Lithium secondary battery

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: The lithium secondary battery includes a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; and an electrolyte. The electrode density of the negative electrode is 1.7 g / cc or more. The electrolyte 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. (An explanation of the chemical formulas is as described in the specification.)SELECTED DRAWING: Figure 1

Description

[Technical field]

[0001] This relates to lithium secondary batteries. [Background technology]

[0002] Recently, with the rapid spread of electronic devices that use batteries, such as mobile phones, laptops, and electric vehicles, the demand for secondary batteries with high energy density and capacity is rapidly increasing. As a result, research and development to improve the performance of lithium secondary batteries is being actively conducted.

[0003] A lithium secondary battery is a battery that contains positive and negative electrodes, which include active materials capable of intercalating and deintercalating lithium ions, and an electrolyte. It produces electrical energy through oxidation and reduction reactions that occur when lithium ions are intercalated and deintercalated at the positive and negative electrodes.

[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. Summary of the Invention [Problem to be solved by the invention]

[0005] One embodiment is directed to providing a lithium secondary battery that has a high density negative electrode while suppressing an increase in thickness and a decrease in lifespan of the battery. [Means for solving the problem]

[0006] One embodiment provides a lithium secondary battery comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolyte, wherein the electrode density of the negative electrode is 1.7 g / cc or more, and the electrolyte 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:

[0007] [ka]

[0008] [ka] Effect of the Invention

[0009] In the lithium secondary battery according to an embodiment, the negative electrode can be densified while suppressing an increase in the thickness of the battery and a decrease in its lifespan. [Brief description of the drawings]

[0010] [Figure 1] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. [Diagram 2] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. [Diagram 3] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. [Figure 4] 1 is a schematic diagram illustrating a lithium secondary battery according to an embodiment. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0011] 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.

[0012] 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.

[0013] 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."

[0014] As used herein, "combinations thereof" can mean mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0015] In this specification, the "electrode 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.

[0016] Unless otherwise defined in a chemical formula herein, when no chemical bond is drawn at a position where a chemical bond should be drawn, it means that a hydrogen atom is bonded to that position.

[0017] 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.

[0018] 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.

[0019] (lithium secondary battery) One embodiment provides a lithium secondary battery comprising: a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; and an electrolyte, wherein the electrode density of the negative electrode is 1.7 g / cc or more, and the electrolyte 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:

[0020] [ka]

[0021] [ka]

[0022] The first additive and the second additive each function as a surfactant having both a hydrophilic group and a hydrophobic group in one molecule.

[0023] The first additive contains an ether group at the center and a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms on both sides thereof. 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.

[0024] The second additive contains a ketone group at the center and a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms on both sides of the ketone group. Here, the ketone group is a hydrophilic group, and the fluorine atom or the fluoroalkyl group having 1 to 10 carbon atoms is a hydrophobic group.

[0025] Therefore, when the electrolyte solution containing the first additive and the second additive is used, the wettability of the positive electrode and the negative electrode 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.

[0026] Therefore, when an electrolyte solution containing the first additive and the second additive is used, the electrode density of the negative electrode can be increased to 1.7 g / cc or more, while suppressing an increase in the thickness of the battery and a decrease in its lifespan.

[0027] Hereinafter, a lithium secondary battery according to an embodiment will be described in more detail.

[0028] Negative electrode density Generally known lithium secondary batteries use a negative electrode having an electrode density of less than 1.7 g / cc, but the lithium secondary battery according to one embodiment uses a negative electrode having an electrode density of 1.7 g / cc or more.

[0029] The upper limit for the electrode density of the negative electrode 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.

[0030] Charging voltage limit According to an embodiment, the lithium secondary battery uses an electrolyte solution including both the first additive and the second additive, thereby preventing an increase in thickness and a decrease in lifespan of the battery even when the negative electrode is densified.

[0031] For example, the lithium secondary battery may have an upper limit charging voltage of 4.5 V or higher.

[0032] First Additive In the above 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 can be represented by the following formula 1-1:

[0036] [ka]

[0037] 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 R is a fluorine atom; 21 ~R 27 are each a hydrogen atom or a fluorine atom, 21 ~R 27At least one of the groups is a fluorine atom.

[0038] Representative examples of the first additive are as follows:

[0039] [ka]

[0040] Second Additive In the above formula 2, R 3 and R 4 are each independently a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms.

[0041] For example, R 3 may be a fluoroalkyl or perfluoroalkyl group having 2 carbon atoms.

[0042] Also, R 4 may be a fluoroalkyl or perfluoroalkyl group having 3 carbon atoms.

[0043] The second additive can be represented by the following formula 2-1:

[0044] [ka]

[0045] In the above chemical formula 2-1, R 31 ~R 35 are each a hydrogen atom or a fluorine atom, 31 ~R 35 At least one of R is a fluorine atom; 41 ~R 47 are each a hydrogen atom or a fluorine atom, 41 ~R 47 At least one of the groups is a fluorine atom.

[0046] Representative examples of the second additive are as follows:

[0047] [ka]

[0048] Content of the first additive and the second additive The first additive may be included in an amount of 1 to 20 wt %, 1 to 15 wt %, or 1 to 10 wt % based on the total amount of the electrolyte solution.

[0049] If the first additive is included 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 first additive is included in a small amount 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 wt %, 1 to 7 wt %, or 1 to 5 wt % based on the total amount of the electrolyte solution.

[0051] If the second additive is included 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 less than the above range, the effect as a surfactant may be insignificant.

[0052] The weight ratio of the first additive and the second additive may be from 20:1 to 2:1, from 15:1 to 2:1, or from 10:1 to 2:1.

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

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

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

[0059] The propionate-based solvent may account for 70% by volume or more of 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 The lithium salt is dissolved in an organic solvent and serves as a source of lithium ions in a battery to enable basic operation of a lithium secondary battery, and promotes the movement of lithium ions between a positive electrode and a negative electrode.

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

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

[0064] positive electrode 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.

[0065] The composite oxide may be a lithium transition metal composite oxide, and specific examples thereof 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.

[0066] As an example, a compound represented by any one of the following chemical formulas can be used: 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, 0.001b≦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).

[0067] In the above 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.

[0068] For example, the positive electrode active material may be a high-nickel-based positive electrode active material in which the nickel content 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 relative to 100 mol% of metals excluding lithium in a lithium transition metal composite oxide. The high-nickel-based positive electrode active material can achieve high capacity and can be applied to high-capacity, high-density lithium secondary batteries.

[0069] 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 oxide represented by the following Chemical Formula 14, or a combination thereof.

[0070] Li a1 Ni x1 M 1 y1 M 2 z1 O 2-b1 X b1 [Chemical formula 11]

[0071] In the above 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. 2are 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.

[0072] In the above 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 may be satisfied.

[0073] Li a2 Co x2 M 3 y2 O 2-b2 X b2 [Chemical formula 12]

[0074] In the above 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.

[0075] Li a3 Fe x3 M 4 y3 PO 4-b3 X b3 [Chemical formula 13]

[0076] In the above 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.

[0077] Li a4 Ni x4 Mn y4 M 5 z4 O 2-b4 X b4 [Chemical formula 14]

[0078] 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、および0≦b4≦0.1であり、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.

[0079] In particular, the electrolyte solution of 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.

[0080] 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.

[0081] For example, the positive electrode may further include an additive that can act as a sacrificial positive electrode.

[0082] The content of the positive electrode active material may be 90% by weight to 99.5% by weight based on 100% by weight of the positive electrode active material layer, and the contents of the binder and the conductive material may be 0.5% by weight to 5% by weight, respectively, based on 100% by weight of the positive electrode active material layer.

[0083] The binder serves to adhere the positive electrode active material particles to each other and to adhere the positive electrode active material 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.

[0084] 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 fiber, carbon nanofiber, and carbon nanotube; metal-based materials containing copper, nickel, aluminum, silver, and the like in the form of metal powder or metal fiber; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0085] The current collector may be made of Al, but is not limited thereto.

[0086] 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.

[0087] Examples of substances that can reversibly intercalate / deintercalate lithium ions include carbon-based negative electrode active materials, which can include, for example, crystalline carbon, amorphous carbon, or combinations thereof. Examples of the crystalline carbon include graphite such as amorphous, plate-like, flaky, 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, and the like.

[0088] As the alloy of the 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.

[0089] As the substance that can be doped and undoped with lithium, an Si-based negative electrode active material or an 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), an Si-Q alloy (where 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 combinations thereof. The Sn-based negative electrode active material may be Sn, SnO2, an Sn-based alloy, or combinations thereof.

[0090] 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 can 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 can be coated with amorphous carbon. The secondary particles may be dispersed in an amorphous carbon matrix.

[0091] The silicon-carbon composite may further include crystalline carbon. For example, the silicon-carbon composite may include a core including crystalline carbon and silicon particles and an amorphous carbon coating layer located on the core.

[0092] The Si-based or Sn-based negative electrode active material may be used in combination with a carbon-based negative electrode active material.

[0093] 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 includes a negative electrode active material, and may further include a binder and / or a conductive material.

[0094] 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.

[0095] The binder serves to adhere the negative 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.

[0096] The non-aqueous binder may be polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or a combination thereof.

[0097] 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. When the negative electrode binder is an aqueous binder, it may further include a cellulose-based compound capable of imparting viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li.

[0098] The dry binder is a polymeric material that can be fiberized, and may be, for example, polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, polyethylene oxide, or a combination thereof.

[0099] The conductive material is used to impart conductivity to the electrodes, and any material that does not cause a chemical change in the battery 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 fiber, carbon nanofiber, and carbon nanotube; metal-based materials in the form of metal powder or metal fiber, including copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0100] The negative electrode current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0101] Separator Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such separators can be polyethylene, polypropylene, polyvinylidene fluoride, or multi-layered films of two or more of these, and of course mixed multi-layered films such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator can also be used.

[0102] The separator may include a porous substrate and a coating layer, including an organic material, an inorganic material, or a combination thereof, located on one or both sides of the porous substrate.

[0103] 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, and polytetrafluoroethylene, or a copolymer or mixture of two or more of these polymers.

[0104] The organic material may include a polyvinylidene fluoride-based polymer or a (meth)acrylic-based polymer.

[0105] The inorganic material may include, but is 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.

[0106] 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.

[0107] 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 an 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 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 shown in FIG. 1. Also, in FIG. 2, the lithium secondary battery 100 may 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 taps 70, i.e., a positive electrode tap 71 and a negative electrode tap 72, which serve as electrical paths for conducting the current generated in the electrode assembly 40 to the outside.

[0108] 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.

[0109] Examples and comparative examples of the present invention will be described below. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples. EXAMPLES

[0110] Examples and Comparative Examples An electrolyte and a lithium secondary battery were manufactured as follows.

[0111] 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 10 wt % of the first additive and 1 wt % of the second additive.

[0112] The first additive was represented by the following formula 1-1-1, and the second additive was represented by the following formula 2-1-1:

[0113] [ka]

[0114] 1,1,2,2-Tetrafluoroethyl2,2,3,3-tetrafluoropropylether (CAS No.: 16627-68-2)

[0115] [ka]

[0116] Perfluoro(2-methyl-3-pentanone)(CAS No.:756-13-8)

[0117] (2) Manufacture of lithium secondary batteries A positive electrode active material slurry was prepared by mixing LiCoO2 as a positive electrode active material, polyvinylidene fluoride as a binder, and acetylene black as a conductive material in a weight ratio of 96:3:1 and dispersing the mixture in N-methylpyrrolidone.

[0118] 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.

[0119] Artificial graphite was used as the negative active material, and the negative 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 active material slurry.

[0120] 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, the electrode density of which was 1.7 g / cc.

[0121] The positive and negative electrodes were assembled with a 10 μm thick polyethylene separator to prepare an electrode assembly, and the electrolyte was injected into the electrode assembly to prepare a lithium secondary battery.

[0122] Example 2 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 10 wt % of the first additive and 2 wt % of the second additive were added during the preparation of the electrolyte.

[0123] Example 3 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 10 wt % of the first additive and 5 wt % of the second additive were added during the preparation of the electrolyte.

[0124] Example 4 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 10 wt % of the first additive and 2 wt % of the second additive were added when preparing the electrolyte, and the electrode density was formed to be 1.75 g / cc when preparing the negative electrode.

[0125] Example 5 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 10 wt % of the first additive and 2 wt % of the second additive were added when preparing the electrolyte, and the electrode density was formed to be 1.8 g / cc when preparing the negative electrode.

[0126] Comparative Example 1 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that no additives were added when the electrolyte was prepared, and the electrode density was set to 1.65 g / cc when the negative electrode was prepared.

[0127] Comparative Example 2 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that no additives were added when the electrolyte was prepared, and the electrode density was set to 1.67 g / cc when the negative electrode was prepared.

[0128] Comparative Example 3 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that no additives were added when the electrolyte was prepared, and the electrode density was set to 1.7 g / cc when the negative electrode was prepared.

[0129] Comparative Example 4 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 15 wt % of the first additive was added when the electrolyte was prepared, and the electrode density was set to 1.7 g / cc when the negative electrode was prepared.

[0130] Comparative Example 5 An electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that 10 wt. % of the second additive was added when the electrolyte was prepared, and the electrode density was set to 1.7 g / cc when the negative electrode was prepared.

[0131] Evaluation example The negative electrodes and lithium secondary batteries were evaluated by the following methods.

[0132] Evaluation 1: Impregnation of the electrolyte into the negative electrode The negative electrode according to Example 1 was prepared into a test piece with a width x length of 3 cm x length of 5 cm. 1 g of the electrolyte according to Example 1 was dropped onto the test piece and left for 20 minutes. Then, the amount of electrolyte dropped onto the test piece and immersed in the test piece out of 100% by weight of the electrolyte was evaluated into a value of 0 to 5 according to the following criteria, and the evaluation results are shown in Table 1 below.

[0133] 0: The amount of electrolyte immersed in the specimen is 0% or more but less than 10% by weight. 1: When the amount of electrolyte immersed in the specimen is 10% by weight or more but less than 20% by weight 2: When the amount of electrolyte immersed in the specimen is 20% by weight or more but less than 40% by weight 3: When the amount of electrolyte immersed in the specimen is 40% or more but less than 60% by weight 4: When the amount of electrolyte immersed in the specimen is 60% or more but less than 80% by weight 5: When the amount of electrolyte immersed in the specimen is 80% or more but less than 100% by weight

[0134] Examples 2 to 5 and Comparative Examples 1 to 5 were also evaluated in the same manner, and the evaluation results are shown in Table 1 below.

[0135] [Table 1]

[0136] Evaluation 2: Room temperature charge / discharge cycle characteristics evaluation The lithium secondary battery was subjected to 400 charge / discharge cycles at 125°C and 2.0C charge (CC / CV, 4.53V, 0.025C cut-off) / 1.0C discharge (CC, 3V cut-off).

[0137] 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.

[0138] Thickness increase rate = {(fully charged thickness after 400 cycles) - (fully charged thickness after 1 cycle)} / (fully charged thickness after 1 cycle) * 100 [Formula 1]

[0139] In the above formula 1, "fully charged thickness" means the thickness of a lithium secondary battery measured after charging to SOC100% (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.

[0140] Capacity retention rate = (discharge capacity after 400 cycles / discharge capacity after 1 cycle)*100 [Formula 2]

[0141] [Table 2]

[0142] General Referring to Tables 1 and 2, when an electrolyte solution containing no additives is used (Comparative Examples 1 to 3), as the electrode density of the negative electrode increases from 1.65 g / cc to 1.7 g / cc, the impregnation of the electrolyte solution into the negative electrode decreases, the thickness of the battery increases, and the lifespan decreases.

[0143] However, when the electrode density of the negative electrode is the same at 1.7 g / cc, when an electrolyte solution containing both the first additive and the second additive is used (Examples 1 to 3), the impregnation of the electrolyte into the negative electrode is increased, the thickness of the battery is reduced, and the lifespan is increased, compared to when an electrolyte solution containing no additive is used (Comparative Example 3).

[0144] On the other hand, when an electrolyte solution containing only one of the first additive and the second additive is used (Comparative Examples 4 and 5), an SEI film is formed unstably on the negative electrode surface, causing lithium dendrites to grow, resulting in a sharp drop in the capacity retention rate during charging and discharging. Therefore, it is necessary to use an electrolyte solution containing both the first additive and the second additive.

[0145] Furthermore, when an electrolyte solution containing the first additive and the second additive simultaneously is used, even if the electrode density of the negative electrode is increased from 1.7 g / cc to 1.8 g / cc (Examples 4 and 5), the increase in the thickness of the battery and the decrease in its lifespan are suppressed.

[0146] Although a preferred embodiment of the present invention has been described above, the present invention is not limited thereto, and can be embodied in various modified forms within the scope of the claims, the detailed description of the invention, and the attached drawings, which of course also fall within the scope of the present invention. [Explanation of symbols]

[0147] 100: Lithium secondary battery 10: Positive electrode 11: Positive lead tap 12: Positive terminal 20: Negative electrode 21: Negative lead tap 22: Negative terminal 30: Separator 40: Electrode assembly 50: Case 60: Sealing material 70: Electrode tap 71: Positive tap 72: Negative tap

Claims

1. a positive electrode comprising a positive electrode active material; a negative electrode comprising a negative electrode active material; and Contains an electrolyte, The electrode density of the negative electrode is 1.7 g / cc or more, The electrolyte is Lithium salts; non-aqueous organic solvent; A first additive represented by the following formula 1: The second additive represented by the following formula 2 is included: Lithium secondary battery: 【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; 【Chemistry 2】 In the above chemical formula 2, R 3 and R 4 are each independently a fluorine atom or a fluoroalkyl group having 1 to 10 carbon atoms.

2. The lithium secondary battery of claim 1, wherein the first additive is represented by the following 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 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 lithium secondary battery of claim 1, wherein the first additive is represented by the following formula 1-1-1: 【Chemistry 4】

4. The lithium secondary battery of claim 1, wherein the second additive is represented by the following formula 2-1: 【Chemistry 5】 In the above chemical formula 2-1, R 31 ~R 35 are each a hydrogen atom or a fluorine atom, 31 ~R 35 at least one of is a fluorine atom; R 41 ~R 47 are each a hydrogen atom or a fluorine atom, 41 ~R 47 At least one of the groups is a fluorine atom.

5. The lithium secondary battery of claim 1, wherein the second additive is represented by the following formula 2-1-1: 【Chemistry 6】

6. 2. The lithium secondary battery according to claim 1, wherein the first additive is contained in an amount of 1 to 20% by weight based on the total amount of the electrolyte.

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

8. 2. The lithium secondary battery according to claim 1, wherein a weight ratio of the first additive and the second additive is from 20:1 to 2:

1.

9. 2. The 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 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 lithium secondary battery according to claim 1 ,

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

13. 2. The lithium secondary battery according to claim 1, 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.

14. 2. The lithium secondary battery of claim 1, 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.

15. 2. The lithium secondary battery according to claim 1, wherein the upper limit charging voltage of the lithium secondary battery is 4.5 V or higher.

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