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

Abstract

The present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery comprising the same, wherein the electrolyte for the lithium secondary battery comprises 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: [Chemical Formula 1] [Chemical Formula 2]. (The description of each of the above chemical formulas is in accordance with the specification.)

Description

Technology Field

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

[0003] With the recent rapid proliferation of battery-powered electronic devices such as mobile phones, laptop computers, and electric vehicles, the demand for high-energy-density, high-capacity rechargeable batteries is increasing rapidly. Accordingly, research and development to improve the performance of lithium-ion batteries is actively underway.

[0004] A lithium secondary battery is a battery comprising a positive electrode and a negative electrode containing an active material capable of lithium ion intercalation and deintercalation, and an electrolyte, and produces electrical energy through oxidation and reduction reactions when lithium ions are intercalated / deintercalated from the positive electrode and the negative electrode.

[0005] One of the recent development directions for lithium-ion batteries is to increase the density of the anode. However, increasing the density of the anode reduces the void volume, which increases the amount of electrolyte impregnated into the anode, leading to increased battery thickness and reduced lifespan. The problem to be solved

[0007] One embodiment provides an electrolyte for a lithium secondary battery that suppresses an increase in battery thickness and a decrease in lifespan even when the negative electrode is densified.

[0008] Another embodiment provides a lithium secondary battery comprising the above-mentioned electrolyte for the lithium secondary battery. means of solving the problem

[0010] One embodiment provides an electrolyte for a lithium secondary battery 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:

[0011] [Chemical Formula 1]

[0012]

[0013] [Chemical Formula 2]

[0014] .

[0015] Another embodiment provides a lithium secondary battery comprising a positive electrode including a positive active material; a negative electrode including a negative active material; and the electrolyte. Effects of the invention

[0017] An electrolyte for a lithium secondary battery according to one embodiment can suppress an increase in battery thickness and a decrease in lifespan even when the negative electrode is made denser. Brief explanation of the drawing

[0019] FIGS. 1 to 4 are schematic diagrams illustrating a lithium secondary battery according to one embodiment. Specific details for implementing the invention

[0020] Hereinafter, embodiments of the present invention will be described in detail. However, these are presented as examples and are not intended to limit the present invention, and the present invention is defined only by the scope of the claims set forth below.

[0021] Unless otherwise specifically stated in this specification, when a part such as a layer, film, region, plate, etc. is described as being "on" another part, this includes not only cases where it is "immediately on" another part, but also cases where there is another part in between.

[0022] Unless otherwise specified in this specification, a singular form may also include a plural form. Additionally, unless otherwise specified, "A or B" may mean "including A, including B, or including A and B."

[0023] In this specification, "combination of these" may mean a mixture of components, a laminate, a composite, a copolymer, an alloy, a blend, and a reaction product, etc.

[0024] In this specification, "density of the cathode composition" refers to a value calculated by dividing the weight of the components (active material, conductive material, binder, etc.) excluding the current collector from the cathode by the volume.

[0025] Unless otherwise defined in the chemical formulas within this specification, if a chemical bond is not drawn at a position where a chemical bond should be drawn, it means that a hydrogen atom is bonded at said position.

[0026] In this specification, "fluoroalkyl group" means an alkyl group in which some or all of the hydrogen atoms are substituted with fluorine atoms.

[0027] In this specification, "perfluoroalkyl group" means an alkyl group in which all hydrogen atoms are substituted with fluorine atoms.

[0029] (Electrolyte)

[0030] One embodiment provides an electrolyte for a lithium secondary battery 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:

[0031] [Chemical Formula 1]

[0032]

[0033] [Chemical Formula 2]

[0034] .

[0036] The first additive and the second additive each function as surfactants having both hydrophilic and hydrophobic groups within a single molecule.

[0037] The first additive above comprises an ether group at the center and fluorine atoms or fluoroalkyl groups having 1 to 10 carbon atoms on both sides. Here, the ether group is a hydrophilic group, and the fluorine atoms or fluoroalkyl groups having 1 to 10 carbon atoms are hydrophobic groups.

[0038] In addition, the second additive comprises an ether group at the center and, on both sides, a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms. 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.

[0039] (However, the above Chemical Formula 1 and the above Chemical Formula 2 are different.)

[0040] Accordingly, when an electrolyte containing the first additive and the second additive simultaneously is used, the wettability to the anode and cathode is improved, and lithium cations (Li + ) is uniformly formed, and a stable SEI film is formed at the interface between the cathode and the electrolyte, thereby suppressing the precipitation of lithium dendrites.

[0041] Therefore, by using an electrolyte containing both the first additive and the second additive, it is possible to suppress the increase in thickness and the decrease in lifespan of the battery even when the negative electrode is made denser.

[0043] Below, an electrolyte according to one embodiment is described in more detail.

[0045] First additive

[0046] In the above chemical formula 1, R 1 and R 2 Each is independently a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms.

[0047] For example, R 1 It can be a fluoroalkyl group having 2 carbon atoms.

[0048] Also, R 2 It can be a fluoroalkyl group having 3 carbon atoms.

[0050] The above first additive can be represented by the following chemical formula 1-1:

[0051] [Chemical Formula 1-1]

[0052]

[0053] In the above chemical formula 1-1, R 11 to R 15 are each a hydrogen atom or a fluorine atom, wherein R 11 to R 15 At least one of them is a fluorine atom; R 21 to R 27 Each is a hydrogen atom or a fluorine atom, wherein R 21 to R 27 At least one of them is a fluorine atom.

[0055] Representative examples of the above-mentioned first additive are as follows:

[0056] [Chemical Formula 1-1-1]

[0057] .

[0059] Second additive

[0060] In the above chemical formula 2, R 3 and R 4 Each is independently a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms.

[0061] For example, R 3 It can be a fluoroalkyl group or a perfluoroalkyl group having 3 carbon atoms.

[0062] Also, R 4 can be a carbon-1 alkyl group or a fluoroalkyl group.

[0064] The above second additive can be represented by the following chemical formula 2-1:

[0065] [Chemical Formula 2-1]

[0066]

[0067] In the above chemical formula 2-1, R 31 to R 37 Each is a hydrogen atom or a fluorine atom, wherein R 31 to R 37 At least one of them is a fluorine atom; R41 to R 43 Each is a hydrogen atom or a fluorine atom.

[0069] Representative examples of the above second additive are as follows:

[0070] [Chemical Formula 2-1-1]

[0071] .

[0073] Content of the first additive and the second additive

[0074] The first additive may be included in an amount of 1 to 10 weight%, specifically 1 to 5 weight%, more specifically 1 to 3 weight% with respect to the total amount of the electrolyte.

[0075] If the first additive is included in an excessive amount exceeding the above range, the viscosity of the electrolyte containing the first additive increases excessively, and the wettability to the anode and cathode may actually decrease. On the other hand, if the content of the first additive is included in a small amount below the above range, the effect as a surfactant may be negligible.

[0077] The second additive may be included in an amount of 1 to 10 weight%, specifically 1 to 5 weight%, more specifically 1 to 3 weight% with respect to the total amount of the electrolyte.

[0078] If the second additive is included in an excessive amount exceeding the above range, the viscosity of the electrolyte containing the second additive increases excessively, and the wettability to the anode and cathode may actually decrease. On the other hand, if the content of the second additive is included in a small amount below the above range, the effect as a surfactant may be negligible.

[0080] The weight ratio of the first additive and the second additive may be 10:1 to 1:10, specifically 5:1 to 1:5, more specifically 3:1 to 1:3.

[0081] Within this range, the effects of the first additive and the second additive can be harmonized.

[0083] Non-aqueous organic solvents

[0084] The above-mentioned non-aqueous organic solvent serves as a medium through which ions involved in the electrochemical reaction of the battery can move.

[0085] The above-mentioned non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, a non-protic solvent, or a combination thereof.

[0086] The above carbonate-based solvents may include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methylpropyl carbonate (MPC), ethylpropyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), etc. As ester-based solvents, methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, valerolactone, caprolactone, etc. As ether-based solvents, dibutyl ether, tetraglame, diglame, dimethoxyethane, 2-methyltetrahydrofuran, 2,5-dimethyltetrahydrofuran, tetrahydrofuran, etc. may be used. Additionally, as ketone-based solvents, cyclohexanone, etc. may be used. As alcohol-based solvents, ethyl alcohol, isopropyl alcohol, etc. may be used, and as aprotic solvents, nitriles such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms and may include a double bond, an aromatic ring, or an ether group); amides such as dimethylformamide; dioxolanes such as 1,3-dioxolane, 1,4-dioxolane; sulfolanes, etc. may be used.

[0088] The above-mentioned non-aqueous organic solvent can be used alone or in a mixture of two or more types.

[0089] In the latter case, the above-mentioned non-aqueous organic solvent may include carbonate-based solvents and propionate-based solvents.

[0090] Here, the entire propionate-based solvent may be included in an amount of 70 volume% or more relative to the total amount of the non-aqueous organic solvent. In this case, the high voltage and / or high temperature characteristics of the lithium secondary battery can be improved.

[0091] For example, the above-mentioned non-aqueous organic solvent may be a mixed solvent of ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propyl propionate (PP).

[0093] lithium salt

[0094] The above lithium salt is a material that is dissolved in an organic solvent and acts as a source of lithium ions within the battery, enabling the operation of a basic lithium secondary battery and facilitating the movement of lithium ions between the positive and negative electrodes.

[0095] LiPF6 can be used as the above lithium salt.

[0096] The concentration of the lithium salt may be 0.1M to 2.0M.

[0098] (Lithium secondary battery)

[0099] Another embodiment provides a lithium secondary battery comprising a positive electrode including a positive active material; a negative electrode including a negative active material; and the electrolyte.

[0101] Cathode composite density

[0102] Generally known lithium secondary batteries use a negative electrode with a composite density of less than 1.68 g / cc, but a lithium secondary battery according to one embodiment uses a negative electrode with a composite density of 1.68 g / cc or more.

[0103] The upper limit for the density of the cathode mixture is not specifically limited, but it may be 2.0 g / cc or less, 1.9 g / cc or less, or 1.8 g / cc or less.

[0105] Charging upper limit voltage

[0106] A lithium secondary battery according to one embodiment includes the electrolyte of the above-described embodiment, so that an increase in thickness and a decrease in lifespan can be suppressed even if the negative electrode is made denser.

[0107] For example, the above lithium secondary battery may have a charging upper limit voltage of 4.4 V or higher.

[0109] Hereinafter, descriptions that overlap with the above-mentioned content will be omitted, and a lithium secondary battery according to one embodiment will be described in more detail.

[0112] positive electrode active material

[0113] As a positive electrode active material, a compound capable of reversible intercalation and deintercalation of lithium (a lithated intercalation compound) may be used. Specifically, one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0114] The above composite oxide may be a lithium transition metal composite oxide, and specific examples include 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.

[0115] As an example, a compound represented by any one of the following chemical formulas may be used. 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 FePO4(0.90≤a≤1.8).

[0116] 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.

[0117] For example, the above-mentioned positive electrode active material may be a high-nickel positive electrode active material in which the nickel content relative to 100 mol% of the metal excluding lithium in the 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.

[0118] 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.

[0119] [Chemical Formula 11]

[0120] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1

[0121] In the above 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, and M 1 and M 2 Each is 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.

[0122] In the above chemical formula 1, 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.

[0123] [Chemical Formula 12]

[0124] Li a2 Co x2 M 3 y2 O 2-b2 X b2

[0125] In the above 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, and 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.

[0126] [Chemical Formula 13]

[0127] Li a3 Fe x3 M 4 y3 PO 4-b3 X b3

[0128] In the above chemical formula 13, 0.9≤a3≤1.8, 0.6≤x3≤1, 0≤y3≤0.4, and 0≤b3≤0.1, and 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.

[0129] [Chemical Formula 14]

[0130] Li a4 Ni x4 Mn y4 M 5 z4 O 2-b4 X b4

[0131] In the above 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, 및 0≤b4≤0.1이고 M 5 is 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.

[0132] In particular, the electrolyte of the above-described embodiment can significantly improve the high voltage and / or high temperature characteristics of a battery using a lithium cobalt-based oxide represented by the above chemical formula 12.

[0134] anode

[0135] A 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 may include a positive electrode active material and may further include a binder and / or a conductive material.

[0136] For example, the above anode may further include an additive that can serve as a sacrificial anode.

[0137] The content of the positive active material is 90% to 99.5% by weight with respect to 100% by weight of the positive active material layer, and the content of the binder and the conductive material may each be 0.5% to 5% by weight with respect to 100% by weight of the positive active material layer.

[0138] The above binder serves to adhere the positive active material particles well to each other and also to adhere the positive active material well to the current collector. Representative examples of binders include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, 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.

[0139] The above conductive material is used to impart conductivity to the electrode, and any electronically conductive material that does not cause chemical changes can be used in the battery being constructed. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanofiber, carbon nanotube; metal-based materials in the form of metal powder or metal fibers containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0140] Al may be used as the current collector mentioned above, but is not limited thereto.

[0142] cathode active material

[0143] 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 doping and dedoping lithium, or a transition metal oxide.

[0144] A material capable of reversibly intercalating / deintercalating the above lithium ions may be a carbon-based negative electrode active material, such as crystalline carbon, amorphous carbon, or a combination thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flake-like, spherical, or fibrous natural graphite or artificial graphite, and examples of the amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0145] As the above lithium metal alloy, an alloy of lithium and a metal selected from Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, and Sn may be used.

[0146] As a material capable of doping and undoping the above lithium, a Si-based negative electrode active material or a Sn-based negative electrode active material may be used. The Si-based negative electrode active material may be silicon, a silicon-carbon composite, SiOx (0 < x < 2), a Si-Q alloy (wherein 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.

[0147] The silicon-carbon composite may be a composite of silicon and amorphous carbon. According to one embodiment, the silicon-carbon composite may be in the form of silicon particles and amorphous carbon coated on the surface of the silicon particles. For example, it may include a secondary particle (core) assembled from silicon primary particles and an amorphous carbon coating layer (shell) located on the surface of the secondary particle. The amorphous carbon may also be located between the silicon primary particles, so that, for example, the silicon primary particles may be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0148] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core comprising crystalline carbon and silicon particles and an amorphous carbon coating layer located on the surface of the core.

[0149] The above Si-based or Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0151] cathode

[0152] A negative electrode for a lithium secondary battery comprises a current collector and a negative electrode active material layer located on the current collector. The negative electrode active material layer comprises a negative electrode active material and may further comprise a binder and / or a conductive material.

[0153] For example, the negative electrode active material layer may comprise 90% to 99% by weight of negative electrode active material, 0.5% to 5% by weight of binder, and 0% to 5% by weight of conductive material.

[0154] The above binder serves to effectively bond the negative electrode active material particles to each other and also to effectively bond the negative electrode active material to the current collector. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof may be used.

[0155] Examples of the above-mentioned non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide imide, polyimide, or combinations thereof.

[0156] The above-mentioned water-based 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, ethylenepropylenediene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0157] When a water-based binder is used as the above-mentioned cathode binder, a cellulose-based compound capable of imparting viscosity may be further included. As this cellulose-based compound, one or more types such as carboxymethyl cellulose, hydroxypropylmethyl cellulose, methyl cellulose, or alkali metal salts thereof may be mixed and used. Na, K, or Li may be used as the alkali metal.

[0158] The above dry binder is a polymer material capable of fiberization, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0159] The above conductive material is used to impart conductivity to the electrode, and any electronically conductive material that does not cause chemical changes can be used in the battery being constructed. Specific examples include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjenblack, carbon fiber, carbon nanofiber, carbon nanotube; metal-based materials in the form of metal powder or metal fibers including copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; or mixtures thereof.

[0160] As the above-mentioned cathode current collector, a material 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 may be used.

[0162] separator

[0163] 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 multilayer films of two or more layers thereof may be used, and of course, mixed multilayer films such as polyethylene / polypropylene two-layer separators, polyethylene / polypropylene / polyethylene three-layer separators, and polypropylene / polyethylene / polypropylene three-layer separators may be used.

[0164] The above separator may 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.

[0165] The porous substrate may be a polymer membrane formed from any one of the following: polyolefins such as polyethylene and polypropylene; polyesters such as polyethylene terephthalate and polybutylene terephthalate; polyacetal; polyamide; polyimide; polycarbonate; polyetherketone; polyaryletherketone; polyetherimide; polyamideimide; polybenzimidazole; polyethersulfone; polyphenylene oxide; cyclic olefin copolymer; polyphenylene sulfide; polyethylene naphthalate; glass fiber; Teflon; and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0166] The above organic material may include a polyvinylidene fluoride-based antibody or a (meth)acrylic-based polymer.

[0167] The above inorganic materials are Al2O3, SiO2, TiO2, SnO2, CeO2, MgO, NiO, CaO, GaO, ZnO, ZrO2, Y2O3, It may include, but is not limited to, inorganic particles selected from SrTiO3, BaTiO3, Mg(OH)2, boehmite, and combinations thereof.

[0168] The above organic and inorganic materials may exist mixed in a single coating layer, or may exist in a stacked form with a coating layer containing organic materials and a coating layer containing inorganic materials.

[0170] lithium secondary battery

[0171] Lithium secondary batteries can be classified into cylindrical, prismatic, pouch, coin, etc., depending on their shape. FIGS. 1 to 4 are schematic diagrams illustrating lithium secondary batteries according to one embodiment, where FIG. 1 is a cylindrical battery, FIG. 2 is a prismatic battery, and FIGS. 3 and 4 are pouch-type batteries. Referring to FIGS. 1 to 4, the lithium secondary battery (100) may include an electrode assembly (40) having a separator (30) interposed between a positive electrode (10) and a negative electrode (20), and a case (50) in which the electrode assembly (40) is housed. The positive electrode (10), the negative electrode (20), and the separator (30) may be impregnated with an electrolyte (not shown). The lithium secondary battery (100) may include a sealing member (60) that seals the case (50) as in FIG. 1. In addition, in FIG. 2, the lithium secondary battery (100) may include a positive lead tab (11) and a positive terminal (12), a negative lead tab (21) and a negative terminal (22). As shown in FIG. 3 and FIG. 4, the lithium secondary battery (100) may include an electrode tab (70), namely a positive tab (71) and a negative tab (72), which serve as an electrical path to guide the current formed in the electrode assembly (40) to the outside.

[0173] A lithium secondary battery according to one embodiment of the present invention may be applied to automobiles, mobile phones, and / or various types of electric devices, etc., but the present invention is not limited thereto.

[0175] Examples and comparative examples of the present invention are described below. However, the following examples are merely one example of the present invention, and the present invention is not limited to the following examples.

[0177] Examples and Comparative Examples

[0178] The electrolyte and lithium secondary battery were manufactured in the following manner.

[0180] Example 1

[0181] (1) Preparation of electrolyte

[0182] An electrolyte was prepared by dissolving 1.3 M LiPF6 in a non-aqueous organic solvent mixed with ethylene carbonate (EC), propylene carbonate (PC), ethyl propionate (EP), and propyl propionate (PP) in a volume ratio of 10:15:30:45, and adding 1 wt% of a first additive and 1 wt% of a second additive.

[0183] As the first additive, the one represented by the following chemical formula 1-1-1 was used, and as the second additive, the one represented by the following chemical formula 2-1-1 was used:

[0184] [Chemical Formula 1-1-1]

[0185] .

[0186] 1,1,2,2-Tetrafluoroethyl2,2,3,3-tetrafluoropropylether (CAS No.: 16627-68-2)

[0187] [Chemical Formula 2-1-1]

[0188] .

[0189] 1,1,1,2,2,3,3-Heptafluoro-3-methoxypropane (CAS No.: 375-03-1)

[0190] (2) Manufacturing of lithium secondary batteries

[0191] LiCoO2 as the positive active material, polyvinylidene fluoride as the binder, and acetylene black as the conductive material are mixed in a weight ratio of 96:3:1, respectively. N - A positive electrode active material slurry was prepared by dispersing in methylpyrrolidone.

[0192] The above positive active material slurry was coated onto an Al foil with a thickness of 15 μm, dried at 100°C, and then pressed to produce a positive electrode.

[0193] Artificial graphite was used as the cathode active material, and the cathode active material, styrene-butadiene rubber binder, and carboxymethylcellulose were mixed in a weight ratio of 98:1:1, respectively, and dispersed in distilled water to prepare a cathode active material slurry.

[0194] The above cathode active material slurry was coated onto a 10㎛ thick Cu foil, dried at 100℃, and then pressed to manufacture a cathode. At this time, the composite density of the cathode was set to 1.68 g / cc.

[0195] An electrode assembly was manufactured by assembling the above positive electrode and the above negative electrode with a separator made of polyethylene material with a thickness of 10 μm, and a lithium secondary battery was manufactured by injecting the above electrolyte.

[0197] Example 2

[0198] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 1 weight% of the first additive and 3 weight% of the second additive were added when preparing the electrolyte.

[0200] Example 3

[0201] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 1 weight% of the first additive and 5 weight% of the second additive were added when preparing the electrolyte.

[0203] Example 4

[0204] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 3 weight% of a first additive and 3 weight% of a second additive were added when preparing the electrolyte.

[0206] Example 5

[0207] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that 5 weight% of a first additive and 5 weight% of a second additive were added when preparing the electrolyte.

[0209] Example 6

[0210] The electrolyte and lithium secondary battery were prepared in the same manner as in Example 1, except that the density of the composite material was 1.75 g / cc when manufacturing the cathode.

[0212] Example 7

[0213] The electrolyte and lithium secondary battery were prepared in the same manner as in Example 1, except that the composite density was formed to be 1.8 g / cc when manufacturing the cathode.

[0215] Comparative Example 1 (Ref.)

[0216] The electrolyte and lithium secondary battery were prepared in the same manner as in Example 1, except that no additives were added during the preparation of the electrolyte.

[0218] Comparative Example 2

[0219] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the second additive was not added during the preparation of the electrolyte, and 1 weight% of the first additive was added.

[0221] Comparative Example 3

[0222] The electrolyte and the lithium secondary battery were prepared in the same manner as in Example 1, except that the second additive was not added during the preparation of the electrolyte, and 3 weight% of the first additive was added.

[0224] Comparative Example 4

[0225] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive was not added during the preparation of the electrolyte, and 1 weight% of the second additive was added.

[0227] Comparative Example 5

[0228] An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the first additive was not added during the preparation of the electrolyte, and 3 weight% of the second additive was added.

[0230] Evaluation example

[0231] The cathode and lithium secondary battery were evaluated in the following manner.

[0233] Evaluation 1: Electrolyte impregnation to the cathode

[0234] A cathode according to Example 1 was prepared as a specimen with dimensions of width * length = 3 cm * 5 cm. 0.1 g of the electrolyte according to Example 1 was dropped onto the specimen and left for 20 minutes. Subsequently, the amount of electrolyte immersed in the specimen was evaluated as a value from 0 to 5 based on the following criteria, out of 100% by weight of the electrolyte dropped onto the specimen, and the evaluation results were recorded in Table 1 below.

[0235] 0: When the amount of electrolyte immersed in the specimen is 0 wt% or more and less than 10 wt%

[0236] 1: When the amount of electrolyte immersed in the specimen is 10 wt% or more and less than 20 wt%

[0237] 2: When the amount of electrolyte immersed in the specimen is 20 wt% or more and less than 40 wt%

[0238] 3: When the amount of electrolyte immersed in the specimen is 40 wt% or more and less than 60 wt%

[0239] 4: When the amount of electrolyte immersed in the specimen is 60 wt% or more and less than 80 wt%

[0240] 5: When the amount of electrolyte immersed in the specimen is 80 wt% or more and less than 100 wt%

[0241] Examples 2 to 7 and Comparative Examples 1 to 5 were also evaluated in the same way, and the evaluation results are listed in Table 1 below.

[0243] Cathode composite density (g / cc) Content of additives in electrolyte (weight%) Impregnation of the electrolyte into the cathode First additive Second additive Comparative Example 1 1.68 0 0 0 Comparative Example 2 1.68 1 0 1 Comparative Example 3 1.68 3 0 2 Comparative Example 4 1.68 0 1 1 Comparative Example 5 1.68 0 3 2 Example 1 1.68 1 1 5 Example 2 1.68 1 3 5 Example 3 1.68 1 5 5 Example 4 1.68 3 3 5 Example 5 1.68 5 5 5 Example 6 1.75 1 1 5 Example 7 1.8 1 1 4

[0245] Evaluation 2: Evaluation of Room Temperature Charge / Discharge Cycle Characteristics

[0246] The lithium secondary battery was subjected to 200 charge and discharge cycles under conditions of 25℃, 2.0C charging (CC / CV, 4.47V, 0.025C Cut-off) / 1.0C discharging (CC, 3V Cut-off).

[0247] The thickness increase rate was calculated according to Formula 1 below, and the capacity retention rate was calculated according to Formula 2 below, and the results were listed in Table 2 below.

[0248] [Equation 1]

[0249] Thickness increase rate = {(n th {Full thickness after cycle) - (Full thickness after 1 cycle)} / (Full thickness after 1 cycle) * 100

[0250] In the above Equation 1, "full charge thickness" refers to the thickness of a lithium secondary battery measured after charging to SOC 100% (a state where the total charge capacity of the battery is set to 100%, and the battery is charged to 100% charge capacity).

[0251] [Equation 2]

[0252] Capacity retention rate = (n th Discharge capacity after cycle / Discharge capacity after 1 cycle) * 100

[0254] Room temperature charge / discharge characteristics of lithium secondary batteries Thickness increase rate (%) @200cyc Capacity retention rate @ 50 cyc.(%) Capacity retention rate @ 100 cycles.(%) Capacity retention rate @ 200 cyc.(%) Comparative Example 1 17.7 85 0 0 Comparative Example 2 15.0 88 0 0 Comparative Example 3 12.5 90 86 0 Comparative Example 4 12.5 88 86 78 Comparative Example 5 10.7 89 87 78 Example 1 7.3 91 89 87 Example 2 7.0 91 89 86 Example 3 7.1 91 88 86 Example 4 7.0 92 90 87 Example 5 7.1 91 89 85 Example 6 7.9 91 87 80 Example 7 8.2 90 85 78

[0256] synthesis

[0257] Referring to Tables 1 and 2 above, when an electrolyte with no additives added is used (Comparative Example 1), the thickness of the battery containing the high-density negative electrode increases and the lifespan drops sharply.

[0258] However, when the composite density of the cathode is the same at 1.68 g / cc, compared to the case where an electrolyte without any additives is used (Comparative Example 1), when an electrolyte containing both the first additive and the second additive is used (Examples 1 to 5), the impregnation of the electrolyte into the cathode is increased, the thickness of the battery is reduced, and the lifespan is increased.

[0259] Meanwhile, when an electrolyte containing only one of the first additive and the second additive is used (Comparative Examples 2 and 3), the electrolyte impregnation characteristics of the negative electrode are inferior, and as a result, the amount of free liquid increases, causing the battery life to drop sharply, comparable to when an electrolyte with no additive added is used (Comparative Example 1). Therefore, it is necessary to use an electrolyte containing both the first additive and the second additive simultaneously.

[0260] Furthermore, when using an electrolyte containing both the first additive and the second additive, even if the composite density of the negative electrode is increased from 1.68 g / cc to 1.8 g / cc (Examples 6 and 7), the increase in thickness and decrease in lifespan of the battery are suppressed.

[0262] Although preferred embodiments of the present invention have been described above, the present invention is not limited thereto and can be implemented with various modifications within the scope of the claims, the detailed description of the invention, and the attached drawings, and it is obvious that such modifications also fall within the scope of the present invention. Explanation of the symbols

[0264] 100: Lithium secondary battery 10: Positive electrode 11: Positive lead tab 12: Positive terminal 20: Cathode 21: Cathode lead tab 22: Negative terminal 30: Separator 40: Electrode assembly 50: Case 60: Sealing member 70: Electrode tab 71: Positive tab 72: Negative tab

Claims

Claim 1 Electrolyte for a lithium secondary battery 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, wherein the weight ratio of the first additive to the second additive is 10:1 to 1:10: [Chemical Formula 1] In the above chemical formula 1, R 1 and R 2 Each is independently a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms; [Chemical Formula 2] In the above chemical formula 2, R 3 and R 4 Each is independently a fluorine atom, an alkyl group having 1 to 10 carbon atoms, or a fluoroalkyl group having 1 to 10 carbon atoms; and the above formula 1 and the above formula 2 are different. Claim 2 In claim 1, the first additive is an electrolyte for a lithium secondary battery represented by the following chemical formula 1-1: [Chemical Formula 1-1] In the above chemical formula 1-1, R 11 to R 15 are each a hydrogen atom or a fluorine atom, wherein R 11 to R 15 At least one of them is a fluorine atom;R 21 to R 27 Each is a hydrogen atom or a fluorine atom, wherein R 21 to R 27 At least one of them is a fluorine atom. Claim 3 In claim 1, the first additive is an electrolyte for a lithium secondary battery represented by the following chemical formula 1-1-1: [Chemical Formula 1-1-1] . Claim 4 In claim 1, the second additive is an electrolyte for a lithium secondary battery represented by the following chemical formula 2-1: [Chemical Formula 2-1] In the above chemical formula 2-1, R 31 to R 37 Each is a hydrogen atom or a fluorine atom, wherein R 31 to R 37 At least one of them is a fluorine atom;R 41 to R 43 Each is a hydrogen atom or a fluorine atom. Claim 5 In claim 1, the second additive is an electrolyte for a lithium secondary battery represented by the following chemical formula 2-1-1: [Chemical Formula 2-1-1] . Claim 6 In claim 1, the first additive is an electrolyte for a lithium secondary battery included in an amount of 1 to 10 weight percent relative to the total amount of the electrolyte. Claim 7 In claim 1, the second additive is an electrolyte for a lithium secondary battery included in an amount of 1 to 10 weight% with respect to the total amount of the electrolyte. Claim 8 delete Claim 9 In claim 1, the above-mentioned non-aqueous organic solvent is an electrolyte for a lithium secondary battery comprising a carbonate-based solvent and a propionate-based solvent. Claim 10 In claim 9, the above propionate-based solvent is an electrolyte for a lithium secondary battery containing at least 70 volume% of the total amount of the above non-aqueous organic solvent. Claim 11 In claim 1, the lithium salt is an electrolyte for a lithium secondary battery, which is LiPF6. Claim 12 In claim 1, the electrolyte for a lithium secondary battery, wherein the concentration of the lithium salt is 0.1M to 2.0M. Claim 13 A lithium secondary battery comprising: a positive electrode comprising a positive active material; a negative electrode comprising a negative active material; and an electrolyte according to any one of claims 1 to 7 and claims 9 to 12. Claim 14 A lithium secondary battery according to claim 13, wherein the composite density of the cathode is 1.68 g / cc or higher. Claim 15 In claim 13, the positive active material comprises a lithium secondary battery comprising 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. Claim 16 In paragraph 13, the negative electrode active material comprises a carbon-based negative electrode active material, a Si-based negative electrode active material, or a combination thereof, in a lithium secondary battery. Claim 17 In paragraph 13, the lithium secondary battery has a charging upper limit voltage of 4.4 V or higher.

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