Lithium secondary battery

Abstract

The present invention relates to a lithium secondary battery including a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by Chemical Formula 1, and a second additive represented by Chemical Formula 2, a positive electrode including a positive electrode active material containing a lithium iron phosphate-based composite oxide, a negative electrode including a negative electrode active material, and a separator interposed between the positive electrode and the negative electrode.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2021-0182299, filed with the Korean Intellectual Property Office on December 20, 2021, the entire contents of which are incorporated herein by reference.

[0002] The present invention relates to a lithium secondary battery that includes an electrolyte solution containing a specific combination of additives and a lithium iron phosphate-based positive electrode material. [Background technology]

[0003] Lithium secondary batteries are generally manufactured by forming an electrode assembly by interposing a separator between a positive electrode including a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode including a negative electrode active material capable of storing lithium ions, inserting the electrode assembly into a battery case, injecting an electrolyte serving as a medium for transferring lithium ions, and then sealing the battery case.

[0004] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and therefore are applied in various fields such as mobile devices, electronic products, electric vehicles, etc. As the application fields of lithium secondary batteries become more diverse, the required physical properties are also becoming more stringent. In particular, there is a demand for the development of lithium secondary batteries that can be operated stably even under high temperature conditions.

[0005] At high temperatures, lithium salts such as LiPF6 in the electrolyte are converted to PF6 - Thermal decomposition of anions can generate Lewis acids such as PF5, which react with moisture to generate HF. The decomposition products, such as PF5 and HF, as well as unstable structural changes in the positive electrode caused by charging and discharging, can cause the transition metals in the positive electrode material to leach into the electrolyte. In particular, when a battery contains a lithium iron phosphate (LFP) positive electrode, the structural stability of the positive electrode can be improved, but iron leaching can cause electrolyte decomposition and severe degradation of battery performance, so improvements are needed. Summary of the Invention [Problem to be solved by the invention]

[0006] The present invention aims to solve the above problems and aims to improve the high-temperature performance of lithium secondary batteries containing LFP-based positive electrode active materials by introducing a nonaqueous electrolyte containing a combination of two specific additives. [Means for solving the problem]

[0007] According to one embodiment, the present invention comprises: a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2; a positive electrode including a positive electrode active material including a lithium iron phosphate composite oxide; a negative electrode including a negative electrode active material; and a separator interposed between the positive electrode and the negative electrode.

[0008] [ka]

[0009] In the above Chemical Formula 1, R1 and R2 are each independently hydrogen or an alkyl group having 1 to 5 carbon atoms; [ka] In the above Chemical Formula 2, A is a heterocyclic group having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R3 is an alkylene group having 1 to 3 carbon atoms. [Effects of the Invention]

[0010] The lithium secondary battery according to the present invention includes a non-aqueous electrolyte solution containing a specific combination of additives, thereby improving the high-temperature life and resistance characteristics of a lithium secondary battery containing a lithium iron phosphate-based positive electrode active material. DETAILED DESCRIPTION OF THE INVENTION

[0011] The present invention will now be described in more detail.

[0012] Generally, anions contained in lithium salts such as LiPF6, which are widely used in electrolytes for lithium secondary batteries, form decomposition products such as hydrogen fluoride (HF) and PF5 due to thermal decomposition or moisture, and this phenomenon becomes more severe when the battery is operated under high temperature conditions. The decomposition products have acidic properties, which deteriorate the surface properties of the electrodes inside the battery.

[0013] The resulting decomposition products and structural changes in the positive electrode due to repeated charging and discharging easily cause transition metals in the positive electrode to leach into the electrolyte, and the leached transition metals are further redeposited on the positive electrode, increasing the resistance of the positive electrode. Furthermore, if the leached transition metals migrate to the negative electrode via the electrolyte, they are electrodeposited on the negative electrode, causing destruction of the solid electrolyte interphase (SEI) film and further electrolyte decomposition reactions, resulting in problems such as lithium ion consumption and increased resistance.

[0014] In particular, batteries containing lithium iron phosphate (LFP)-based cathode materials, which have low discharge potential and strong water adsorption properties, can be significantly affected by decomposition products of water in the electrodes and electrolyte during initial charge and discharge. Furthermore, since capacity is generated by phase boundary movement due to lithium ion diffusion during charge and discharge, if the lithium migration channel is blocked by iron elution, irreversible capacity loss can occur.

[0015] In order to solve this problem, the present inventors have found that by adding a first additive represented by the following chemical formula 1 and a second additive represented by the following chemical formula 2 to a non-aqueous electrolyte solution, the elution of iron ions is suppressed and a strengthened SEI film is formed, thereby improving battery performance.

[0016] Each of the components constituting the present invention will be described in more detail below.

[0017] non-aqueous electrolyte The lithium secondary battery according to the present invention includes a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2.

[0018] Each component of the non-aqueous electrolyte will be specifically described below.

[0019] (1) First additive and second additive The non-aqueous electrolyte of the present invention contains a first additive represented by the following chemical formula 1.

[0020] [ka]

[0021] In the above Chemical Formula 1, R1 and R2 are each independently hydrogen or an alkyl group having 1 to 5 carbon atoms.

[0022] In one embodiment of the present invention, R1 and R2 are each hydrogen, and specifically, the compound represented by Chemical Formula 1 may be represented by the following Chemical Formula 1A:

[0023] [ka]

[0024] The first additive contains primary and secondary amines in its structure, which can more effectively neutralize the Lewis acidity of the electrolyte, thereby reducing electrolyte decomposition reactions and transition metal elution. Furthermore, the amino groups of the first additive form nitrogen-based solid electrolyte interface (SEI) and cathode electrolyte interface (CEI) films on the electrodes, contributing to thermal stability.

[0025] In one embodiment of the present invention, the content of the first additive may be 0.05 wt % to 2 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.2 wt % to 0.5 wt %, based on the total weight of the non-aqueous electrolyte. If an excessive amount of the first additive is added, it may be excessively involved in the decomposition reaction at the interface between the electrode and the electrolyte, causing the resistance of the coating to become too high, which may result in an increase in the resistance of the battery. Therefore, the content of the first additive is preferably 2 wt % or less.

[0026] The non-aqueous electrolyte of the present invention also contains a second additive represented by the following chemical formula 2.

[0027] [ka]

[0028] In the above Chemical Formula 2, A is a heterocyclic group having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R3 is an alkylene group having 1 to 3 carbon atoms.

[0029] The second additive represented by Formula 2 contains a propargyl functional group, which is reductively decomposed to form a highly passivating SEI film on the surface of the anode, improving the high-temperature durability of the anode itself and preventing electrodeposition of transition metals on the anode surface. Furthermore, by binding with PF5, a decomposition product of the electrolyte, it suppresses the generation of HF, thereby preventing destruction of the cathode electrolyte interphase (CEI) film formed on the surface of the cathode by the first additive and suppressing further decomposition of the electrolyte.

[0030] In one embodiment of the present invention, A in Chemical Formula 2 may be a nitrogen-containing heterocyclic group having 3 to 5 carbon atoms or a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms, preferably a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms, and more preferably imidazole.

[0031] Specifically, the second additive may be represented by the following chemical formula 2-1.

[0032] [ka]

[0033] In the above chemical formula 2-1, R3 is as defined in Chemical Formula 2 above.

[0034] In one embodiment of the present invention, R3 in Chemical Formula 2 may be a linear or branched alkylene group having 1 to 3 carbon atoms, preferably a linear alkylene group having 1 to 3 carbon atoms, and more preferably a methylene group.

[0035] In one embodiment of the present invention, the second additive may be represented by the following formula 2A:

[0036] [ka]

[0037] In one embodiment of the present invention, the content of the second additive may be 0.05 wt % to 2 wt %, preferably 0.1 wt % to 1 wt %, and more preferably 0.3 wt % to 0.5 wt %, based on the total weight of the non-aqueous electrolyte. If the content of the second additive is too high, it may be excessively involved in the decomposition reaction at the interface between the electrode and the electrolyte, causing the resistance of the coating to become too high, which may result in an increase in the resistance of the battery. Therefore, the content of the second additive is preferably 2 wt % or less.

[0038] In the nonaqueous electrolyte according to the present invention, the use of both the first and second additives is more advantageous than the use of the first additive alone in that it forms a low-resistance anode coating. This improves initial resistance and strengthens the anode coating, improving high-temperature performance. Furthermore, the first additive has a higher bond energy with PF5 than the second additive, making it more effective at suppressing HF generation. Therefore, the combination of the two additives can further enhance the effect of suppressing iron ion leaching from the LFP cathode.

[0039] In one embodiment of the present invention, the weight ratio of the first additive to the second additive in the non-aqueous electrolyte may be 1:0.5 to 1:2.5, preferably 1:1 to 1:2.5, which is preferable because it minimizes an increase in initial resistance and maximizes the effect of improving the high-temperature performance of the battery.

[0040] (2) Third additive In one embodiment of the present invention, the non-aqueous electrolyte may include, as a third additive, one or more compounds selected from carbonate-based compounds, sultone-based compounds, sulfate-based compounds, phosphate-based or phosphite-based compounds, nitrile-based compounds, amine-based compounds, silane-based compounds, benzene-based compounds, and lithium salt-based compounds.

[0041] The carbonate compound may be one or more of vinylene carbonate (VC), vinylethylene carbonate (VEC), and fluoroethylene carbonate (FEC), and preferably vinylene carbonate (VC).

[0042] The sultone-based compound is a material capable of forming a stable SEI film on the surface of the negative electrode through a reduction reaction, and may be one or more selected from 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, and preferably 1,3-propane sultone (PS).

[0043] The sulfate-based compound is a material that can be electrically decomposed on the surface of the negative electrode to form a stable SEI film that does not crack even when stored at high temperatures, and may be one or more selected from ethylene sulfate (Esa), trimethylene sulfate (TMS), and methyl trimethylene sulfate (MTMS), and is preferably ethylene sulfate (Esa).

[0044] The phosphate-based or phosphite-based compound may be one or more selected from lithium difluoro(bisoxalato)phosphate, lithium difluorophosphate, tris(trimethylsilyl)phosphate, tris(trimethylsilyl)phosphite, tris(2,2,2-trifluoroethyl)phosphate, and tris(trifluoroethyl)phosphite.

[0045] The nitrile compound may be one or more selected from succinonitrile (SN), adiponitrile (ADN), acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluorophenylacetonitrile, ethylene glycol bis(2-cyanoethyl)ether (ASA3), 1,3,6-hexanetricarbonitrile (HTCN), 1,4-dicyano-2-butene (DCB), and 1,2,3-tris(2-cyanoethyl)propane (TCEP).

[0046] The amine compound may be one or more selected from triethanolamine and ethylenediamine, and the silane compound may be tetravinylsilane.

[0047] The benzene-based compound may be one or more selected from monofluorobenzene, difluorobenzene, trifluorobenzene, and tetrafluorobenzene.

[0048] The lithium salt-based compound is a compound different from the lithium salt contained in the non-aqueous electrolyte solution, and may be one or more compounds selected from lithium difluorophosphate (LiDFP; LiPO2F2), lithium bis(oxalato)borate (LiBOB; LiB(C2O4)2), lithium tetrafluoroborate (LiBF4), lithium tetraphenylborate, and lithium difluoro(bis(oxalato)phosphate) (LiDFOP).

[0049] Preferably, the non-aqueous electrolyte of the present invention may further contain one or more third additives selected from the group consisting of vinylene carbonate (VC), 1,3-propane sultone (PS), and ethylene sulfate (ESA), which has the advantage of forming a stable coating on the negative electrode, thereby improving the high-temperature performance of the battery.

[0050] In one embodiment of the present invention, the content of the third additive may be 0.05 wt % to 5 wt %, preferably 0.1 wt % to 3 wt %, based on the total weight of the non-aqueous electrolyte solution. The content of the third additive is preferably 5 wt % or less in terms of reducing the initial resistance.

[0051] (3) Organic solvent The non-aqueous electrolyte of the present invention contains an organic solvent.

[0052] The organic solvent may be any of various organic solvents commonly used in lithium electrolytes without limitation. For example, the organic solvent may be a cyclic carbonate solvent, a linear carbonate solvent, a linear ester solvent, a cyclic ester solvent, a nitrile solvent, or a mixture thereof, and preferably a mixture of a cyclic carbonate solvent and a linear carbonate solvent.

[0053] The cyclic carbonate solvent is a highly viscous organic solvent that has a high dielectric constant and therefore easily dissociates the lithium salt in the electrolyte. The cyclic carbonate solvent may be one or more solvents 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 preferably contains ethylene carbonate (EC).

[0054] The linear carbonate solvent is an organic solvent having low viscosity and low dielectric constant, and may be one or more 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. Preferably, the linear carbonate solvent contains dimethyl carbonate (DMC) and ethyl methyl carbonate (EMC). When DMC is used alone, it increases ionic conductivity and improves resistance at room temperature. However, it is unstable at high temperatures, resulting in significant gas generation due to reduction reactions. This results in a high freezing point and significantly reduced low-temperature performance. Therefore, using EMC in combination with DMC can suppress gas generation and improve low-temperature performance.

[0055] The organic solvent is preferably a mixture of a cyclic carbonate solvent and a linear carbonate solvent in order to produce an electrolyte solution having high ionic conductivity.

[0056] The linear ester solvent may be one or more selected from methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate, and preferably methyl propionate, ethyl propionate, or propyl propionate.

[0057] The cyclic ester solvent may be one or more selected from γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0058] The nitrile solvent may be one or more selected from succinonitrile, acetonitrile, propionitrile, butyronitrile, valeronitrile, caprylonitrile, heptanenitrile, cyclopentanecarbonitrile, cyclohexanecarbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, and 4-fluorophenylacetonitrile, and preferably succinonitrile.

[0059] In the total weight of the nonaqueous electrolyte, the remainder excluding other components other than the organic solvent, such as the first to third additives and the lithium salt, is the organic solvent unless otherwise specified.

[0060] (4) Lithium salt The non-aqueous electrolyte of the present invention contains a lithium salt.

[0061] The lithium salt may be any one that is commonly used in electrolytes for lithium secondary batteries, and may be any one of those containing Li as a cation. + and as an anion, F - , Cl - , Br - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , B 10 Cl 10 - , AlCl4 - , AlO4 - , PF6 - , CF3SO3 - , CH3CO2 - , CF3CO2 - , AsF6 - , SbF6 - , CH3SO3 - , (CF3CF2SO2)2N - , (CF3SO2)2N - , (FSO2)2N -, BF2C2O4 - , BC4O8 - , BF2C2O4CHF - , PF4C2O4 - , PF2C4O8 - , PO2F2 - , (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , C4F9SO3 - , CF3CF2SO3 - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , CF3(CF2)7SO3 - , and SCN - It may include any one or more selected from the following.

[0062] Specifically, the lithium salts include LiPF6, LiClO4, LiBF4, LiN(FSO2)2 (LiFSI), LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO3CF3, LiPO2F2, lithium bis(oxalate)borate (LiBOB), lithium difluoro(oxalate)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFOP), lithium tetrafluoro(oxalate)phosphate (LiTFOP), and lithium fluoromalonato(difluoro)borate (LiTFOP). The lithium ion battery may be one or more selected from the group consisting of LiF, LiFePO4, LiF2PO4, LiF3PO4, LiF4PO4, LiF6 ...

[0063] In one embodiment of the present invention, the concentration of the lithium salt in the non-aqueous organic solution containing the lithium salt and the organic solvent may be 0.5 M to 4.0 M, specifically 0.5 M to 3.0 M, more specifically 0.8 M to 2.0 M. When the concentration of the lithium salt is within the above range, the effects of improving low-temperature output and cycle characteristics are sufficiently ensured, and excessive increases in viscosity and surface tension are prevented, resulting in appropriate electrolyte impregnation.

[0064] positive electrode The positive electrode according to the present invention includes a lithium iron phosphate (LFP)-based composite oxide. Specifically, the positive electrode includes a positive electrode current collector and a positive electrode active material layer formed on the positive electrode current collector, and the positive electrode active material included in the positive electrode active material layer may include the lithium iron phosphate-based composite oxide.

[0065] LFP-based positive electrodes have an olivine structure and offer superior structural stability and long lifespan compared to layered positive electrodes, such as NCM-based positive electrodes, which are at risk of structural collapse. However, LFP-based positive electrodes have high moisture sensitivity and voltage dependence, and are vulnerable to metal ion elution. If these issues can be resolved by combining the additives of the present invention, batteries with superior stability and lifespan compared to NCM-based positive electrodes can be obtained.

[0066] The positive electrode active material layer may be prepared by coating a positive electrode slurry containing a positive electrode active material, a binder, a conductive material, and a solvent on a positive electrode current collector, followed by drying and rolling.

[0067] The positive electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has conductivity. For example, stainless steel; aluminum; nickel; titanium; baked carbon; or aluminum or stainless steel whose surface has been surface-treated with carbon, nickel, titanium, silver, or the like may be used.

[0068] The lithium iron phosphate composite oxide may be represented by the following chemical formula 3.

[0069] [Chemical formula 3] Life 1-x M x PO4

[0070] In the above Chemical Formula 3, M is one or more selected from Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V; 0≦x<1.

[0071] In one embodiment of the present invention, the lithium iron phosphate composite oxide may be LiFePO4.

[0072] The positive electrode active material may be included in an amount of 80 wt% to 99 wt%, specifically 90 wt% to 99 wt%, based on the total weight of the solid content in the positive electrode slurry. In this case, if the content of the positive electrode active material is 80 wt% or less, the energy density may be reduced, and the capacity may be reduced.

[0073] The binder is a component that aids in binding the active material and conductive material, etc., and in binding to the current collector, and is typically added in an amount of 1 to 30 wt % based on the total weight of the solids in the positive electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0074] The conductive material is a substance that imparts conductivity to the battery without causing any chemical change to the battery, and may be added in an amount of 0.5% by weight to 20% by weight based on the total weight of the solid content in the positive electrode slurry.

[0075] Examples of the conductive material include carbon blacks such as acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; graphite powders such as natural graphite, artificial graphite, carbon nanotubes, and graphite; conductive fibers such as carbon fibers and metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives; or combinations thereof.

[0076] The solvent for the positive electrode slurry may include an organic solvent such as N-methyl-2-pyrrolidone (NMP), and may be used in an amount that provides a suitable viscosity when the positive electrode active material, binder, conductive material, etc. are contained. For example, the positive electrode slurry may contain the organic solvent so that the solids concentration in the positive electrode slurry containing the positive electrode active material, binder, and conductive material is 40 wt % to 90 wt %, preferably 50 wt % to 80 wt %.

[0077] negative electrode The lithium secondary battery according to the present invention includes a negative electrode including a negative electrode active material, and the negative electrode can be manufactured by coating a negative electrode current collector with a negative electrode slurry including a negative electrode active material, a binder, a conductive material, a solvent, and the like, followed by drying and rolling.

[0078] The negative electrode current collector generally has a thickness of 3 μm to 500 μm. There are no particular limitations on the negative electrode current collector, so long as it does not cause chemical changes in the battery and has high conductivity. For example, copper; stainless steel; aluminum; nickel; titanium; calcined carbon; copper or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, or the like; or an aluminum-cadmium alloy may be used. Furthermore, as with the positive electrode current collector, the bonding strength of the negative electrode active material may be strengthened by forming fine irregularities on the surface, and the negative electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0079] The negative electrode active material may include one or more selected from a carbon-based material; a metal or an alloy of such a metal and lithium; a metal composite oxide; a material capable of doping and dedoping lithium; lithium metal; and a transition metal oxide, and preferably includes a carbon-based material.

[0080] The carbon-based material can be any material capable of reversibly intercalating / deintercalating lithium ions, and representative examples thereof include crystalline carbon, amorphous carbon, or a combination of these. Examples of crystalline carbon include amorphous, plate-like, flake-like, spherical, or fibrous graphite, such as natural graphite and artificial graphite. Examples of amorphous carbon include soft carbon (low-temperature calcined carbon), hard carbon, mesophase pitch carbide, and calcined coke. Preferably, a mixture of natural graphite and artificial graphite can be used.

[0081] The metal or the alloy of such a metal with lithium may be a 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, and Sn, or an alloy of such a metal with lithium.

[0082] Examples of the metal composite oxide include PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5, Li x Fe2O3 (0 ≦ x ≦ 1), Li x WO2 (0 ≦ x ≦ 1), and Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, elements of Group 1, Group 2, Group 3 of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8), and one or more selected from the group consisting thereof can be used.

[0083] Examples of the substance capable of doping and undoping lithium include Si, SiO x (0 < x < 2), Si - Y alloy (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Sn, SnO2, Sn - Y (where Y is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, transition metals, rare earth elements, and combinations thereof, and is not Sn), etc. Further, at least one of these and SiO2 may be mixed and used. The element Y may be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db (dubnium), Cr, Mo, W, Sg, Tc, Re, Bh, Fe, Pb, Ru, Os, Hs, Rh, Ir, Pd, Pt, Cu, Ag, Au, Zn, Cd, B, Al, Ga, Sn, In, Ge, P, As, Sb, Bi, S, Se, Te, Po, and combinations thereof.

[0084] Examples of the transition metal oxide include lithium - containing titanium composite oxide (LTO), vanadium oxide, lithium vanadium oxide, etc.

[0085] The negative electrode active material may be contained at 60% to 99% by weight based on the total weight of the solid content in the negative electrode slurry.

[0086] The binder is a component that aids in bonding between the conductive material, active material, and current collector, and may be added in an amount of 1 to 30 wt % based on the total weight of the solids in the negative electrode slurry. Examples of such binders include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, sulfonated ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, and various copolymers thereof.

[0087] The conductive material is a component for further improving the conductivity of the negative electrode active material and may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solid content in the negative electrode slurry. The conductive material is not particularly limited as long as it does not cause chemical changes in the battery and has conductivity, and examples thereof include carbon black such as acetylene black, ketjen black, channel black, furnace black, lamp black, or thermal black; graphite powder such as natural graphite, artificial graphite, carbon nanotubes, or graphite, which have a highly developed crystalline structure; conductive fibers such as carbon fibers or metal fibers; conductive powders such as carbon fluoride powder, aluminum powder, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives; or combinations thereof.

[0088] The solvent for the negative electrode slurry may include water or an organic solvent such as NMP or alcohol, and may be used in an amount that provides a suitable viscosity when containing the negative electrode active material, binder, conductive material, etc. For example, the solvent may be included so that the solids concentration in the slurry containing the negative electrode active material, binder, and conductive material is 30 wt % to 80 wt %, preferably 40 wt % to 70 wt %.

[0089] Separator The lithium secondary battery according to the present invention includes a separator between the positive electrode and the negative electrode.

[0090] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator that is commonly used as a separator in a lithium secondary battery can be used without any particular limitation. In particular, a separator that has low resistance to the movement of ions in the electrolyte, excellent electrolyte impregnation ability, and safety is preferred.

[0091] Specifically, the separator may be a 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; or a laminate structure of two or more layers thereof. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a coated separator containing a ceramic component or a polymer material may also be used, and may be used in a single-layer or multi-layer structure.

[0092] 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 electric vehicles such as hybrid electric vehicles (HEVs).

[0093] 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.

[0094] 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.

[0095] 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.

[0096] 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.

[0097] The present invention will be specifically described below with reference to specific examples.

[0098] <Example> Example 1 (Production of non-aqueous electrolyte) Ethylene carbonate (EC), dimethyl carbonate (DMC), and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 30:40:30, and LiPF6 was dissolved therein to prepare a non-aqueous organic solution to a concentration of 1.0 M. 0.2 wt% of the compound represented by Chemical Formula 1A, 0.3 wt% of the compound represented by Chemical Formula 2A, 2.5 wt% of vinylene carbonate (VC), 0.5 wt% of 1,3-propane sultone (PS), 0.7 wt% of ethylene sulfate (ESa), and the remainder of the non-aqueous organic solution were mixed together to prepare a 100 wt% non-aqueous electrolyte.

[0099] (Lithium secondary battery manufacturing) A cathode slurry (solid content 67.5 wt%) was prepared by adding LiFePO4 as a cathode active material, carbon black as a conductive material, polyvinylidene fluoride as a binder, and nitrile-butadiene rubber to N-methyl-2-pyrrolidone (NMP) as a solvent in a weight ratio of 95.86:0.8:2.2:1.14. The cathode slurry was applied to a 15 μm thick cathode current collector (Al thin film), dried, and roll-pressed to prepare a cathode.

[0100] Graphite (artificial graphite:natural graphite (weight ratio 8:2)) as the negative electrode active material, styrene-butadiene rubber-carboxymethyl cellulose (SBR-CMC) as the binder, carbon black as the conductive material, and sodium carboxymethyl cellulose (CMC) as the thickener were mixed with distilled water as the solvent in a weight ratio of 96.0:2.3:0.7:1, and then mixed with distilled water as the solvent to prepare a negative electrode active material slurry with a solids content of 47.0 wt%. The negative electrode active material slurry was applied to an 8 μm-thick negative electrode current collector (Cu thin film), dried, and roll-pressed to prepare a negative electrode.

[0101] The positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a negative electrode were sequentially stacked to prepare an electrode assembly.

[0102] The electrode assembly assembled as described above was housed in a pouch-shaped battery case, and the non-aqueous electrolyte prepared as described above was poured into the case, thereby producing a lithium secondary battery.

[0103] Example 2. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 2A was changed to 0.5 wt % when preparing the non-aqueous electrolyte.

[0104] Example 3. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the content of the compound represented by Chemical Formula 1A was changed to 0.5 wt % and the content of the compound represented by Chemical Formula 2A was changed to 0.5 wt % during the preparation of the non-aqueous electrolyte.

[0105] Comparative Example 1 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 2A was not added during the preparation of the non-aqueous electrolyte solution.

[0106] Comparative Example 2 A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by Chemical Formula 1A was not added during the preparation of the non-aqueous electrolyte solution.

[0107] Comparative Example 3. Instead of the slurry described in Example 1, a positive electrode active material slurry containing Li[Ni 0.8 Co 0.1 Mn 0.1 A lithium secondary battery was fabricated in the same manner as in Example 1, except that a cathode active material slurry (solid content 67.5 wt %) was used, which was prepared by mixing 102, carbon black as a conductive material, and polyvinylidene fluoride (PVDF) as a binder in a weight ratio of 94:3:3 and adding the mixture to N-methyl-2-pyrrolidone (NMP) as a solvent.

[0108] Comparative Example 4. A lithium secondary battery was manufactured in the same manner as in Example 1, except that a compound (imidazole) represented by the following chemical formula B1 was used instead of the compound represented by the chemical formula 1A when preparing the non-aqueous electrolyte.

[0109] [ka]

[0110] Comparative Example 5. A lithium secondary battery was manufactured in the same manner as in Example 1, except that the compound represented by the following chemical formula B2 was used instead of the compound represented by the chemical formula 1A when preparing the non-aqueous electrolyte.

[0111] [ka]

[0112] <Experimental Example> Experimental Example 1: Evaluation of high temperature (60°C) storage characteristics Each of the lithium secondary batteries prepared in the Examples and Comparative Examples was fully charged to 100% SOC at 4.2 V (0.05 C cut-off) under CC / CV conditions at 0.33 C at 25° C. The fully charged lithium secondary batteries were then stored at high temperature (60° C.) for 12 weeks, after which the capacity retention rate and resistance increase rate were measured, and the results are shown in Table 1 below.

[0113] In this case, the capacity retention rate was calculated by substituting the discharge capacity of the lithium secondary battery measured before and after high-temperature storage using a PNE-0506 charger / discharger (manufacturer: PNE Solution Co., Ltd., 5V, 6A) into the following formula (1):

[0114] The resistance increase rate was calculated by substituting the initial resistance value measured before high-temperature storage and the resistance value measured after high-temperature storage into the following formula (2).

[0115] Equation (1): Capacity retention rate (%) = (discharge capacity after high-temperature storage / discharge capacity before high-temperature storage) × 100 Equation (2): Resistance increase rate (%) = {(resistance value after high-temperature storage - initial resistance value) / initial resistance value} × 100

[0116] Experimental example 2: Evaluation of high temperature (45°C) life characteristics Each of the lithium secondary batteries manufactured in the examples and comparative examples was activated at 0.1 C / C and then degassed.

[0117] The battery was then charged at 0.33 C CC to 3.6 V under constant current-constant voltage (CC-CV) charging conditions at 25°C, followed by a 0.05 C current cut and discharge under CC conditions at 0.33 C to 2.5 V. Next, the battery was charged at 0.33 C CC to 3.6 V under constant current-constant voltage (CC-CV) charging conditions at 45°C, followed by a 0.05 C current cut and discharge under CC conditions at 0.33 C to 2.5 V.

[0118] The above charge / discharge cycle was counted as one cycle, and while charging / discharging was performed at a high temperature (45° C.), the discharge capacity and resistance were measured after one cycle and after 400 cycles using a charge / discharger (5 V, 6 A).

[0119] The measured discharge capacity and resistance were substituted into the following formulas (3) and (4), respectively, to calculate the capacity retention and resistance increase rate, and the results are shown in Table 1 below.

[0120] Equation (3): Capacity retention rate (%) = (discharge capacity after 400 cycles / discharge capacity after 1 cycle) × 100 Equation (4): Resistance increase rate (%) = {(resistance after 400 cycles - resistance after 1 cycle) / resistance after 1 cycle} × 100

[0121] [Table 1]

[0122] From the results in Table 1, it can be seen that when the first additive and the second additive of the present invention are both included as electrolyte additives, both the high-temperature life and storage characteristics are excellent.

[0123] Specifically, it can be seen that Examples 1 to 3 have superior high-temperature life and storage characteristics compared to Comparative Examples 1 and 2, which used an electrolyte containing only one of the first additive and the second additive, as well as Comparative Example 3, which used an NCM cathode material, which is generally known to have superior capacity characteristics compared to an LFP cathode material.

[0124] Furthermore, even when the second additive is used, Examples 1 to 3 are confirmed to have excellent high-temperature life and storage characteristics compared to when the compound represented by Chemical Formula 1 is replaced with the compound represented by Chemical Formula B1 having an imidazole structure with no substituted amino group (Comparative Example 4) and the compound represented by Chemical Formula B2 having a phenylimidazole structure (Comparative Example 5).

[0125] This confirms that the effects of the present invention can be achieved only when both the first additive and the second additive according to the present invention are contained.

Claims

1. a non-aqueous electrolyte solution containing a lithium salt, an organic solvent, a first additive represented by the following chemical formula 1, and a second additive represented by the following chemical formula 2; a positive electrode including a positive electrode active material including a lithium iron phosphate composite oxide; a negative electrode including a negative electrode active material; a separator interposed between the positive electrode and the negative electrode. 【Chemical 1】 (In the above Chemical Formula 1, R1 and R2 are each independently hydrogen or an alkyl group having 1 to 5 carbon atoms; 【Chemistry 2】 In the above Chemical Formula 2, A is a heterocyclic group having 3 to 5 carbon atoms or a heteroaryl group having 3 to 5 carbon atoms, R3 is an alkylene group having 1 to 3 carbon atoms.

2. 2. The lithium secondary battery of claim 1, wherein R1 and R2 in Formula 1 are each hydrogen.

3. 2. The lithium secondary battery according to claim 1, wherein A in Chemical Formula 2 is a nitrogen-containing heterocyclic group having 3 to 5 carbon atoms or a nitrogen-containing heteroaryl group having 3 to 5 carbon atoms.

4. 2. The lithium secondary battery according to claim 1, wherein the non-aqueous electrolyte further comprises one or more third additives selected from the group consisting of vinylene carbonate, 1,3-propane sultone, and ethylene sulfate.

5. 2. The lithium secondary battery of claim 1, wherein the content of the first additive is 0.05 wt % to 2 wt % based on the total weight of the non-aqueous electrolyte solution.

6. 2. The lithium secondary battery of claim 1, wherein the content of the second additive is 0.05 wt % to 2 wt % based on the total weight of the non-aqueous electrolyte.

7. 7. The lithium secondary battery according to claim 1, wherein the weight ratio of the first additive to the second additive in the non-aqueous electrolyte solution is 1:0.5 to 1:2.

5.

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

9. 2. The lithium secondary battery according to claim 1, wherein the lithium iron phosphate composite oxide is represented by the following chemical formula 3: [Chemical formula 3] LiFe １－ｘ M ｘ PO ４ (In the above chemical formula 3, M is one or more selected from Ni, Co, Mn, Al, Mg, Y, Zn, In, Ru, Sn, Sb, Ti, Te, Nb, Mo, Cr, Zr, W, Ir, and V; 0≦x<1.)

10. The lithium secondary battery according to claim 1 , wherein the negative electrode active material comprises a carbon-based material.

11. A lithium secondary battery as described in claim 1, wherein the content of the first additive is 0.1 wt % to 1 wt % based on the total weight of the non-aqueous electrolyte, the content of the second additive is 0.1 wt % to 1 wt % based on the total weight of the non-aqueous electrolyte, and the weight ratio of the first additive to the second additive in the non-aqueous electrolyte is 1:1 to 1:2.5.

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