Electrolyte for lithium secondary battery and lithium secondary battery including the same

Abstract

To provide a lithium secondary battery that suppresses increase in battery thickness and decrease in battery life even when a negative electrode is made denser.SOLUTION: With regard to the electrolyte for a lithium secondary battery and a lithium secondary battery including the same, the electrolyte for a lithium secondary battery includes a lithium salt, a non-aqueous organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2. (A detailed explanation of chemical formula 1 and chemical formula 2 is as described in the specification.)SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. [Background technology]

[0002] Recently, the demand for high-energy-density, high-capacity secondary batteries has been growing rapidly with the rapid spread of electronic devices that use batteries, such as mobile phones, laptops, and electric vehicles. This has led to active research and development into improving the performance of lithium secondary batteries.

[0003] A lithium secondary battery is a battery that includes a positive electrode, a negative electrode, and an electrolyte, each of which includes an active material capable of intercalating and deintercalating lithium ions, and produces electrical energy through oxidation and reduction reactions that occur when lithium ions are intercalated / deintercalated from the positive electrode and the negative electrode.

[0004] Recently, one of the development trends of lithium secondary batteries is to densify the negative electrode, but densifying the negative electrode reduces the void volume and increases the amount of electrolyte impregnated in the negative electrode, which increases the thickness of the battery and shortens its lifespan. [Prior art documents] [Patent documents]

[0005] [Patent Document 1] Korean Patent Publication No. 10-2019-0058134 Summary of the Invention [Problem to be solved by the invention]

[0006] An embodiment of the present invention provides an electrolyte for a lithium secondary battery that suppresses an increase in thickness and a decrease in lifespan of the battery even when the negative electrode is densified.

[0007] An embodiment of the present invention provides a lithium secondary battery including the above-mentioned electrolyte for lithium secondary batteries. [Means for solving the problem]

[0008] An electrolyte for a lithium secondary battery according to one embodiment of the present invention includes a lithium salt, a non-aqueous organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2.

[0009] [Chemical formula 1] [ka]

[0010] [Chemical formula 2] [ka]

[0011] A lithium secondary battery according to an embodiment of the present invention includes a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and the above-mentioned electrolyte for lithium secondary batteries. Effect of the Invention

[0012] The electrolyte for a lithium secondary battery according to an embodiment of the present invention can suppress an increase in thickness and a decrease in lifespan of a battery even if the negative electrode is densified. [Brief description of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram showing a lithium secondary battery according to an embodiment. [Diagram 2] FIG. 1 is a schematic diagram showing a lithium secondary battery according to an embodiment. [Diagram 3] FIG. 1 is a schematic diagram showing a lithium secondary battery according to an embodiment. [Figure 4] FIG. 1 is a schematic diagram showing a lithium secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0014] DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Hereinafter, preferred embodiments of the present invention will be described in detail, but they are given by way of example only and are not intended to limit the present invention, which is defined only by the scope of the following claims.

[0015] Unless otherwise specified in this specification, when a part such as a layer, film, region, plate, etc. is said to be "on" another part, this includes not only the case where it is "directly on" the other part, but also the case where there is another part in between.

[0016] Unless otherwise specified herein, the singular can also include the plural. At the same time, unless otherwise specified, "A or B" can mean "including A, including B, or including A and B."

[0017] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0018] In this specification, the "mixture density of the negative electrode" is a value calculated by dividing the weight of the components (active material, conductive material, binder, etc.) excluding the current collector in the negative electrode by the volume.

[0019] Unless otherwise defined in the chemical formulas herein, when no chemical bond is drawn at a position where a chemical bond should be drawn, this means that a hydrogen atom is bonded at that position.

[0020] As used herein, the term "fluoroalkyl group" refers to an alkyl group in which some or all of the hydrogen atoms have been substituted with fluorine atoms.

[0021] As used herein, the term "perfluoroalkyl group" refers to an alkyl group in which all of the hydrogen atoms have been replaced with fluorine atoms.

[0022] (Electrolyte for lithium secondary batteries) One embodiment provides an electrolyte solution for a lithium secondary battery (hereinafter, simply referred to as "electrolyte solution"), comprising a lithium salt, a non-aqueous organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2.

[0023] [Chemical formula 1] [ka]

[0024] [Chemical formula 2] [ka]

[0025] The first additive and the second additive each function as a surfactant having both a hydrophilic group and a hydrophobic group in one molecule.

[0026] The first additive contains an ether group at the center and, on both sides of the ether group, a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms. Here, the ether group is a hydrophilic group, and the fluorine atom or the fluoroalkyl group having 1 to 10 carbon atoms is a hydrophobic group.

[0027] The second additive contains an ether group at the center and a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms on both sides of the ether group. Here, the ether group is a hydrophilic group, and the fluorine atom, the alkyl group having 1 to 10 carbon atoms, or the fluoroalkyl group having 1 to 10 carbon atoms is a hydrophobic group.

[0028] Note that Chemical Formula 1 and Chemical Formula 2 are different.

[0029] Therefore, when an electrolyte solution containing both the first and second additives is used, the wettability of the positive and negative electrodes is improved, and the lithium cations (Li + ) is formed uniformly, and a stable SEI film is formed at the interface between the negative electrode and the electrolyte, while the precipitation of lithium dendrites is suppressed.

[0030] Therefore, by using an electrolyte solution containing both the first additive and the second additive, it is possible to suppress an increase in the thickness of the battery and a decrease in its lifespan even if the negative electrode is densified.

[0031] Hereinafter, the electrolyte for a lithium secondary battery according to an embodiment will be described in more detail.

[0032] (First additive) In chemical formula 1, R 1 and R 2 are each independently a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms.

[0033] For example, R 1 may be a fluoroalkyl group having 2 carbon atoms.

[0034] Also, R 2 may be a fluoroalkyl group having 3 carbon atoms.

[0035] The first additive is represented by the following chemical formula 1-1.

[0036] [Chemical formula 1-1] [ka]

[0037] In chemical formula 1-1, R 11 ~R 15 are each a hydrogen atom or a fluorine atom, and R 11 ~R 15 At least one of R is a fluorine atom. 21 ~R 27 are each a hydrogen atom or a fluorine atom, and R 21 ~R 27 At least one of the groups is a fluorine atom.

[0038] A representative example of the first additive is represented by the following chemical formula 1-1-1.

[0039] [Chemical formula 1-1-1] [ka] .

[0040] (Second additive) In chemical formula 2, R 3 and R 4 each independently represents a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms.

[0041] For example, R 3 may be a fluoroalkyl or perfluoroalkyl group having 3 carbon atoms.

[0042] Also, R 4 may be an alkyl group or a fluoroalkyl group having one carbon atom.

[0043] The second additive is represented by the following chemical formula 2-1.

[0044] [Chemical formula 2-1] [ka]

[0045] In chemical formula 2-1, R 31 ~R 37 are each a hydrogen atom or a fluorine atom, and R 31 ~R 37 At least one of R is a fluorine atom. 41 ~R 43 are each a hydrogen atom or a fluorine atom.

[0046] A representative example of the second additive is represented by the following chemical formula 2-1-1.

[0047] [Chemical formula 2-1-1] [ka] .

[0048] (Content of the first additive and the second additive) The first additive may be included in an amount of 1 to 10% by weight, specifically 1 to 5% by weight, and more specifically 1 to 3% by weight, based on the total amount of the electrolyte.

[0049] If the first additive is contained in an excessive amount exceeding the above range, the viscosity of the electrolyte containing the first additive may be excessively increased, and the wettability to the positive and negative electrodes may be decreased. On the other hand, if the content of the first additive is small below the above range, the effect as a surfactant may be insignificant.

[0050] The second additive may be included in an amount of 1 to 10% by weight, specifically 1 to 5% by weight, and more specifically 1 to 3% by weight, based on the total amount of the electrolyte.

[0051] If the second additive is contained in an excessive amount exceeding the above range, the viscosity of the electrolyte containing the second additive may be excessively increased, and the wettability to the positive and negative electrodes may be decreased. On the other hand, if the content of the second additive is small below the above range, the effect as a surfactant may be insignificant.

[0052] The weight ratio of the first additive to the second additive may be from 10:1 to 1:10, specifically from 5:1 to 1:5, and more specifically from 3:1 to 1:3.

[0053] Within the above content range, the effects of the first additive and the second additive can be balanced.

[0054] (Non-aqueous organic solvent) The non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can migrate.

[0055] The non-aqueous organic solvent may be a carbonate, ester, ether, ketone, or alcohol solvent, an aprotic solvent, or a combination thereof.

[0056] Examples of the carbonate solvent include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. Examples of ether solvents that can be used include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, and tetrahydrofuran. Examples of ketone solvents that can be used include cyclohexanone. Examples of alcohol solvents that can be used include ethyl alcohol and isopropyl alcohol. Examples of aprotic solvents that can be used include nitriles such as R-CN (R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain a double bond, an aromatic ring, or an ether group), amides such as dimethylformamide, dioxolanes such as 1,3-dioxolane and 1,4-dioxolane, and sulfolanes.

[0057] The non-aqueous organic solvents can be used alone or in combination of two or more kinds.

[0058] When used in a mixture of two or more, the non-aqueous organic solvent may include a carbonate-based solvent and a propionate-based solvent.

[0059] The total amount of the propionate-based solvent may be 70% by volume or more based on the total amount of the non-aqueous organic solvent, which can improve the high voltage and / or high temperature characteristics of the lithium secondary battery.

[0060] For example, the non-aqueous organic solvent may be a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP) and propyl propionate (PP).

[0061] (Lithium salts) Lithium salts are substances that dissolve in organic solvents and act as a source of lithium ions within the battery to enable basic lithium secondary battery operation and to facilitate the movement of lithium ions between the positive and negative electrodes.

[0062] As the lithium salt, LiPF6 can be used.

[0063] The concentration of the lithium salt may be from 0.1M to 2.0M.

[0064] (lithium secondary battery) One embodiment provides a lithium secondary battery including a positive electrode including a positive electrode active material, a negative electrode including a negative electrode active material, and the above-described lithium secondary battery electrolyte.

[0065] (Negative electrode mix density) Generally known lithium secondary batteries use a negative electrode with a composite density of less than 1.68 g / cc, but the lithium secondary battery according to one embodiment uses a negative electrode with a composite density of 1.68 g / cc or more.

[0066] The upper limit for the negative electrode mix density is not particularly limited, but may be 2.0 g / cc or less, 1.9 g / cc or less, or 1.8 g / cc or less.

[0067] (Upper charging voltage) The lithium secondary battery according to the embodiment includes the electrolyte for a lithium secondary battery according to the embodiment described above, so that an increase in thickness and a decrease in lifespan can be suppressed even if the negative electrode is densified.

[0068] For example, the lithium secondary battery may have an upper limit charging voltage of 4.4 V or higher.

[0069] Hereinafter, the description overlapping with the above description will be omitted, and a lithium secondary battery according to an embodiment will be described in more detail.

[0070] (Cathode active material) The positive electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium (lithiated intercalation compound). Specifically, at least one of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0071] The composite oxide may be a lithium transition metal composite oxide, and specific examples include lithium nickel-based oxides, lithium cobalt-based oxides, lithium manganese-based oxides, lithium iron phosphate-based compounds, cobalt-free nickel-manganese-based oxides, or combinations thereof.

[0072] As an example, Li a A 1-b X b O 2-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05), Li a Mn 2-b X b O 4-c D c (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.05), Li a Ni 1-b-c Co b X c O 2-α D α(0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2), Li a Ni 1-b-c Mn b X c O 2-α D α (0.90≦a≦1.8, 0≦b≦0.5, 0≦c≦0.5, 0<α<2), Li a Ni b Co c L 1 d G e O2 (0.90≦a≦1.8, 0≦b≦0.9, 0≦c≦0.5, 0≦d≦0.5, 0≦e≦0.1), Li a NiG b O2 (0.90≦a≦1.8, 0.001≦b≦0.1), Li a CoG b O2 (0.90≦a≦1.8, 0.001≦b≦0.1), Li a Mn 1-b G b O2 (0.90≦a≦1.8, 0.001≦b≦0.1), Li a Mn2G b O4 (0.90≦a≦1.8, 0.001≦b≦0.1), Li a Mn 1-g G g PO4(0.90≦a≦1.8, 0≦g≦0.5), Li (3-f) Fe2(PO4)3(0≦f≦2), Li a Any one of the compounds represented by the chemical formula FePO4 (0.90≦a≦1.8) can be used.

[0073] In the above chemical formula, A is Ni, Co, Mn, or a combination thereof; X is Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, a rare earth element, or a combination thereof; D is O, F, S, P, or a combination thereof; G is Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, or a combination thereof; L 1 is Mn, Al or a combination thereof.

[0074] As an example, the positive electrode active material may be a high-nickel positive electrode active material in which the nickel content relative to 100 mol% of metals excluding lithium in a lithium transition metal composite oxide is 80 mol% or more, 85 mol% or more, 90 mol% or more, 91 mol% or more, or 94 mol% or more and 99 mol% or less. The high-nickel positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0075] The positive electrode active material may include, for example, a lithium nickel-based oxide represented by the following Chemical Formula 11, a lithium cobalt-based oxide represented by the following Chemical Formula 12, a lithium iron phosphate-based compound represented by the following Chemical Formula 13, a cobalt-free lithium nickel-manganese-based oxide represented by the following Chemical Formula 14, or a combination thereof.

[0076] [Chemical formula 11] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1

[0077] In Chemical Formula 11, 0.9≦a1≦1.8, 0.3≦x1≦1, 0≦y1≦0.7, 0≦z1≦0.7, 0.9≦x1+y1+z1≦1.1, and 0≦b1≦0.1; M 1 and M. 2 are each independently one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Fe, Mg, Mn, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0078] In Chemical Formula 11, 0.6≦x1≦1, 0≦y1≦0.4, and 0≦z1≦0.4, or 0.8≦x1≦1, 0≦y1≦0.2, and 0≦z1≦0.2.

[0079] [Chemical formula 12] Li a2Co x2 M 3 y2 O 2-b2 X b2

[0080] In Chemical Formula 12, 0.9≦a2≦1.8, 0.7≦x2≦1, 0≦y2≦0.3, 0.9≦x2+y2≦1.1, and 0≦b2≦0.1; M 3 is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Cr, Cu, Fe, Mg, Mn, Mo, Ni, Se, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0081] [Chemical formula 13] Li a3 Fe x3 M 4 y3 PO 4-b3 X b3

[0082] In Chemical Formula 13, 0.9≦a3≦1.8, 0.6≦x3≦1, 0≦y3≦0.4, and 0≦b3≦0.1; M 4 is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Co, Cr, Cu, Mg, Mn, Mo, Ni, Se, Si, Sn, Sr, Ti, V, W, Y, Zn, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0083] [Chemical formula 14] Li a4 Ni x4 Mn y4 M 5 z4 O 2-b4 X b4

[0084] In chemical formula 14, 0.9≦a2≦1.8, 0.8≦x4<1, 0 <y4≦0.2、0≦z4≦0.2、0.9≦x4+y4+z4≦1.1、および0≦b4≦0.1であり、M 5is one or more elements selected from the group consisting of Al, B, Ba, Ca, Ce, Cr, Fe, Mg, Mo, Nb, Si, Sn, Sr, Ti, V, W, and Zr, and X is one or more elements selected from the group consisting of F, P, and S.

[0085] In particular, the electrolyte for a lithium ion secondary battery according to the above embodiment can significantly improve the high voltage and / or high temperature characteristics of a battery using the lithium cobalt-based oxide represented by Chemical Formula 12.

[0086] (positive electrode) The positive electrode for a lithium secondary battery may include a current collector and a positive electrode active material layer formed on the current collector. The positive electrode active material layer includes a positive electrode active material, and may further include a binder and / or a conductive material.

[0087] As an example, the positive electrode can further include an additive that can act as a sacrificial positive electrode.

[0088] The content of the positive electrode active material may be 90% by weight to 99.5% by weight relative to 100% by weight of the positive electrode active material layer, and the contents of the binder and conductive material may each be 0.5% by weight to 5% by weight relative to 100% by weight of the positive electrode active material layer.

[0089] The binder serves to make the positive electrode active material particles adhere well to each other and to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0090] The conductive material is used to impart electrical conductivity to the electrodes, and any material that does not cause chemical changes in the battery that is constructed and is electronically conductive can be used. Examples of the conductive material include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, carbon nanofibers, and carbon nanotubes, metal-based materials in the form of metal powders or metal fibers containing copper, nickel, aluminum, or silver, conductive polymers such as polyphenylene derivatives, or mixtures thereof.

[0091] The current collector may be made of Al, but is not limited to this.

[0092] (Negative electrode active material) The negative electrode active material includes a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, an alloy of lithium metal, a material capable of being doped and dedoped with lithium, or a transition metal oxide.

[0093] The material capable of reversibly intercalating / deintercalating lithium ions may be a carbon-based negative electrode active material, and may include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, and examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, or calcined coke.

[0094] As the alloy of lithium metal, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn can be used.

[0095] As the substance that can be doped and de-doped with lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material can be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (Q is selected from alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements (excluding Si), Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof), or a combination thereof. The Sn-based negative electrode active material may be Sn, SnO2, a Sn-based alloy, or a combination thereof.

[0096] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in a form in which silicon particles are coated with amorphous carbon on the surface of the silicon particles. For example, it may include secondary particles (cores) formed by granulating primary silicon particles and an amorphous carbon coating layer (shell) located on the surface of the secondary particles. The amorphous carbon may also be located between the primary silicon particles. For example, the primary silicon particles may be coated with amorphous carbon. The secondary particles may be dispersed and present in an amorphous carbon matrix.

[0097] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core containing crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of the core.

[0098] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used by mixing with a carbon-based negative electrode active material.

[0099] (Negative electrode) The negative electrode for a lithium secondary battery includes a current collector and a negative electrode active material layer located on the current collector. The negative electrode active material layer contains a negative electrode active material and may further contain a binder and / or a conductive material.

[0100] For example, the negative electrode active material layer can contain 90% to 99% by weight of the negative electrode active material, 0.5% to 5% by weight of the binder, and 0% to 5% by weight of the conductive material.

[0101] The binder serves to adhere the negative electrode active material particles to each other and to the current collector well. The binder may be a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof.

[0102] Non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymers, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0103] The aqueous binder may be selected from styrene-butadiene rubber, (meth)acrylated styrene-butadiene rubber, (meth)acrylonitrile-butadiene rubber, (meth)acrylic rubber, butyl rubber, fluororubber, polyethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, poly(meth)acrylonitrile, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0104] When an aqueous binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included. As the cellulose-based compound, one or more of carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof may be mixed and used. As the alkali metal, Na, K, or Li may be used.

[0105] The dry binder may be a polymeric material capable of being fiberized, such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0106] The conductive material is used to impart electrical conductivity to the electrodes, and any material that does not cause chemical changes in the battery that is constructed and is electronically conductive can be used. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fibers, carbon nanofibers, and carbon nanotubes, metal-based materials in the form of metal powder or metal fibers including copper, nickel, aluminum, or silver, conductive polymers such as polyphenylene derivatives, or mixtures thereof.

[0107] The negative electrode current collector may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0108] (Separator) Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. As such a separator, polyethylene, polypropylene, polyvinylidene fluoride, or a multi-layer film of two or more layers of these may be used, and it is of course possible to use a mixed multi-layer film such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator.

[0109] The separator can include a porous substrate and a coating layer comprising an organic material, an inorganic material, or a combination thereof located on one or both sides of the porous substrate.

[0110] The porous substrate may be a polymer membrane formed from any one polymer selected from polyolefins such as polyethylene and polypropylene, polyesters such as polyethylene terephthalate and polybutylene terephthalate, polyacetal, polyamide, polyimide, polycarbonate, polyether ketone, polyaryl ether ketone, polyetherimide, polyamide imide, polybenzimidazole, polyethersulfone, polyphenylene oxide, cyclic olefin copolymer, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon (registered trademark), and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0111] The organic material may include a polyvinylidene fluoride-based polymer or a (meth)acrylic-based polymer.

[0112] The inorganics can include, but are not limited to, inorganic particles selected from Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0113] The organic material and the inorganic material may be mixed in one coating layer, or a coating layer containing an organic material and a coating layer containing an inorganic material may be laminated.

[0114] (lithium secondary battery) Lithium secondary batteries can be classified into cylindrical, square, pouch, coin, and other types according to their shapes. FIGS. 1 to 4 are schematic diagrams showing a lithium secondary battery according to one embodiment, in which FIG. 1 shows a cylindrical battery, FIG. 2 shows a square battery, and FIGS. 3 and 4 show a pouch battery. Referring to FIGS. 1 to 4, the lithium secondary battery 100 can include an electrode assembly 40 between a positive electrode 10 and a negative electrode 20, with a separator 30 interposed therebetween, and a case 50 in which the electrode assembly 40 is built in. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated with an electrolyte (not shown). As shown in FIG. 1, the lithium secondary battery 100 can include a sealing member 60 that seals the case 50. In addition, in FIG. 2, the lithium secondary battery 100 can include a positive electrode lead tab 11, a positive electrode terminal 12, a negative electrode lead tab 21, and a negative electrode terminal 22. As shown in FIGS. 3 and 4, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0115] The lithium secondary battery according to an embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electric devices, but the present invention is not limited thereto.

[0116] Examples and comparative examples of the present invention will be described below. However, the examples described below are merely examples of the present invention, and the present invention is not limited to the examples described below. EXAMPLES

[0117] Examples and Comparative Examples An electrolyte solution and a lithium secondary battery were produced in the following manner.

[0118] <Example 1> (1) Electrolyte production An electrolyte solution was prepared by dissolving 1.3 M LiPF6 in a non-aqueous organic solvent in which ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propyl propionate (PP) were mixed in a volume ratio of 10:15:30:45, and adding 1 wt % of the first additive and 1 wt % of the second additive.

[0119] The first additive was represented by the following chemical formula 1-1-1, and the second additive was represented by the following chemical formula 2-1-1.

[0120] [Chemical formula 1-1-1] [ka] . 1,1,2,2-Tetrafluoroethyl2,2,3,3-tetrafluoropropylether(CAS No.:16627-68-2)

[0121] [Chemical formula 2-1-1] [ka] . 1,1,1,2,2,3,3-Heptafluoro-3-methoxypropane (CAS No.:375-03-1)

[0122] (2) Manufacture of lithium secondary batteries LiCoO2 as a positive electrode active material, polyvinylidene fluoride as a binder, and acetylene black as a conductive material were mixed in a weight ratio of 96:3:1, and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0123] The positive electrode active material slurry was coated on an aluminum foil having a thickness of 15 μm, dried at 100° C., and pressed to prepare a positive electrode.

[0124] Artificial graphite was used as the negative electrode active material, and the negative electrode active material was mixed with a styrene-butadiene rubber binder and carboxymethyl cellulose in a weight ratio of 98:1:1, respectively, and dispersed in distilled water to prepare a negative electrode active material slurry.

[0125] The negative electrode active material slurry was coated on a copper foil having a thickness of 10 μm, dried at 100° C., and pressed to prepare a negative electrode, so that the density of the negative electrode mixture was 1.68 g / cc.

[0126] The prepared positive and negative electrodes were assembled with a 10 μm thick polyethylene separator to prepare an electrode assembly, and an electrolyte was injected to prepare a lithium secondary battery.

[0127] <Example 2> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 1 wt % of the first additive and 3 wt % of the second additive were added during the preparation of the electrolyte.

[0128] <Example 3> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 1 wt % of the first additive and 5 wt % of the second additive were added during the preparation of the electrolyte.

[0129] <Example 4> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 3 wt % of the first additive and 3 wt % of the second additive were added during the preparation of the electrolyte.

[0130] <Example 5> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 5 wt % of the first additive and 5 wt % of the second additive were added during the preparation of the electrolyte.

[0131] <Example 6> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the density of the mixture was set to 1.75 g / cc when manufacturing the negative electrode.

[0132] <Example 7> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the negative electrode was formed so that the mixture density was 1.8 g / cc.

[0133] <Comparative Example 1> An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that no additive was added during preparation of the electrolyte.

[0134] <Comparative Example 2> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the second additive was not added and 1 wt % of the first additive was added during the preparation of the electrolyte.

[0135] <Comparative Example 3> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the second additive was not added and 3 wt % of the first additive was added during the preparation of the electrolyte.

[0136] <Comparative Example 4> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the first additive was not added and 1 wt % of the second additive was added during the preparation of the electrolyte.

[0137] <Comparative Example 5> An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the first additive was not added and 3 wt % of the second additive was added during the preparation of the electrolyte.

[0138] (Evaluation example) The negative electrode and the lithium secondary battery were evaluated by the following methods.

[0139] <Evaluation 1: Impregnation of electrolyte into negative electrode> The negative electrode according to Example 1 was prepared into a test piece measuring 3 cm x 5 cm in length. 1 g of the electrolyte according to Example 1 was dropped onto the test piece and left for 20 minutes. After that, the amount of electrolyte immersed in the test piece out of 100% by weight of the electrolyte dropped onto the test piece was evaluated into a value of 0 to 5 according to the following criteria, and the evaluation results are shown in Table 1 below. 0: When the amount of electrolyte immersed in the test piece is 0% by weight or more and less than 10% by weight 1: When the amount of electrolyte immersed in the test piece is 10% by weight or more but less than 20% by weight 2: When the amount of electrolyte immersed in the test piece is 20% by weight or more but less than 40% by weight 3: When the amount of electrolyte immersed in the test piece is 40% by weight or more but less than 60% by weight 4: When the amount of electrolyte immersed in the test piece is 60% by weight or more but less than 80% by weight 5: When the amount of electrolyte immersed in the test piece is 80% by weight or more but less than 100% by weight

[0140] Examples 2 to 7 and Comparative Examples 1 to 5 were also evaluated in the same manner, and the evaluation results are shown in Table 1 below.

[0141] [Table 1]

[0142] <Evaluation 2: Evaluation of charge / discharge cycle characteristics at room temperature> The lithium secondary battery was subjected to 200 charge / discharge cycles at 25°C and 2.0C charge (CC / CV, 4.47V, 0.025C cut-off) / 1.0C discharge (CC, 3V cut-off).

[0143] The thickness increase rate was calculated according to the following formula 1, and the capacity retention rate was calculated according to the following formula 2. The results are shown in Table 2 below.

[0144] [Formula 1] Thickness increase rate = {(n th (fully charged thickness after 1 cycle)-(fully charged thickness after 1 cycle) / (fully charged thickness after 1 cycle)×100

[0145] In Equation 1, "fully charged thickness" means the thickness of a lithium secondary battery measured after charging to SOC 100% (a state in which the battery is charged to 100% charge capacity when the entire battery charge capacity is set to 100%) after each cycle.

[0146] [Formula 2] Capacity retention rate=(n th Discharge capacity after cycle / Discharge capacity after 1 cycle)×100

[0147] [Table 2]

[0148] (Overall) Referring to Tables 1 and 2, when an electrolyte solution without any additive was used (Comparative Example 1), the thickness of the battery including the high-density negative electrode increased and the lifespan dropped sharply.

[0149] When comparing cases where the negative electrode mix density was the same at 1.68 g / cc, when an electrolyte solution containing both the first and second additives was used (Examples 1 to 5), the impregnation of the electrolyte into the negative electrode was improved, the thickness of the battery was reduced, and the lifespan was increased, compared to when an electrolyte solution without any additives was used (Comparative Example 3).

[0150] On the other hand, when an electrolyte solution containing only one of the first additive and the second additive is used (Comparative Examples 2 and 3), the impregnation characteristics of the electrolyte solution at the negative electrode deteriorate, and as the amount of free liquid increases, the battery life drops sharply to a degree comparable to that of an electrolyte solution containing no additive at all (Comparative Example 1). Therefore, it is necessary to use an electrolyte solution containing both the first additive and the second additive.

[0151] Furthermore, when an electrolyte solution containing both the first and second additives is used, even if the negative electrode mix density is increased from 1.68 g / cc to 1.8 g / cc (Examples 6 and 7), the increase in battery thickness and the decrease in battery life are suppressed.

[0152] Although the above describes a preferred embodiment of the present invention, the present invention is not limited thereto, and various modifications and variations can be made within the scope of the claims, the detailed description of the invention, and the accompanying drawings, which of course also fall within the scope of the present invention. [Explanation of symbols]

[0153] 100 Lithium secondary battery 10 positive electrode 11 Positive electrode lead tab 12 Positive terminal 20 negative electrode 21 Negative electrode lead tab 22 Negative terminal 30 Separator 40 Electrode assembly 50 cases 60 Sealing material 70 Electrode tab 71 Positive tab 72 Negative electrode tab

Claims

1. A lithium salt; A non-aqueous organic solvent; A first additive represented by the following chemical formula 1; An electrolyte for a lithium secondary battery comprising a second additive represented by the following chemical formula 2: [Chemical formula 1] 【Chemistry 1】 In the above Chemical Formula 1, R １ and R ２ each independently represents a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms, [Chemical formula 2] 【Chemistry 2】 In the above Chemical Formula 2, R ３ and R ４ each independently represents a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms, The formula 1 and the formula 2 are different.

2. The electrolyte for a lithium secondary battery according to claim 1, wherein the first additive is represented by the following formula 1-1: [Chemical formula 1-1] 【Chemistry 3】 In the above Chemical Formula 1-1, R １１ ~R １５ are each a hydrogen atom or a fluorine atom, １１ ~R １５ At least one of the groups is a fluorine atom, R ２１ ~R ２７ are each a hydrogen atom or a fluorine atom, ２１ ~R ２７ At least one of the groups is a fluorine atom.

3. The electrolyte for a lithium secondary battery according to claim 1, wherein the first additive is represented by the following chemical formula 1-1-1: [Chemical formula 1-1-1] 【Chemistry 4】 。

4. The electrolyte for a lithium secondary battery according to claim 1, wherein the second additive is represented by the following formula 2-1: [Chemical formula 2-1] 【Chemistry 5】 In the above Chemical Formula 2-1, R ３１ ~R ３７ are each a hydrogen atom or a fluorine atom, ３１ ~R ３７ At least one of the groups is a fluorine atom, R ４１ ~R ４３ are each a hydrogen atom or a fluorine atom.

5. The electrolyte for a lithium secondary battery according to claim 1, wherein the second additive is represented by the following chemical formula 2-1-1: [Chemical formula 2-1-1] 【Chemistry 6】 。

6. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the first additive is contained in an amount of 1 to 10 wt % based on the total amount of the electrolyte for a lithium secondary battery.

7. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the second additive is contained in an amount of 1 to 10 wt % based on the total amount of the electrolyte for a lithium secondary battery.

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

10.

9. 2. The electrolyte for a lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent comprises a carbonate-based solvent and a propionate-based solvent.

10. 10. The electrolyte solution for a lithium secondary battery according to claim 9, wherein the propionate-based solvent is contained in an amount of 70% by volume or more based on the total amount of the non-aqueous organic solvent.

11. The lithium salt is LiPF ６ The electrolyte for a lithium secondary battery according to claim 1,

12. 2. The electrolyte for lithium secondary batteries according to claim 1, wherein the concentration of the lithium salt is 0.1M to 2.0M.

13. a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; A lithium secondary battery comprising the electrolyte solution for lithium secondary batteries according to any one of claims 1 to 12.

14. 14. The lithium secondary battery according to claim 13, wherein the negative electrode has a mix density of 1.68 g / cc or more.

15. 14. The lithium secondary battery according to claim 13, wherein the positive electrode active material comprises a lithium nickel-based oxide, a lithium cobalt-based oxide, a lithium manganese-based oxide, a lithium iron phosphate-based compound, a cobalt-free nickel-manganese-based oxide, or a combination thereof.

16. 14. The lithium secondary battery of claim 13, wherein the negative electrode active material comprises a carbon-based negative electrode active material, a Si-based negative electrode active material, or a combination thereof.

17. The lithium secondary battery according to claim 13, wherein the upper limit charging voltage of the lithium secondary battery is 4.4 V or more.

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