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 negative electrode active material, the negative electrode active material 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. 10-2022-0182374, filed December 22, 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 contains a nonaqueous electrolyte containing an additive that can form a strong SEI coating on the positive and negative electrodes, thereby suppressing an increase in initial resistance and improving output characteristics and life performance. [Background technology]

[0003] The development of the information society has led to the development of personal IT devices and computer networks, which has increased society's overall dependence on electrical energy. This has led to a demand for the development of technologies to efficiently store and utilize electrical energy.

[0004] Of the technologies currently being developed, secondary batteries are the most suitable for a variety of applications, and among these secondary batteries, interest is growing in lithium-ion batteries, which not only can be miniaturized to a degree suitable for use in 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] 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 in recent years, the use of silicon-based active materials has been considered 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 and shortens its lifespan. Furthermore, during initial activation, a solid electrolyte interface layer (SEI film) forms on the surface of the negative electrode. Silicon-based active materials experience significant volume expansion, which can lead to cracking of the SEI film and the continuous generation of new negative electrode surface layers. This can accelerate electrolyte side reactions due to the continuous formation of the SEI film, leading to problems such as an increased thickness of the SEI film and increased resistance. Summary of the Invention [Problem to be solved by the invention]

[0009] 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, and that has improved output characteristics and life characteristics by forming a strong and low-resistance SEI coating on the negative electrode. [Means for solving the problem]

[0010] 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 negative electrode active material, the negative electrode active material comprising a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1:

[0011] [ka]

[0012] In the above chemical formula 1, n is an integer of 0-18. [Effects of the Invention]

[0013] The present invention relates to a lithium secondary battery in which the negative electrode contains a silicon-based active material and the non-aqueous electrolyte contains, as an additive, a compound represented by Chemical Formula 1. The compound represented by Chemical Formula 1 contains a propargyl group (-C≡C-) and a fluorine-substituted alkyl group within its structure, and is therefore reduced before the organic solvent, allowing a low-resistance SEI coating containing a fluorocarbon component to be formed on the surface of the negative electrode. The SEI coating formed on the negative electrode by the compound represented by Chemical Formula 1 is strong and has low resistance, preventing cracking of the SEI coating due to volume expansion of the silicon-based active material. The low resistance can improve both 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

[0014] First, before describing the present invention, the terms and words used in this specification and claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a manner that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventors can appropriately define the concepts of terms in order to best explain their invention.

[0015] However, 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.

[0016] As used herein, the terms "comprises," "comprises," or "has" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0017] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0018] Before describing the present invention, in the description of "number of carbon atoms a to b" in the specification, "a" and "b" refer to the number of carbon atoms contained in a specific functional group. That is, the functional group may contain "a" to "b" carbon atoms.

[0019] In addition, in this specification, unless otherwise defined, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom is substituted with an element other than hydrogen, for example, with an alkyl group having 1 to 5 carbon atoms or a fluorine atom.

[0020] 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, for example, by the laser diffraction method. The laser diffraction method is generally capable of measuring particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0021] The present invention will now be described in more detail.

[0022] 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 negative electrode active material, the negative electrode active material comprising a silicon-based active material, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1:

[0023] [ka]

[0024] In the above chemical formula 1, n is an integer of 0-18.

[0025] The present invention relates to a lithium secondary battery in which the negative electrode contains a silicon-based active material and the non-aqueous electrolyte contains, as an additive, a compound represented by Chemical Formula 1. The compound represented by Chemical Formula 1 contains a propargyl group (-C≡C-) and a fluorine-substituted alkyl group within its structure, and is therefore reduced before the organic solvent, allowing a low-resistance SEI coating containing a fluorocarbon component to be formed on the surface of the negative electrode. The SEI coating formed on the negative electrode by the compound represented by Chemical Formula 1 is strong and has low resistance, preventing cracking of the SEI coating due to volume expansion of the silicon-based active material, and its low resistance can improve both the life and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery.

[0026] 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 may be manufactured by placing 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 the non-aqueous electrolyte.

[0027] (1) Negative electrode The negative electrode includes a negative electrode active material.

[0028] The negative electrode active material includes a silicon-based active material. While silicon-based active materials exhibit higher capacity than carbon-based active materials, they suffer from the problem of large volume expansion / contraction during charge / discharge. However, when the silicon-based active material is used in combination with a nonaqueous electrolyte (described below), a strong, low-resistance SEI coating can be formed on the negative electrode, preventing electrolyte side reactions and enabling the realization of a lithium secondary battery with long life and output characteristics.

[0029] The silicon-based active material may include a compound represented by the following chemical formula A:

[0030] [Chemical formula A] SiOx(0≦x<2)

[0031] In the chemical formula A, since SiO2 does not react with lithium ions and therefore cannot store lithium, x is preferably within the above range. Specifically, x in the chemical formula A may be 0.5 to 1.5. More specifically, the silicon-based active material may be SiO.

[0032] The average particle size (D 50 ) may be 1 μm to 20 μm, preferably 3 μm to 10 μm, from the viewpoint of achieving structural stability of the active material during charge and discharge, more smoothly forming a conductive network for maintaining electrical conductivity, and facilitating access to the binder for binding the active material and the current collector.

[0033] The negative electrode active material may further include a carbon-based active material in addition to the silicon-based active material. By using the silicon-based active material together with the carbon-based active material, it is possible to achieve high energy density and capacity and prevent a decrease in life performance.

[0034] The carbon-based active material may include at least one selected from the group consisting of graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and may specifically include graphite. The graphite may include at least one selected from the group consisting of artificial graphite and natural graphite.

[0035] The average particle size (D 50 ) may be 5 μm to 35 μm, preferably 10 μm to 20 μm, in order to ensure structural stability during charge and discharge and reduce side reactions with the electrolyte.

[0036] When the negative electrode active material further includes a carbon-based active material in addition to the silicon-based active material, the weight ratio of the silicon-based active material to the carbon-based active material may be 1:99 to 30:70, specifically 5:95 to 20:80. When the weight ratio is within this range, the effect of the silicon-based active material on volume expansion can be reduced, sufficient capacity of the negative electrode can be secured, and a high-loading negative electrode can be realized.

[0037] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material may be contained in the negative electrode active material layer.

[0038] 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, baked carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0039] The negative electrode current collector usually has a thickness of 3 μm to 500 μm.

[0040] 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, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0041] 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.

[0042] The negative electrode active material may be contained in the negative electrode active material layer in an amount of 60% by weight to 99% by weight, preferably 80% by weight to 97% by weight.

[0043] The negative electrode active material layer may further include a conductive material, a binder, and / or a thickener in addition to the negative electrode active material.

[0044] 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.

[0045] The binder may include at least one selected from the group consisting of polyvinylidene fluoride (PVDF), styrene butadiene rubber (SBR), 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), in order to further improve electrode adhesive strength and provide sufficient resistance to volume expansion / contraction of the active material. Specifically, the binder may include styrene butadiene rubber (SBR).

[0046] The binder may be contained in the negative electrode active material layer in an amount of 0.1 wt % to 10 wt %. When the binder is contained in the negative electrode active material layer in the amount within the above range, it is possible to realize a negative electrode having excellent capacity in addition to improving adhesive strength and controlling thickness expansion of the negative electrode, which is preferable.

[0047] The conductive material is used to assist and improve the conductivity of the secondary battery, and is not particularly limited as long as it does not cause chemical changes and is conductive. Specifically, the conductive material may 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 may include carbon black in order to achieve high conductivity.

[0048] The conductive material may be contained in the negative electrode active material layer in an amount of 1% by weight to 20% by weight, and when the amount is within the above range, it is preferable because it can mitigate the increase in resistance caused by the binder and form an excellent conductive network.

[0049] The negative electrode active material layer may further include a thickener, and the thickener may include carboxymethyl cellulose (CMC).

[0050] The thickener may be contained in the negative electrode active material layer in an amount of 0.5 wt % to 5 wt %, but is not limited thereto.

[0051] The thickness of the negative electrode active material layer may be 10 μm to 300 μm, preferably 50 μm to 200 μm, in order to achieve a high energy density.

[0052] The negative electrode may be fabricated 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.

[0053] 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 including distilled water, from the viewpoint of facilitating the dispersion of the negative electrode active material, binder, and / or conductive material.

[0054] The solvent for forming the negative electrode slurry may be included in the negative electrode slurry such that the concentration of the solid content including the negative electrode active material and optionally the binder and conductive material is 15% by weight to 45% by weight, considering the viscosity, coating property, dispersibility, etc. of the negative electrode slurry.

[0055] (2) Positive electrode The positive electrode includes a positive electrode active material.

[0056] The positive electrode active material is a compound capable of reversible intercalation and deintercalation of lithium, specifically, a lithium transition metal composite oxide including at least one transition metal selected from nickel, cobalt, manganese, and aluminum and lithium, preferably including a lithium transition metal composite oxide including a transition metal including nickel, cobalt, and manganese and lithium.

[0057] For example, as the lithium transition metal composite oxide, 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 CoZ1 O4 (where 0 < Z1 < 2, etc.), lithium-nickel-manganese-cobalt-based oxide (e.g., Li(Ni p Co q Mn r1 )O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of independent elements, 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, p2 + q2 + r3 + s2 = 1), etc.), and any one or two or more of these compounds may be included. Among them, from the viewpoint of enhancing 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 significance of the improvement effect by controlling the types and content ratios of the constituent elements forming the lithium transition metal composite oxide, the lithium transition metal composite oxide is Li(Ni 0.6 Mn 0.2Co 0.2 )O2, Li(Ni 0.5 Mn 0.3 Co 0.2 )O2, Li(Ni 0.7 Mn 0.15 Co 0.15 )O2, or Li(Ni 0.8 Mn 0.1 Co 0.1 )O2, etc., and any one or a mixture of two or more of these can be used.

[0058] More specifically, the positive electrode active material may be a lithium transition metal composite oxide containing 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 may be a lithium transition metal composite oxide containing nickel and at least one selected from manganese, cobalt, and aluminum, and containing 60 mol% or more, specifically 60 mol% to 90 mol%, of the transition metals based on the total number of moles of the transition metals. When such a lithium transition metal composite oxide containing a high content of nickel is used together with the nonaqueous electrolyte solution, it is preferable because it can reduce by-products in the gas generated by structural collapse.

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

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

[0061] 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 each independent element, and 0≦x≦0.2, 0.50≦a<1, 0 <b≦0.25、0<c≦0.25、0≦d≦0.1、a+b+c+d=1である。

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

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

[0064] Furthermore, the a, b, c, and d may be 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 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. In this case, the positive electrode active material layer may include the positive electrode active material described above.

[0066] The positive electrode current collector usually has a thickness of 3 μm to 500 μm.

[0067] The positive electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0068] 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.

[0069] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80% by weight to 99% by weight, taking into consideration the sufficient capacity of the positive electrode active material.

[0070] The positive electrode active material layer may further contain a binder and / or a conductive material in addition to the positive electrode active material.

[0071] The binder is a component that assists in binding the active material and conductive material, etc., and in binding them to the current collector, and specifically may contain 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, and preferably polyvinylidene fluoride.

[0072] The binder may be contained in the positive electrode active material layer in an amount of 0.1 wt % to 20 wt % in order to ensure sufficient binding strength between components such as the positive electrode active material.

[0073] The conductive material is used to assist and improve the conductivity of the secondary battery, and is not particularly limited as long as it does not cause chemical changes and is conductive. Specifically, the positive electrode conductive material may 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, and preferably includes carbon nanotubes in order to improve conductivity.

[0074] The conductive material may be contained in the positive electrode active material layer in an amount of 0.1% by weight to 20% by weight in order to ensure sufficient electrical conductivity.

[0075] The thickness of the positive electrode active material layer may be 10 μm to 300 μm, and preferably 40 μm to 150 μm.

[0076] The positive electrode may be fabricated 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.

[0077] The solvent for forming the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and 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.

[0078] (3) Separator The separator may be interposed between the positive electrode and the negative electrode.

[0079] The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminate, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, etc. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be selectively used as a single-layer or multi-layer structure.

[0080] (4) Nonaqueous electrolyte 1) Lithium salt First, the lithium salt will be described as follows.

[0081] 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 any limitation. For example, Li + 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). The lithium salt may specifically 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 may include LiPF6.

[0082] The lithium salt may be varied as appropriate within a range that is normally usable, but in order to obtain the optimum effect of forming a corrosion prevention coating on the electrode surface, it may be contained in the electrolyte at a concentration of 0.8 M to 3.0 M, specifically 1.0 M to 3.0 M.

[0083] When the concentration of the lithium salt satisfies the above range, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, improving the mobility of lithium ions and improving the capacity characteristics and cycle characteristics of the lithium secondary battery.

[0084] 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 minimizes decomposition due to oxidation reactions during charging and discharging of the secondary battery.

[0085] Specifically, the organic solvent may include a cyclic carbonate organic solvent and a linear carbonate organic solvent.

[0086] The cyclic carbonate organic solvent is a highly viscous organic solvent that has a high dielectric constant and easily dissociates 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.

[0087] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and specifically may include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0088] The organic solvent may be a mixture of a cyclic carbonate organic solvent and a linear carbonate organic solvent, in which the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed in a volume ratio of 10:90 to 50:50, specifically 15:85 to 40:60.

[0089] Meanwhile, the organic solvent may further include, as needed, any organic solvent commonly used in non-aqueous electrolytes, for example, at least one of an ester-based organic solvent, an ether-based organic solvent, a glyme-based solvent, and a nitrile-based organic solvent.

[0090] 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.

[0091] 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 of these, but is not limited thereto.

[0092] 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, and may include at least one selected from the group consisting of dimethoxyethane (glyme, DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME), but is not limited thereto.

[0093] The nitrile solvent may 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.

[0094] On the other hand, in the non-aqueous electrolyte, the remainder excluding the lithium salt and the additive is an organic solvent unless otherwise specified.

[0095] (3) Additives The non-aqueous electrolyte of the present invention contains an additive.

[0096] The additive includes a compound represented by the following Chemical Formula 1:

[0097] [ka]

[0098] In the above chemical formula 1, n is an integer of 0-18.

[0099] Specifically, the compound represented by Formula 1 contains a propargyl functional group within its structure, and thus can be 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 nonaqueous 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 additional reductive decomposition of the electrolyte caused by the instability of the SEI film.

[0100] Furthermore, the compound represented by Chemical Formula 1 contains an alkyl group substituted with a fluorine atom at the structural terminal, which forms an oxidation-resistant coating on the surface of the positive electrode, thereby suppressing the elution of transition metals from the positive electrode and the electrodeposition and precipitation of the eluted transition metals on the negative electrode, thereby preventing internal short circuits. Furthermore, the fluorine-substituted alkyl group, which has excellent flame retardancy and non-flammability, contained in the molecular structure, acts as a radical scavenger due to the fluorine atom and can form a passivation coating on the surface of the positive electrode that can ensure excellent oxidation resistance. As a result, side reactions between the electrode and the electrolyte are suppressed, and a lithium secondary battery with improved life characteristics at room temperature and low temperatures can be provided.

[0101] Furthermore, since the compound represented by Chemical Formula 1 of the present invention contains an ethylene group (-CH2-CH2-) between the acrylate functional group and the terminal fluorine-substituted alkyl group, the flexibility of the compound is increased due to the increase in the molecular chain of the linking group portion compared to a compound containing a methylene group (-CH2-) between the acrylate functional group and the terminal fluorine-substituted alkyl group. As a result, a coating derived from such a compound can be formed on the surface of a negative electrode with improved durability.

[0102] In addition, since the compound represented by Chemical Formula 1 contains two oxygen atoms in its molecular structure, it can improve oxidation safety, high voltage stability, and electrolyte durability compared to compounds containing three or more oxygen atoms.

[0103] As described above, the compound represented by Chemical Formula 1 contains a fluorine-containing alkyl group and a propargyl group, which have excellent flame retardancy and non-flammability, and therefore forms a low-resistance, strong SEI coating on the negative electrode, which not only suppresses the additional reduction decomposition reaction of the electrolyte but also prevents the self-discharge reaction of the negative electrode, thereby improving the life performance, suppressing an increase in initial resistance, and providing a lithium secondary battery with improved room temperature and low temperature output characteristics.

[0104] In the above Chemical Formula 1, n may be an integer of 0 to 18, specifically an integer of 1 to 10, and more specifically an integer of 2 to 8.

[0105] When n is within the above range, the thermal properties of the compound itself can be improved, and the stability of the coating formed therefrom can be expected. In Formula 1, when n is greater than 18, the fluorine element is contained in an excessive amount, which increases the viscosity and non-polarity of the material and reduces the solubility in the electrolyte, thereby reducing ionic conductivity and possibly causing deterioration of battery performance.

[0106] Preferably, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1-1 to 1-4, and more preferably, may include at least one selected from the group consisting of compounds represented by Chemical Formulas 1-1 to 1-2.

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[0111] Meanwhile, the compound represented by Chemical Formula 1 may be included in the non-aqueous electrolyte in an amount of 0.05 wt % to 8 wt %.

[0112] When the content of the compound represented by Chemical Formula 1 is within the above range, defects such as side reactions, capacity reduction, and resistance increase due to additives 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 decomposition reactions of the electrolyte, and preventing self-discharge reactions of the negative electrode.

[0113] Specifically, when the content of the compound represented by Chemical Formula 1 is 0.05 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 8 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, further increasing ionic conductivity within the battery, and preventing a decrease in output characteristics.

[0114] Specifically, the compound represented by Chemical Formula 1 may be contained in the non-aqueous electrolyte solution in an amount of 0.1% by weight to 5% by weight, more specifically 0.3% by weight to 3% by weight.

[0115] Meanwhile, the additive may contain other additional additives in addition to the compound represented by Formula 1, as needed, 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 the effect of suppressing battery expansion at high temperatures.

[0116] Examples of such additional additives include at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate or phosphite compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0117] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinylethylene carbonate.

[0118] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0119] 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.

[0120] The sulfate compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0121] The phosphate-based or phosphite-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0122] Examples of the borate-based compounds include tetraphenylborate, lithium difluoro(oxalato)borate (LiODFB), and lithium bisoxalatoborate (LiB(C2O4)2, LiBOB).

[0123] 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.

[0124] 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.

[0125] 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.

[0126] Among these additional additives, when at least one selected from the group consisting of vinylene carbonate, vinylethylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, ethylene sulfate, succinonitrile, and lithium difluoro(oxalato)borate, specifically at least one selected from the group consisting of vinylene carbonate, 1,3-propane sultone, and ethylene sulfate, is used, a stronger SEI coating can be formed on the surface of the negative electrode during the initial activation step of the secondary battery.

[0127] The additional additive may be a combination 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 wt % to 20 wt %, more specifically 0.05 wt % to 10 wt %, based on the total weight of the non-aqueous electrolyte. When the total content of the additives satisfies the above range, the low-temperature output characteristics, high-temperature storage characteristics, and high-temperature life characteristics of the battery may be improved more effectively, and side reactions in the battery due to residual additives after the reaction may be prevented.

[0128] 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).

[0129] 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.

[0130] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0131] 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.

[0132] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in a medium- to large-sized battery module containing a large number of battery cells.

[0133] The present invention will be specifically described below with reference to examples.

[0134] In this regard, 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 described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0135] The present invention will be specifically described below with reference to specific examples.

[0136] Example Example 1 (Production of non-aqueous electrolyte) An organic solvent was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70.

[0137] LiPF6 as a lithium salt was dissolved in the organic solvent to a molar concentration of 1.0M.

[0138] In addition, the compound represented by Formula 1-1, vinylene carbonate, 1,3-propane sultone, and ethylene sulfate were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0139] The compound represented by Chemical Formula 1-1 was contained in the non-aqueous electrolyte at 0.5 wt %, the vinylene carbonate was contained in the non-aqueous electrolyte at 1 wt %, the 1,3-propane sultone was contained in the non-aqueous electrolyte at 0.5 wt %, and the ethylene sulfate was contained in the non-aqueous electrolyte at 1 wt %.

[0140] (Secondary battery manufacturing) Cathode active material (LiNi 0.85 Co 0.05 Mn 0.07 Al 0.03 A cathode mixture slurry (solid content 75 wt%) was prepared by adding O2, a conductive material (carbon nanotubes), and a binder (polyvinylidene fluoride) in a weight ratio of 97.5:1.0:1.5 to a solvent, N-methyl-2-pyrrolidone (NMP). The cathode mixture slurry was applied to one side of a 15 μm-thick cathode current collector (Al thin film) to provide a current density of 4.1 mAh / cm. 2 The coating was then dried and roll pressed to prepare a positive electrode (thickness of the positive electrode active material: 130 μm).

[0141] Anode active material, styrene-butadiene rubber (SBR) as a binder, carbon black as a conductive material, and carboxymethyl cellulose (CMC) as a thickener were added to water as a solvent in a weight ratio of 96:2:1:1 to prepare anode slurry. The anode active material was a silicon-based active material, SiO (average particle size (D 50 : 7 μm) and graphite (average particle size (D 50The negative electrode slurry was applied to a copper (Cu) thin film, which was a 6 μm-thick negative electrode current collector, and the negative electrode current collector was charged to 4.3 mAh / cm. 2 After drying, a negative electrode was manufactured by roll pressing (thickness of negative electrode active material: 140 μm).

[0142] The positive electrode, a polyolefin-based porous separator, and a negative electrode were stacked in this order to prepare an electrode assembly.

[0143] The assembled electrode assembly was placed in a battery case, and the prepared non-aqueous electrolyte was then poured into the battery case to manufacture a lithium secondary battery.

[0144] 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 Chemical Formula 1-2 was added to the non-aqueous electrolyte at a content of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0145] 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 1-3 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

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

[0147] Example 5 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 1-1 was added to the non-aqueous electrolyte in an amount of 0.1 wt %.

[0148] Example 6 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 1-1 was added to the non-aqueous electrolyte in an amount of 5 wt %.

[0149] 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 compound represented by Chemical Formula 1-1 was not added.

[0150] Comparative Example 2 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 3 (a=20) was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

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[0152] Comparative Example 3 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 4 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

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[0154] 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 5 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

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[0156] Experimental example Experimental Example 1: Evaluation of initial capacity and capacity retention rate during high-temperature storage The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 prepared above were initially charged at 25°C under CC / CV 1 / 3C conditions to 4.2V, 0.05C, and then discharged at CC 1 / 3C to 2.5V, thereby carrying out an initial charge-discharge cycle. They were then charged at 25°C under CC / CV 1 / 3C conditions to 4.2V, 0.05C, and then stored at 60°C for 12 weeks. After storage, the secondary batteries were charged at 25°C under CC / CV 1 / 3C conditions to 4.2V, 0.05C, and then discharged at CC 1 / 3C to 2.5V, and the discharge capacities were measured.

[0157] The capacity retention rate was evaluated using the following formula, and the results are shown in Table 1 below.

[0158] Capacity retention rate (%) = (discharge capacity after 12 weeks of storage / initial discharge capacity) x 100

[0159] Experimental Example 2: Evaluation of initial resistance and resistance increase rate during high temperature storage The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 prepared above were initially charged and discharged in the same manner as in Experimental Example 1. After confirming the capacity at room temperature, they were charged to 50% SOC based on the discharge capacity and discharged at a current of 0.33 C for 10 seconds, and the resistance was measured from the difference in voltage drop at this time to obtain the initial resistance. After storing at 60°C for 12 weeks, the resistance was measured in the same manner to obtain the final resistance, and the resistance increase rate was calculated using the following formula. The results are shown in Table 1 below.

[0160] Resistance increase rate (%) = (final resistance - initial resistance) / (initial resistance) x 100

[0161] Experimental Example 3: Evaluation of volume increase rate during high temperature storage The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 manufactured above were initially charged and discharged in the same manner as in Experimental Example 1. For each battery, the volume was measured at an SOC of 50% based on the discharge capacity, and this was defined as the initial volume. In addition, the volume measured after high-temperature storage at 60°C for 12 weeks at an SOC of 100% was defined as the final volume, and the volume increase rate of the battery was calculated using the following formula. The results are shown in Table 1 below.

[0162] Volume increase rate (%) = (final volume - initial volume) / (initial volume) x 100

[0163] [Table 1]

[0164] Referring to Table 1, it can be seen that the lithium secondary batteries of Examples 1 to 6 according to the present invention exhibit excellent life performance, a low resistance increase rate, and a low battery volume increase rate 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 includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive; The additive for a lithium secondary battery includes a compound represented by the following Chemical Formula 1: 【Chemistry 1】 In the above Chemical Formula 1, n is an integer from 0 to 18.

2. 2. The lithium secondary battery according to claim 1, wherein n is an integer of 1 to 10 in Chemical Formula 1.

3. 2. The lithium secondary battery according to claim 1, wherein n is an integer of 2 to 8 in Chemical Formula 1.

4. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 includes at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1-1 to 1-4: 【Chemistry 2】 【Transformation 3】 【Chemistry 4】 【Transformation 5】

5. 5. 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 0.05% by weight to 8% by weight.

6. 5. The lithium secondary battery according to 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.

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

8. 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] SiOx (0≦x<2)

9. The lithium secondary battery according to claim 1 , wherein the negative electrode active material further comprises a carbon-based active material.

10. 10. The lithium secondary battery according to claim 9, wherein the weight ratio of the silicon-based active material to the carbon-based active material is 1:99 to 30:70.

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