Lithium secondary battery

Abstract

To provide a lithium secondary battery having improved life and high-temperature storage characteristics.SOLUTION: A 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 electrolyte contains a nitrile additive, and the positive electrode active material includes at least one of the compounds represented by the following chemical formulas 1 and 2. [Chemical Formula 1] Lia1Fex1By1 PO4-b1. [Chemical Formula 2] Lia2Mnz2Fex2By2 PO4-b2. The definitions of chemical formulas 1 and 2 are as described in the specification.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] An embodiment of the present invention relates to an electrolyte for a lithium secondary battery and a lithium secondary battery including the same. [Background technology]

[0002] In recent years, the demand for high-energy-density, high-capacity secondary batteries has been increasing rapidly with the rapid spread of battery-powered electronic devices such as mobile phones, laptops, and electric vehicles. Therefore, research and development efforts to improve the performance of lithium secondary batteries have been actively conducted.

[0003] A lithium secondary battery is a battery that includes a positive electrode and a negative electrode, each containing an active material capable of intercalating and deintercalating lithium ions, and an electrolyte, and generates electrical energy through oxidation and reduction reactions that occur when lithium ions are intercalated and deintercalated at the positive electrode and the negative electrode. Summary of the Invention [Problem to be solved by the invention]

[0004] An object of one embodiment of the present invention is to provide a lithium secondary battery having improved life and high-temperature storage characteristics. [Means for solving the problem]

[0005] 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 an electrolyte, wherein the electrolyte includes a nitrile-based additive, and the positive electrode active material may include at least one of compounds represented by the following Chemical Formulas 1 and 2:

[0006] [Chemical formula 1] Li a1 Fe x1 B y1 PO 4-b1

[0007] In Chemical Formula 1, 0.8≦a1≦1.2, 0.9≦x1≦1.1, 0≦y1≦0.05, and 0≦b1≦0.05.

[0008] [Chemical formula 2] Li a2 Mn z2 Fe x2 B y2 PO 4-b2

[0009] In Chemical Formula 2, 0.8≦a2≦1.2, 0.5≦z2≦0.9, 0.1≦x2≦0.5, 0≦y2≦0.05, 0≦b2≦0.05, and 0.9≦z2+x2≦1.2.

[0010] In Chemical Formula 1 and Chemical Formula 2, B may be at least one element selected from the group consisting of Ti, Mg, V, and Nb.

[0011] The nitrile additive may include a compound represented by the following Chemical Formula 5:

[0012] [Chemical formula 5] [ka]

[0013] In Chemical Formula 5, l, m, and n may each independently be an integer of 0 to 10. [Effects of the Invention]

[0014] The lithium secondary battery according to one embodiment of the present invention has improved life characteristics and can suppress an increase in battery resistance during high-temperature storage. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic conceptual diagram of a lithium secondary battery according to one embodiment of the present invention. [Figure 2] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 3]1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 4] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present invention. [Figure 5] 1 is a cross-sectional view schematically illustrating a lithium secondary battery according to an embodiment of the present invention. DETAILED DESCRIPTION OF THE INVENTION

[0016] In order to fully understand the configuration and effects of the present invention, preferred embodiments of the present invention will be described with reference to the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, and can be embodied in various forms and can be modified in various ways. However, the description of the embodiments is provided to ensure complete disclosure of the present invention and to fully convey the scope of the invention to those skilled in the art.

[0017] In this specification, when a component is referred to as being on top of another component, this means that the component may be formed directly on the other component, or may be formed with a third component interposed therebetween. Also, in the drawings, the thickness of the components may be exaggerated for the purpose of effectively explaining the technical content. Throughout the specification, parts designated with the same reference numerals refer to the same components.

[0018] Unless otherwise specified herein, the singular may refer to the plural. Additionally, unless otherwise specified, "A or B" may mean "including A," "including B," or "including A and B." As used herein, "comprises" and / or "comprising" do not exclude the presence or addition of one or more other elements to the referenced element.

[0019] As used herein, "combinations thereof" may refer to mixtures, laminates, composites, copolymers, alloys, blends, reaction products, and the like of compositions.

[0020] In this specification, unless otherwise defined, the term "substituted" may mean that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a hydroxyl group, an amino group, a C1 to C30 amine group, a nitro group, a C1 to C40 silyl group, a C1 to C30 alkyl group, a C1 to C10 alkylsilyl group, a C6 to C30 arylsilyl group, a C3 to C30 cycloalkyl group, a C3 to C30 heterocycloalkyl group, a C6 to C30 aryl group, a C2 to C30 heteroaryl group, a C1 to C20 alkoxy group, a C1 to C10 fluoroalkyl group, a cyano group, or a combination thereof.

[0021] Specifically, "substituted" can mean that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1 to C30 alkyl group, a C1 to C10 alkylsilyl group, a C6 to C30 arylsilyl group, a C3 to C30 cycloalkyl group, a C3 to C30 heterocycloalkyl group, a C6 to C30 aryl group, a C2 to C30 heteroaryl group, a C1 to C10 fluoroalkyl group, or a cyano group. For example, "substituted" can mean that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1 to C20 alkyl group, a C6 to C30 aryl group, a C1 to C10 fluoroalkyl group, or a cyano group. Alternatively, "substituted" can mean that at least one hydrogen atom in a substituent or compound is replaced with deuterium, a halogen group, a C1 to C5 alkyl group, a C6 to C18 aryl group, a C1 to C5 fluoroalkyl group, or a cyano group. As an example, "substituted" may mean that at least one hydrogen in a substituent or compound is replaced with deuterium, a cyano group, a halogen group, a methyl group, an ethyl group, a propyl group, a butyl group, a phenyl group, a biphenyl group, a terphenyl group, a trifluoromethyl group, or a naphthyl group.

[0022] 1 is a schematic conceptual diagram of a lithium secondary battery according to one embodiment of the present invention. Referring to FIG. 1, the lithium secondary battery may include a positive electrode 10, a negative electrode 20, a separator 30, and an electrolyte solution ELL.

[0023] The positive electrode 10 and the negative electrode 20 may be separated from each other by a separator 30. The separator 30 may be disposed between the positive electrode 10 and the negative electrode 20. The positive electrode 10, the negative electrode 20, and the separator 30 may be in contact with an electrolyte solution ELL. The positive electrode 10, the negative electrode 20, and the separator 30 may be impregnated in the electrolyte solution ELL.

[0024] The electrolyte ELL may be a medium for transferring lithium ions between the positive electrode 10 and the negative electrode 20. In the electrolyte ELL, the lithium ions can pass through the separator 30 and move toward the positive electrode 10 or the negative electrode 20.

[0025] (positive electrode 10) The lithium secondary battery positive electrode 10 may include a current collector COL1 and a positive electrode active material layer AML1 formed on the current collector COL1. The positive electrode active material layer AML1 includes a positive electrode active material and may further include a binder and / or a conductive material.

[0026] As an example, the cathode 10 may further include an additive that can function as a sacrificial cathode.

[0027] The content of the positive electrode active material in the positive electrode active material layer AML1 may be 90% by weight to 99.5% by weight, based on 100% by weight of the positive electrode active material layer AML1. The contents of the binder and the conductive material may be 0.5% by weight to 5% by weight, based on 100% by weight of the positive electrode active material layer AML1.

[0028] The binder functions to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material to the current collector COL1. Representative examples of binders 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)acrylic styrene-butadiene rubber, epoxy resin, (meth)acrylic resin, polyester resin, nylon, etc.

[0029] The conductive material is used to impart conductivity to the electrode, and any material that is electron-conductive and does not cause a chemical change in the battery that is constructed 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 in the form of metal powder or metal fiber containing copper, nickel, aluminum, silver, etc.; conductive polymers such as polyphenylene derivatives; and mixtures thereof.

[0030] The current collector COL1 can be made of Al, but is not limited to this.

[0031] (Cathode active material) The positive electrode active material in the positive electrode active material layer AML1 may be a compound capable of reversibly inserting and extracting lithium (lithiated intercalation compound). Specifically, one or more composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof may be used.

[0032] The composite oxide may be a lithium transition metal composite oxide, but specific examples include lithium nickel-based oxide, lithium cobalt-based oxide, lithium manganese-based oxide, lithium iron phosphate-based oxide, cobalt-free nickel-manganese-based oxide, or a combination thereof.

[0033] As an example, a compound represented by any one of the following chemical formulas can 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 Mn2Gb 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).

[0034] 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; and L 1 is Mn, Al, or a combination thereof.

[0035] For example, the positive electrode active material may include a positive electrode active material having an olivine structure. More specifically, the positive electrode active material may include at least one of compounds represented by the following Formula 1 or 2:

[0036] [Chemical formula 1] Li a1 Fe x1 B y1 PO 4-b1

[0037] In Chemical Formula 1, 0.8≦a1≦1.2, 0.9≦x1≦1.1, 0≦y1≦0.05, and 0≦b1≦0.05.

[0038] [Chemical formula 2] Li a2 Mn z2 Fe x2 B y2 PO 4-b2

[0039] In Chemical Formula 2, 0.8≦a2≦1.2, 0.5≦z2≦0.9, 0.1≦x2≦0.5, 0≦y2≦0.05, 0≦b2≦0.05, and 0.9≦z2+x2≦1.2.

[0040] In Chemical Formula 1 and Chemical Formula 2, B may be at least one element selected from the group consisting of Ti, Mg, V, and Nb, and may be a dopant that is doped into the positive electrode active material particles.

[0041] Positive electrode active materials with an olivine structure are not only less expensive than other materials, but also have excellent stability and lifespan characteristics. Furthermore, the use of a nitrile-based additive (described below) in combination with an olivine-based positive electrode active material can further improve the lifespan and storage characteristics of secondary batteries compared to the use of other positive electrode active materials. This may be due to the excellent reactivity of the Fe element and CN groups in the olivine-based positive electrode active material.

[0042] (Negative electrode 20) The lithium secondary battery positive electrode 20 may include a current collector COL2 and a negative electrode active material layer AML2 formed on the current collector COL2. The negative electrode active material layer AML2 includes a negative electrode active material and may further include a binder and / or a conductive material.

[0043] For example, the negative electrode active material layer AML2 may 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.

[0044] The binder functions to firmly adhere the negative electrode active material particles to each other and to firmly adhere the negative electrode active material to the current collector COL 2. As the binder, a non-aqueous binder, an aqueous binder, a dry binder, or a combination thereof can be used.

[0045] Examples of non-aqueous binders include polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene propylene copolymer, polystyrene, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, or combinations thereof.

[0046] The water-based binder may be selected from styrene-butadiene rubber, (meth)acrylate 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, polyether resin, (meth)acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0047] When an aqueous binder is used as the negative electrode binder, it may further contain a cellulose-based compound to impart viscosity. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be sodium, potassium, or lithium.

[0048] The dry binder is a fiberizable polymeric material, such as polytetrafluoroethylene, polyvinylidene fluoride, polyvinylidene fluoride-hexafluoropropylene copolymer, ethylene oxide, or combinations thereof.

[0049] The conductive material is used to impart conductivity to the electrode, and any material that does not cause a chemical change in the constructed 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.

[0050] As the current collector COL2, it is possible to use those 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.

[0051] (Negative electrode active material) The negative electrode active material in the negative electrode active material layer AML2 includes a material capable of reversibly inserting / desorbing lithium ions, lithium metal, an alloy of lithium metal, a material capable of doping or undoping lithium, or a transition metal oxide.

[0052] The material capable of reversibly inserting / desorbing lithium ions is a carbon-based negative electrode active material, which may include, for example, crystalline carbon, amorphous carbon, or a combination thereof. Examples of crystalline carbon include graphite such as amorphous, plate-like, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of amorphous carbon include soft carbon or hard carbon, mesophase pitch carbide, calcined coke, etc.

[0053] As the alloy of lithium metal, 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 can be used.

[0054] As the material capable of doping or undoping 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.

[0055] 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 the surfaces of the silicon particles coated with amorphous carbon. For example, the composite may include secondary particles (cores) formed by combining primary silicon particles, and an amorphous carbon coating layer (shell) located on the surfaces of the secondary particles. 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 in an amorphous carbon matrix.

[0056] The silicon-carbon composite may further comprise crystalline carbon. For example, the silicon-carbon composite may comprise a core comprising crystalline carbon and silicon particles and an amorphous carbon coating layer disposed on the core.

[0057] The Si-based negative electrode active material or the Sn-based negative electrode active material can be used in combination with a carbon-based negative electrode active material.

[0058] (Separator 30) Depending on the type of lithium secondary battery, a separator 30 may be present between the positive electrode 10 and the negative electrode 20. As such a separator 30, polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more layers thereof can be used, but it goes without saying that mixed multilayer 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.

[0059] Separator 30 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.

[0060] The porous substrate may be a polymer membrane made of 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 copolymers, polyphenylene sulfide, polyethylene naphthalate, glass fiber, Teflon, and polytetrafluoroethylene, or a copolymer or mixture of two or more of these.

[0061] The organic material may include a polyvinylidene fluoride polymer or a (meth)acrylic polymer.

[0062] 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.

[0063] 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 stacked.

[0064] (Electrolyte ELL) The electrolyte ELL for lithium secondary batteries contains a non-aqueous organic solvent and a lithium salt.

[0065] The non-aqueous organic solvent functions as a medium through which ions involved in the electrochemical reaction of the battery can migrate.

[0066] The non-aqueous organic solvent may be a carbonate-based, ester-based, ether-based, ketone-based, or alcohol-based solvent, a non-quantum solvent, or a combination thereof.

[0067] Examples of carbonate solvents that can be used include dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl pyrrolyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), and butylene carbonate (BC).

[0068] Examples of ester solvents that can be used include methyl acetate, ethyl acetate, n-propyl acetate, dimethyl acetate, methyl propionate, ethyl propionate, propyl propionate, decanolide, mevalonolactone, valerolactone, and caprolactone.

[0069] 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 non-quantum solvents that can be used include nitriles such as R-CN (R is a hydrocarbon group having a linear, branched, or cyclic structure and having 2 to 20 carbon atoms, and 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.

[0070] The non-aqueous organic solvents can be used alone or in combination of two or more.

[0071] When a carbonate-based solvent is used, a cyclic carbonate and a chain carbonate may be mixed and used, and the cyclic carbonate and the chain cyclic carbonate may be mixed in a volume ratio of 1:1 to 1:9.

[0072] Lithium salts are substances dissolved in organic solvents and act as a source of lithium ions within the battery, enabling basic lithium secondary battery operation and facilitating the movement of lithium ions between the positive and negative electrodes. Representative examples of lithium salts include LiPF6, LiBF4, LiSbF6, LiAsF6, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiCl, LiI, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI), LiC4F9SO3, and LiN(C x F 2x+1 SO2)(C y F 2y+1 SO2) (x and y are positive numbers from 1 to 20), lithium trifluoromethanesulfonate, lithium tetrafluoroethanesulfonate, lithium difluorobis(oxalato)phosphate (LiDFOB), and lithium bis(oxalato)borate (LiBOB).

[0073] (lithium secondary battery) Lithium secondary batteries may be classified into cylindrical, prismatic, pouch, and coin types depending on their shape. FIGS. 2 to 5 are schematic diagrams showing a lithium secondary battery according to an embodiment of the present invention, with FIG. 2 illustrating a cylindrical battery, FIG. 3 illustrating a prismatic battery, and FIGS. 4 and 5 illustrating pouch battery types. Referring to FIGS. 2 to 4, a 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 housing 50 in which the electrode assembly 40 is embedded. 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 housing 50, as shown in FIG. 2. Also, in FIG. 3, 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. 4 and 5, the lithium secondary battery 100 may include electrode tabs 70, i.e., a positive electrode tab 71 and a negative electrode tab 72, which function as electrical paths for conducting the current generated in the positive electrode assembly 40 to the outside.

[0074] The electrolyte solution for a lithium secondary battery according to the embodiment of the present invention will be described in more detail below.

[0075] An electrolyte for a lithium secondary battery according to an embodiment of the present invention may include a lithium salt, a non-aqueous organic solvent, and a nitrile-based additive. The nitrile-based additive may include at least one of compounds represented by the following Formulas 3 to 5:

[0076] [Chemical formula 3] [ka]

[0077] In Chemical Formula 3, R is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms.

[0078] [Chemical formula 4] [ka]

[0079] In Chemical Formula 4, k may be an integer from 0 to 10.

[0080] [Chemical formula 5] [ka]

[0081] In Chemical Formula 5, l, m, and n may each independently be an integer from 0 to 10, and l, m, and n may each be a different integer.

[0082] The nitrile-based compound forms a coating on the surface of the electrode during the activation process and initial charge / discharge process, thereby reducing side reactions between the electrode and the electrolyte. The formation of a stable coating on the surface of the negative electrode effectively inhibits the reductive decomposition of the non-aqueous organic solvent on the surface of the negative electrode during battery storage, particularly at high temperatures. This inhibits an increase in battery resistance during high-temperature storage, improving battery life.

[0083] Furthermore, the nitrile compound forms a complex with the transition metal when the transition metal is eluted, thereby making it possible to suppress the reduction in life and gas generation due to the elution of the transition metal.

[0084] Among the compounds represented by Chemical Formula 5, 1,3,6-HTCN has three CN groups, allowing it to form multiple coordinate bonds. HTCN can also form coordinate bonds that surround metal ions three-dimensionally, forming stable coordinate bond compounds. This reduces side reactions between the electrode active material and the electrolyte and more effectively removes eluted transition metals.

[0085] The additive may be included in an amount of 0.01 wt % to 5 wt %, 0.05 wt % to 5 wt %, 0.05 wt % to 4 wt %, 0.1 wt % to 4 wt %, or 0.1 wt % to 3 wt % based on the total amount of the electrolyte.

[0086] When the content of the additive satisfies the above range, the electrolyte solution has a suitable viscosity and satisfies the wettability for the positive and negative electrodes.When the content of the additive satisfies the above range, the additive can exert its effect as a surfactant.

[0087] The non-aqueous organic solvent according to one embodiment of the present invention may be a mixed solvent of ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC).

[0088] As an example, ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) may be mixed in a volume ratio of 1:a:b, where a and b may each be 1 to 3.

[0089] As another example, the ethylene carbonate (EC) solvent may be present in an amount of 5 to 30 volume %, 10 to 30 volume %, or 10 to 20 volume % relative to the total amount of the non-aqueous organic solvent. The ethyl methyl carbonate (EMC) solvent may be present in an amount of 20 to 60 volume %, 20 to 50 volume %, or 30 to 50 volume % relative to the total amount of the non-aqueous organic solvent. The dimethyl carbonate (DMC) solvent may be present in an amount of 20 to 60 volume %, 20 to 50 volume %, or 30 to 50 volume % relative to the total amount of the non-aqueous organic solvent.

[0090] The electrolyte according to one embodiment of the present invention can use LiPF6 as the lithium salt.

[0091] The concentration of the lithium salt may be 0.1 M to 2.0 M. Specifically, the concentration of the lithium salt may be 0.5 M or more, or 1.0 M or more. The concentration of the lithium salt may be 2.0 M or less, 1.7 M or less, or 1.5 M or less. In the present invention, when the concentration of the lithium salt is 0.1 M to 2.0 M, the conductivity and viscosity of the electrolyte can be appropriately maintained. [Example]

[0092] Examples of the present invention and comparative examples are 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.

[0093] Examples and Comparative Examples An electrolyte and a lithium secondary battery were manufactured as follows.

[0094] Example 1 (1) Electrolyte production An electrolyte solution was prepared by dissolving 1.5 M LiPF6 in a non-aqueous organic solvent containing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 20:40:40, and adding 0.5 wt% of an additive.

[0095] The additive used was the one represented by the following chemical formula 5-1.

[0096] [Chemical formula 5-1] [ka]

[0097] (2) Manufacture of lithium secondary batteries LiFePO4 (LFP) as a positive electrode active material, polyvinylidene fluoride as a binder, and carbon black as a conductive material were mixed in a weight ratio of 98:1:1 and dispersed in N-methylpyrrolidone to prepare a positive electrode active material slurry.

[0098] The positive electrode active material slurry was coated on an Al foil having a thickness of 20 μm, dried at 100° C., and then pressed to prepare a positive electrode.

[0099] As the negative electrode active material, graphite, styrene-butadiene rubber binder, and carboxymethyl cellulose were mixed in a weight ratio of 98:1:1, and dispersed in distilled water to prepare a negative electrode active material slurry.

[0100] The negative electrode active material slurry was coated on a Cu foil having a thickness of 10 μm, dried at 100° C., and then rolled to prepare a negative electrode.

[0101] The 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.

[0102] Example 2 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 wt % of a compound represented by the following Formula 4-1 was added as an additive when preparing the electrolyte solution.

[0103] [Chemical formula 4-1] [ka]

[0104] Example 3 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 wt % of a compound represented by the following Formula 3-1 was added as an additive when preparing the electrolyte solution.

[0105] [Chemical formula 3-1] [ka]

[0106] Example 4 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 wt % of a compound represented by the following Formula 3-2 was added as an additive when preparing the electrolyte solution.

[0107] [Chemical formula 3-2] [ka]

[0108] Example 5 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.5 wt % of a compound represented by the following Formula 3-3 was added as an additive when preparing the electrolyte solution.

[0109] [Chemical formula 3-3] [ka]

[0110] Example 6 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.1 wt % of the compound represented by Formula 5-1 was added as an additive when preparing the electrolyte solution.

[0111] Example 7 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 0.3 wt % of the compound represented by Formula 5-1 was added as an additive when preparing the electrolyte solution.

[0112] Example 8 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 1 wt % of the compound represented by Formula 5-1 was added as an additive when preparing the electrolyte solution.

[0113] Example 9 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 3 wt % of the compound represented by Formula 5-1 was added as an additive when preparing the electrolyte solution.

[0114] Example 10 An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that 5 wt % of the compound represented by Formula 5-1 was added as an additive when preparing the electrolyte solution.

[0115] Example 11 LiFe is used as a positive electrode active material in manufacturing lithium secondary batteries. 0.4 Mn 0.6 An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that PO4(LMFP) was used.

[0116] Example 12 LiFe is used as a positive electrode active material in manufacturing lithium secondary batteries. 0.4 Mn 0.6 An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 6, except that PO4 was used.

[0117] Example 13 LiFe is used as a positive electrode active material in manufacturing lithium secondary batteries. 0.4 Mn 0.6 An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 9, except that PO4 was used.

[0118] (Comparative Example 1) An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 1, except that no additives were added when preparing the electrolyte solution.

[0119] (Comparative Example 2) An electrolyte solution and a lithium secondary battery were prepared in the same manner as in Example 11, except that no additives were added when preparing the electrolyte solution.

[0120] (Comparative Example 3) LiNi is used as a positive electrode active material in manufacturing lithium secondary batteries. 0.8 Co 0.1 Al 0.1 An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that O2(NCA) was used.

[0121] Comparative Example 4 LiNi is used as a positive electrode active material in manufacturing lithium secondary batteries. 0.7 Co 0.2 Mn 0.1 An electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that O2(NCM) was used.

[0122] (Evaluation example) The electrolyte solution and the lithium secondary battery were evaluated by the following methods.

[0123] (Evaluation 1: Evaluation of room temperature life characteristics) The secondary batteries of Examples 1 to 13 and Comparative Examples 1 to 4 were continuously charged and discharged up to 800 cycles at room temperature (25°C) under 0.5C charge and 0.5C discharge conditions, and then the capacity retention rate and resistance increase rate (hereinafter sometimes referred to as "DC-IR increase rate") after 800 cycles were evaluated. The evaluation results are shown in Tables 1 and 2 below.

[0124] The capacity retention rate was calculated according to the following formula 1, and the resistance increase rate was calculated according to the following formula 2.

[0125] [Formula 1] Capacity retention rate after 800 cycles (%) = (discharge capacity at 800 cycles / discharge capacity at 1 cycle) × 100

[0126] [Formula 2] DC-IR increase rate (%) = (DC-IR at 800 cycles (mΩ) / DC-IR at 1 cycle (mΩ)) × 100

[0127] [Table 1]

[0128] Referring to Table 1, it can be seen that the lithium batteries according to Examples 1 to 13 have higher capacity retention rates and lower DC-IR increase rates than the lithium batteries according to Comparative Examples 1 and 2. In particular, it can be seen that the lithium batteries according to Examples 1 and 8 have high capacity retention rates of up to 90% after 800 cycles and low resistance increase rates of 114% or less.

[0129] [Table 2]

[0130] Referring to Table 2, it can be seen that the capacity retention rates of the lithium secondary batteries of Examples 1 and 11, which used an electrolyte containing an additive of Formula 5-1 together with an olivine-based positive electrode active material (LFP, LMFP), were higher than those of the lithium secondary batteries of Comparative Examples 1 and 2, which did not contain an additive.

[0131] In addition, the DC-IR increase rate of Example 1 was lower than that of Comparative Example 1, and the DC-IR increase rate of Example 11 was lower than that of Comparative Example 2. This confirms that the use of an electrolyte solution containing the additive of Chemical Formula 5-1 significantly suppresses the increase in resistance.

[0132] Further referring to Table 2, it can be seen that the capacity retention rates of the lithium secondary batteries of Examples 1 and 11, which used an electrolyte containing an additive of Chemical Formula 5-1 together with an olivine-based positive electrode active material (LFP, LMFP), were higher than those of the lithium secondary batteries of Comparative Examples 3 and 4, which used an electrolyte containing an additive of Chemical Formula 5-1 together with a nickel-based positive electrode active material (NCA, NCM).

[0133] In addition, it can be seen that the DC-IR increase rates of Examples 1 and 11 are lower than those of Comparative Examples 3 and 4, demonstrating a superior effect of suppressing the increase in resistance. In other words, when the additive of the present invention is used with an olivine-based positive electrode active material, it is possible to obtain a significant improvement in the lifespan and storage characteristics.

[0134] (Evaluation 2: High-temperature storage characteristics) The secondary batteries of Examples 1 to 13 and Comparative Examples 1 to 4 were charged to 100% SOC, and then left at 60° C. for 60 days, and the capacity retention rate and resistance increase rate of the batteries were measured. The measurement results are shown in Tables 3 and 4 below.

[0135] The capacity retention rate was calculated according to the following formula 3, and the resistance increase rate was calculated according to the following formula 4.

[0136] [Formula 3] Capacity retention rate (%) = (discharge capacity after 60 days / discharge capacity immediately before storage) x 100

[0137] [Formula 4] DC-IR increase rate (%) = (DC-IR after 60 days (mΩ) / DC-IR immediately before storage (mΩ)) × 100

[0138] [Table 3]

[0139] Referring to Table 3, it can be seen that the lithium batteries of Examples 1 to 13 containing a nitrile-based additive have higher high-temperature capacity retention and lower high-temperature DC-IR increase rates than the lithium batteries of Comparative Examples 1 and 2 containing no additive. In particular, it can be seen that the lithium batteries of Examples 1 and 8 have high capacity retention rates of up to 92% when left at 60°C for 60 days and low resistance increase rates of 116% or less.

[0140] [Table 4]

[0141] Referring to Table 4, it can be seen that the capacity retention rates of the lithium secondary batteries of Examples 1 and 11, which used an electrolyte containing an additive of Formula 5-1 together with an olivine-based positive electrode active material (LFP, LMFP), were higher than those of the lithium secondary batteries of Comparative Examples 1 and 2, which did not contain an additive.

[0142] Furthermore, since the high-temperature DC-IR increase rate of Example 1 is lower than that of Comparative Example 1, and the high-temperature DC-IR increase rate of Example 11 is lower than that of Comparative Example 2, it can be confirmed that the use of an electrolyte containing the additive of Chemical Formula 5-1 significantly suppresses the increase in high-temperature resistance.

[0143] Further referring to Table 4, it can be seen that the capacity retention rates of the lithium secondary batteries of Examples 1 and 11, which used an electrolyte containing an additive of Chemical Formula 5-1 together with an olivine-based positive electrode active material (LFP, LMFP), were higher than those of the lithium secondary batteries of Comparative Examples 3 and 4, which used an electrolyte containing an additive of Chemical Formula 5-1 together with a nickel-based positive electrode active material (NCA, NCM).

[0144] Furthermore, it can be seen that the high-temperature DC-IR increase rates of Examples 1 and 11 are lower than those of Comparative Examples 3 and 4, demonstrating a superior effect of suppressing the increase in high-temperature resistance. In other words, when the additive of the present invention is used with an olivine-based positive electrode active material, it is possible to obtain a significant improvement in high-temperature life and high-temperature storage characteristics.

[0145] Although the preferred embodiment of the present invention has been described above, the present invention is not limited thereto, and can be embodied in various modifications within the scope of the claims, the embodiments for implementing the invention, and the accompanying drawings, and it is of course understood that these also fall within the scope of the present invention. [Explanation of symbols]

[0146] 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: Cabinet 60: Sealing material 70: Electrode tab 71: Positive electrode tab 72: Negative electrode tab

Claims

1. a positive electrode including a positive electrode active material; a negative electrode including a negative electrode active material; an electrolyte; and The electrolyte contains a nitrile-based additive, The positive electrode active material includes at least one of compounds represented by the following Formulas 1 and 2: [Chemical formula 1] Li ａ１ Fe ｘ１ B ｙ１ PO ４－ｂ１ In the formula 1, 0.8≦a1≦1.2, 0.9≦x1≦1.1, 0≦y1≦0.05, and 0≦b1≦0.05; [Chemical formula 2] Li ａ２ Mn ｚ２ Fe ｘ２ B ｙ２ 2O ４－ｂ２ In Chemical Formula 2, 0.8≦a2≦1.2, 0.5≦z2≦0.9, 0.1≦x2≦0.5, 0≦y2≦0.05, 0≦b2≦0.05, and 0.9≦z2+x2≦1.2; In the formulas 1 and 2, B is at least one element selected from the group consisting of Ti, Mg, V, and Nb.

2. 2. The lithium secondary battery according to claim 1, wherein said B is Ti.

3. The nitrile-based additive includes at least one compound represented by the following formulas 3 and 5: [Chemical formula 3] 【Chemical 1】 In Formula 3, R is a substituted or unsubstituted alkyl group having 1 to 10 carbon atoms; [Chemical formula 4] 【Chemistry 2】 In Formula 4, k is an integer from 0 to 10; [Chemical formula 5] 【Chemistry 3】 2. The lithium secondary battery of claim 1, wherein in Formula 5, l, m, and n each independently represent an integer of 0 to 10.

4. The lithium secondary battery according to claim 3 , wherein l, m, and n are integers different from one another.

5. 2. The lithium secondary battery of claim 1, wherein the nitrile additive comprises at least one of 1,3,6-HTCN (1,3,6-hexanetricyanamide) and 1,2,6-HTCN (1,2,6-hexanetricyanamide).

6. The lithium secondary battery of claim 1 , wherein the nitrile additive is contained in an amount of 0.01 to 5 wt % based on the total weight of the electrolyte solution.

7. The electrolyte solution is a non-aqueous organic solvent; The lithium secondary battery according to claim 1 , further comprising: a lithium salt.

8. 8. The lithium secondary battery according to claim 7, wherein the non-aqueous organic solvent comprises a carbonate-based solvent.

9. The carbonate-based solvent is Ethylene carbonate (EC), Ethyl methyl carbonate (EMC), 9. The lithium secondary battery of claim 8, comprising:

10. the ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) have a volume ratio of 1:a:b; a is 1 to 3, 10. The lithium secondary battery according to claim 9, wherein b is an integer of 1 to 5.

11. The lithium salt is LiPF ６ 8. The lithium secondary battery according to claim 7, wherein

12. 8. The lithium secondary battery according to claim 7, wherein the concentration of the lithium salt is 0.1M to 2.0M.

13. The lithium secondary battery according to claim 1 , wherein the negative electrode active material comprises a carbon-based negative electrode active material, a Si-based negative electrode active material, a Sn-based negative electrode active material, or a combination thereof.

14. The lithium secondary battery according to claim 1 , wherein the surface of the negative electrode further comprises a solid electrolyte interface (SEI) film.

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