Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery including a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, in which the negative electrode includes a negative electrode active material including a silicon-based active material having an average particle diameter (D50) of 1 [mu] m to 20 [mu] m, the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, the organic solvent includes fluoroethylene carbonate and diethyl carbonate in a volume ratio of 5: 95 to 25: 75, and the additive includes a compound represented by a specific chemical formula.

Description

[0001] Cross - reference to related applications

[0002] This application claims priority and the benefit of Korean Patent Application No. 10 - 2022 - 0182375, filed on December 22, 2022, the entire content of which is incorporated herein by reference. Technical field

[0003] The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery including a non - aqueous electrolyte, the non - aqueous electrolyte including an additive capable of forming a stable SEI layer on a positive electrode / negative electrode, thereby suppressing an increase in initial resistance and improving output characteristics and life performance. Background art

[0004] With the increasing dependence on electric energy in our society and the development of personal IT devices and computer networks in the developing information society, there is a need to develop technologies for efficiently storing and utilizing electric energy.

[0005] Among the developed technologies, secondary batteries are considered most suitable for various purposes, and among secondary batteries, lithium secondary batteries have attracted great attention because they can be miniaturized enough to be applied to personal IT devices and have the highest energy density.

[0006] Generally, a lithium secondary battery is manufactured by injecting a non - aqueous electrolyte into an electrode assembly composed of a positive electrode, a negative electrode, and a porous separator or impregnating the electrode assembly with a non - aqueous electrolyte.

[0007] As the positive electrode active material of a lithium secondary battery, lithium - containing cobalt oxides, LiMnO2 having a layered crystal structure, LiMn2O4 having a spinel crystal structure, lithium - containing nickel oxides (LiNiO2), lithium nickel cobalt manganese transition metal oxides, etc. are considered.

[0008] Meanwhile, as the negative electrode active material, carbon - based active materials such as graphite have been used, but recently, due to having a higher capacity than carbon - based active materials, silicon - based active materials are being considered.

[0009] The silicon - based active material has a high capacity, but there is a problem that the volume expansion / contraction during charging and discharging is very large. This large volume expansion / contraction significantly reduces the conductivity of the negative electrode, resulting in a reduction in life performance. In addition, during initial activation, a solid electrolyte interphase layer (hereinafter referred to as the SEI layer) is formed on the negative electrode surface, but the silicon - based active material has a large volume expansion, which causes cracks in the SEI layer and continuous generation on the negative electrode surface, and thus as the SEI layer formation reaction continuously occurs, the electrolyte side reaction intensifies, and as the SEI layer thickens, the resistance increases. Summary of the invention

[0010] [Technical Problem]

[0011] The present invention aims to solve the above problems and provides a lithium secondary battery including a silicon-based active material as a negative electrode active material, which has improved output characteristics and life characteristics by forming a stable low-resistance SEI layer on the negative electrode.

[0012] [Technical Solution]

[0013] One aspect of the present invention provides a lithium secondary battery including a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein the negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, and an average particle diameter (D 50 ) of the silicon-based active material is from 1 μm to 20 μm, the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, the organic solvent contains fluoroethylene carbonate and diethyl carbonate in a volume ratio of 5:95 to 25:75, and the additive contains at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 3.

[0014] [Chemical Formula 1]

[0015]

[0016] In Chemical Formula 1, R1 is hydrogen or a C1-C3 alkyl group, and n is an integer from 1 to 10.

[0017] [Chemical Formula 2]

[0018]

[0019] In Chemical Formula 2, m is an integer from 0 to 18.

[0020] [Chemical Formula 3]

[0021]

[0022] In Chemical Formula 3, R2 and R3 are each independently a C1-C 10 alkylene group, and R4 is a C1-C 20 alkyl group substituted with one or more fluorine atoms.

[0023] [Advantageous Effects]

[0024] The present invention relates to a lithium secondary battery, wherein the negative electrode includes a certain average particle diameter (D 50The silicon-based active material, the non-aqueous electrolyte includes fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a certain volume ratio as organic solvents, and includes at least one selected from the group consisting of the compounds represented by Chemical Formulas 1 to 3 as an additive. The silicon-based active material has an average particle diameter in the micron order, and since an organic / inorganic composite SEI layer can be easily formed on the negative electrode due to the composition of the non-aqueous electrolyte, the SEI layer can be prevented from cracking due to the volume change of the silicon-based active material, and thus the life performance at high temperature and room temperature can be significantly improved. Detailed Description of the Invention

[0025] Before describing the present invention, the terms and words used in this specification and claims should not be construed as limited to their conventional or dictionary meanings, but should be interpreted as meanings and concepts consistent with the technical idea of the present invention based on the principle that the inventor can appropriately define the terms to best describe his invention.

[0026] The terms used herein are only for describing exemplary embodiments and are not intended to limit the present invention. Singular expressions include plural expressions unless the context clearly indicates otherwise.

[0027] In this specification, the terms "comprising", "providing" or "having" are intended to indicate the presence of the implemented features, numbers, steps, elements or combinations thereof, and it should be understood that they do not preclude the presence or addition of other features, numbers, steps, elements or combinations thereof in advance.

[0028] In this specification, "%" means weight percentage unless otherwise clearly specified.

[0029] Before describing the present invention, in the description herein of "C a to C b ", "a" and "b" are the number of carbon atoms contained in a specific functional group. In other words, the functional group may contain "a" to "b" carbon atoms.

[0030] In this specification, unless otherwise defined, the term "substituted" means that at least one hydrogen bonded to carbon is substituted by an element other than hydrogen, such as being substituted by a C1 to C5 alkyl group or a fluorine atom.

[0031] In this specification, the average particle diameter (D 50 ) can be defined as the particle diameter corresponding to 50% of the cumulative volume in the particle diameter distribution curve. The average particle diameter (D 50 ) can be measured, for example, using the laser diffraction method. The laser diffraction method is generally used to measure the particle diameter from the submicron region to several millimeters, and results with high reproducibility and high resolution can be obtained.

[0032] Hereinafter, the present invention will be described in detail.

[0033] Lithium secondary battery

[0034] The present invention relates to a lithium secondary battery, which includes 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, and the average particle diameter (D 50 ) of the silicon-based active material is 1 μm to 20 μm. The non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive. The organic solvent contains fluoroethylene carbonate and diethyl carbonate in a volume ratio of 5:95 to 25:75, and the additive includes at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 3.

[0035] [Chemical Formula 1]

[0036]

[0037] In Chemical Formula 1, R1 is hydrogen or a C1 to C3 alkyl group, and n is an integer from 1 to 10.

[0038] [Chemical Formula 2]

[0039]

[0040] In Chemical Formula 2, m is an integer from 0 to 18.

[0041] [Chemical Formula 3]

[0042]

[0043] In Chemical Formula 3, R2 and R3 are each independently a C1 to C 10 alkylene group, and R4 is a C1 to C 20 alkyl group substituted with one or more fluorine atoms.

[0044] The present invention relates to a lithium secondary battery, wherein the negative electrode includes a silicon-based active material having a certain average particle diameter (D 50 ), the non-aqueous electrolyte includes fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) in a certain volume ratio as an organic solvent, and includes at least one selected from the group consisting of compounds represented by Chemical Formulas 1 to 3 as an additive. The silicon-based active material has an average particle diameter in the micron range, and since an organic / inorganic composite SEI layer can be easily formed on the negative electrode by the composition of the non-aqueous electrolyte, the SEI layer can be prevented from cracking due to the volume change of the silicon-based active material. Therefore, the life performance at high temperature and room temperature can be significantly improved.

[0045] The lithium secondary battery includes: a positive electrode; a negative electrode; a separator; and a non-aqueous electrolyte. Specifically, the lithium secondary battery includes: a negative electrode; a positive electrode opposite to the negative electrode; a separator disposed between the positive electrode and the negative electrode; and a non-aqueous electrolyte. The lithium secondary battery can be manufactured by accommodating an electrode assembly in a battery case and then injecting a non-aqueous electrolyte, where the electrode assembly includes: a negative electrode, a positive electrode opposite to the negative electrode, and a separator disposed between the positive electrode and the negative electrode.

[0046] (1) Negative electrode

[0047] The negative electrode includes a silicon-based active material. The silicon-based active material has a higher capacity than a carbon-based active material, but the problem is that the volume expansion / contraction during charge and discharge is very large. However, when the silicon-based active material is used together with the non-aqueous electrolyte described below, a highly flexible and durable SEI layer can be formed on the negative electrode, so that electrolyte side reactions can be prevented and a lithium secondary battery with improved life performance and output characteristics can be provided.

[0048] The silicon-based active material may include a compound represented by Chemical Formula A below.

[0049] [Chemical Formula A]

[0050] SiOx (0 ≤ x < 2)

[0051] In Chemical Formula A, considering that SiO2 does not react with lithium ions and thus cannot store lithium, x is preferably within the above range.

[0052] The silicon-based active material may be silicon (Si). The advantage of Si is that its capacity is about 2.5 to 3 times that of silicon oxides (such as SiOx (0 < x < 2)), but the volume expansion / contraction caused by the charge and discharge of Si is much larger than that of silicon oxides, so Si is not easily commercialized. However, by applying the non-aqueous electrolyte described below, the lithium secondary battery of the present invention can have high life performance and output characteristics.

[0053] The average particle diameter (D 50 ) of the silicon-based active material is 1 μm to 20 μm. When the average particle diameter (D 50 ) of the silicon-based active material is less than 1 μm, the surface area is too high, so the fluoroethylene carbonate described later reacts and is completely consumed, so physical properties such as ionic conductivity change significantly, resulting in an imbalance in lithium ion transfer and an increase in resistance, which leads to a reduction in life performance. When the average particle diameter (D 50 ) of the silicon-based active material is greater than 20 μm, the volume expansion of the silicon-based active material becomes too large, making it difficult to maintain a conductive network, and the life performance and resistance characteristics are significantly reduced.

[0054] Specifically, the average particle diameter (D50 ) It can be from 2 μm to 10 μm, specifically from 3 μm to 8 μm. Within the above range, when combined with the non-aqueous electrolyte described below, an organic / inorganic composite SEI layer can be well formed, thereby preventing the rupture of the SEI layer and the consumption of the non-aqueous electrolyte, and thus the life performance can be improved to the desired level.

[0055] The negative electrode may include: a negative electrode current collector; and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. The silicon-based active material may be included in the negative electrode active material layer.

[0056] There is no particular limitation on the negative electrode current collector as long as it has high conductivity without causing chemical changes in the battery. Specifically, the negative electrode current collector may be copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc.

[0057] The negative electrode current collector generally may have a thickness of 3 to 500 μm.

[0058] The negative electrode current collector may have fine irregularities formed on its surface to enhance the adhesion 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, net, porous material, foam, and non-woven material.

[0059] The negative electrode active material layer may be provided on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be provided on one or both surfaces of the negative electrode current collector.

[0060] In order to fully realize the high capacity of the silicon-based active material in the secondary battery and at the same time minimize the influence of volume expansion / contraction on the battery, the silicon-based active material may be included in the negative electrode active material layer in an amount of 60 wt% to 99 wt%, preferably 70 wt% to 85 wt%.

[0061] In addition to the silicon-based active material, the negative electrode active material layer may further include a carbon-based active material.

[0062] The carbon-based active material may be, for example, graphite, hard carbon, or soft carbon. Specifically, the carbon-based active material may be graphite. The graphite may be artificial graphite, natural graphite, or a mixture thereof.

[0063] When the negative electrode active material layer contains a silicon-based active material and a carbon-based active material, based on the total weight of the negative electrode active material, the total weight of the silicon-based active material and the carbon-based active material may be 60 wt% to 99 wt%, preferably 70 wt% to 98 wt%. When the negative electrode active material layer contains a silicon-based active material and a carbon-based active material, the weight ratio of the silicon-based active material to the carbon-based active material may be from 1:99 to 30:70, specifically from 2:98 to 20:80.

[0064] In addition to the silicon-based active material, the negative electrode active material layer may further include a conductive material and / or a binder.

[0065] The binder can be used to improve the adhesion between the negative electrode active material layer and the negative electrode current collector (which will be described later), or to improve the adhesion between the silicon-based active materials.

[0066] In order to further improve electrode adhesion and provide sufficient resistance to the volume expansion / contraction of the silicon-based active material, the binder may include at least one selected from styrene-butadiene rubber (SBR), nitrile 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 nitrile rubber.

[0067] 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%. Within the above range, by more tightly bonding the silicon-based active materials, the volume expansion of the active material can be minimized, and when preparing a slurry for the negative electrode active material layer, the binder can be easily dispersed, and the coatability and phase stability of the slurry can be improved.

[0068] The conductive material can be used to assist and improve the conductivity in the secondary battery, and there is no particular limitation as long as it has conductivity without causing chemical changes. Specifically, the conductive material may include at least one selected from the following: graphite, including natural graphite or artificial graphite; carbon black, including acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, including carbon fibers and metal fibers; conductive tubes, including carbon nanotubes; fluorocarbons; metal powders, including aluminum powder and nickel powder; conductive whiskers, including zinc oxide and potassium titanate; conductive metal oxides, including titanium oxide; and polyphenylene derivatives, and preferably carbon black may be included to achieve high conductivity.

[0069] The conductive material may be included in the negative electrode active material layer in an amount of 1 wt% to 20 wt%, preferably 8 wt% to 15 wt%. When the content of the conductive material is within the above range, it is preferred because the increase in resistance caused by the binder can be reduced and an excellent conductive network can be formed.

[0070] The thickness of the negative electrode active material layer may be 5 μm to 500 μm, preferably 5 μm to 100 μm.

[0071] The loading amount of the negative electrode active material layer may be 3 mAh / cm 2 to 15 mAh / cm 2, preferably 8 mAh / cm 2 to 13 mAh / cm 2 .

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

[0073] The solvent for forming the negative electrode paste may include, for example, at least one selected from distilled water, ethanol, methanol, and isopropyl alcohol to easily disperse the negative electrode active material, the binder, and / or the conductive material, and distilled water is preferred.

[0074] Considering the viscosity, coatability, and dispersibility of the negative electrode paste, the solvent for forming the negative electrode paste may be contained in the negative electrode paste such that the solid content including the negative electrode active material and optionally the binder and the conductive material is 15 wt% to 45 wt%.

[0075] (2) Positive electrode

[0076] The positive electrode includes a positive electrode active material.

[0077] The positive electrode active material is a compound capable of reversibly intercalating / deintercalating lithium ions, and may specifically include a lithium transition metal composite oxide including at least one transition metal composed of nickel, cobalt, manganese, and aluminum and lithium, and preferably a lithium transition metal composite oxide containing a transition metal and lithium including nickel, cobalt, and manganese.

[0078] For example, the lithium transition metal composite oxide may be: lithium-manganese-based oxides (such as LiMnO2, LiMn2O4, etc.), lithium-cobalt-based oxides (such as LiCoO2, etc.), lithium-nickel-based oxides (such as LiNiO2, etc.), lithium-nickel-manganese-based oxides (such as LiNi 1-Y Mn Y O2 (where 0 < Y < 1), LiMn 2-Z Ni z O4 (where 0 < Z < 2), etc.), lithium-nickel-cobalt-based oxides (such as LiNi 1- Y1 Co Y1 O2 (where 0 < Y1 < 1), etc.), lithium-manganese-cobalt-based oxides (such as LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1), LiMn 2- Z1 Co z1 O4 (where 0 < Z1 < 2), etc.), lithium-nickel-manganese-cobalt-based oxides (such as Li(Ni p Co q Mn r1)O2 (where 0 < p < 1, 0 < q < 1, 0 < r1 < 1, and p + q + r1 = 1) or Li(Ni p1 Co q1 Mn r2 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r2 < 2, and p1 + q1 + r2 = 2), etc.), or lithium-nickel-cobalt-transition metal (M) oxide (e.g., Li(Ni p2 Co q2 Mn r3 M S2 )O2 (where M is selected from the group consisting of Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, and p2, q2, r3, and s2 are atomic fractions of various independent elements, where 0 < p2 < 1, 0 < q2 < 1, 0 < r3 < 1, 0 < s2 < 1, and p2 + q2 + r3 + s2 = 1), etc.), and may include any one or two or more of these compounds. In terms of improving the capacity characteristics and stability of the battery, the lithium transition metal composite oxide can be 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., LiNi 0.8 Co 0.15 Al 0.05 O2, etc.), and considering the significant improvement effect brought by the type and content ratio of each component 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, and any one or a mixture of two or more of them can be used.

[0079] More specifically, the positive electrode active material is a lithium transition metal composite oxide, and based on the total number of moles of transition metals contained in the lithium transition metal composite oxide, it may contain 60 mol% or more of nickel. Specifically, the positive electrode active material is a lithium transition metal composite oxide, and the transition metals include nickel and at least one selected from manganese, cobalt, and aluminum, and may include 60 mol% or more, specifically 60 mol% to 90 mol% of nickel based on the total number of moles of transition metals. When this lithium transition metal composite oxide with a high nickel content is used together with the above non-aqueous electrolyte, by-products in the gas generated due to structural collapse can be reduced, and thus it is preferred.

[0080] The positive electrode active material may include a lithium transition metal composite oxide represented by the following Chemical Formula B.

[0081] [Chemical Formula B]

[0082] Li 1+x (Ni a Co b Mn c M d )O2

[0083] In 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, and a + b + c + d = 1.

[0084] Preferably, a, b, c, and d may respectively satisfy 0.70 ≤ a ≤ 0.95, 0.025 ≤ b ≤ 0.20, 0.025 ≤ c ≤ 0.20, and 0 ≤ d ≤ 0.05.

[0085] In addition, a, b, c, and d may respectively satisfy 0.80 ≤ a ≤ 0.95, 0.025 ≤ b ≤ 0.15, 0.025 ≤ c ≤ 0.15, and 0 ≤ d ≤ 0.05.

[0086] In addition, a, b, c, and d may respectively satisfy 0.85 ≤ a ≤ 0.90, 0.05 ≤ b ≤ 0.10, 0.05 ≤ c ≤ 0.10, and 0 ≤ d ≤ 0.03.

[0087] The positive electrode may include: a positive electrode current collector; and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. The positive electrode active material layer may contain the above positive electrode active material.

[0088] The positive electrode current collector generally may have a thickness of 3 to 500 μm.

[0089] The positive electrode current collector may have fine irregularities formed on its surface to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector can be used in various forms, such as films, sheets, foils, meshes, porous materials, foams, and nonwoven materials.

[0090] The positive electrode active material layer can be disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer can be disposed on one or both surfaces of the positive electrode current collector.

[0091] Considering that the positive electrode active material exhibits sufficient capacity, the positive electrode active material can be included in the positive electrode active material layer in an amount of 80% to 99% by weight.

[0092] In addition to the above positive electrode active material, the positive electrode active material layer may further include a binder and / or a conductive material.

[0093] The binder is a component that helps to bond the active material and the conductive material as well as bond the current collector, and can specifically include at least one selected from polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, among which polyvinylidene fluoride is preferred.

[0094] The binder can be included in the positive electrode active material layer in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, to ensure sufficient adhesion between the components including the positive electrode active material.

[0095] The conductive material can be used to assist and improve the conductivity in the secondary battery and is not particularly limited as long as it has conductivity without causing chemical changes. Specifically, the positive electrode conductive material can include at least one selected from the group consisting of: graphite, including natural graphite or artificial graphite; carbon black, including acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, including carbon fibers and metal fibers; conductive tubes, including carbon nanotubes; fluorocarbons; metal powders, including aluminum powder and nickel powder; conductive whiskers, including zinc oxide and potassium titanate; conductive metal oxides, including titanium oxide; and polyphenylene derivatives; preferably carbon nanotubes to improve conductivity.

[0096] The conductive material can be included in the positive electrode active material layer in an amount of 1% to 20% by weight, preferably 1.2% to 10% by weight, to ensure sufficient conductivity.

[0097] The thickness of the positive electrode active material layer can be 5 μm to 500 μm, preferably 100 μm to 200 μm.

[0098] The loading amount of the positive electrode active material layer can be 2 mAh / cm 2 to 6 mAh / cm 2 , preferably 4 mAh / cm 2 to 5 mAh / cm 2 .

[0099] The positive electrode can be manufactured by coating a positive electrode current collector with a positive electrode paste containing a positive electrode active material and optionally a binder, a conductive material, and a solvent for forming the positive electrode paste, and then drying and rolling.

[0100] The solvent for forming the positive electrode paste may include an organic solvent such as N-methyl-2-pyrrolidone (NMP). The solid content of the positive electrode paste can be 40 wt% to 90 wt%, specifically 50 wt% to 80 wt%.

[0101] (3) Separator

[0102] The separator can be placed between the positive electrode and the negative electrode.

[0103] The separator may include a common porous polymer film that is conventionally used as a separator. For example, a porous polymer film prepared from an olefin-based polymer (such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, etc.) can be used alone or in a laminated manner, or a commonly used porous nonwoven material such as a nonwoven material made of high melting point glass fiber and polyethylene terephthalate fiber, etc. can be used, but it is not limited thereto. In addition, in order to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer can be used, and optionally a single-layer or multi-layer structure can be used.

[0104] (4) Non-aqueous electrolyte

[0105] 1) Lithium salt

[0106] The lithium salt is described as follows.

[0107] In the non-aqueous electrolyte for a lithium secondary battery according to an embodiment of the present invention, the lithium salt can be a lithium salt commonly used in electrolytes for lithium secondary batteries without limitation. For example, the lithium salt can include Li + as a cation, and includes at least one selected from the following 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 includes at least one selected from the following: 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, LiDFOB (LiB(C2O4)F2) and LiBETI (LiN(SO2CF2CF3)2). The lithium salt may specifically include one substance selected from LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), LiDFOB (LiB(C2O4)F2) and LiBETI (LiN(SO2CF2CF3)2) or a mixture of two or more thereof, and more specifically includes LiPF6.

[0108] The lithium salt can be appropriately changed within the generally used range. However, in order to obtain the best effect of forming an anti-corrosion film on the electrode surface, the lithium salt can be included in the electrolyte at a concentration of 0.8 M to 3.0 M, specifically at a concentration of 1.0 M to 3.0 M.

[0109] When the lithium salt concentration satisfies the above range, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal impregnation, and the capacity characteristics and cycle characteristics of the lithium secondary battery can be improved by increasing the mobility of lithium ions.

[0110] 2) Organic solvents

[0111] The organic solvents include fluoroethylene carbonate (FEC) and diethyl carbonate (DEC).

[0112] The volume ratio of fluoroethylene carbonate (FEC) to diethyl carbonate (DEC) is 5:95 to 25:75. Fluoroethylene carbonate (FEC) can decompose and form a SEI layer containing inorganic components (such as LiF) on the negative electrode. Since the adsorption of inorganic components (such as LiF) on the negative electrode is much better than that of the polymer-type organic SEI layer, it is very effective in preventing electrolyte side reactions of the negative electrode. At the same time, since diethyl carbonate has a low viscosity and a low dielectric constant, it can improve electrolyte impregnation when used together with fluoroethylene carbonate, so it is preferred.

[0113] As described below, the lithium secondary battery of the present invention requires the use of an additive containing the compound represented by Chemical Formula 1 and an organic solvent. When the additive is not used, it is difficult to effectively form an inorganic SEI layer including inorganic components (such as LiF) on the negative electrode, and when the decomposition of FEC continues, an imbalance in the transfer of lithium ions on the negative electrode occurs, which leads to an increase in resistance and a reduction in life performance due to the permanent loss of lithium.

[0114] When the volume content of fluoroethylene carbonate is less than 5% by volume and the volume content of diethyl carbonate is greater than 95% by volume in the total volume of fluoroethylene carbonate and diethyl carbonate, since the inorganic SEI layer formed by fluoroethylene carbonate is insufficient, the above effects of the present invention cannot be achieved. On the other hand, when the volume content of fluoroethylene carbonate is greater than 25% by volume and the volume content of diethyl carbonate is less than 75% by volume in the total volume of fluoroethylene carbonate and diethyl carbonate, it is difficult to easily form the above inorganic SEI layer because fluoroethylene carbonate has a solvation structure that is difficult to effectively generate inorganic components (such as LiF).

[0115] Specifically, the volume ratio of fluoroethylene carbonate to diethyl carbonate can be from 7:93 to 20:80.

[0116] The organic solvent can be used by adding organic solvents commonly used in non-aqueous electrolytes, but is not limited thereto.

[0117] The organic solvent can further include at least one organic solvent selected from cyclic carbonate organic solvents, chain carbonate organic solvents, ester organic solvents, ether organic solvents, glycol diether organic solvents, and nitrile organic solvents.

[0118] The cyclic carbonate organic solvent does not include fluoroethylene carbonate (FEC), and can specifically 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.

[0119] The chain carbonate organic solvent does not include diethyl carbonate (DEC), and can specifically include at least one selected from dimethyl carbonate (DMC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate.

[0120] The ester organic solvent can include at least one selected from 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.

[0121] The ether solvent can be any one or a mixture of two or more selected from 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), but is not limited thereto.

[0122] Compared with chain carbonate organic solvents, glyme solvents have a high dielectric constant and a low surface tension, and their reactivity with metals is low. Glyme solvents may include at least one selected from dimethoxyethane (glyme dimethyl ether, DME), diethoxyethane, diglyme dimethyl ether, triglyme dimethyl ether, and tetraethylene glycol dimethyl ether (TEGDME), but are not limited thereto.

[0123] Nitrile solvents may be one or more selected from acetonitrile, propionitrile, butyronitrile, valeronitrile, octanenitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but are not limited thereto.

[0124] Unless otherwise specified, the remainder of the non-aqueous electrolyte except for the lithium salt and additives may be an organic solvent.

[0125] 3) Additives

[0126] The non-aqueous electrolyte of the present invention includes additives.

[0127] The additives include at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 3.

[0128] [Chemical Formula 1]

[0129]

[0130] In Chemical Formula 1, R1 is hydrogen or a C1 to C3 alkyl group, and n is an integer from 1 to 10.

[0131] [Chemical Formula 2]

[0132]

[0133] In Chemical Formula 2, m is an integer from 0 to 18.

[0134] [Chemical Formula 3]

[0135]

[0136] In Chemical Formula 3, R2 and R3 are each independently a C1 to C 10 alkylene group, and R4 is a C1 to C 20 alkyl group substituted with one or more fluorine atoms.

[0137] Specifically, when at least one of the compounds represented by Chemical Formulas 1 to 3 is used together with the above-mentioned organic solvent, an inorganic SEI layer including an inorganic component (e.g., LiF) can be formed more efficiently. In addition, at least one of the compounds represented by Chemical Formulas 1 to 3 prevents the continuous decomposition of fluoroethylene carbonate and forms an inorganic SEI layer on the negative electrode to ensure the balance of lithium ion transfer on the negative electrode, thereby significantly improving the life performance of the lithium secondary battery.

[0138] Since at least one of the compounds represented by Chemical Formulas 1 to 3 includes a vinyl group or a propargyl-containing functional group in its structure, the compound can be easily reduced and decomposed on the surface of the negative electrode containing a silicon-based active material to form an SEI layer with low resistance and high passivation ability. Therefore, when using a non-aqueous electrolyte containing at least one of the compounds represented by Chemical Formulas 1 to 3 as an electrolyte additive, the self-discharge of the negative electrode caused by the additional reductive decomposition of the electrolyte due to the instability of the SEI layer can be prevented.

[0139] Because at least one of the compounds represented by Chemical Formulas 1 to 3 includes a fluorinated carbon functional group substituted with one or more fluorine atoms at the structural end, an antioxidant film is formed on the surface of the positive electrode, suppressing the elution of transition metals from the positive electrode and suppressing the electrodeposition and precipitation of the eluted transition metals on the negative electrode, thereby preventing internal short circuits.

[0140] Since at least one of the compounds represented by Chemical Formulas 1 to 3 includes a fluoroalkyl group having excellent flame retardancy and non-flammability and a vinyl group or a propargyl group, a stable low-resistance SEI layer is formed on the negative electrode, which not only suppresses the additional reductive decomposition reaction of the electrolyte but also prevents the self-discharge of the negative electrode. Therefore, the life characteristics can be improved, the increase in the initial resistance can be suppressed, and a lithium secondary battery having improved output characteristics at room temperature and low temperature can be provided.

[0141] In Chemical Formula 1, R1 can be hydrogen or a C1 to C3 alkyl group, and specifically, it can be oxygen.

[0142] In Chemical Formula 1, n can be an integer from 1 to 10, specifically an integer from 3 to 8. When n is 0, the compound acts like a non-solvent, making it difficult to effectively generate an inorganic SEI layer containing an inorganic component (e.g., LiF). When n is an integer greater than 10, the miscibility with an organic solvent or the like decreases, making it difficult to effectively generate an inorganic SEI layer, and thus the required inorganic SEI layer may not be generated.

[0143] The compound represented by Chemical Formula 1 can include at least one selected from the group consisting of the compounds represented by Chemical Formulas 1-1 to 1-3, and more specifically, it can include the compound represented by the following Chemical Formula 1-1.

[0144] [Chemical Formula 1-1]

[0145]

[0146] [Chemical Formula 1-2]

[0147]

[0148] [Chemical Formula 1-3]

[0149]

[0150] In Chemical Formula 2, m can be an integer from 0 to 18, specifically an integer from 1 to 10, and more specifically an integer from 2 to 8.

[0151] When m satisfies the above range, the thermal properties of the compound itself can be improved, and it is expected that the film formed therefrom is stable. In Chemical Formula 2, when m exceeds 18, due to the excessive fluorine atoms contained, the viscosity and nonpolarity increase, the solubility in the electrolyte decreases, and the ionic conductivity decreases, so the battery performance may be reduced.

[0152] Preferably, the compound represented by Chemical Formula 2 may include at least one selected from the compounds represented by the following Chemical Formulas 2-1 to 2-4, and more preferably includes at least one selected from the compounds represented by Chemical Formula 2-1 and Chemical Formula 2-2.

[0153] [Chemical Formula 2-1]

[0154]

[0155] [Chemical Formula 2-2]

[0156]

[0157] [Chemical Formula 2-3]

[0158]

[0159] [Chemical Formula 2-4]

[0160]

[0161] In Chemical Formula 3, R2 and R3 are each independently a C1 to C5 alkylene group, and R4 is a C3 to C 20 alkyl group substituted with one or more fluorine atoms.

[0162] In Chemical Formula 3, R2 and R3 are each independently a C1 to C3 alkylene group, and R4 is a C3 to C 15 alkyl group substituted with one or more fluorine atoms.

[0163] Specifically, in Chemical Formula 3, R4 may be a C4 to C8 alkyl group substituted with one or more fluorine atoms.

[0164] Preferably, the compound represented by Chemical Formula 3 may be a compound represented by the following Chemical Formula 3-1.

[0165] [Chemical Formula 3-1]

[0166]

[0167] At least one of the compounds represented by Chemical Formulas 1 to 3 may be included in the non-aqueous electrolyte in an amount of 0.01% by weight to 10.0% by weight.

[0168] When the content of at least one of the compounds represented by Chemical Formulas 1 to 3 is within the above range, disadvantages such as side reactions caused by additives, capacity degradation, and increased resistance are minimized as much as possible, and the lithium transfer effect in the layer can be improved by effectively forming an inorganic SEI layer including an inorganic component (such as LiF) on the negative electrode surface. And self-discharge of the negative electrode can be prevented by suppressing additional reductive decomposition of the electrolyte.

[0169] Specifically, when the content of at least one of the compounds represented by Chemical Formulas 1 to 3 is 0.01% by weight or more, a stable film is formed during the operation of the battery, and a low-resistance SEI layer is formed on the surface of the negative electrode to improve the output performance of the battery. In addition, when the content of at least one of the compounds represented by Chemical Formulas 1 to 3 is 10.0% by weight or less, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal impregnation, an increase in battery resistance due to decomposition of the additive can be effectively suppressed, the ionic conductivity of the battery can be further increased, and thus a reduction in output characteristics can be prevented.

[0170] Specifically, at least one of the compounds represented by Chemical Formulas 1 to 3 may be included in the non-aqueous electrolyte in an amount of 0.05% by weight to 6.0% by weight, specifically 0.1% by weight to 5.0% by weight.

[0171] The non-aqueous electrolyte may further include additional additives. When the non-aqueous electrolyte further includes additional additives, at least one of the compounds represented by Chemical Formulas 1 to 3 may be referred to as the "first additive", and the additional additive may be referred to as the "second additive".

[0172] Specifically, if necessary, in addition to the compound represented by Chemical Formula 1, the additive may further include other additional additives to prevent the non-aqueous electrolyte from decomposing under high-power conditions and causing negative electrode collapse, improve low-temperature high-rate discharge characteristics and high-temperature stability, prevent overcharging, and further improve the effect of reducing battery swelling at high temperatures.

[0173] The additional additive may be at least one selected from, for example, cyclic carbonate compounds, halogenated carbonate compounds, sultone compounds, sulfate / ester compounds, phosphate / ester or phosphite / ester compounds, borate / ester compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds or lithium salt compounds.

[0174] The cyclic carbonate compounds may be, for example, vinylene carbonate (VC) or ethylene vinyl carbonate.

[0175] The halogenated carbonate compounds may be, for example, fluoroethylene carbonate (FEC).

[0176] The sultone compounds may be at least one compound selected from the group consisting of, for example, 1,3 - propane sultone (PS), 1,4 - butane sultone, ethylene sultone, 1,3 - propene sultone (PRS), 1,4 - butene sultone and 1 - methyl - 1,3 - propene sultone.

[0177] The sulfate / ester compounds may be, for example, ethylene sulfite (Esa), trimethylene sulfate (TMS) or methyl trimethylene sulfate (MTMS).

[0178] The phosphate / ester or phosphite / ester compounds may be at least one compound selected from the group consisting of, for example, lithium difluoro(oxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2 - trifluoroethyl)phosphate and tris(trifluoroethyl)phosphite.

[0179] The borate / ester compounds may be tetraphenylborate, lithium oxalodifluoroborate (LiODFB, LiB(C2O4)F2) or lithium bis(oxalato)borate (LiB(C2O4)2, LiBOB).

[0180] The nitrile compounds may be at least one compound selected from the group consisting of, for example, succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2 - fluorobenzonitrile, 4 - fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2 - fluorophenylacetonitrile and 4 - fluorophenylacetonitrile.

[0181] The benzene compounds may be, for example, fluorobenzene, the amine compounds may be triethanolamine or ethylenediamine, and the silane compounds may be tetravinylsilane.

[0182] The lithium salt compounds are compounds different from the lithium salts contained in the non - aqueous electrolyte, and may be lithium difluorophosphate (LiPO2F2) or LiBF4.

[0183] Among these additional additives, when at least one selected from vinylene ethylene carbonate, 1,3 - propane sultone, vinylene carbonate, succinonitrile, and lithium oxalyl difluoroborate is used, a more stable SEI layer can be formed on the surface of the negative electrode during the initial activation process of the secondary battery.

[0184] As the additional additive, a combination of two or more compounds can be used, and based on the total weight of the non - aqueous electrolyte, the total content of the compound represented by Chemical Formula 1 and the additional additive can be 50 wt% or less, specifically 0.05 to 20 wt%, and more specifically 0.05 to 10 wt%. When the total content of the additive satisfies the above range, the low - temperature output characteristics of the battery can be improved, the high - temperature storage characteristics and high - temperature life characteristics can be improved more effectively, and side reactions caused by the additive remaining in the battery after the reaction can be prevented.

[0185] The lithium secondary battery of the present invention described above can be usefully used in portable devices (such as mobile phones, laptop computers, and digital cameras) and electric vehicles (such as hybrid electric vehicles (HEV)).

[0186] According to another embodiment of the present invention, there are provided a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module.

[0187] The battery module or the battery pack can be used as a power source for one or more of the following medium - to - large - sized devices: power tools, electric vehicles (EV) (including hybrid electric vehicles, plug - in hybrid electric vehicles (PHEV)), or power storage systems.

[0188] The external shape of the lithium secondary battery of the present invention is not particularly limited, but can be cylindrical, prismatic, pouch - shaped, or coin - shaped using a can.

[0189] The lithium secondary battery of the present invention can be used not only in a single cell as a power source for a small device, but also preferably as a unit cell in a medium - to - large - sized battery module including a plurality of single cells.

[0190] Hereinafter, the present invention will be described in detail through examples.

[0191] 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 in detail below. The embodiments of the present invention are provided to more completely illustrate the present invention to those of ordinary skill in the art.

[0192] Hereinafter, the present invention will be described in detail through specific examples.

[0193] Examples and Comparative Examples

[0194] Example 1

[0195] (Preparation of non-aqueous electrolyte)

[0196] Ethylene fluorosulfite (FEC) and diethyl carbonate (DEC) were mixed at a volume ratio of 10:90 to prepare an organic solvent.

[0197] LiPF6 and LiFSI were used as lithium salts and dissolved in the organic solvent. LiPF6 was dissolved in the prepared non-aqueous electrolyte to a molar concentration of 1.0 M, and LiFSI was dissolved in the prepared non-aqueous electrolyte to a molar concentration of 0.5 M.

[0198] The compound represented by Chemical Formula 1-1 and vinylene carbonate were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0199] The compound represented by Chemical Formula 1-1 was included in the non-aqueous electrolyte in an amount of 2% by weight. Vinylene carbonate was included in the non-aqueous electrolyte in an amount of 2% by weight.

[0200] (Manufacture of secondary battery)

[0201] The positive electrode active material (LiNi 0.86 Co 0.05 Mn 0.07 Al 0.02 O2), a conductive material (carbon nanotubes), and a binder (polyvinylidene fluoride) were added to the solvent N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97:1:2 to prepare a positive electrode mixture slurry (solid content: 78% by weight). The positive electrode mixture slurry was coated on one side of a positive electrode current collector (Al film) having a thickness of 12 μm at a loading amount of 4.5 mAh / cm 2 , dried, and roll-pressed to prepare a positive electrode.

[0202] Si (average particle diameter (D 50 ) of 5 μm) as the negative electrode active material, nitrile butadiene rubber (NBR) as the binder, and carbon black as the conductive material were added to water as the solvent at a weight ratio of 80:10:10 to prepare a negative electrode slurry (solid content: 48% by weight). The negative electrode slurry was coated on a negative electrode current collector copper (Cu) film having a thickness of 15 μm at a loading amount of 10.7 mAh / cm 2 , dried, and roll-pressed to prepare a negative electrode.

[0203] An electrode assembly was manufactured by sequentially stacking the positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and the negative electrode.

[0204] After the manufactured electrode assembly was housed in a battery case, the prepared non-aqueous electrolyte was injected to manufacture a lithium secondary battery.

[0205] Example 2

[0206] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 1-2 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0207] Example 3

[0208] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 1-3 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0209] Example 4

[0210] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 2-1 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0211] Example 5

[0212] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 2-2 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0213] Example 6

[0214] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 2-3 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0215] Example 7

[0216] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 2-4 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0217] Example 8

[0218] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 2 wt% of the compound represented by Chemical Formula 3-1 was added to the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1-1.

[0219] Comparative Example 1

[0220] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the average particle size of the silicon-based active material (Si) used in the negative electrode was 0.05 μm (= 50 nm).

[0221] Comparative Example 2

[0222] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the average particle size of the silicon-based active material (Si) used in the negative electrode was 0.7 μm (= 700 nm).

[0223] Comparative Example 3

[0224] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the average particle size of the silicon-based active material (Si) used in the negative electrode was 40 μm.

[0225] Comparative Example 4

[0226] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) with a volume ratio of 3:97 was used as the organic solvent.

[0227] Comparative Example 5

[0228] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a mixture of fluoroethylene carbonate (FEC) and diethyl carbonate (DEC) with a volume ratio of 30:70 was used as the organic solvent.

[0229] Comparative Example 6

[0230] 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 1-1 was not included in the non-aqueous electrolyte.

[0231] Comparative Example 7

[0232] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 10:90 was used as the organic solvent.

[0233] Experimental Examples

[0234] Experimental Example 1: Evaluation of the cycle capacity retention rate at room temperature

[0235] Using an electrochemical charge and discharge device, the lithium secondary batteries fabricated in the above Examples 1 to 8 and Comparative Examples 1 to 7 were charged to 4.25 V at 0.05 C under CC / CV conditions of 0.33 C and 25 °C, and then discharged to 2.5 V under CC conditions of 0.33 C. This was regarded as one cycle, and 100 charge / discharge cycles were carried out. Then, the capacity retention rate was measured.

[0236] The capacity retention rate was calculated according to the following formula, and the results are shown in Table 1 below.

[0237] Capacity retention rate (%) = {(discharge capacity after 100 cycles / discharge capacity after 1 cycle)} × 100

[0238] Experimental Example 2: Evaluation of cycle capacity retention rate at high temperature

[0239] Using an electrochemical charge and discharge device, the lithium secondary batteries fabricated in the above Examples 1 to 8 and Comparative Examples 1 to 7 were charged to 4.25 V at 0.05 C under CC / CV conditions of 0.33 C and 45 °C, and then discharged to 2.5 V under CC conditions of 0.33 C. This was regarded as one cycle, and 100 charge / discharge cycles were carried out. Then, the capacity retention rate was measured.

[0240] The capacity retention rate was calculated according to the following formula, and the results are shown in Table 1 below.

[0241] Capacity retention rate (%) = {(discharge capacity after 100 cycles / discharge capacity after 1 cycle)} × 100

[0242] [Table 1]

[0243]

[0244]

[0245] Referring to Table 1, it can be seen that compared with Comparative Examples 1 to 6, the lithium secondary batteries of Examples 1 to 8 of the present invention exhibit excellent room temperature life performance and high temperature life performance.

[0246] Reference Examples

[0247] Reference Example 1

[0248] A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Comparative Example 1, except that a mixture of ethylene carbonate (EC) and diethyl carbonate (DEC) with a volume ratio of 10:90 was used as the organic solvent.

[0249] Reference Experimental Example 1: Evaluation of cycle capacity retention rate at room temperature

[0250] For the lithium secondary batteries of Comparative Example 1 and Reference Example 1, the 100-cycle capacity retention rate was measured in the same manner as in Experimental Example 1.

[0251] Reference Experimental Example 2: Evaluation of the cycle capacity retention rate at high temperatures

[0252] For the lithium secondary batteries of Comparative Example 1 and Reference Example 1, the 100-cycle capacity retention rate was measured in the same manner as in Experimental Example 2.

[0253] [Table 2]

[0254]

[0255] Referring to Table 2, it can be seen that Reference Example 1, which uses EC instead of FEC, has a higher cycle capacity retention rate at room temperature and high temperatures compared to Comparative Example 1. In other words, it can be seen that when the average particle size (D 50 ) of the silicon-based active material (Si) is less than 1 μm, using FEC as an organic solvent has a negative impact on the performance of the lithium secondary battery.

Claims

1. A lithium secondary battery, which includes a negative electrode, a positive electrode, a separator, and a non-aqueous electrolyte, wherein, The negative electrode includes a negative electrode active material, the negative electrode active material includes a silicon-based active material, and the average particle size D of the silicon-based active material 50 is from 1 μm to 20 μm, The non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive. The organic solvent contains fluoroethylene carbonate and diethyl carbonate in a volume ratio of 5:95 to 25:

75. The additive contains at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1 to 3. [Chemical Formula 1] In Chemical Formula 1, R1 is hydrogen or a C1-C3 alkyl group, and n is an integer from 1 to 10. [Chemical Formula 2] In Chemical Formula 2, m is an integer from 0 to 18. [Chemical Formula 3] In Chemical Formula 3, R2 and R3 are each independently a C1 to C 10 alkylene group, and R4 is a C1 to C alkyl group substituted with one or more fluorine atoms. 20 ​ 2. The lithium secondary battery according to claim 1, wherein In Chemical Formula 1, n is an integer from 3 to 7.

3. The lithium secondary battery according to claim 1, wherein, In Chemical Formula 1, R1 is hydrogen.

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 1-3: [Chemical Formula 1-1] [Chemical Formula 1-2] [Chemical Formula 1-3] 5. The lithium secondary battery according to claim 1, wherein In Chemical Formula 2, m is an integer from 2 to 8.

6. The lithium secondary battery according to claim 1, wherein, The compound represented by Chemical Formula 2 includes at least one selected from the group consisting of compounds represented by the following Chemical Formulas 2-1 to 2-4: [Chemical Formula 2-1] [Chemical Formula 2-2] [Chemical Formula 2-3] [Chemical Formula 2-4] 7. The lithium secondary battery according to claim 1, wherein, In Chemical Formula 3, R2 and R3 are each independently a C1 to C5 alkylene group, and R4 is a C3 to C alkyl group substituted with one or more fluorine atoms. 20 alkyl group.

8. The lithium secondary battery according to claim 1, wherein, In Chemical Formula 3, R4 is a C4-C8 alkyl group substituted with one or more fluorine atoms.

9. The lithium secondary battery according to claim 1, wherein, The compound represented by Chemical Formula 3 includes the compound represented by the following Chemical Formula 3-1: [Chemical Formula 3-1] 10. The lithium secondary battery according to claim 1, wherein, At least one selected from the group consisting of compounds represented by Chemical Formulas 1 to 3 is contained in the non-aqueous electrolyte in an amount of 0.01 wt% to 10 wt%.

11. The lithium secondary battery according to claim 1, wherein, The lithium salt includes at least one selected from the group consisting of LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), and LiBETI (LiN(SO2CF2CF3)2).

12. The lithium secondary battery according to claim 1, wherein, The lithium salt is contained in the non-aqueous electrolyte at a concentration of 0.8 M to 3.0 M.

13. The lithium secondary battery according to claim 1, wherein, The silicon-based active material includes the compound represented by the following Chemical Formula A: [Chemical Formula A] SiOx, where 0 ≤ x < 2.

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

Images