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. 10-2022-0182373, filed December 22, 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 contains a nonaqueous electrolyte containing an additive that can form a strong SEI coating 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, which has increased society's overall dependence on electrical energy. This has led to a demand for the development of technologies to efficiently store and utilize electrical energy.

[0004] Of the technologies currently being developed, secondary batteries are the most suitable for a variety of applications, and among these secondary batteries, interest is growing in lithium-ion batteries, which not only can be miniaturized to a degree suitable for use in 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] Carbon-based active materials, silicon-based active materials, etc. are considered as negative electrode active materials for such lithium secondary batteries, while lithium-containing cobalt oxide, LiMnO2 with a layered crystal structure, LiMn2O4 with a spinel crystal structure, lithium-containing nickel oxide (LiNiO2), etc. are considered as positive electrode active materials.

[0007] In recent years, lithium iron phosphate (for example, LiFePO4)-based compounds 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 - The thermal decomposition of anions can produce Lewis acids such as PF5, which can react with water to produce HF. Such substances, such as PF5 or HF, can destroy the coating formed on the surface of the electrode and potentially 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 leaching of Fe destabilizes the lattice structure of the lithium iron phosphate, which can generate active oxygen and accelerate the decomposition of the organic solvent in the non-aqueous electrolyte, potentially 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 layer). The process of regenerating the destroyed SEI layer can consume additional lithium ions, resulting in increased resistance and reduced capacity. Summary of the Invention [Problem to be solved by the invention]

[0009] 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 side reactions caused by Fe eluted from the lithium iron phosphate are suppressed and a low-resistance SEI coating is formed on the negative electrode, thereby improving both the output characteristics and the life characteristics. [Means for solving the problem]

[0010] 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, the additive comprising a compound represented by the following Chemical Formula 1:

[0011] [ka]

[0012] In the above chemical formula 1, n is an integer of 0-18. [Effects of the Invention]

[0013] The present invention relates to a lithium secondary battery in which a positive electrode contains a positive electrode active material including lithium iron phosphate particles, and a non-aqueous electrolyte contains, as an additive, a 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 an organic solvent, forming a low-resistance SEI coating containing a fluorocarbon component on the surface of the negative electrode. The SEI coating formed on the negative electrode using 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 improving the life characteristics, high-temperature storage performance, and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery. DETAILED DESCRIPTION OF THE INVENTION

[0014] First, before describing the present invention, the terms and words used in this specification and claims should not be interpreted in a limited manner to their ordinary or dictionary meanings, but should be interpreted in a manner that is consistent with the technical idea of ​​the present invention, in accordance with the principle that the inventor can appropriately define the concept of the term in order to best explain his or her invention.

[0015] Meanwhile, the terms used in this specification are used only 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.

[0016] As used herein, the terms "comprises," "comprises," or "having" are intended to specify the presence of embodied features, numbers, steps, components, or combinations thereof, and should be understood as not precluding the presence or possible addition of one or more other features, numbers, steps, components, or combinations thereof.

[0017] In this specification, "%" means % by weight unless expressly indicated otherwise.

[0018] Before describing the present invention, in the description of "number of carbon atoms a to b" in the specification, "a" and "b" refer to the number of carbon atoms contained in a specific functional group. That is, the functional group may contain "a" to "b" carbon atoms.

[0019] In addition, in this specification, unless otherwise defined, the term "substituted" means that at least one hydrogen atom bonded to a carbon atom is substituted with an element other than hydrogen, for example, with an alkyl group having 1 to 5 carbon atoms or a fluorine atom.

[0020] 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 by, for example, the laser diffraction method. The laser diffraction method is generally capable of measuring particle sizes from the submicron range to several mm, and can provide results with high reproducibility and high resolution.

[0021] The present invention will now be described in more detail.

[0022] 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, the additive comprising a compound represented by the following Chemical Formula 1:

[0023] [ka]

[0024] In the above chemical formula 1, n is an integer of 0-18.

[0025] The present invention relates to a lithium secondary battery in which a positive electrode contains a positive electrode active material including lithium iron phosphate particles, and a non-aqueous electrolyte contains, as an additive, a 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 an organic solvent, forming a low-resistance SEI coating containing a fluorocarbon component on the surface of the negative electrode. The SEI coating formed on the negative electrode using 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 improving the life characteristics, high-temperature storage performance, and output characteristics (specifically, room-temperature output characteristics and low-temperature output characteristics) of the lithium secondary battery.

[0026] 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 the separator interposed between the positive electrode and the negative electrode in a battery case and then injecting the non-aqueous electrolyte.

[0027] (1) Positive electrode The positive electrode includes a positive electrode active material, which includes lithium iron phosphate particles.

[0028] The lithium iron phosphate particles may include a compound represented by the following chemical formula A:

[0029] [Chemical formula A] Li 1+a Fe 1-s M s (PO 4-b )X b

[0030] 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.

[0031] The chemical formula A can be specifically represented as LiFePO4 (a=0, s=0, and b=0).

[0032] 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.

[0033] At this time, the primary particles have an average particle size (D 50 ) may be 0.2 μm to 3.0 μm, specifically 0.2 μm to 2.0 μm, and more specifically 0.3 μm to 1.5 μm, and the secondary particles may have an average particle size (D 50 ) may be 7 μm to 25 μm, specifically 10 μm to 20 μm.

[0034] The positive electrode active material may further include a carbon coating layer disposed on the lithium iron phosphate particles for the purposes of protecting the lithium iron phosphate particles and improving electrical conductivity.

[0035] 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. In this case, the positive electrode active material layer may include the positive electrode active material described above.

[0036] The positive electrode current collector usually has 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, sheet, foil, mesh, porous material, foam, or 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% by weight 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 contain a binder and / or a conductive material in addition to the positive electrode active material.

[0041] The binder is a component that assists in binding the active material and conductive material, etc., and in binding them to the current collector, and specifically may contain 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, and 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 is used to assist and improve the conductivity of the 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 may 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 may include carbon black 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 100 μm to 400 μm, and preferably 150 μm to 300 μm.

[0046] The loading amount of the positive electrode active material layer is 2 mAh / cm 2 ~5mAh / cm 2 , preferably 3.5mAh / cm 2 ~4.0mAh / cm 2 It may be.

[0047] The positive electrode may be fabricated 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 N-methyl-2-pyrrolidone (NMP), and 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 faces 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 contained 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, baked carbon, copper or stainless steel whose surface is treated with carbon, nickel, titanium, silver, or the like, or an aluminum-cadmium alloy.

[0053] The negative electrode current collector usually has 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, sheet, foil, mesh, porous material, foam, or 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 is a material capable of reversibly intercalating / deintercalating lithium ions, and may include at least one selected from the group consisting of a carbon-based active material, a (quasi)metal-based active material, and lithium metal, and specifically may include at least one selected from a carbon-based active material and a (quasi)metal-based active material.

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

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

[0060] Specifically, the (quasi-)metallic active material may include at least one (quasi-)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 (quasi-)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 (quasi-)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 (quasi)metallic active material may include a silicon-based active material.

[0062] The silicon-based active material is SiO x(0≦x<2). SiO2 does not react with lithium ions and therefore cannot store lithium. Therefore, x is preferably within the above range, and more preferably, the silicon-based oxide may be SiO.

[0063] The average particle size (D 50 ) may be 1 μm to 30 μm, preferably 2 μm to 15 μm, from the viewpoint of improving structural stability during charge and discharge and reducing side reactions with the electrolyte.

[0064] The negative electrode active material may be contained in the negative electrode active material layer in an amount of 60% to 99% by weight, preferably 75% 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 is conductive. For example, 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 conductive materials such as polyphenylene derivatives may be used.

[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 100 μm to 300 μm, and preferably 150 μm to 200 μm.

[0071] The loading amount of the negative electrode active material layer is 2 mAh / cm 2 ~5mAh / cm 2 , preferably 3.5mAh / cm 2 ~4.0mAh / cm 2 It may be.

[0072] The negative electrode may be prepared by coating at least one surface of a negative electrode current collector with a negative electrode slurry containing 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, N-methyl-2-pyrrolidone (NMP), 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 an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, either alone or in a laminate, or a conventional porous nonwoven fabric, such as a nonwoven fabric made of a high-melting point glass fiber, a polyethylene terephthalate fiber, etc. Furthermore, to ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may be used, and may be selectively used as a single-layer or multi-layer structure.

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

[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 any limitation. For example, Li + 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, may include LiPF6.

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

[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, resulting in improved 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 minimizes 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 easily dissociates 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. The linear carbonate organic solvent may specifically include ethyl methyl carbonate and dimethyl carbonate, more specifically, ethyl methyl carbonate and dimethyl carbonate 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, in which the cyclic carbonate organic solvent and the linear carbonate organic solvent may be mixed in a volume ratio of 10:90 to 50:50, specifically 15:85 to 40:60.

[0085] Meanwhile, the organic solvent may further include, as needed, any organic solvent commonly used in non-aqueous electrolytes, for example, at least one of an ester-based organic solvent, an ether-based organic solvent, a glyme-based solvent, and a nitrile-based organic solvent.

[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 of these, 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, and may include at least one selected from the group consisting of dimethoxyethane (glyme, DME), diethoxyethane, diglyme, triglyme, and tetraglyme (TEGDME), but is not limited thereto.

[0089] The nitrile solvent may be one or more 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, in the non-aqueous electrolyte, the remainder excluding the lithium salt and the additive is 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, n is an integer of 0-18.

[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, forming an SEI coating 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 coating, 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 damaging the SEI coating of the negative electrode. Therefore, when a nonaqueous electrolyte containing the compound represented by Chemical Formula 1 as an electrolyte additive is used with a positive electrode using the lithium iron phosphate particles as the positive electrode active material, it prevents the self-discharge reaction of the negative electrode, which is caused by additional reductive decomposition of the electrolyte due to the instability of the SEI coating.

[0096] Furthermore, the compound represented by Chemical Formula 1 contains one or more fluorine-substituted alkyl groups at its structural terminals, thereby forming an oxidation-resistant coating on the surface of the positive electrode, thereby inhibiting the elution of Fe from the lithium iron phosphate of the positive electrode and inhibiting the electrodeposition and precipitation of the eluted Fe on the negative electrode, thereby preventing internal short circuits. Furthermore, the fluorine-substituted alkyl groups contained in the molecular structure, which have excellent flame retardancy and non-combustibility, function as radical scavengers originating from the fluorine elements and can form a passivation coating on the surface of the positive electrode that can ensure excellent oxidation resistance. As a result, side reactions between the electrode and the electrolyte are suppressed, providing a lithium secondary battery with improved life characteristics at room temperature and low temperatures.

[0097] Furthermore, since the compound represented by Chemical Formula 1 of the present invention contains an ethylene group (-CH2-CH2-) between the acrylate functional group and the terminal fluorine-substituted alkyl group, the flexibility of the compound is increased due to the increase in the molecular chain of the linking group portion compared to a compound containing a methylene group (-CH2-) between the acrylate functional group and the terminal fluorine-substituted alkyl group. As a result, a coating derived from such a compound can be formed on the surface of a negative electrode with improved durability.

[0098] In addition, since the compound represented by Chemical Formula 1 contains two oxygen atoms in its molecular structure, it can improve oxidation safety, high voltage stability, and electrolyte durability compared to compounds containing three or more oxygen atoms.

[0099] As described above, the compound represented by Chemical Formula 1 contains a fluorine-containing alkyl group and a propargyl group, which have excellent flame retardancy and non-flammability, and therefore forms a strong SEI coating with low resistance, which not only suppresses the additional reduction decomposition reaction of the electrolyte but also prevents the self-discharge reaction of the negative electrode, thereby suppressing an increase in initial resistance and providing a lithium secondary battery with improved room temperature and low temperature output characteristics.

[0100] In the above Chemical Formula 1, n may be an integer of 0 to 18, specifically an integer of 1 to 10, and more specifically an integer of 2 to 8.

[0101] When n is within the above range, the thermal properties of the compound itself can be improved, and the stability of the coating formed therefrom can be expected. In Formula 1, when n is greater than 18, the fluorine element is contained in an excessive amount, which increases the viscosity and non-polarity of the material and reduces the solubility in the electrolyte, thereby reducing ionic conductivity and possibly causing deterioration of battery performance.

[0102] Preferably, the compound represented by Chemical Formula 1 may include at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1-1 to 1-4, and more preferably, may include at least one selected from the group consisting of compounds represented by Chemical Formulas 1-1 to 1-2.

[0103] [ka]

[0104] [ka]

[0105] [ka]

[0106] [ka]

[0107] Meanwhile, the compound represented by Chemical Formula 1 may be included in the non-aqueous electrolyte in an amount of 0.05 wt % to 8 wt %.

[0108] When the content of the compound represented by Chemical Formula 1 is within the above range, defects such as side reactions, capacity reduction, and resistance increase due to additives 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 decomposition reactions of the electrolyte, and preventing self-discharge reactions of the negative electrode.

[0109] Specifically, when the content of the compound represented by Chemical Formula 1 is 0.05 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 8 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, further increasing ionic conductivity within the battery, and preventing a decrease in output characteristics.

[0110] Specifically, the compound represented by Chemical Formula 1 may be contained in the non-aqueous electrolyte solution in an amount of 0.1 wt % to 5 wt %, more specifically 0.3 wt % to 3 wt %.

[0111] Meanwhile, the additive may contain other additional additives in addition to the compound represented by Formula 1, as needed, 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 the effect of suppressing battery expansion at high temperatures.

[0112] Examples of such additional additives include at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sultone compounds, sulfate compounds, phosphate or phosphite compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0113] The cyclic carbonate compound may be, for example, vinylene carbonate (VC) or vinylethylene carbonate.

[0114] The halogen-substituted carbonate compound may be, for example, fluoroethylene carbonate (FEC).

[0115] 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.

[0116] The sulfate compound may be, for example, ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0117] The phosphate-based or phosphite-based compound may be, for example, one or more compounds selected from the group consisting of lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0118] Examples of the borate-based compounds include tetraphenylborate, lithium difluoro(oxalato)borate (LiODFB), and lithium bisoxalatoborate (LiB(C2O4)2, LiBOB).

[0119] 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.

[0120] 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.

[0121] 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.

[0122] Among these additional additives, when at least one selected from the group consisting of vinylene carbonate, vinylethylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, ethylene sulfate, succinonitrile, and lithium difluoro(oxalato)borate, specifically at least one selected from the group consisting of vinylene carbonate and ethylene sulfate, is used, a stronger SEI coating can be formed on the surface of the negative electrode during the initial activation step of the secondary battery.

[0123] The additional additive may be a combination 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 %, 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 the above range, the low-temperature output characteristics, high-temperature storage characteristics, and high-temperature life characteristics of the battery may be improved more effectively, and side reactions in the battery due to residual additives after the reaction may be prevented.

[0124] 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).

[0125] 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.

[0126] The battery module or battery pack can be used as a power source for one or more medium- to large-sized devices, such as power tools; electric vehicles (EVs), hybrid electric vehicles, and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0127] 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.

[0128] The lithium secondary battery according to the present invention can be used not only as a battery cell used as a power source for small devices, but also as a unit battery in a medium- to large-sized battery module containing a large number of battery cells.

[0129] The present invention will be specifically described below with reference to examples.

[0130] In this regard, 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 described below. The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art.

[0131] The present invention will be specifically described below with reference to specific examples.

[0132] 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 30:55:15.

[0133] LiPF6 as a lithium salt was dissolved in the organic solvent to a molar concentration of 1.0M.

[0134] In addition, the compound represented by Formula 1-1, vinylene carbonate, and ethylene sulfate were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0135] The compound represented by Chemical Formula 1-1 was contained in the non-aqueous electrolyte at 0.5 wt %, the vinylene carbonate was contained in the non-aqueous electrolyte at 3 wt %, and the ethylene sulfate was contained in the non-aqueous electrolyte at 1 wt %.

[0136] (Secondary battery manufacturing) Lithium iron phosphate particles (LiFePO4, D 50A positive electrode slurry (solid content 65 wt%) was prepared by adding a 96:1:3 weight ratio of carbon nanotubes as a conductive material and polyvinylidene fluoride as a binder to N-methyl-2-pyrrolidone (NMP) as a solvent. The positive electrode slurry was applied to a 15 μm-thick positive electrode current collector (Al thin film) at 3.7 mAh / cm. 2 After drying, a positive electrode was manufactured by roll pressing (positive electrode active material thickness: 200 μm).

[0137] Anode active material graphite, binder SBR-CMC, and conductive material carbon black were mixed in a weight ratio of 95.5:4.0:0.5 with water as a solvent to prepare anode slurry (solid content: 50 wt%). The anode slurry was applied to an 8 μm-thick copper (Cu) thin film as anode current collector, and the resulting anode slurry provided 3.9 mAh / cm 2 After drying, a negative electrode was manufactured by roll pressing (thickness of negative electrode active material: 160 μm).

[0138] The positive electrode, a polyolefin-based porous separator, and a negative electrode were stacked in this order to prepare an electrode assembly.

[0139] The assembled electrode assembly was placed in a battery case, and the prepared non-aqueous electrolyte was injected into the battery case to manufacture a lithium secondary battery.

[0140] 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 Chemical Formula 1-2 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0141] 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 1-3 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0142] Example 4 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 1-4 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0143] Example 5 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 1-1 was added to the non-aqueous electrolyte in an amount of 0.1 wt %.

[0144] Example 6 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 1-1 was added to the non-aqueous electrolyte in an amount of 5 wt %.

[0145] Comparative Example 1 A non-aqueous electrolyte and a lithium secondary battery were produced in the same manner as in Example 1, except that the compound represented by Chemical Formula 1-1 was not added.

[0146] Comparative Example 2 A non-aqueous electrolyte and a lithium secondary battery were manufactured in the same manner as in Example 1, except that a compound represented by the following Chemical Formula 3 (a=20) was added to the non-aqueous electrolyte at a content of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0147] [ka]

[0148] Comparative Example 3 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 4 was added to the non-aqueous electrolyte at a content of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0149] [ka]

[0150] 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 5 was added to the non-aqueous electrolyte in an amount of 0.5 wt % instead of the compound represented by Chemical Formula 1-1.

[0151] [ka]

[0152] Experimental example Experimental Example 1: Evaluation of initial capacity and capacity retention rate when stored at room temperature The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 prepared above were initially charged to 3.65 V, 0.05 C under CC / CV, 1 / 3 C conditions at 25°C, and then discharged to 2.5 V at 1 / 3 C. Subsequently, they were charged to 3.65 V, 0.05 C under CC / CV, 1 / 3 C conditions at 25°C, and then stored at 25°C for 12 weeks. After storage, the secondary batteries were charged to 3.65 V, 0.05 C under CC / CV, 1 / 3 C conditions at 25°C, and then discharged to 2.5 V at 1 / 3 C, and the discharge capacities were measured.

[0153] The capacity retention rate was evaluated using the following formula, and the results are shown in Table 1 below.

[0154] Capacity retention rate (%) = (discharge capacity after 12 weeks of storage / initial discharge capacity) x 100

[0155] Experimental Example 2: Evaluation of initial resistance and resistance increase rate during high temperature storage The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 prepared above were initially charged to 3.65 V at 0.05 C under CC / CV, 1 / 3 C conditions at 25°C, and then discharged at 1 / 3 C to 2.5 V. After confirming the capacity at room temperature, they were charged to 50% SOC based on the discharge capacity and discharged at a current of 2.5 C for 10 seconds. The resistance was measured from the difference in voltage drop at this time and recorded as the initial resistance. After storing the batteries at 60°C for 12 weeks, the resistance was measured in the same manner and recorded as the final resistance. The resistance increase rate was calculated using the following formula. The results are shown in Table 1 below.

[0156] Resistance increase rate (%) = (final resistance - initial resistance) / (initial resistance) x 100

[0157] Experimental Example 3: Evaluation of volume increase rate during high temperature storage The lithium secondary batteries of Examples 1 to 6 and Comparative Examples 1 to 4 prepared above were initially charged and discharged at 25°C under CC / CV, 1 / 3C conditions to 3.65V, 0.05C, and then discharged at 1 / 3C to 2.5V. The volume of each battery was measured at an SOC of 50% based on the discharge capacity, and this was defined as the initial volume. Furthermore, the volume measured after 12 weeks of high-temperature storage at 60°C with an SOC of 100% was defined as the final volume, and the volume increase rate of the battery was calculated using the following formula. The results are shown in Table 1 below.

[0158] Volume increase rate (%) = (final volume - initial volume) / (initial volume) x 100

[0159] [Table 1]

[0160] Referring to Table 1, it can be seen that the lithium secondary batteries of Examples 1 to 6 according to the present invention exhibit excellent life performance, a low resistance increase rate, and a low battery volume increase rate 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 includes lithium iron phosphate particles, the non-aqueous electrolyte contains a lithium salt, an organic solvent, and an additive; The additive comprises a compound represented by the following Chemical Formula 1: 【Chemistry 1】 (In the above chemical formula 1, n is an integer from 0 to 18.

2. 2. The lithium secondary battery according to claim 1, wherein n is an integer of 1 to 10 in Chemical Formula 1.

3. 2. The lithium secondary battery according to claim 1, wherein n is an integer of 2 to 8 in Chemical Formula 1.

4. The lithium secondary battery according to claim 1, wherein the compound represented by Chemical Formula 1 includes at least one selected from the group consisting of compounds represented by the following Chemical Formulas 1-1 to 1-4: 【Chemistry 2】 【Transformation 3】 【Chemistry 4】 【Transformation 5】

5. 5. 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 0.05% by weight to 8% by weight.

6. 5. 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 0.3% by weight to 3% by weight.

7. 5. 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.

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

9. the cyclic carbonate organic solvent contains ethylene carbonate, 9. The lithium secondary battery according to claim 8, wherein the linear carbonate organic solvent includes ethyl methyl carbonate and dimethyl carbonate.

10. The lithium secondary battery according to claim 1 , 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, X is F, S, or N, and 0≦s≦0.5; −0.5≦a≦+0.5; and 0≦b≦0.1.)

11. 5. The lithium secondary battery according to claim 1, wherein the positive electrode active material further comprises a carbon coating layer located on lithium iron phosphate particles.

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