Lithium secondary battery

Abstract

The present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprising lithium iron phosphate particles, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by a specific chemical formula.

Description

[Technical Field]

[0001] This application claims the benefit of priority based on Korean Patent Application No. 10-2022-0146435, filed November 4, 2022, and all contents disclosed in the documents of this Korean patent application are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery, and more specifically to a lithium secondary battery that includes a nonaqueous electrolyte containing an additive capable of forming a strong SEI film on the positive and negative electrodes, thereby suppressing an increase in initial resistance and improving output characteristics and life performance. [Background technology]

[0003] The development of the information society has led to the development of personal IT devices and computer networks, and as a result, society as a whole has become more dependent on electrical energy, which has created a demand for the development of technologies to efficiently store and utilize electrical energy.

[0004] Of all the technologies developed, secondary batteries are the most suitable for a variety of applications. Among these secondary batteries, interest is growing in lithium secondary batteries, which not only can be miniaturized to a degree that makes them suitable for personal IT devices, but also have the highest energy density.

[0005] Generally, a lithium secondary battery is manufactured by injecting or impregnating a non-aqueous electrolyte into an electrode assembly including a positive electrode, a negative electrode, and a porous separator.

[0006] Possible negative electrode active materials for such lithium secondary batteries include carbon-based active materials and silicon-based active materials, while possible positive electrode active materials include lithium-containing cobalt oxide, LiMnO2 with a layered crystal structure, LiMn2O4 with a spinel crystal structure, and lithium-containing nickel oxide (LiNiO2).

[0007] Recently, lithium iron phosphate (for example, LiFePO4)-based active materials have been considered for use as positive electrode active materials because they have excellent thermal stability and are relatively inexpensive.

[0008] On the other hand, lithium salts such as LiPF6 contained in non-aqueous electrolytes are converted into PF6 - Thermal decomposition of anions can form Lewis acids such as PF5, which can react with water to produce HF. Substances such as PF5 or HF can not only destroy the coating formed on the electrode surface but also cause decomposition of the organic solvent. In particular, when lithium iron phosphate is used as the positive electrode active material, there is a problem of Fe leaching from the surface of the positive electrode active material exposed by the HF and PF5. This Fe leaching destabilizes the lattice structure of the lithium iron phosphate, which generates active oxygen, accelerating the decomposition of the organic solvent in the non-aqueous electrolyte and accelerating gas generation. Furthermore, the leached Fe migrates to the negative electrode through the non-aqueous electrolyte and is electrodeposited on the negative electrode surface, destroying the solid electrolyte interface layer (SEI). The process of rebuilding the destroyed SEI layer can cause further lithium ion depletion, resulting in increased resistance and reduced capacity. [Prior art documents] [Patent documents]

[0009] [Patent Document 1] Korean Patent Publication No. 2017-0012308 Summary of the Invention [Problem to be solved by the invention]

[0010] An object of the present invention is to solve the above-mentioned problems and to provide a lithium secondary battery containing lithium iron phosphate particles as a positive electrode active material, in which a low-resistance SEI film is formed on the negative electrode by suppressing side reactions caused by Fe eluted from the lithium iron phosphate, thereby improving both output characteristics and life characteristics. [Means for solving the problem]

[0011] The present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprising lithium iron phosphate particles, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1: [ka] In the above chemical formula 1, R1 and R2 are each independently an alkylene group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms. [Effects of the Invention]

[0012] The present invention relates to a lithium secondary battery in which the positive electrode contains a positive electrode active material including lithium iron phosphate particles, and the non-aqueous electrolyte contains, as an additive, the compound represented by Chemical Formula 1. The compound represented by Chemical Formula 1 contains, in its structure, a propargyl group (—C≡C—) and a fluorocarbon functional group substituted with one or more fluorine elements, and is therefore reduced before the organic solvent, allowing it to form a low-resistance SEI film containing a fluorocarbon component on the surface of the negative electrode. The SEI film formed on the negative electrode by the compound represented by Chemical Formula 1 not only prevents side reactions caused by Fe eluted from the lithium iron phosphate particles, but also has low resistance, thereby simultaneously improving the life and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0013] First, before describing the present invention, the terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that inventors can appropriately define the concepts of terms in order to best describe their inventions.

[0014] Meanwhile, the terms used in this specification are merely used to describe exemplary embodiments and are not intended to limit the present invention. The singular expressions include the plural expressions unless the context clearly indicates otherwise.

[0015] In this specification, the terms "comprises," "includes," "has," and the like are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, but are not intended to preclude the possible presence or addition of one or more different features, numbers, steps, components, or combinations thereof.

[0016] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0017] Before describing the present invention, in the description of "number of carbon atoms a to b" in the specification, "a" and "b" mean the number of carbon atoms contained in a specific functional group. That is, the functional group can contain "a" to "b" carbon atoms.

[0018] In addition, in this specification, unless otherwise defined, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom has been substituted with an element other than hydrogen, for example, with an alkyl group having 1 to 5 carbon atoms or with a fluorine atom.

[0019] In this specification, the average particle size (D 50 The average particle size (D) can be defined as the particle size corresponding to 50% of the cumulative volume in the particle size distribution curve. 50 ) can be measured using, for example, a laser diffraction method. The laser diffraction method generally enables measurement of particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0020] The present invention will now be described in further detail.

[0021] Lithium secondary battery The present invention provides a lithium secondary battery comprising a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode comprises a positive electrode active material, the positive electrode active material comprising lithium iron phosphate particles, the non-aqueous electrolyte comprises a lithium salt, an organic solvent, and an additive, and the additive comprises a compound represented by the following Chemical Formula 1:

[0022] [ka]

[0023] In the above chemical formula 1, R1 and R2 are each independently an alkylene group having 1 to 10 carbon atoms, and R3 is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms.

[0024] The present invention relates to a lithium secondary battery in which the positive electrode contains a positive electrode active material including lithium iron phosphate particles, and the non-aqueous electrolyte contains, as an additive, the compound represented by Chemical Formula 1. The compound represented by Chemical Formula 1 contains, in its structure, a propargyl group (—C≡C—) and a fluorocarbon functional group substituted with one or more fluorine elements, and is therefore reduced before the organic solvent, allowing it to form a low-resistance SEI film containing a fluorocarbon component on the surface of the negative electrode. The SEI film formed on the negative electrode by the compound represented by Chemical Formula 1 not only prevents side reactions caused by Fe eluted from the lithium iron phosphate particles, but also has low resistance, thereby simultaneously improving the life and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery.

[0025] The lithium secondary battery includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Specifically, the lithium secondary battery includes a positive electrode, a negative electrode facing the positive electrode, a separator interposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. The lithium secondary battery may be manufactured by placing an electrode assembly including the positive electrode, the negative electrode facing the positive electrode, and a separator interposed between the positive electrode and the negative electrode in a battery case and then injecting the non-aqueous electrolyte.

[0026] (1) Positive electrode The positive electrode includes a positive electrode active material, which includes lithium iron phosphate particles.

[0027] The lithium iron phosphate particles may include a compound represented by the following chemical formula A:

[0028] [Chemical formula A] Li 1+a Fe 1-s M s (PO 4-b )X b

[0029] In the chemical formula A, M is one or more elements selected from Co, Ni, Al, Mg, Ti, and V, and X is F, S, or N, where 0≦s≦0.5; −0.5≦a≦+0.5; and 0≦b≦0.1.

[0030] The chemical formula A can be specifically represented as LiFePO4 (a=0, s=0, and b=0).

[0031] The lithium iron phosphate particles may be composed of primary particles, secondary particles formed by agglomeration of two or more primary particles, or a mixture of primary particles and secondary particles formed by agglomeration of two or more primary particles.

[0032] Here, the primary particles have an average particle size (D 50 ) can be 0.2 μm to 3.0 μm, specifically 0.2 μm to 1.0 μm, and more specifically 0.3 μm to 0.8 μm, and the secondary particles can have an average particle size (D50 ) may be 7 μm to 25 μm, particularly 10 μm to 20 μm.

[0033] The positive electrode active material may further include, but is not limited to, a carbon coating layer disposed on the lithium iron phosphate particles for purposes such as protecting the lithium iron phosphate particles and improving electrical conductivity.

[0034] The positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on at least one surface of the positive electrode current collector, wherein the positive electrode active material layer may include the above-described positive electrode active material.

[0035] The positive electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the positive electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0036] The positive electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0037] The positive electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the positive electrode active material. For example, the positive electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous body, a foam, or a nonwoven fabric.

[0038] The positive electrode active material layer may be disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one or both surfaces of the positive electrode current collector.

[0039] The positive electrode active material may be contained in the positive electrode active material layer in an amount of 80 to 99% by weight, taking into consideration the sufficient capacity of the positive electrode active material.

[0040] The positive electrode active material layer may further include a binder and / or a conductive material in addition to the positive electrode active material.

[0041] The binder is a component that helps bind the active material and conductive material together and to the current collector, and specifically includes at least one selected from the group consisting of polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer (EPDM), sulfonated EPDM, styrene-butadiene rubber, and fluororubber, preferably polyvinylidene fluoride.

[0042] The binder may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight, in order to ensure sufficient binding strength between components such as the positive electrode active material.

[0043] The conductive material can be used to assist and improve the conductivity of a secondary battery, and is not particularly limited as long as it does not cause chemical changes and has conductivity. Specifically, the positive electrode conductive material can include at least one selected from the group consisting of graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and polyphenylene derivatives. Preferably, the positive electrode conductive material can include carbon nanotubes in order to improve conductivity.

[0044] In order to ensure sufficient electrical conductivity, the conductive material may be contained in the positive electrode active material layer in an amount of 1 to 20% by weight, preferably 1.2 to 10% by weight.

[0045] The thickness of the positive electrode active material layer may be 10 μm to 500 μm, and preferably 200 μm to 400 μm.

[0046] The loading amount of the positive electrode active material layer is 2.5 mAh / cm 2 ~5.0mAh / cm 2 , preferably 3mAh / cm 2 ~4mAh / cm 2 It can be.

[0047] The positive electrode may be manufactured by coating a positive electrode slurry containing a positive electrode active material and, optionally, a binder, a conductive material, and a solvent for forming the positive electrode slurry on the positive electrode current collector, followed by drying and rolling.

[0048] The solvent for forming the positive electrode slurry may include an organic solvent such as NMP (N-methyl-2-pyrrolidone). The solid content of the positive electrode slurry may be 40% by weight to 90% by weight, specifically 50% by weight to 80% by weight.

[0049] (2) Negative electrode The negative electrode can face the positive electrode.

[0050] The negative electrode includes a negative electrode active material.

[0051] The negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one surface of the negative electrode current collector, wherein the negative electrode active material may be included in the negative electrode active material layer.

[0052] The negative electrode current collector is not particularly limited as long as it does not cause chemical changes in the battery and has high conductivity. Specifically, the negative electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0053] The negative electrode current collector can usually have a thickness of 3 μm to 500 μm.

[0054] The negative electrode current collector may have a surface with fine irregularities to enhance the bonding strength of the negative electrode active material. For example, the negative electrode current collector may be used in various forms such as a film, a sheet, a foil, a mesh, a porous material, a foam, or a nonwoven fabric.

[0055] The negative electrode active material layer may be disposed on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both surfaces of the negative electrode current collector.

[0056] The negative electrode active material layer may include a negative electrode active material.

[0057] The negative electrode active material may include at least one material selected from the group consisting of a carbon-based active material, a (semi)metal-based active material, and lithium metal, as a material capable of reversibly inserting / extracting lithium ions. Specifically, the negative electrode active material may include at least one material selected from the group consisting of a carbon-based active material and a (semi)metal-based active material.

[0058] The carbon-based active material may include at least one selected from the group consisting of artificial graphite, natural graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and preferably includes at least one selected from the group consisting of artificial graphite and natural graphite.

[0059] The average particle size (D 50) can be 10 μm to 30 μm, preferably 15 μm to 25 μm, in order to ensure structural stability and reduce side reactions with the electrolyte during charge and discharge.

[0060] Specifically, the (semi)metallic active material may include at least one (semi)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an alloy of at least one (semi)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; an oxide of at least one (semi)metal selected from the group consisting of Cu, Ni, Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al, V, Ti, and Sn; lithium titanium oxide (LTO); lithium vanadium oxide; and the like.

[0061] More specifically, the (semi)metallic active material can include a silicon-based active material.

[0062] The silicon-based active material is SiO x (0≦x<2) In the case of SiO2, since it does not react with lithium ions and therefore cannot store lithium, it is preferable that x is within the above range, and more preferably, the silicon-based active material can be SiO.

[0063] The average particle size (D 50 ) can be 1 μm to 30 μm, preferably 2 μm to 15 μm, in order to ensure structural stability and reduce side reactions with the electrolyte during charge and discharge.

[0064] The negative electrode active material may be contained in the negative electrode active material layer in an amount of 60% by weight to 99% by weight, preferably 75% by weight to 95% by weight.

[0065] The negative electrode active material layer may further include a binder and / or a conductive material in addition to the negative electrode active material.

[0066] The binder is used to improve the adhesive strength between the negative electrode active material layer and the negative electrode current collector, thereby improving battery performance. For example, the binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0067] The binder may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 10% by weight, preferably 1% by weight to 5% by weight.

[0068] The conductive material is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity, and examples thereof include graphite such as natural graphite and artificial graphite; carbon black such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; conductive tubes such as carbon nanotubes; fluorocarbons; metal powders such as aluminum and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives.

[0069] The conductive material may be contained in the negative electrode active material layer in an amount of 0.5% by weight to 10% by weight, preferably 1% by weight to 5% by weight.

[0070] The thickness of the negative electrode active material layer may be 5 μm to 500 μm, and preferably 100 μm to 300 μm.

[0071] The loading amount of the negative electrode active material layer is 3.0 mAh / cm 2 ~5.5mAh / cm 2 , preferably 3.5mAh / cm 2 ~4.5mAh / cm 2 It can be.

[0072] The negative electrode may be manufactured by coating at least one surface of a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, and / or a solvent for forming the negative electrode slurry, followed by drying and rolling.

[0073] The solvent for forming the negative electrode slurry may include, for example, at least one selected from the group consisting of distilled water, NMP (N-methyl-2-pyrrolidone), ethanol, methanol, and isopropyl alcohol, preferably distilled water, in order to facilitate dispersion of the negative electrode active material, binder, and / or conductive material. The solid content of the negative electrode slurry may be 30% by weight to 80% by weight, specifically 40% by weight to 70% by weight.

[0074] (3) Separator The separator may be interposed between the positive electrode and the negative electrode.

[0075] The separator may be a conventional porous polymer film, such as a porous polymer film made of a polyolefin polymer such as ethylene homocopolymer, propylene homocopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, either alone or in a laminate, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. Furthermore, to ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be used in a single-layer or multi-layer structure.

[0076] (4) Nonaqueous electrolyte 1) Lithium salt First, the lithium salt will be described below.

[0077] In the non-aqueous electrolyte for a lithium secondary battery according to one embodiment of the present invention, the lithium salt may be any of those commonly used in electrolytes for lithium secondary batteries, without limitation. For example, a lithium salt containing Li as a cation may be used. + and as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , AlO4 - , AlCl4 - , PF6 - , SbF6 - , AsF6 - , B 10 Cl 10 - , BF2C2O4 - , BC4O8 - , PF4C2O4 - , PF2C4O8 - , (CF3)2PF4 - , (CF3)3PF3 -, (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , C4F9SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CH3SO3 - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - Specifically, the lithium salt may be at least one selected from the group consisting of LiCl, LiBr, LiI, LiBF4, LiClO4, LiAlO4, LiAlCl4, LiPF6, LiSbF6, LiAsF6, LiB 10 Cl 10 , LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), LiCH3SO3, LiCF3CO2, LiCH3CO2, and LiBETI (LiN(SO2CF2CF3)2). Specifically, the lithium salt may include a single substance or a mixture of two or more substances selected from the group consisting of LiBF4, LiClO4, LiPF6, LiBOB (LiB(C2O4)2), LiCF3SO3, LiTFSI (LiN(SO2CF3)2), LiFSI (LiN(SO2F)2), and LiBETI (LiN(SO2CF2CF3)2), and more specifically, LiPF6.

[0078] The lithium salt can be appropriately changed within a range that is normally usable, but in order to obtain the optimum effect of forming a corrosion prevention coating on the surface of the electrode, it can be contained in the electrolyte at a concentration of 0.8M to 3.0M, specifically 1.0M to 3.0M.

[0079] When the concentration of the lithium salt satisfies the above range, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal impregnation, and the mobility of lithium ions can be improved, thereby improving the capacity characteristics and cycle characteristics of the lithium secondary battery.

[0080] 2) Organic solvents The organic solvent is not particularly limited as long as it is a non-aqueous solvent commonly used in lithium secondary batteries and can minimize decomposition due to oxidation reactions during charging and discharging of the secondary battery.

[0081] Specifically, the organic solvent may include a cyclic carbonate organic solvent and a linear carbonate organic solvent.

[0082] The cyclic carbonate organic solvent is a highly viscous organic solvent that has a high dielectric constant and can effectively dissociate the lithium salt in the electrolyte. Specifically, the cyclic carbonate organic solvent may include at least one organic solvent selected from the group consisting of ethylene carbonate (EC), propylene carbonate (PC), 1,2-butylene carbonate, 2,3-butylene carbonate, 1,2-pentylene carbonate, 2,3-pentylene carbonate, and vinylene carbonate, and more specifically, may include ethylene carbonate.

[0083] The linear carbonate organic solvent is an organic solvent having low viscosity and low dielectric constant, and may specifically include at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methyl propyl carbonate, and ethyl propyl carbonate. In the case of a positive electrode containing lithium iron phosphate particles as a positive electrode active material, the thickness must be increased to achieve the required energy density. Therefore, in order to further improve electrolyte impregnation, the linear carbonate organic solvent may specifically include dimethyl carbonate and ethyl methyl carbonate, and more specifically, dimethyl carbonate and ethyl methyl carbonate may be included in a volume ratio of 60:40 to 90:10.

[0084] The organic solvent may be a mixture of a cyclic carbonate organic solvent and a linear carbonate organic solvent, wherein the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed in a volume ratio of 10:90 to 40:60, specifically, a volume ratio of 15:85 to 35:65.

[0085] Meanwhile, the organic solvent may be any organic solvent commonly used in non-aqueous electrolytes, without limitation, if necessary, and may further include at least one organic solvent selected from the group consisting of ester-based organic solvents, ether-based organic solvents, glyme-based organic solvents, and nitrile-based organic solvents.

[0086] The ester-based organic solvent may include at least one selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, butyl propionate, γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0087] The ether solvent may be any one selected from the group consisting of dimethyl ether, diethyl ether, dipropyl ether, methyl ethyl ether, methyl propyl ether, ethyl propyl ether, 1,3-dioxolane (DOL), and 2,2-bis(trifluoromethyl)-1,3-dioxolane (TFDOL), or a mixture of two or more thereof, but is not limited thereto.

[0088] The glyme-based solvent has a higher dielectric constant and lower surface tension than linear carbonate-based organic solvents, and is less reactive with metals. The glyme-based solvent may include, but is not limited to, at least one selected from the group consisting of dimethoxyethane (DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME).

[0089] The nitrile solvent may be at least one selected from the group consisting of acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but is not limited thereto.

[0090] On the other hand, the remainder of the non-aqueous electrolyte other than the lithium salt and the additive may be an organic solvent unless otherwise specified.

[0091] (3) Additives The non-aqueous electrolyte of the present invention contains an additive.

[0092] The additive includes a compound represented by the following Chemical Formula 1:

[0093] [ka]

[0094] In the above Chemical Formula 1, R1 and R2 each independently represent an alkylene group having 1 to 10 carbon atoms; R3 is an alkyl group having 1 to 20 carbon atoms and substituted with one or more fluorine atoms.

[0095] Specifically, the compound represented by Chemical Formula 1 contains a propargyl functional group within its structure, which allows it to be easily reductively decomposed on the surface of the negative electrode to form an SEI film with low resistance and high passivation ability, thereby improving the durability of the negative electrode itself. Furthermore, when a component derived from the compound represented by Chemical Formula 1 is included in the SEI film, it significantly prevents the problem of Fe eluted from lithium iron phosphate particles of the positive electrode being electrodeposited on the surface of the negative electrode, thereby destroying the negative electrode SEI film. Therefore, when a nonaqueous electrolyte containing the compound represented by Chemical Formula 1 as an electrolyte additive is used with a positive electrode that uses lithium iron phosphate particles as the positive electrode active material, it prevents the self-discharge reaction of the negative electrode, which is caused by further reductive decomposition of the electrolyte due to the instability of the SEI film.

[0096] In addition, the compound represented by Chemical Formula 1 contains fluorocarbon functional groups substituted with one or more fluorine elements at the ends of its structure, thereby forming an oxidation-resistant coating on the surface of the positive electrode, thereby suppressing the elution of Fe from the lithium iron phosphate in the positive electrode and suppressing the electrodeposition and precipitation of the eluted Fe on the negative electrode, thereby preventing internal short circuits.

[0097] As described above, the compound represented by Chemical Formula 1 contains a fluorocarbon functional group substituted with at least one fluorine element, which has excellent flame retardancy and non-flammability, and a propargyl group. This allows the compound to form a strong SEI coating with low resistance, thereby suppressing the additional reduction decomposition reaction of the electrolyte and preventing the self-discharge reaction of the anode, thereby suppressing an increase in initial resistance, and providing a lithium secondary battery with improved room temperature and low temperature output characteristics.

[0098] Meanwhile, in the above Chemical Formula 1, R1 and R2 may each independently be an alkylene group having 1 to 5 carbon atoms, and R3 may be an alkyl group having 3 to 20 carbon atoms substituted with one or more fluorine atoms.

[0099] In addition, in the above chemical formula 1, R1 and R2 may each independently be an alkylene group having 1 to 3 carbon atoms, and R3 may be an alkyl group having 3 to 15 carbon atoms substituted with one or more fluorine atoms.

[0100] Specifically, in the above Chemical Formula 1, R3 may be an alkyl group having 4 to 8 carbon atoms substituted with one or more fluorine atoms.

[0101] Preferably, the compound represented by Chemical Formula 1 may be a compound represented by Chemical Formula 1a below.

[0102] [ka]

[0103] Meanwhile, the compound represented by Chemical Formula 1 may be included in the non-aqueous electrolyte in an amount of 0.01 wt % to 10.0 wt %.

[0104] When the content of the compound represented by Chemical Formula 1 is within the above range, defects such as side reactions due to additives, a decrease in capacity, and an increase in resistance can be minimized, and a low-resistance SEI coating can be formed on the surface of the negative electrode, improving the lithium migration effect in the coating, suppressing further reduction and decomposition reactions of the electrolyte, and preventing self-discharge reactions of the negative electrode.

[0105] Specifically, when the content of the compound represented by Chemical Formula 1 is 0.01 wt % or more, a stable coating is formed during the battery's operating time, forming a low-resistance SEI coating on the surface of the negative electrode, thereby improving battery output performance. Also, when the content of the compound represented by Chemical Formula 1 is 10.0 wt % or less, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, effectively suppressing an increase in battery resistance due to decomposition of additives, and further increasing ionic conductivity within the battery, thereby preventing a decrease in output characteristics.

[0106] Specifically, the compound represented by Chemical Formula 1 may be contained in the non-aqueous electrolyte in an amount of 0.05 wt % to 6.0 wt %, specifically 0.08 wt % to 0.5 wt %.

[0107] Meanwhile, the additive may include, as needed, additional additives in addition to the compound represented by Chemical Formula 1 to prevent the non-aqueous electrolyte from being decomposed and causing the collapse of the negative electrode in a high-power environment, or to further improve low-temperature high-rate discharge characteristics, high-temperature stability, overcharge prevention, and suppression of battery expansion at high temperatures.

[0108] Examples of such additional additives include at least one selected from the group consisting of cyclic carbonate-based compounds, halogen-substituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, nitrile-based compounds, benzene-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

[0109] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinylethylene carbonate.

[0110] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0111] The sultone compound may be, for example, at least one compound selected from the group consisting of 1,3-propane sultone (PS), 1,4-butane sultone, ethene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone.

[0112] The sulfate-based compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0113] The phosphate- or phosphite-based compound can be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalate)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0114] Examples of the borate-based compound include tetraphenylborate, lithium difluoro(oxalate)borate (LiODFB), and lithium bisoxalateborate (LiB(C2O4)2, LiBOB).

[0115] The nitrile compound may be, for example, at least one compound selected from the group consisting of succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile.

[0116] The benzene-based compound may be, for example, fluorobenzene, the amine-based compound may be, for example, triethanolamine or ethylenediamine, and the silane-based compound may be, for example, tetravinylsilane.

[0117] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte, and examples thereof include lithium difluorophosphate (LiPO2F2) and LiBF4.

[0118] When at least one selected from the group consisting of vinyl ethylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, succinonitrile, and lithium difluoro(oxalate)borate is contained among these additional additives, a stronger SEI coating can be formed on the surface of the negative electrode during the initial activation step of the secondary battery.

[0119] The additional additive may be a mixture of two or more compounds, and the total content of the compound represented by Formula 1 and the additional additive may be 50 wt % or less, specifically 0.05 wt % to 20 wt %, and more specifically 0.05 wt % to 10 wt %, based on the total weight of the non-aqueous electrolyte. When the total content of the additives satisfies this range, the low-temperature output characteristics, high-temperature storage characteristics, and high-temperature life characteristics of the battery may be improved, and side reactions in the battery due to residual additives after the reaction may be prevented.

[0120] The lithium secondary battery according to the present invention can be usefully used in portable devices such as mobile phones, notebook computers, and digital cameras, and in electric vehicles such as hybrid electric vehicles (HEVs).

[0121] Therefore, according to another embodiment of the present invention, there is provided a battery module including the lithium secondary battery as a unit cell, and a battery pack including the same.

[0122] The battery module or battery pack may be used as a power source for one or more medium- to large-sized devices, including power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); or power storage systems.

[0123] The external shape of the lithium secondary battery of the present invention is not particularly limited, and may be a cylindrical shape using a can, a square shape, a pouch shape, a coin shape, or the like.

[0124] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for a small device, but also as a unit battery in a medium- to large-sized battery module including a large number of battery cells.

[0125] The present invention will be specifically described below with reference to examples.

[0126] Here, the embodiments of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the embodiments detailed below. The embodiments of the present invention are provided to more completely explain the present invention to those having average knowledge in the art.

[0127] The present invention will be specifically described below with reference to specific examples.

[0128] Example Example 1 (Production of non-aqueous electrolyte) An organic solvent was prepared by mixing ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) in a volume ratio of 30:10:60.

[0129] LiPF6 was dissolved as a lithium salt in the organic solvent to a molar concentration of 1.0M.

[0130] In addition, the compound represented by Formula 1a and additional additives were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0131] The compound represented by Chemical Formula 1a was contained in the non-aqueous electrolyte at 0.1 wt %.

[0132] The additional additives used were vinyl ethylene carbonate (VEC), 1,3-propane sultone (PS), fluoroethylene carbonate (FEC), succinonitrile (SN), and LiODFB. 0.5 wt% of vinyl ethylene carbonate (VEC), 1.0 wt% of 1,3-propane sultone (PS), 5.0 wt% of fluoroethylene carbonate (FEC), 1.0 wt% of succinonitrile (SN), and 0.5 wt% of LiODFB were added to the non-aqueous electrolyte.

[0133] (Secondary battery manufacturing) A positive electrode slurry (solid content: 50 wt%) was prepared by adding lithium iron phosphate particles (LiFePO4) as a positive electrode active material, carbon nanotubes as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 94:3:3 to N-methyl-2-pyrrolidone (NMP) as a solvent. The positive electrode slurry was applied to a 20 μm-thick positive electrode current collector (Al thin film) at a capacity of 3.7 mAh / cm. 2 After coating and drying, a roll press was performed to prepare a positive electrode (thickness of the positive electrode active material: 220 μm).

[0134] Graphite as the negative electrode active material, SBR-CMC as the binder, and carbon black as the conductive material were added to water as the solvent in a weight ratio of 96:3:1 to prepare a negative electrode slurry (solid content: 60 wt%). The negative electrode slurry was applied to a 10 μm-thick copper (Cu) thin film as a negative electrode current collector, and the resulting negative electrode slurry had a capacity of 4.2 mAh / cm. 2 After drying, a roll press was carried out to prepare a negative electrode (thickness of negative electrode active material: 170 μm).

[0135] The positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a negative electrode were sequentially stacked to prepare an electrode assembly.

[0136] The assembled electrode assembly was placed in a battery case, and the prepared non-aqueous electrolyte was poured into the battery case to prepare a lithium secondary battery.

[0137] Example 2 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by Formula 1a was added in an amount of 1.0 wt % based on the weight of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0138] Example 3 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by Formula 1a was added in an amount of 5.0 wt % based on the weight of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0139] Comparative Example 1 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that the non-aqueous electrolyte was manufactured without adding the compound represented by Chemical Formula 1a.

[0140] Comparative Example 2 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 2 was added in an amount of 0.1 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0141] [ka]

[0142] Comparative Example 3 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by Chemical Formula 2 was added in an amount of 1.0 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0143] Comparative Example 4 A non-aqueous electrolyte and a lithium secondary battery were prepared in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 3 was added in an amount of 0.1 wt % based on the weight of the non-aqueous electrolyte instead of the compound represented by Chemical Formula 1a to prepare a non-aqueous electrolyte.

[0144] [ka]

[0145] Experimental example Experimental Example 1: Evaluation of initial capacity and cycle capacity retention rate The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as described above were charged to 3.65 V, 0.05 C under CC / CV conditions at 25° C. and 0.33 C using an electrochemical charger / discharger, and then discharged to 2.5 V under CC conditions at 0.33 C for 300 cycles, and the capacity retention rate was measured.

[0146] The capacity retention rate was calculated using the following formula, and the results are shown in Table 1 below.

[0147] Capacity retention rate (%) = {(discharge capacity after 300 cycles / discharge capacity after 1 cycle)} × 100

[0148] The discharge capacity (initial capacity) after one cycle and the capacity retention rate after 300 cycles are shown in Table 1 below.

[0149] Experimental Example 2: Evaluation of initial resistance The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared above were charged to 3.65 V, 0.05 C under CC / CV, 0.1 C conditions at 25° C., and then discharged to 2.5 V under CC, 0.1 C conditions using an electrochemical charger / discharger, with one cycle being the charge / discharge.

[0150] After one charge / discharge cycle, the discharge capacity after one cycle was measured using an electrochemical charge / discharge device. After adjusting the SOC to 50%, a 2.5C pulse was applied for 10 seconds, and the initial resistance was calculated from the difference in voltage before and after the pulse application. The results are shown in Table 1 below.

[0151] Experimental Example 3: Evaluation of metal elution amount The lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as described above were charged to 3.65 V, 0.05 C under CC / CV conditions at 25° C. and 0.33 C using an electrochemical charger / discharger, and then discharged to 2.5 V under CC conditions at 0.33 C for 300 cycles, and the capacity retention rate was measured.

[0152] Next, the concentration of Fe dissolved in the electrolyte was measured using an inductively coupled plasma optical emission spectrophotometer (ICP-OES). The amount of Fe measured using ICP analysis is shown in Table 1 below.

[0153] [Table 1]

[0154] Referring to Table 1, it can be seen that the lithium secondary batteries of Examples 1 to 3 according to the present invention have excellent initial capacity, capacity retention rate, and resistance characteristics, and exhibit a low amount of Fe elution, compared to Comparative Examples 1 to 4.

Claims

1. a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte; the positive electrode includes a positive electrode active material, the positive electrode active material contains lithium iron phosphate particles, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive; The additive for a lithium secondary battery includes a compound represented by the following Chemical Formula 1: 【Chemical 1】 In the above Chemical Formula 1, R １ and R ２ are each independently an alkylene group having 1 to 10 carbon atoms, R ３ is an alkyl group having 1 to 20 carbon atoms substituted with one or more fluorine atoms.

2. In the above formula 1, R １ and R ２ are each independently an alkylene group having 1 to 5 carbon atoms, and R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 3 to 20 carbon atoms substituted with one or more fluorine atoms.

3. In the above formula 1, R １ and R ２ are each independently an alkylene group having 1 to 3 carbon atoms, and R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 3 to 15 carbon atoms substituted with one or more fluorine atoms.

4. In the above formula 1, R ３ 2. The lithium secondary battery according to claim 1, wherein is an alkyl group having 4 to 8 carbon atoms substituted with one or more fluorine atoms.

5. The lithium secondary battery of claim 1 , wherein the compound represented by Chemical Formula 1 is a compound represented by Chemical Formula 1a: 【Chemistry 2】

6. 6. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 is contained in the non-aqueous electrolyte in an amount of 0.01% by weight to 10.0% by weight.

7. 6. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 is contained in an amount of 0.05 wt % to 6.0 wt % based on the total weight of the non-aqueous electrolyte solution for the lithium secondary battery.

8. 6. The lithium secondary battery according to claim 1, wherein the additive comprises at least one additional additive selected from the group consisting of halogen-substituted or unsubstituted carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, borate-based compounds, nitrile-based compounds, amine-based compounds, silane-based compounds, and lithium salt-based compounds.

9. 6. The lithium secondary battery according to claim 1, wherein the organic solvent includes a cyclic carbonate organic solvent and a linear carbonate organic solvent.

10. the cyclic carbonate organic solvent contains ethylene carbonate, 10. The lithium secondary battery according to claim 9, wherein the linear carbonate organic solvent includes ethyl methyl carbonate and dimethyl carbonate.

11. The lithium secondary battery according to any one of claims 1 to 5, wherein the lithium iron phosphate particles contain a compound represented by the following chemical formula A: [Chemical formula A] Li １＋ａ Fe １－ｓ M ｓ (PO ４－ｂ )X ｂ In the chemical formula A, M is one or more elements selected from Co, Ni, Al, Mg, Ti, and V, and X is F, S, or N, and 0≦s≦0.5; −0.5≦a≦+0.5; and 0≦b≦0.1.

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