separator interposed between the negative electrode and the positive electrode in a battery case and then injecting a non-aqueous electrolyte.

[0026] (1) Negative electrode The negative electrode contains a silicon-based active material. The silicon-based active material exhibits a higher capacity compared to the carbon-based active material, but has a problem in that the degree of volume expansion / contraction due to charge / discharge is large. However, when both the silicon-based active material and the non-aqueous electrolyte described later are used, it is possible to form a strong and low-resistance SEI film on the negative electrode, prevent side reactions of the electrolyte, and realize a lithium secondary battery having high life performance and output characteristics.

[0027] The silicon-based active material may contain a compound represented by the following chemical formula A.

[0028] [Chemical formula A] SiO x (0≦x<2)

[0029] In the chemical formula A, in the case of SiO2, since it does not react with lithium ions, lithium cannot be stored, so x is preferably within the above range.

[0030] The silicon-based active material can be Si (silicon). Si is advantageous in that its capacity is about 2.5 to 3 times higher than that of silicon oxide (for example, SiO x (0<x<2)), but the degree of volume expansion / contraction due to charge / discharge of Si is much larger than that of silicon oxide, so it is not easy to commercialize. However, the lithium secondary battery of the present invention can have high life performance and output characteristics by applying the non-aqueous electrolyte described later.

[0031] The average particle size (D 50 ) can be 0.5 μm to 20 μm, preferably 1 μm to 8 μm, in order to achieve structural stability of the active material during charge and discharge, to more smoothly form a conductive network for maintaining electrical conductivity, and to facilitate access to the binder for binding the active material and the current collector.

[0032] 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 silicon-based active material may be included in the negative electrode active material layer.

[0033] 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.

[0034] The negative electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0035] 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.

[0036] 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.

[0037] The silicon-based active material may be contained in the negative electrode active material layer in an amount of 60 wt % to 99 wt %, preferably 70 wt % to 85 wt %, in order to minimize the effect of volume expansion / contraction on the battery and to fully realize the high capacity of the silicon-based active material in the secondary battery.

[0038] The negative electrode active material layer may further include a conductive material and / or a binder in addition to the silicon-based active material.

[0039] The binder may be used to improve the adhesive strength between the negative electrode active material layer and a negative electrode current collector (described later) or to improve the binding strength between silicon-based active materials.

[0040] Specifically, the binder can further improve electrode adhesive strength and provide sufficient resistance to the volume expansion / contraction of the silicon-based active material. To this end, the binder can include at least one selected from the group consisting of styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), acrylonitrile butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl alcohol (PVA), polyacrylic acid (PAA), polyethylene glycol (PEG), polyacrylonitrile (PAN), and polyacrylamide (PAM), and preferably includes nitrile butadiene rubber.

[0041] The binder may be included in the negative electrode active material layer in an amount of 1 wt % to 30 wt %, preferably 7 wt % to 15 wt %, and within this range, the binder can better bind the silicon-based active material, minimizing the problem of volume expansion of the active material, and facilitating dispersion of the binder during preparation of a slurry for forming the negative electrode active material layer, thereby improving coatability and phase stability of the slurry.

[0042] 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 carbon black, 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; metal powders such as fluorocarbon, 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 order to achieve high conductivity.

[0043] The conductive material may be contained in the negative electrode active material layer in an amount of 1 wt % to 20 wt %, preferably 8 wt % to 15 wt %, and when the amount is in this range, it is preferable in that an increase in resistance due to the binder can be mitigated and an excellent conductive network can be formed.

[0044] The thickness of the negative electrode active material layer can be 5 μm to 500 μm, and preferably 5 μm to 100 μm.

[0045] The loading amount of the negative electrode active material layer is 3 mAh / cm 2 ~15mAh / cm 2 , preferably 8mAh / cm 2 ~13mAh / cm 2 It can be.

[0046] The negative electrode may be manufactured by coating a negative electrode slurry containing a negative electrode active material, and optionally a binder, a conductive material, and a solvent for forming the negative electrode slurry, on the negative electrode current collector, followed by drying and rolling.

[0047] The solvent for forming the negative electrode slurry may include, for example, at least one selected from the group consisting of distilled water, ethanol, methanol, and isopropyl alcohol, preferably distilled water, in order to facilitate dispersion of the negative electrode active material, binder, and / or conductive material.

[0048] The solvent for forming the negative electrode slurry may be included in the negative electrode slurry so that the concentration of solids including the negative electrode active material, and optionally the binder and conductive material, is 15 wt % to 45 wt %, taking into consideration the viscosity, coating property, dispersibility, etc. of the negative electrode slurry.

[0049] (2) Positive electrode The positive electrode includes a positive electrode active material.

[0050] The positive electrode active material can contain, as a compound capable of reversible intercalation and deintercalation of lithium, specifically, a lithium transition metal composite oxide containing lithium and at least one transition metal selected from nickel, cobalt, manganese, and aluminum, preferably a lithium transition metal composite oxide containing lithium and a transition metal including nickel, cobalt, and manganese.

[0051] Examples of the lithium transition metal composite oxide include lithium-manganese oxides (e.g., LiMnO2, LiMn2O4, etc.), lithium-cobalt oxides (e.g., LiCoO2, etc.), lithium-nickel oxides (e.g., LiNiO2, etc.), and lithium-nickel-manganese oxides (e.g., LiNi 1-Y Mn Y O2 (where 0 <Y<1)、LiMn 2-z Ni zO4 (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) oxides (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 any 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), or lithium nickel cobalt aluminum oxide (e.g., Li(Ni 0.8 Co 0.15 Al 0.05 )O2, etc.), and in consideration of the remarkable improvement effect by controlling the type and content ratio of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide can be 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.

[0052] 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 with the above-mentioned nonaqueous electrolyte.

[0053] The positive electrode active material may include a lithium composite transition metal oxide represented by the following chemical formula B:

[0054] [Chemical formula B] Li 1+x (Ni a Co b Mn c M d)O2

[0055] In the chemical formula B, 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。

[0056] 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.

[0057] Furthermore, the a, b, c, and d may be within the ranges 0.80≦a≦0.95, 0.025≦b≦0.15, 0.025≦c≦0.15, and 0≦d≦0.05, respectively.

[0058] 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.

[0059] 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, wherein the positive electrode active material layer may include the positive electrode active material described above.

[0060] 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 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.

[0061] The positive electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0062] 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, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0063] The positive electrode active material layer may be disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one or both surfaces of the positive electrode current collector.

[0064] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80 to 99% by weight, taking into consideration the sufficient capacity of the positive electrode active material.

[0065] The positive electrode active material layer may further include a binder and / or a conductive material in addition to the positive electrode active material.

[0066] The binder is a component that helps bind the active material and conductive material together and 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.

[0067] 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.

[0068] 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 cause chemical changes and has conductivity. Specifically, the positive electrode 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 carbon black, 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 positive electrode conductive material can include carbon nanotubes in order to improve conductivity.

[0069] 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.

[0070] The thickness of the positive electrode active material layer can be 5 μm to 500 μm, and preferably 100 μm to 200 μm.

[0071] The loading amount of the positive electrode active material layer is 2 mAh / cm 2 ~6mAh / cm 2 , preferably 4mAh / cm 2 ~5mAh / cm 2 It can be.

[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). The solid content of the positive electrode slurry may be 40% by weight to 90% by weight, specifically 50% by weight to 80% by weight.

[0074] (3) Separator The separator may be interposed between the positive electrode and the negative electrode.

[0075] The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer such as ethylene homocopolymer, propylene homocopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, either alone or in a laminate, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. Furthermore, to ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be used in a single-layer or multi-layer structure.

[0076] (4) Nonaqueous electrolyte 1) Lithium salt First, the lithium salt will be described below.

[0077] In the non-aqueous electrolyte for a lithium secondary battery according to one embodiment of the present invention, the lithium salt may be any of those commonly used in electrolytes for lithium secondary batteries, without limitation. For example, a lithium salt containing Li as a cation may be used. + and as an anion, 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 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CH3SO3 - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - Specifically, the lithium salt may be at least one selected from the group consisting of LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt may include a single substance or a mixture of two or more substances 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), and more specifically, LiPF6.

[0078] The lithium salt can be appropriately changed within a range that is normally usable, but in order to obtain the optimum effect of forming a corrosion prevention coating on the surface of the electrode, it can be contained in the electrolyte at a concentration of 0.8M to 3.0M, specifically 1.0M to 3.0M.

[0079] When the concentration of the lithium salt satisfies the above range, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal impregnation, and the mobility of lithium ions can be improved, thereby improving the capacity characteristics and cycle characteristics of the lithium secondary battery.

[0080] 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.

[0081] Specifically, the organic solvent may include a cyclic carbonate organic solvent and a linear carbonate organic solvent.

[0082] The cyclic carbonate organic solvent is a highly viscous organic solvent that has a high dielectric constant and can effectively dissociate the lithium salt 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), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and more specifically, may include ethylene carbonate.

[0083] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and specifically includes 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. The linear carbonate organic solvent specifically includes dimethyl carbonate and ethyl methyl carbonate, and more specifically, includes dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 60:40 to 90:10.

[0084] The organic solvent may be a mixture of a cyclic carbonate organic solvent and a linear carbonate organic solvent, wherein the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed in a volume ratio of 10:90 to 40:60, specifically, a volume ratio of 15:85 to 35:65.

[0085] 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 ester-based organic solvents, ether-based organic solvents, glyme-based organic solvents, and nitrile-based organic solvents.

[0086] The ester-based 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, butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0087] 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.

[0088] 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).

[0089] 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.

[0090] On the other hand, the remainder of the non-aqueous electrolyte other than the lithium salt and the additive may be an organic solvent unless otherwise specified.

[0091] (3) Additives The non-aqueous electrolyte of the present invention contains an additive.

[0092] The additive includes a compound represented by the following Chemical Formula 1:

[0093] [ka]

[0094] In the above Chemical Formula 1, R1 and R2 each independently represent an alkylene group having 1 to 10 carbon atoms; R3 is an alkyl group having 1 to 20 carbon atoms and substituted with one or more fluorine atoms.

[0095] Specifically, the compound represented by Formula 1 contains a propargyl functional group within its structure, and is therefore easily reductively decomposed on the surface of a negative electrode containing a silicon-based active material to form an SEI film with low resistance and high passivation ability. Therefore, when a non-aqueous electrolyte containing the compound represented by Formula 1 as an electrolyte additive is used, it is possible to prevent a self-discharge reaction of the negative electrode due to further reductive decomposition of the electrolyte caused by the instability of the SEI film.

[0096] In addition, the compound represented by Chemical Formula 1 contains a fluorocarbon functional group substituted with one or more fluorine atoms at the end of its structure, thereby forming an oxidation-resistant coating on the surface of the positive electrode, thereby inhibiting the elution of transition metals from the positive electrode and inhibiting the eluted transition metals from being electrodeposited and precipitated on the negative electrode, thereby preventing internal short circuits.

[0097] As described above, the compound represented by Chemical Formula 1 contains a fluorocarbon functional group substituted with at least one fluorine element, which has excellent flame retardancy and non-flammability, and a propargyl group. This allows the compound to form a low-resistance, strong SEI coating on the negative electrode, suppress further reduction and decomposition reactions of the electrolyte, and prevent self-discharge reactions of the negative electrode. This improves the lifespan of a lithium secondary battery, suppresses an increase in initial resistance, and improves room-temperature and low-temperature output characteristics.

[0098] Meanwhile, in the above Chemical Formula 1, R1 and R2 may each independently be an alkylene group having 1 to 5 carbon atoms, and R3 may be an alkyl group having 3 to 20 carbon atoms substituted with one or more fluorine atoms.

[0099] In addition, in the above Chemical Formula 1, R1 and R2 may each independently be an alkylene group having 1 to 3 carbon atoms, and R3 may be an alkyl group having 3 to 15 carbon atoms substituted with one or more fluorine atoms.

[0100] Specifically, in the above Chemical Formula 1, R3 may be an alkyl group having 4 to 8 carbon atoms substituted with one or more fluorine atoms.

[0101] Preferably, the compound represented by Chemical Formula 1 may be a compound represented by Chemical Formula 1a below.

[0102] [ka]

[0103] Meanwhile, the compound represented by Chemical Formula 1 may be included in the non-aqueous electrolyte in an amount of 0.01 wt % to 10.0 wt %.

[0104] When the content of the compound represented by Chemical Formula 1 is within the above range, defects such as side reactions due to additives, a decrease in capacity, and an increase in resistance can be minimized, and a low-resistance SEI coating can be formed on the surface of the negative electrode, improving the lithium migration effect in the coating, suppressing further reduction and decomposition reactions of the electrolyte, and preventing self-discharge reactions of the negative electrode.

[0105] Specifically, when the content of the compound represented by Chemical Formula 1 is 0.01 wt % or more, a stable coating is formed during the battery's operating time, forming a low-resistance SEI coating on the surface of the negative electrode, thereby improving battery output performance. Also, when the content of the compound represented by Chemical Formula 1 is 10.0 wt % or less, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, effectively suppressing an increase in battery resistance due to decomposition of additives, and further increasing ionic conductivity within the battery, thereby preventing a decrease in output characteristics.

[0106] Specifically, the compound represented by Chemical Formula 1 may be contained in the non-aqueous electrolyte solution in an amount of 0.05 wt % to 6.0 wt %, specifically 0.1 wt % to 5.0 wt %, and more specifically 0.5 wt % to 2 wt %.

[0107] Meanwhile, the additive may include, as needed, additional additives in addition to the compound represented by Chemical Formula 1 to prevent the non-aqueous electrolyte from being decomposed and causing the collapse of the negative electrode in a high-power environment, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and suppression of battery expansion at high temperatures.

[0108] Examples of such additional additives include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, nitrile-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0109] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinylethylene carbonate.

[0110] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0111] The sultone compound may be, for example, at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone.

[0112] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0113] The phosphate- or phosphite-based compound can be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0114] Examples of the borate-based compound include tetraphenylborate, lithium difluoro(oxalate)borate (LiODFB), and lithium bisoxalateborate (LiB(C2O4)2, LiBOB).

[0115] The nitrile compound may be, for example, at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0116] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0117] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and examples thereof include lithium difluorophosphate (LiPO2F2) and LiBF4.

[0118] When at least one selected from the group consisting of vinyl ethylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, succinonitrile, and lithium difluoro(oxalate)borate is contained among these additional additives, a stronger SEI coating can be formed on the surface of the negative electrode during the initial activation step of the secondary battery.

[0119] The additional additive may be a mixture of two or more compounds, and the total content of the compound represented by Formula 1 and the additional additive may be 50 wt % or less, specifically 0.05 to 20 wt %, and more specifically 0.05 to 10 wt %, based on the total weight of the non-aqueous electrolyte. When the total content of the additives satisfies this range, the low-temperature output characteristics of the battery may be improved, and the high-temperature storage characteristics and high-temperature life characteristics may be more effectively improved, and side reactions in the battery due to the additives remaining after the reaction may be prevented.

[0120] The lithium secondary battery according to the present invention can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0121] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0122] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0123] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0124] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery in a medium- to large-sized battery module including a large number of battery cells.

[0125] The present invention will be specifically described below with reference to examples.

[0126] Here, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments detailed below. The embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0127] The present invention will be specifically described below with reference to specific examples.

[0128] [Example] Example 1 (Production of non-aqueous electrolyte) An organic solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:10:60.

[0129] LiPF6 was dissolved as a lithium salt in the organic solvent to a molar concentration of 1.0M.

[0130] In addition, the compound represented by Formula 1a and additional additives were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0131] The compound represented by Chemical Formula 1a was contained in the non-aqueous electrolyte at 0.1 wt %.

[0132] The additional additives used were vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), fluoroethylene carbonate (FEC), succinonitrile (SN), and LiODFB. 0.5 wt% of vinyl ethylene carbonate (VEC), 1.0 wt% of 1,3-propane sultone (PS), 5.0 wt% of fluoroethylene carbonate (FEC), 1.0 wt% of succinonitrile (SN), and 0.5 wt% of LiODFB were added to the non-aqueous electrolyte.

[0133] (Secondary battery manufacturing) Cathode active material (LiNi 0.85 Co 0.05 Mn 0.07 Al 0.03O2), conductive material (carbon nanotubes), and binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 97:1:2 to prepare a cathode mixture slurry (solid content 78 wt%). The cathode mixture slurry was applied to one side of a 12 μm-thick cathode current collector (Al thin film) at a rate of 4.5 mAh / cm. 2 The coating was applied in a loading amount of 1000 ppm, dried, and roll pressed to prepare a positive electrode.

[0134] Anode active material Si, binder NBR, and conductive material carbon black were mixed in a weight ratio of 80:10:10 with water as a solvent to prepare anode slurry (solid content: 48 wt%). The anode slurry was applied to a 15 μm-thick copper (Cu) thin film as anode current collector to obtain a current capacity of 10.7 mAh / cm. 2 After drying, roll pressing was carried out to prepare a negative electrode.

[0135] The positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and the negative electrode were sequentially stacked to prepare an electrode assembly.

[0136] The assembled electrode assembly was placed in a battery case, and the prepared non-aqueous electrolyte was poured into the battery case to prepare a lithium secondary battery.

[0137] Example 2 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 Formula 1a was added in an amount of 1.0 wt % based on the weight of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0138] Example 3 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 Formula 1a was added in an amount of 5.0 wt % based on the weight of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0139] Comparative Example 1 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was manufactured without adding the compound represented by Chemical Formula 1a.

[0140] Comparative Example 2 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 2 was added in an amount of 0.1 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0141] [ka]

[0142] Comparative Example 3 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 Chemical Formula 2 was added in an amount of 1.0 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0143] Comparative Example 4 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 3 was added in an amount of 0.1 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0144] [ka]

[0145] [Experimental Example] Experimental Example 1: Evaluation of initial capacity and cycle capacity retention rate The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as described above were charged to 4.25 V, 0.05 C under CC / CV conditions at 25° C. and 0.33 C using an electrochemical charger / discharger, and then discharged to 2.5 V under CC conditions at 0.33 C, with 200 charge / discharge cycles being defined as one cycle, and the capacity retention rate was measured.

[0146] The capacity retention rate was calculated using the following formula, and the results are shown in Table 1 below.

[0147] Capacity retention rate (%) = {(discharge capacity after 200 cycles / discharge capacity after 1 cycle)} × 100

[0148] The discharge capacity (initial capacity) after one cycle and the capacity retention rate after 200 cycles are shown in Table 1 below.

[0149] Experimental Example 2: Evaluation of initial resistance The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared above were charged and discharged using an electrochemical charger / discharger at 25°C under CC / CV and 0.33C conditions up to 4.25V and 0.05C, and then discharged under CC and 0.33C conditions down to 2.5V, with this being one cycle.

[0150] After one charge / discharge cycle, the discharge capacity after one cycle was measured using an electrochemical charge / discharge device. After adjusting the SOC to 50%, a 2.5C pulse was applied for 10 seconds, and the initial resistance was calculated from the difference in voltage before and after the pulse application. The results are shown in Table 1 below.

[0151] [Table 1]

[0152] Referring to Table 1, it can be seen that the lithium secondary batteries of Examples 1 to 3 according to the present invention have superior initial capacity, capacity retention rate, and resistance characteristics compared to Comparative Examples 1 to 4.

Claims

1. a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte; the negative electrode contains a silicon-based active material, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive; The additive comprises a compound represented by the following Chemical Formula 1: 【Chemical 1】 In the above Chemical Formula 1, R １ and R ２ are each independently an alkylene group having 1 to 10 carbon atoms, R ３ is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms.

2. In the above formula 1, R １ and R ２ are each independently an alkylene group having 1 to 5 carbon atoms, and R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 3 to 20 carbon atoms substituted with one or more fluorine atoms.

3. In the above formula 1, R １ and R ２ are each independently an alkylene group having 1 to 3 carbon atoms, and R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 3 to 15 carbon atoms substituted with one or more fluorine atoms.

4. In the above formula 1, R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 4 to 8 carbon atoms substituted with one or more fluorine atoms.

5. The lithium secondary battery of claim 1 , wherein the compound represented by Chemical Formula 1 is a compound represented by Chemical Formula 1a: 【Chemistry 2】

6. 2. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 is contained in the non-aqueous electrolyte in an amount of 0.01 wt % to 10.0 wt %.

7. 2. The lithium secondary battery of claim 1, wherein the compound represented by Chemical Formula 1 is contained in an amount of 0.05 to 6.0 wt % based on the total weight of the non-aqueous electrolyte solution for the lithium secondary battery.

8. 2. The lithium secondary battery of claim 1, wherein the additive comprises at least one additional additive selected from the group consisting of halogen-substituted or unsubstituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, nitrile-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

9. 2. The lithium secondary battery according to claim 1, wherein the organic solvent includes a cyclic carbonate organic solvent and a linear carbonate organic solvent.

10. the cyclic carbonate organic solvent contains ethylene carbonate, 10. The lithium secondary battery according to claim 9, wherein the linear carbonate organic solvent includes ethyl methyl carbonate and dimethyl carbonate.

11. The lithium secondary battery according to claim 1 , wherein the silicon-based active material comprises a compound represented by the following chemical formula A: [Chemical formula A] Yes ｘ (0≦+<2).

12. The lithium secondary battery according to claim 1 , wherein the silicon-based active material is Si.

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