Lithium secondary battery

Abstract

Provided is a lithium secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, in which the positive electrode includes a positive electrode active material including lithium iron phosphate particles, the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, and the additive includes a compound represented by a specific formula.

Description

[0001] Cross - reference to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10 - 2022 - 0146435, filed with the Korean Intellectual Property Office on November 4, 2022, the disclosure of which is incorporated herein by reference. Technical field

[0003] The present invention relates to a lithium secondary battery, and more particularly, to a lithium secondary battery capable of suppressing an increase in initial resistance and improving output characteristics and life performance by including a non - aqueous electrolyte containing an additive, the additive being capable of forming a strong SEI layer on a positive electrode / negative electrode. Background art

[0004] With the development of the information society, personal IT devices and computer networks have also been continuously developed, and the dependence of the entire society on electric energy has also increased accordingly. Therefore, technologies for efficiently storing and using electric energy need to be developed.

[0005] Among the developed technologies, secondary batteries are the most suitable for various applications. Among secondary batteries, lithium secondary batteries have attracted much attention because they can be miniaturized to the extent that they can be applied to personal IT devices and the like and have the highest energy density.

[0006] Generally, a lithium secondary battery is prepared by injecting a non - aqueous electrolyte into an electrode assembly composed of a positive electrode, a negative electrode, and a porous separator, or impregnating the electrode assembly with a non - aqueous electrolyte.

[0007] As the negative electrode active material of such a lithium secondary battery, carbon - based active materials, silicon - based active materials, etc. are considered. At the same time, as the positive electrode active material, lithium cobalt oxide, LiMnO with a layered crystal structure 2 , LiMn with a spinel crystal structure 2 O 4 , lithium nickel oxide (LiNiO 2 ), etc. are considered.

[0008] Recently, lithium iron phosphate (e.g., LiFePO 4 ) - based compounds having excellent thermal stability and relatively low prices are considered as positive electrode active materials.

[0009] At the same time, when a lithium salt (e.g., LiPF 6 ) contained in the non - aqueous electrolyte thermally decomposes to generate PF 6 - anions, Lewis acids such as PF 5 may be formed, and PF 5 may react with moisture to generate HF. Such materials (e.g., PF 5HF) can not only damage the film formed on the electrode surface, but also may cause the decomposition reaction of the organic solvent. In particular, if lithium iron phosphate is used as the positive electrode active material, there is a limitation in the elution of Fe from the surface of the positive electrode active material exposed due to the above HF and PF 5 And there is a limitation in the elution of Fe from the surface of the positive electrode active material exposed as a result. The elution of Fe will damage the stability of the lithium iron phosphate lattice structure, thereby generating reactive oxygen species and promoting the decomposition of the organic solvent in the non-aqueous electrolyte, thus accelerating the generation of gas. In addition, the eluted Fe can move to the negative electrode through the non-aqueous electrolyte and be electrodeposited on the surface of the negative electrode, thereby damaging the solid electrolyte interface layer (hereinafter referred to as the SEI layer), and causing additional lithium ion consumption during the regeneration process of the damaged SEI layer, resulting in an increase in resistance and a decrease in capacity.

[0010] [Prior Art Documents]

[0011] [Patent Documents]

[0012] Korean Patent Application Publication No. 2017-0012308 Summary of the Invention

[0013] [Technical Problem]

[0014] One aspect of the present invention provides a lithium secondary battery including lithium iron phosphate particles as a positive electrode active material, the lithium secondary battery having improved output characteristics and life characteristics by forming a low-resistance SEI layer on the negative electrode to suppress side reactions caused by Fe eluted from the lithium iron phosphate.

[0015] [Technical Solution]

[0016] According to one aspect of the present invention, there is provided a lithium secondary battery including a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material, the positive electrode active material includes lithium iron phosphate particles, the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, and the additive includes a compound represented by the following Formula 1:

[0017] [Formula 1]

[0018]

[0019] In the above Formula 1, R 1 And R 2 Are each independently an alkylene group having 1 to 10 carbon atoms, and R 3 Is an alkyl group having 1 to 20 carbon atoms substituted with at least one fluorine.

[0020] [Advantageous Effects]

[0021] The present invention relates to a lithium secondary battery, the positive electrode of which contains a positive electrode active material including lithium iron phosphate particles, and contains a compound represented by Formula 1 as an additive in a non-aqueous electrolyte. Since the compound represented by Formula 1 includes a fluorocarbon functional group substituted with at least one fluorine element and a propargyl group (-C≡C-) in its structure, the compound is reduced before the organic solvent is reduced, and a low-resistance SEI layer including a fluorocarbon component can be formed on the surface of the negative electrode. The SEI layer formed on the negative electrode by the compound represented by Formula 1 can not only prevent side reactions caused by Fe eluted from the lithium iron phosphate particles, but also has a low resistance, thus improving the life characteristics and output characteristics (specifically, output characteristics at room temperature and output characteristics at low temperature) of the lithium secondary battery. Detailed Description

[0022] First, before describing the present invention, it should be understood that the words or terms used in the specification and claims should not be construed as having the meanings defined in a common dictionary. It should be further understood that based on the principle that the inventor can appropriately define the meanings of the words or terms to best explain the present invention, these words or terms should be construed as having meanings consistent with their meanings in the context of the related art and the technical concept of the present invention.

[0023] The terms used in this specification are only used to describe exemplary embodiments and are not intended to limit the present invention. Unless otherwise specified, the terms in the singular form may include the plural form.

[0024] It should also be understood that the terms "comprises", "comprising" or "having" in this specification specify the presence of features, numbers, steps, elements or combinations thereof, but do not exclude the presence or addition of one or more other features, numbers, steps, elements or combinations thereof.

[0025] In this specification, unless otherwise clearly specified, the expression "%" means weight %.

[0026] Before describing the present invention, in the expression "a to b carbon atoms" in the specification, "a" and "b" respectively represent the number of carbon atoms contained in a specific functional group. That is, the functional group may include "a" to "b" carbon atoms.

[0027] In addition, unless otherwise defined in the specification, the expression "substituted" means that at least one hydrogen bonded to carbon is substituted with an element other than hydrogen, for example, substituted with an alkyl group having 1 to 5 carbon atoms or a fluorine element.

[0028] In this specification, the average particle size (D 50 ) can be defined as the particle size at the cumulative volume of 50% in the particle size distribution curve. For example, the average particle size (D 50)Measurement can be carried out using the laser diffraction method. The laser diffraction method can generally measure particle sizes from sub-microns to several millimeters and can obtain results with high reproducibility and high resolution.

[0029] In the following, the present invention will be described in more detail.

[0030] Lithium secondary battery

[0031] The present invention provides a lithium secondary battery, which includes a positive electrode, a negative electrode, a separator, and a non-aqueous electrolyte. Wherein the positive electrode includes a positive electrode active material, the positive electrode active material contains lithium iron phosphate particles, the non-aqueous electrolyte includes a lithium salt, an organic solvent, and an additive, and the additive includes a compound represented by the following formula 1:

[0032] [Formula 1]

[0033]

[0034] In the above formula 1, R 1 and R 2 are each independently an alkylene group having 1 to 10 carbon atoms, and R 3 is an alkyl group having 1 to 20 carbon atoms substituted with at least one fluorine.

[0035] The present invention relates to a lithium secondary battery, the positive electrode of which contains a positive electrode active material containing lithium iron phosphate particles and contains a compound represented by the above formula 1 as an additive in the non-aqueous electrolyte. Since the compound represented by the above formula 1 includes a fluorocarbon functional group substituted with at least one fluorine element and a propargyl group (-C≡C-) in its structure, the compound is reduced before the organic solvent is reduced and can form a low-resistance SEI layer including a fluorocarbon component on the surface of the negative electrode. The SEI layer formed on the negative electrode by the compound represented by the above formula 1 can not only prevent side reactions caused by Fe eluted from the lithium iron phosphate particles, but also has a low resistance, thus improving the life characteristics and output characteristics of the lithium secondary battery (specifically, the output characteristics at room temperature and the output characteristics at low temperature).

[0036] 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 disposed between the negative electrode and the positive electrode; and the non-aqueous electrolyte. The lithium secondary battery can be prepared by accommodating an electrode assembly in a battery case and then injecting the non-aqueous electrolyte. The electrode assembly includes: a positive electrode, a negative electrode facing the positive electrode, and a separator disposed between the negative electrode and the positive electrode.

[0037] (1) Positive electrode

[0038] The positive electrode contains a positive electrode active material. The positive electrode active material contains lithium iron phosphate particles.

[0039] The lithium iron phosphate particles may contain a compound represented by Formula A below:

[0040] [Formula A]

[0041] Li 1+a Fe 1-s M s (PO 4-b )X b

[0042] In Formula A above, M is at least one element selected from Co, Ni, Al, Mg, Ti, and V; X is F, S, or N; 0 ≤ s ≤ 0.5; -0.5 ≤ a ≤ +0.5; 0 ≤ b ≤ 0.1.

[0043] Specifically, the above Formula A may be represented by LiFePO 4 (a = 0, s = 0, b = 0).

[0044] The lithium iron phosphate particles may be composed of primary particles, or may be composed of secondary particles formed by aggregation of two or more primary particles, or may be a mixture of primary particles and secondary particles formed by aggregation of two or more primary particles.

[0045] In this case, the average particle size (D 50 ) of the primary particles may be 0.2 μm to 3.0 μm, specifically 0.2 μm to 1.0 μm, more specifically 0.3 μm to 0.8 μm, and the average particle size (D 50 ) of the secondary particles may be 7 μm to 25 μm, specifically 10 μm to 20 μm.

[0046] The positive electrode active material may further include a carbon coating on the lithium iron phosphate particles, but is not particularly limited thereto. The carbon coating may be introduced for the purpose of protecting the lithium iron phosphate particles and improving conductivity.

[0047] The positive electrode may include: a positive electrode current collector; and a positive electrode active material layer provided on at least one surface of the positive electrode current collector. In this case, the positive electrode active material layer may contain the above positive electrode active material.

[0048] There is no particular limitation on the positive electrode current collector as long as it has high conductivity and does not cause adverse chemical changes in the battery. Specifically, as the positive electrode current collector, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., or aluminum cadmium alloy, etc. may be used.

[0049] The thickness of the positive electrode current collector may generally be 3 μm to 500 μm.

[0050] The positive electrode current collector may form microscopic irregularities on its surface to enhance the adhesion of the positive electrode active material. For example, the positive electrode current collector may be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric bodies.

[0051] The positive electrode active material layer is disposed on at least one surface of the positive electrode current collector. Specifically, the positive electrode active material layer may be disposed on one surface or both surfaces of the positive electrode current collector.

[0052] Considering that the positive electrode active material exhibits sufficient capacity, the content of the positive electrode active material in the positive electrode active material layer may be 80% to 99% by weight.

[0053] In addition to the above positive electrode active material, the positive electrode active material layer may further contain a conductive agent and / or a binder.

[0054] The binder is a component that helps the binding of the active material and the conductive agent and the binding to the current collector. Specifically, it may include at least one selected from 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 includes polyvinylidene fluoride.

[0055] In terms of ensuring sufficient adhesion between components such as the positive electrode active material, the binder content in the positive electrode active material layer may be 1% to 20% by weight, preferably 1.2% to 10% by weight.

[0056] The conductive agent is not particularly limited as long as it can be used to assist and improve the conductivity in the secondary battery, does not cause adverse chemical changes, and has conductivity. Specifically, in terms of improving conductivity, the positive electrode conductive agent may include at least one selected from the group consisting of: graphite, such as natural graphite or artificial graphite; carbon black, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, or thermal cracking carbon black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; fluorocarbons; metal powders, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium oxide; and conductive agents such as polyphenylene derivatives, and preferably includes carbon black.

[0057] In terms of ensuring sufficient conductivity, the conductive agent content in the positive electrode active material layer may be 1% to 20% by weight, preferably 1.2% to 10% by weight.

[0058] The thickness of the positive electrode active material layer may be 10 μm to 500 μm, preferably 200 μm to 400 μm.

[0059] The loading amount of the positive electrode active material layer can be 2.5 mAh / cm 2 to 5.0 mAh / cm 2 and is preferably 3 mAh / cm 2 to 4 mAh / cm 2 .

[0060] The positive electrode can be prepared as follows: coating a positive electrode current collector with a positive electrode paste containing a positive electrode active material, an optional binder, a conductive agent, and a solvent for forming the positive electrode paste, and then drying and rolling the coated positive electrode current collector.

[0061] The solvent for forming the positive electrode paste may include an organic solvent such as N-methyl-2-pyrrolidone (NMP). The solid content in the positive electrode paste can be 40% to 90% by weight, particularly 50% to 80% by weight.

[0062] (2) Negative electrode

[0063] The negative electrode can face the positive electrode.

[0064] The negative electrode contains a negative electrode active material.

[0065] The negative electrode may contain a negative electrode current collector and a negative electrode active material layer provided on at least one surface of the negative electrode current collector. In this case, the negative electrode active material layer may contain a negative electrode active material.

[0066] There is no particular limitation on the negative electrode current collector as long as it has high conductivity without causing adverse chemical changes in the battery. Specifically, copper, stainless steel, aluminum, nickel, titanium, fired carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc., aluminum cadmium alloy, etc. can be used as the negative electrode current collector.

[0067] The thickness of the negative electrode current collector can generally be 3 μm to 500 μm.

[0068] The negative electrode current collector can form micro-irregularities on its surface to enhance the adhesion of the negative electrode active material. For example, the negative electrode current collector can be used in various shapes such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabric bodies.

[0069] The negative electrode active material layer is provided on at least one surface of the negative electrode current collector. Specifically, the negative electrode active material layer can be provided on one surface or both surfaces of the negative electrode current collector.

[0070] The negative electrode active material layer may contain a negative electrode active material.

[0071] The negative electrode active material is a material capable of reversibly inserting / extracting lithium ions, and may include at least one selected from the group consisting of carbonaceous active materials, (metalloid) metallic active materials, and lithium metal. Specifically, it may include at least one selected from the group consisting of carbonaceous active materials or (metalloid) metallic active materials.

[0072] The carbonaceous active materials 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 may preferably include at least one selected from the group consisting of artificial graphite and natural graphite.

[0073] In terms of reducing side reactions with the electrolyte and maintaining structural stability during charging and discharging, the average particle size (D 50 ) of the carbonaceous active materials may be 10 μm to 30 μm, preferably 15 μm to 25 μm.

[0074] Specifically, the (metalloid) metallic active materials may include: at least one (metalloid) metal selected from 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 lithium and at least one (metalloid) metal selected from 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 (metalloid) metal selected from 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 titanate oxide (LTO); lithium vanadium oxide, etc.

[0075] More specifically, the (metalloid) metallic active materials may include silicon-based active materials.

[0076] The silicon-based active materials may include compounds represented by SiO x (0 ≤ x < 2). Since SiO 2 does not react with lithium ions and thus cannot store lithium, x is preferably within the above range, and more preferably the silicon-based oxide may be SiO.

[0077] In terms of reducing side reactions with the electrolyte and maintaining structural stability during charging and discharging, the average particle size (D 50 ) of the silicon-based active materials may be 1 μm to 30 μm, preferably 2 μm to 15 μm.

[0078] Based on the total weight of the negative electrode active material layer, the content of the negative electrode active material can be 60% by weight to 99% by weight, preferably 75% by weight to 95% by weight.

[0079] In addition to the negative electrode active material, the negative electrode active material layer may further contain a conductive agent and / or a binder.

[0080] The binder is used to improve the adhesion between the negative electrode active material layer and the negative electrode current collector and to improve battery performance, and may include, for example, at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, 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 materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0081] The content of the binder in the negative electrode active material layer can be 0.5% by weight to 10% by weight, preferably 1% by weight to 5% by weight.

[0082] There is no particular limitation on the conductive agent as long as it has conductivity and does not cause adverse chemical changes in the battery, and the following conductive materials can be used: for example, graphite, such as natural graphite and artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking carbon black; conductive fibers, such as carbon fibers or metal fibers; conductive tubes, such as carbon nanotubes; carbon fluoride powder; metal powders, such as aluminum powder and nickel powder; conductive whiskers, such as zinc oxide whiskers and potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0083] The content of the conductive agent in the negative electrode active material layer can be 0.5% by weight to 10% by weight, preferably 1% by weight to 5% by weight.

[0084] The thickness of the negative electrode active material layer can be 5 μm to 500 μm, preferably 100 μm to 300 μm.

[0085] The loading amount of the negative electrode active material layer can be 3.0 mAh / cm 2 to 5.5 mAh / cm 2 preferably 3.5 mAh / cm 2 to 4.5 mAh / cm 2 .

[0086] The negative electrode can be prepared as follows: at least one surface of a negative electrode current collector is coated with a negative electrode paste containing a negative electrode active material, a binder, a conductive agent, and a solvent for forming the negative electrode paste, and then the coated negative electrode current collector is dried and roll-pressed.

[0087] In terms of promoting the dispersion of the negative electrode active material, the binder, and / or the conductive agent, the solvent for forming the negative electrode paste 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, and preferably includes distilled water. The solid content in the negative electrode paste may be 30% by weight to 80% by weight, particularly 40% by weight to 70% by weight.

[0088] (3) Separator

[0089] The separator can be disposed between the positive electrode and the negative electrode.

[0090] As the separator, a conventional porous polymer film used as a conventional separator can be used alone or in a laminated manner. For example, a porous polymer film prepared from an olefin-based polymer (such as ethylene homopolymer, propylene homopolymer, ethylene-butene copolymer, ethylene-hexene copolymer, and ethylene-methyl acrylate copolymer) can be used. Additionally, a conventional porous non-woven fabric, such as a non-woven fabric formed from high melting point glass fibers or polyethylene terephthalate fibers, can also be used, but the present invention is not limited thereto. Furthermore, a coated separator containing a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a separator having a single-layer or multi-layer structure can be optionally used.

[0091] (4) Non-aqueous electrolyte

[0092] 1) Lithium salt

[0093] First, the lithium salt will be described as follows.

[0094] In the non-aqueous electrolyte for a lithium secondary battery according to an embodiment of the present invention, any lithium salt commonly used in the electrolyte of a lithium secondary battery can be used as the lithium salt without limitation. For example, the lithium salt may include Li + as a cation, and at least one selected from the group consisting of the following as an anion: F - 、Cl - 、Br - 、I - 、NO 3 - 、N(CN) 2 - 、BF 4 - 、ClO 4 - 、AlO 4 - 、AlCl4 - , PF 6 - , SbF 6 - , AsF 6 - , B 10 Cl 10 - , BF 2 C 2 O 4 - , BC 4 O 8 - , PF 4 C 2 O 4 - , PF 2 C 4 O 8 - , (CF 3 ) 2 PF 4 - , (CF 3 ) 3 PF 3 - , (CF 3 ) 4 PF 2 - , (CF 3 ) 5 PF - , (CF 3 ) 6 P - , CF 3 SO 3 - , C 4 F 9 SO 3 - , CF 3 CF 2 SO 3 - , (CF 3 SO 2 ) 2 N - , (FSO 2 ) 2 N - , CF 3 CF 2 , (CF 3 ) 2 CO - , (CF 3 SO2 ) 2 CH - , CH 3 SO 3 - CF 3 (CF 2 ) 7 SO 3 - CF 3 CO 2 - , CH 3 CO 2 - 、SCN - and (CF 3 CF 2 SO 2 ) 2 N - Specifically, the lithium salt may include at least one selected from the group consisting of: LiCl, LiBr, LiI, LiBF 4 、LiClO 4 、LiAlO 4 、LiAlCl 4 、LiPF 6 、LiSbF 6 、LiAsF 6 , LiB 10 Cl 10 、LiBOB(LiB(C 2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 ) 2 )、LiFSI(LiN(SO 2 F) 2 )、LiCH 3 SO 3 、LiCF 3 CO 2 、LiCH 3 CO 2 and LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ). Specifically, the lithium salt may include a single material or a mixture of two or more selected from the group consisting of: LiBF 4 、LiClO 4 、LiPF 6 、LiBOB(LiB(C2 O 4 ) 2 )、LiCF 3 SO 3 、LiTFSI(LiN(SO 2 CF 3 ) 2 )、LiFSI(LiN(SO 2 F) 2 ) and LiBETI(LiN(SO 2 CF 2 CF 3 ) 2 ), more specifically, may include LiPF 6 .

[0095] The lithium salt may be appropriately changed within a normal use range, but the concentration in the electrolyte may be 0.8 M to 3.0 M, for example, 1.0 M to 3.0 M, to obtain the best effect of forming a film for preventing corrosion of the electrode surface.

[0096] When the concentration of the lithium salt satisfies the above range, the viscosity of the nonaqueous electrolyte can be controlled to achieve optimal impregnation, and the capacity characteristics and cycle characteristics of the lithium secondary battery can be improved by improving the mobility of lithium ions.

[0097] 2) Organic solvents

[0098] The organic solvent is a non-aqueous solvent commonly used in lithium secondary batteries, and is not particularly limited as long as it can minimize decomposition reactions caused by oxidation reactions and the like during charge and discharge of the secondary battery.

[0099] Specifically, the organic solvent may include a cyclic carbonate-based organic solvent and a linear carbonate-based organic solvent.

[0100] Since the cyclic carbonate organic solvent is a high-viscosity organic solvent with a high dielectric constant, it is an organic solvent that can well dissociate the lithium salt in the electrolyte. Specifically, it can 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, vinylene carbonate, and more specifically, ethylene carbonate.

[0101] In addition, linear carbonate organic solvents are organic solvents with 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. When the positive electrode contains lithium iron phosphate particles as the positive electrode active material, its thickness should be increased to achieve the required energy density. Accordingly, in terms of further improving the wettability of the electrolyte, the linear carbonate organic solvent may specifically contain dimethyl carbonate and ethyl methyl carbonate, and more specifically, may contain dimethyl carbonate and ethyl methyl carbonate in a volume ratio of 60:40 to 90:10.

[0102] The organic solvent may be a mixture of cyclic carbonate organic solvents and linear carbonate organic solvents. In this case, 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, in a volume ratio of 15:85 to 35:65.

[0103] Meanwhile, if necessary, any organic solvent commonly used in non-aqueous electrolytes may be additionally used without limitation as the organic solvent. For example, it may further include at least one organic solvent selected from ester organic solvents, ether organic solvents, glycol diether organic solvents, and nitrile organic solvents.

[0104] The ester 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.

[0105] As the ether solvent, 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 may be used, but the ether solvent is not limited thereto.

[0106] The glycol diether solvent is a solvent with a higher dielectric constant and lower surface tension compared to the linear carbonate organic solvent, and has a lower reactivity with metals. It may include at least one selected from the group consisting of dimethoxyethane (ethylene glycol dimethyl ether DME), diethoxyethane, diethylene glycol dimethyl ether, triethylene glycol dimethyl ether, and tetraethylene glycol dimethyl ether (TEGDME), but the glycol diether solvent is not limited thereto.

[0107] The nitrile organic solvents may include at least one selected from the group consisting of: acetonitrile, propionitrile, butyronitrile, valeronitrile, octanenitrile, heptanenitrile, cyclopentanenitrile, cyclohexanenitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, but the nitrile organic solvents are not limited thereto.

[0108] Meanwhile, unless otherwise specified, the remainder of the non-aqueous electrolyte other than the lithium salt and the additive may be an organic solvent.

[0109] (3) Additive

[0110] The non-aqueous electrolyte of the present invention includes an additive.

[0111] The additive includes a compound represented by the following formula 1:

[0112] [Formula 1]

[0113]

[0114] In the above formula 1, R 1 and R 2 are each independently an alkylene group having 1 to 10 carbon atoms, and R 3 is an alkyl group having 1 to 20 carbon atoms substituted with at least one fluorine.

[0115] Specifically, the compound represented by the above formula 1 contains a propargyl functional group in its structure, so it can be easily reduced and decomposed on the surface of the negative electrode containing a silicon-based active material to form an SEI layer with low resistance and high passivation ability, thereby improving the durability of the negative electrode itself. In addition, when the component derived from the compound represented by the above formula 1 is included in the SEI layer, the problem that Fe eluted from the lithium iron phosphate particles of the positive electrode is deposited on the surface of the negative electrode and destroys the negative electrode SEI layer is significantly prevented. Therefore, when a non-aqueous electrolyte containing the compound represented by the above formula 1 as an electrolyte additive is used together with a positive electrode using lithium iron phosphate particles as a positive electrode active material, the self-discharge reaction of the negative electrode caused by an additional reductive decomposition reaction of the electrolyte due to the instability of the SEI layer can be prevented.

[0116] In addition, the compound represented by the above formula 1 includes a fluorocarbon functional group substituted with at least one fluorine element at the end of the compound structure, thereby forming a film on the surface of the positive electrode to ensure antioxidant properties. Therefore, the elution of Fe from the lithium iron phosphate of the positive electrode is inhibited, and the eluted Fe is inhibited from being electrodeposited and precipitated onto the negative electrode, and thus an internal short circuit can be prevented.

[0117] As described above, since the compound represented by Formula 1 above includes a perfluorocarbon functional group substituted with at least one fluorine element and a propargyl group and has excellent flame retardancy and non-combustibility, a strong SEI layer with low resistance is formed, thereby suppressing the additional reductive decomposition reaction of the electrolyte and preventing the self-discharge reaction of the negative electrode. Therefore, a lithium secondary battery can be provided that suppresses an increase in initial resistance and improves room temperature and low temperature output characteristics.

[0118] Meanwhile, in Formula 1 above, R 1 and R 2 can each independently be an alkylene group having 1 to 5 carbon atoms, and R 3 can be an alkyl group having 3 to 20 carbon atoms substituted with at least one fluorine.

[0119] In addition, in Formula 1 above, R 1 and R 2 can each independently be an alkylene group having 1 to 3 carbon atoms, and R 3 can be an alkyl group having 3 to 15 carbon atoms substituted with at least one fluorine.

[0120] Specifically, in Formula 1 above, R 3 can be an alkyl group having 4 to 8 carbon atoms substituted with at least one fluorine.

[0121] Preferably, the compound represented by Formula 1 above can be the compound represented by Formula 1a below:

[0122] [Formula 1a]

[0123]

[0124] Meanwhile, the content of the compound represented by Formula 1 above in the non-aqueous electrolyte can be 0.01% by weight to 10.0% by weight.

[0125] When the content of the compound represented by Formula 1 above is included within the above range, a low-resistance SEI layer is formed on the surface of the negative electrode, while suppressing as much as possible the disadvantages such as side reactions, capacity reduction, and resistance increase caused by the additive, thereby improving the effect of lithium movement in the film and suppressing the additional reductive decomposition reaction of the electrolyte, thereby preventing the self-discharge reaction of the negative electrode.

[0126] Specifically, when the content of the compound represented by Formula 1 above is 0.01% by weight or more, a stable film is formed during the battery driving time, and a low-resistance SEI layer is formed on the negative electrode surface, so that the battery output performance can be improved. In addition, when the content of the compound represented by Formula 1 above is 10.0% by weight or less, the viscosity of the non-aqueous electrolyte can be controlled to achieve optimal wetting, effectively suppressing the increase in battery resistance caused by the decomposition of the additive, and further increasing the ionic conductivity in the battery, thereby preventing the deterioration of the output characteristics.

[0127] Specifically, the content of the compound represented by Formula 1 in the non-aqueous electrolyte can be 0.05% by weight to 6.0% by weight, specifically 0.08% by weight to 0.5% by weight.

[0128] In addition, if necessary, in addition to the compound represented by Formula 1 above, the additive may further include other additional additives to prevent negative electrode collapse due to the decomposition of the non-aqueous electrolyte in a high-power environment, or to further improve the low-temperature high-rate discharge characteristics, high-temperature stability, overcharge protection, and the effect of suppressing battery swelling at high temperatures.

[0129] Examples of the additional additives may include at least one selected from the group consisting of cyclic carbonate compounds, halogenated carbonate compounds, sultone compounds, sulfate / salt compounds, phosphate / salt compounds or phosphite / salt compounds, borate / salt compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds.

[0130] The cyclic carbonate compound may include, for example, vinylene carbonate (VC) or ethylene vinyl carbonate.

[0131] The halogenated carbonate compound may be, for example, fluoroethylene carbonate (FEC), etc.

[0132] 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, ethylene sultone, 1,3-propene sultone (PRS), 1,4-butene sultone, and 1-methyl-1,3-propene sultone.

[0133] For example, the sulfate / salt compound may include ethylene sulfate (Esa), trimethylene sulfate (TMS), or methyl trimethylene sulfate (MTMS).

[0134] The phosphate / phosphate compound or phosphite / phosphite compound may be, for example, at least one compound selected from the group consisting of lithium difluorobis(oxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0135] The borate / borate compound may include tetraphenylborate and lithium difluoro(oxalato)borate (LiODFB) or lithium bis(oxalato)borate (LiB(C 2 O 4 ), 2 LiBOB), etc.

[0136] 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.

[0137] The benzene compound may be, for example, fluorobenzene, the amine compound may be triethanolamine or ethylenediamine, and the silane compound may be tetravinylsilane.

[0138] The lithium salt compound is a compound different from the lithium salt contained in the non-aqueous electrolyte. Among them, the lithium salt compound may include lithium difluorophosphate (LiPO 2 F 2 ) or LiBF 4 , etc.

[0139] Among these additional additives, when at least one selected from vinylene carbonate, 1,3-propane sultone, fluoroethylene carbonate, succinonitrile, and lithium difluoro(oxalate)borate is included, a stronger SEI layer can be formed on the surface of the negative electrode during the initial activation process of the secondary battery.

[0140] The additional additives can be used by mixing two or more compounds. Relative to the total weight of the non-aqueous electrolyte, the total content of the compound represented by the above formula 1 and the additional additives can be 50% by weight or less, specifically 0.05% by weight to 20% by weight, and more specifically 0.05% by weight to 10% by weight. When the total content of the additives satisfies the above range, the low-temperature output characteristics of the battery can be improved, the high-temperature storage characteristics and high-temperature life characteristics can be improved more effectively, and side reactions of the battery due to the remaining additives after the reaction can be prevented.

[0141] The above lithium secondary battery of the present invention can be suitably used for portable devices such as mobile phones, laptop computers, and digital cameras, as well as electric vehicles such as hybrid electric vehicles (HEVs).

[0142] Therefore, another embodiment of the present invention provides a battery pack including the above lithium secondary battery as a unit cell and a battery pack including the battery pack.

[0143] The battery pack or the battery pack can be used as a power source for at least one of the following large and medium-sized devices: power tools; electric vehicles, including electric vehicles (EV), hybrid electric vehicles (HEV), and plug-in hybrid electric vehicles (PHEV); and power storage systems.

[0144] The shape of the lithium secondary battery of the present invention is not particularly limited, but a cylindrical shape using a can, a prismatic shape, a pouch shape, a coin shape, etc. can be used.

[0145] The lithium secondary battery of the present invention can be used not only in battery cells used as power sources for small devices, but also as unit cells of large and medium-sized battery packs including a plurality of battery cells.

[0146] Hereinafter, the present invention will be described in detail according to embodiments.

[0147] In this case, the embodiments of the present invention can be modified in various different forms, and the scope of the present invention should not be construed as being limited to the embodiments described below. On the contrary, the embodiments of the present invention are provided to make the description thorough and complete, and to fully convey the scope of the present invention to those skilled in the art.

[0148] Hereinafter, the present invention will be described in detail with reference to specific embodiments.

[0149] Example

[0150] Example 1

[0151] (Preparation of non-aqueous electrolyte)

[0152] Ethylene carbonate (EC), ethyl methyl carbonate (EMC), and dimethyl carbonate (DMC) were mixed at a volume ratio of 30:10:60 to prepare an organic solvent.

[0153] LiPF as a lithium salt 6 was dissolved in the organic solvent to have a molar concentration of 1.0 M.

[0154] In addition, the compound represented by the above formula 1a and an additional additive were added to the organic solvent in which the lithium salt was dissolved to prepare a non-aqueous electrolyte.

[0155] The content of the compound represented by the formula 1a in the non-aqueous electrolyte was 0.1% by weight.

[0156] As the additional additive, vinylene ethylene carbonate (VEC), 1,3 - propane sultone (PS), fluoroethylene carbonate (FEC), succinonitrile (SN), and LiODFB were used, and 0.5 wt% of vinylene 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.

[0157] (Preparation of secondary battery)

[0158] Lithium iron phosphate particles (LiFePO 4 ) as the positive electrode active material, carbon nanotubes as the conductive agent, and polyvinylidene fluoride (PVdF) as the binder were added to N - methyl - 2 - pyrrolidone (NMP) as the solvent at a weight ratio of 94:3:3 to prepare a positive electrode paste (solid content: 50 wt%). The positive electrode paste was coated on a 20 - μm - thick positive electrode current collector (Al film) at a loading of 3.7 mAh / cm 2 , dried, and then roll - pressed to prepare a positive electrode (thickness of the positive electrode active material: 220 μm).

[0159] Graphite as the negative electrode active material, SBR - CMC as the binder, and carbon black as the conductive agent were added to water as the solvent at a weight ratio of 96:3:1 to prepare a negative electrode paste (solid content: 60 wt%). The negative electrode paste was coated on a 10 - μm - thick copper (Cu) film as the negative electrode current collector at a loading of 4.2 mAh / cm 2 , dried, and then roll - pressed to prepare a negative electrode (thickness of the negative electrode active material: 170 μm).

[0160] The positive electrode, a polyolefin - based porous separator coated with inorganic particles (Al 2 O 3 ) and the negative electrode were stacked in sequence to prepare an electrode assembly.

[0161] After the assembled electrode assembly was accommodated in the battery case, the prepared non - aqueous electrolyte was then injected into the battery case to prepare a lithium secondary battery.

[0162] Example 2

[0163] The non - aqueous electrolyte and the lithium secondary battery were prepared in the same manner as in Example 1, except that the compound represented by the above 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.

[0164] Example 3

[0165] 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% by weight based on the weight of the non-aqueous electrolyte to prepare the non-aqueous electrolyte.

[0166] Comparative Example 1

[0167] 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 not added and the non-aqueous electrolyte was prepared.

[0168] Comparative Example 2

[0169] 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 the following Formula 2 was added in an amount of 0.1% by weight based on the weight of the non-aqueous electrolyte instead of the compound represented by Formula 1a to prepare the non-aqueous electrolyte.

[0170] [Formula 2]

[0171]

[0172] Comparative Example 3

[0173] 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 the following Formula 2 was added in an amount of 1.0% by weight based on the weight of the non-aqueous electrolyte instead of the compound represented by Formula 1a to prepare the non-aqueous electrolyte.

[0174] Comparative Example 4

[0175] 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 the following Formula 3 was added in an amount of 0.1% by weight based on the weight of the non-aqueous electrolyte instead of the compound represented by Formula 1a to prepare the non-aqueous electrolyte.

[0176] [Formula 3]

[0177]

[0178] Experimental example

[0179] Experimental Example 1: Evaluation of Initial Capacity and Cycle Capacity Retention Rate

[0180] Using an electrochemical charge / discharge device, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as above were each charged to 3.65 V and 0.05 C at 25 °C under CC / CV and 0.33 C conditions, and then discharged to 2.5 V under CC and 0.33 C conditions. This was set as one cycle, and 300 charge and discharge cycles were performed to measure the capacity retention rate.

[0181] The capacity retention rate was calculated by the following equation, and the results are shown in Table 1 below.

[0182] Capacity retention rate (%) = {(Discharge capacity after 300 cycles / Discharge capacity after 1 cycle)} × 100

[0183] The discharge capacity after 1 cycle (initial capacity) and the capacity retention rate after 300 cycles are shown in Table 1 below.

[0184] Experimental Example 2: Initial resistance evaluation

[0185] Using an electrochemical charge / discharge device, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as above were each charged to 3.65 V and 0.05 C under the conditions of CC / CV and 0.1 C at 25 °C, and then discharged to 2.5 V under the conditions of CC and 0.1 C. This was set as one cycle, and charging and discharging were carried out.

[0186] After one cycle of charging and discharging, the discharge capacity after one cycle was measured using an electrochemical charge / discharge device, the state of charge (SOC) was adjusted to 50%, and then a pulse of 2.5 C was applied for 10 seconds to calculate the initial resistance from the difference between the voltage before the pulse application and the voltage after the pulse application. The results are shown in Table 1 below.

[0187] Experimental Example 3: Metal elution amount evaluation

[0188] Using an electrochemical charge / discharge device, the lithium secondary batteries of Examples 1 to 3 and Comparative Examples 1 to 4 prepared as above were each charged to 3.65 V and 0.05 C under the conditions of CC / CV and 0.33 C at 25 °C, and then discharged to 2.5 V under the conditions of CC and 0.33 C. This was set as one cycle, and 300 cycles of charging and discharging were carried out to measure the capacity retention rate.

[0189] Thereafter, the concentration of eluted Fe in the electrolyte solution was measured using an inductively coupled plasma optical emission spectrometer (ICP-OES). The amount of Fe measured by ICP analysis is shown in Table 1 below.

[0190]

[0191] Referring to Table 1 above, it can be seen that compared with Comparative Examples 1 to 4, the lithium secondary batteries of Examples 1 to 3 of the present invention have excellent initial capacity, capacity retention rate, and resistance characteristics, and exhibit a low Fe elution amount.

Claims

1. A lithium secondary battery, which comprises: a positive electrode; a negative electrode; a separator; and a non-aqueous electrolyte, wherein the positive electrode includes a positive electrode active material, and 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 contains a compound represented by Formula 1: [Formula 1] wherein, in Formula 1 above, R 1 and R 2 each independently represents an alkylene group having 1 to 10 carbon atoms, and R 3 is an alkyl group having 1 to 20 carbon atoms and substituted with at least one fluorine atom.

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

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

4. The lithium secondary battery according to claim 1, wherein, In Formula 1 above, R 3 is an alkyl group having 4 to 8 carbon atoms substituted with at least one fluorine.

5. The lithium secondary battery according to claim 1, wherein, the compound represented by Formula 1 is a compound represented by Formula 1a: [Formula 1a] 6. The lithium secondary battery according to claim 1, wherein, in the non-aqueous electrolyte, the content of the compound represented by Formula 1 is 0.01% by weight to 10.0% by weight.

7. The lithium secondary battery according to claim 1, wherein, based on the total weight of the non-aqueous electrolyte, the content of the compound represented by Formula 1 is 0.05% by weight to 6.0% by weight.

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

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

11. The lithium secondary battery according to claim 1, wherein, the lithium iron phosphate particles include a compound represented by Formula A: [Formula A] Li 1+a Fe 1-s M s (PO 4-b )X b wherein, in Formula A above, M is at least one element selected from Co, Ni, Al, Mg, Ti, and V; X is F, S, or N; 0 ≤ s ≤ 0.5; -0.5 ≤ a ≤ +0.5; 0 ≤ b ≤ 0.1.

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