Non-aqueous electrolyte for lithium secondary battery and lithium secondary battery containing the same

Abstract

The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. The non-aqueous electrolyte includes: a lithium salt; an organic solvent; a compound represented by formula 1 as a first additive; lithium difluorophosphate as a second additive; and a compound represented by formula 2 as a third additive.

Description

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS

[0002] This application claims the benefit of Korean Patent Application No. 10-2020-0115054 filed on September 9, 2020, in the Korean Intellectual Property Office, the disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same. Background Art

[0004] Lithium secondary batteries are generally prepared by the following method: after an electrode assembly is formed by arranging a separator between a positive electrode and a negative electrode, the electrode assembly is inserted into a battery case, wherein the positive electrode includes a positive electrode active material formed of a lithium-containing transition metal oxide, the negative electrode includes a negative electrode active material capable of storing lithium ions, and a non-aqueous electrolyte as a medium for transmitting lithium ions is injected therein, and then the battery case is sealed.

[0005] Lithium secondary batteries can be miniaturized and have high energy density and operating voltage, and therefore have been used in various fields such as mobile devices, electronic products, and electric vehicles. As the application fields of lithium secondary batteries have become more diverse, lithium secondary batteries are increasingly required to have high physical properties, especially lithium secondary batteries that can operate stably even under high temperature conditions.

[0006] At the same time, when lithium secondary batteries operate under high temperature conditions, the anion PF6 - A lithium salt (e.g., LiPF6) contained in the electrolyte can be thermally decomposed to produce a Lewis acid (e.g., PF5), which reacts with moisture to produce HF. Decomposition products such as PF5 and HF not only destroy the film formed on the electrode surface, but also cause a decomposition reaction of the organic solvent. In addition, the decomposition products can react with the decomposition products of the positive electrode active material to dissolve transition metal ions, and the dissolved transition metal ions can be electrodeposited on the negative electrode to destroy the film formed on the negative electrode surface.

[0007] Therefore, when the electrolyte decomposition reaction continues on the damaged membrane, the performance of the battery is further reduced, and thus there is a need to develop a secondary battery that can maintain excellent performance even under high temperature conditions. Summary of the Invention

[0008] [Technical Issues]

[0009] One aspect of the present invention provides a non-aqueous electrolyte and a lithium secondary battery including the same, wherein the non-aqueous electrolyte facilitates formation of a positive electrode film by including a specific additive combination.

[0010] [Technical solution]

[0011] According to one aspect of the present invention, a non-aqueous electrolyte for a lithium secondary battery is provided, comprising: a lithium salt; an organic solvent; a compound represented by Formula 1 below as a first additive; lithium difluorophosphate as a second additive; and a compound represented by Formula 2 below as a third additive.

[0012] [Formula 1]

[0013]

[0014] In formula 1,

[0015] Ar is a 5-membered or 6-membered nitrogen-containing ring, and

[0016] L1 is a C1-C3 alkylene group,

[0017] [Formula 2]

[0018]

[0019] In formula 2,

[0020] L2 is a direct bond or a C1-C3 alkylene group,

[0021] R4 is a C1-C3 alkyl group substituted with at least one fluorine, and

[0022] R5 is a C1-C3 alkyl group.

[0023] According to another aspect of the present invention, a lithium secondary battery is provided, comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the negative electrode and the positive electrode; and the above-mentioned nonaqueous electrolyte for lithium secondary batteries.

[0024] [Beneficial Effects]

[0025] To address the above limitations, the present invention may provide a non-aqueous electrolyte for a lithium secondary battery, which includes a combination of additives capable of reducing positive electrode dissolution by forming a strong film on the positive electrode surface.

[0026] Furthermore, the present invention can provide a lithium secondary battery having improved high-temperature storage characteristics and life characteristics by including the above-mentioned nonaqueous electrolyte for a lithium secondary battery. DETAILED DESCRIPTION

[0027] Hereinafter, the present invention will be described in detail.

[0028] Generally, the anion of LiPF6, a lithium salt widely used in lithium secondary batteries, forms decomposition products such as hydrogen fluoride (HF) and PF5 by thermal decomposition, moisture, etc. These decomposition products have acidic properties and deteriorate the film or surface of the electrode in the battery.

[0029] For example, these decomposition products easily cause the transition metal constituting the positive electrode to dissolve into the electrolyte, the dissolved transition metal moves to the negative electrode through the electrolyte, and then is electrodeposited onto the solid electrolyte interphase (SEI) formed on the negative electrode to cause further electrolyte decomposition reaction.

[0030] Since such a series of reactions reduces the amount of available lithium ions in the battery, not only does the battery capacity decrease, but further electrolyte decomposition reactions also proceed, resulting in increased resistance.

[0031] The present invention provides a non-aqueous electrolyte for a lithium secondary battery, which contains a combination of additives capable of suppressing the dissolution of transition metals by forming a strong film on the surface of a positive electrode, and a lithium secondary battery including the same.

[0032] The present inventors used a compound represented by the following Chemical Formula 1, lithium difluorophosphate, and a compound represented by the following Chemical Formula 2 as first to third additives to the electrolyte, respectively, and thereby confirmed that these additives have the effect of efficiently forming films on the negative electrode and the positive electrode.

[0033] Specifically, the first additive is a substance that helps improve the durability of the negative electrode and is first reductively decomposed at a relatively high negative electrode potential to form a stable film. This film is polymeric, so the durability of the battery is excellent.

[0034] Specifically, since the compound represented by Formula 1 contains a propargyl functional group, a SEI with high passivation ability is formed on the surface of the negative electrode while the functional group is reductively decomposed, thereby not only improving the high-temperature durability of the negative electrode itself, but also preventing the electrodeposition of transition metals on the surface of the negative electrode. In addition, due to the propargyl group, the compound represented by Formula 1 can act by adsorbing on the surface of the metal impurities contained in the positive electrode to make the impurities difficult to dissolve, and therefore, it is possible to suppress the internal short circuit that may occur due to the deposition of dissolved metal ions on the negative electrode. In addition, since the propargyl group is easily reduced on the surface of the negative electrode, it can form a stable film on the surface of the negative electrode, thereby preventing the self-discharge reaction of the graphite-based and silicon-based negative electrodes caused by the further reductive decomposition reaction of the electrolyte caused by the instability of the SEI.

[0035] The second additive is a substance that does not have a high reaction potential, but it contributes greatly to improving durability by forming an effective film for the positive and negative electrodes. The lithium ion components generated by decomposition during the initial charge can form a stable SEI on the surface of the negative electrode, and the formation of the SEI can not only improve the mobility of Li to the negative electrode, but also reduce the interfacial resistance. In particular, the difluorophosphate anions generated by decomposition during the initial charge are present on the surface of the positive electrode, and thus can stabilize the lattice oxygen of the positive electrode active material present on the surface of the positive electrode and prevent further structural collapse.

[0036] The third additive is a fluorine-containing carbonate solvent and not only strengthens the negative electrode film but also forms a stable positive electrode-electrolyte interface (CEI) represented by metal fluoride on the positive electrode.

[0037] When the first additive is added to form a strong negative electrode film first, further decomposition reactions on the negative electrode surface can be reduced, and thus the consumption of positive electrode film-forming additives such as the second and third additives can be reduced. The additive with an increased residual amount can act more on the positive electrode, which takes more time to form a film than the negative electrode, and therefore, the positive electrode can be enhanced more efficiently. (See J.Phys.Chem.C 2014, 118, 10631-10639, which relates to the mechanism of protecting the positive and negative electrodes using methyl carbonate 2,2,2-trifluoroethyl)

[0038] non-aqueous electrolytes

[0039] The nonaqueous electrolyte of the present invention includes: a lithium salt; an organic solvent; a compound represented by the following formula 1 as a first additive; lithium difluorophosphate as a second additive; and a compound represented by the following formula 2 as a third additive.

[0040] [Formula 1]

[0041]

[0042] In formula 1,

[0043] Ar is a 5-membered or 6-membered nitrogen-containing ring, and

[0044] L1 is a C1-C3 alkylene group,

[0045] [Formula 2]

[0046]

[0047] In formula 2,

[0048] L2 is a direct bond or a C1-C3 alkylene group,

[0049] R4 is a C1-C3 alkyl group substituted with at least one fluorine, and

[0050] R5 is a C1-C3 alkyl group.

[0051] (1) Additives

[0052] In one embodiment of the present invention, Ar in Formula 1 includes at least two nitrogen atoms. More specifically, the compound represented by Formula 1 can be represented by the following Formula 1A.

[0053] [Formula 1A]

[0054]

[0055] In Formula 1A,

[0056] L1 is C1-C3 alkylene, and

[0057] R1 to R3 are each independently hydrogen or C1-C3 alkyl.

[0058] L1 in Formula 1 may be a methylene group or an ethylene group, more specifically a methylene group.

[0059] R1 to R3 in Formula 1 may each independently be hydrogen or a methyl group, and more specifically, each of R1 to R3 may be hydrogen.

[0060] In one embodiment of the present invention, the compound represented by Formula 1 may be 1H-imidazole-1-carboxylic acid propargyl ester.

[0061] In one embodiment of the present invention, L2 in Formula 2 can be a direct bond or a methylene group, more specifically a methylene group.

[0062] R4 in Formula 2 may be a C1 or C2 alkyl group substituted with at least one fluorine, and more specifically a trifluoromethyl group.

[0063] R5 in Formula 2 may be a methyl group or an ethyl group, more specifically a methyl group.

[0064] In one embodiment of the present invention, the compound represented by Formula 2 may be methyl carbonate 2,2,2-trifluoroethyl ester.

[0065] In one embodiment of the present invention, the content of the first additive may be 0.1% to 1% by weight, preferably 0.1% to 0.5% by weight, based on the total weight of the non-aqueous electrolyte. When the content of the first additive is within the above range, the first additive may react in the high-potential negative electrode, thereby forming a strong polymer film on the negative electrode more quickly and stably, thereby improving the lifespan and high-temperature durability of the battery.

[0066] Specifically, if the content of the first additive is less than 0.1 wt %, the effect of adding the first additive may be slight, and if the content of the first additive is greater than 1 wt %, the resistance of the negative electrode film increases, and thus the resistance of the battery also increases significantly, which may cause problems such as reduced output and increased heat generated during operation.

[0067] In one embodiment of the present invention, the content of the second additive may be 0.1 wt % to 1.5 wt %, preferably 0.5 wt % to 1.5 wt %, based on the total weight of the non-aqueous electrolyte. When the content of the second additive is within the above range, the second additive can form a film containing P and O on the surface of the negative electrode, which is effective for improving durability, and can stabilize the oxygen constituting the positive electrode active material on the positive electrode and effectively form CEI, thereby ultimately improving the durability of the battery.

[0068] If the content of the second additive is less than 0.1 wt %, the effect of adding the second additive may be slight, and if the content of the second additive is greater than 1.5 wt %, the second additive is difficult to dissolve in the electrolyte solvent system of the present invention, thereby possibly causing an increase in electrolyte viscosity and a decrease in ionic conductivity, and thus, problems such as reduced battery output and reduced electrode wetting may occur.

[0069] In one embodiment of the present invention, the content of the third additive may be 1 wt % to 5 wt %, preferably 2 wt % to 4 wt %, based on the total weight of the non-aqueous electrolyte. When the content of the third additive is within the above range, the third additive may form a metal fluoride film on the positive electrode and the negative electrode that effectively improves durability.

[0070] Specifically, if the content of the third additive is less than 1 wt %, the effect of adding the third additive may be slight, and if the content of the third additive is greater than 5 wt %, the ionic conductivity of the electrolyte is reduced due to the low dielectric constant of the third additive, and excessive decomposition of the third additive may be the cause of increased battery resistance.

[0071] In one embodiment of the present invention, the non-aqueous electrolyte may further include 1-propylene-1,3-sultone as a fourth additive. 1-propylene-1,3-sultone has excellent film-forming properties on both the positive and negative electrodes and, when used with the first to third additives, can further improve high-temperature durability. The fourth additive has a relatively low negative electrode decomposition voltage compared to the first additive, but decomposes more rapidly than the solvent.

[0072] The content of the fourth additive may be 0.1 wt % to 1 wt %, preferably 0.1 wt % to 0.5 wt %, based on the total weight of the non-aqueous electrolyte. When the content of the fourth additive is within the above range, the fourth additive effectively forms a film on the positive electrode and the negative electrode, thereby having excellent surface protection ability.

[0073] Specifically, if the content of the fourth additive is less than 0.1 wt %, the effect of adding the fourth additive may be slight, and if the content of the fourth additive is greater than 1 wt %, the resistance of the negative electrode film increases and thus the resistance of the battery increases significantly, which may cause problems such as reduced output and increased heat generated during operation.

[0074] In one embodiment of the present invention, the nonaqueous electrolyte may include at least one selected from vinylene carbonate (VC) and 1,3-propane sultone (PS), preferably both vinylene carbonate (VC) and 1,3-propane sultone (PS), as a fifth additive.

[0075] The content of the fifth additive may be 0.1 wt % to 4 wt %, preferably 0.2 wt % to 2 wt %, based on the total weight of the non-aqueous electrolyte. When the content of the fifth additive is within the above range, the fifth additive may help strengthen the positive and negative electrode films to improve durability.

[0076] Specifically, if the content of the fifth additive is less than 0.1 wt %, the effect of adding the fifth additive may be slight, and if the content of the fifth additive is greater than 4 wt %, the resistance of the membrane increases and thus the resistance of the battery increases, so that problems such as reducing the output of the battery and increasing heat generation may occur.

[0077] (2) Organic solvents

[0078] Various organic solvents commonly used in lithium electrolytes can be used as the organic solvent without limitation. For example, the organic solvent can be a cyclic carbonate solvent, a linear carbonate solvent, or a mixture thereof, and preferably, can include a cyclic carbonate solvent and a linear carbonate solvent.

[0079] The cyclic carbonate solvent is a highly viscous organic solvent that can well dissociate lithium salts in the electrolyte due to its high dielectric constant, and can be at least one 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 specifically can include ethylene carbonate (EC).

[0080] In addition, the linear carbonate solvent is an organic solvent with low viscosity and low dielectric constant, and can be at least one selected from the group consisting of dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate, ethyl methyl carbonate (EMC), methylpropyl carbonate and ethylpropyl carbonate, and specifically can include ethyl methyl carbonate (EMC).

[0081] In order to prepare an electrolyte having high ionic conductivity, it is desirable to use a mixture of a cyclic carbonate-based solvent and a linear carbonate-based solvent as the organic solvent.

[0082] Furthermore, the organic solvent may include a linear ester solvent and / or a cyclic ester solvent in addition to the cyclic carbonate solvent and / or the linear carbonate solvent.

[0083] Specific examples of the linear ester-based solvent may include at least one organic solvent selected from the group consisting of methyl acetate, ethyl acetate, propyl acetate, methyl propionate, ethyl propionate, propyl propionate, and butyl propionate.

[0084] In addition, specific examples of the cyclic ester-based solvent may include at least one organic solvent selected from the group consisting of γ-butyrolactone, γ-valerolactone, γ-caprolactone, σ-valerolactone, and ε-caprolactone.

[0085] If necessary, the organic solvent may be used by adding an organic solvent generally used in an electrolyte of a lithium secondary battery without limitation. For example, the organic solvent may further include at least one organic solvent selected from ether solvents, amide solvents, and nitrile solvents.

[0086] Unless otherwise specified, other components other than the organic solvent, such as additives, lithium salts, and the remainder excluding the additive content in the total weight of the non-aqueous electrolyte may be the organic solvent.

[0087] (3) Lithium salt

[0088] Any lithium salt commonly used in lithium secondary battery electrolytes may be used as the lithium salt without limitation. For example, the lithium salt may include Li + as cations, and including those selected from the group consisting of 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 - At least one of the group consisting of is used as an anion.

[0089] Specifically, the lithium salt may include one or a mixture of two or more selected from the group consisting of LiPF6, LiClO4, LiBF4, LiFSI, LiTFSI, lithium bis(pentafluoroethanesulfonyl)imide (LiBETI), LiSO3CF3, LiPO2F2, lithium bis(oxalato)borate (LiBOB), lithium difluoro(oxalato)borate (LiFOB), lithium difluoro(bisoxalato)phosphate (LiDFBP), lithium tetrafluoro(oxalato)phosphate (LiTFOP) and lithium fluoromalonate (difluoro)borate (LiFMDFB), and is preferably at least one selected from LiPF6 and LiN(FSO2)2(LiFSI).

[0090] The lithium salt may be appropriately changed within a generally available range, but may be contained in the electrolyte at a concentration of 0.8 M to 4.0 M, specifically 1.0 M to 3.0 M, to obtain the best effect of forming an anticorrosion film on the electrode surface.

[0091] If the concentration of the lithium salt is less than 0.8 M, the effect of improving the low-temperature output and the cycle characteristics of the lithium secondary battery is slight. If the concentration of the lithium salt is greater than 4.0 M, the electrolyte wettability may decrease due to an increase in the viscosity of the non-aqueous electrolyte solution.

[0092] Lithium secondary battery

[0093] Next, the lithium secondary battery of the present invention will be described.

[0094] The lithium secondary battery of the present invention includes a positive electrode, a negative electrode, a separator disposed between the positive electrode and the negative electrode, and a non-aqueous electrolyte. In this case, the non-aqueous electrolyte is the non-aqueous electrolyte described in the present invention. Since the non-aqueous electrolyte has been described above, its description will be omitted, and other components will be described below.

[0095] (1) Positive electrode

[0096] The positive electrode of the present invention may contain a positive electrode active material and may be prepared by coating a positive electrode current collector with a positive electrode paste containing a positive electrode active material, a binder, a conductive agent, a solvent, etc., and then drying and rolling the coated positive electrode current collector.

[0097] The positive electrode current collector is not particularly limited as long as it has conductivity and does not cause adverse chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, fired carbon, or aluminum or stainless steel surface-treated with one of carbon, nickel, titanium, silver, etc. may be used.

[0098] The positive electrode active material includes a lithium composite transition metal oxide containing nickel (Ni), cobalt (Co), and manganese (Mn), and the lithium composite transition metal oxide can be represented by the following formula 3:

[0099] [Formula 3]

[0100] Li(Ni a Co b Mn c M d )O2

[0101] In formula 3,

[0102] M is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B, or Mo,

[0103] a, b, c, and d are the atomic fractions of each independent element, which satisfy 0.5 ≤ a < 1, 0 < b ≤ 0.3, 0 < c ≤ 0.3, 0 ≤ d ≤ 0.05, and a + b + c + d = 1.

[0104] Specifically, a, b, c, and d in Formula 3 may be 0.70≤a≤0.95, 0.025≤b≤0.20, 0.025≤c≤0.20, and 0≤d≤0.03, respectively.

[0105] More specifically, a, b, c, and d in Formula 3 may be 0.80≤a≤0.95, 0.025≤b≤0.15, 0.025≤c≤0.15, and 0≤d≤0.03, respectively.

[0106] In addition, a, b, c, and d in Formula 3 may be 0.83≤a≤0.90, 0.05≤b≤0.1, 0.05≤c≤0.1, and 0≤d≤0.03, respectively.

[0107] Preferably, M in Formula 3 may be Al.

[0108] In the case of NCM positive electrode active materials containing nickel (Ni), cobalt (Co) and manganese (Mn), the higher the Ni content, the higher the energy density can be achieved, but there is a defect that the surface reactivity and stability of the positive electrode are deteriorated, and the introduction of aluminum (Al) as M can make up for this defect.

[0109] The positive electrode active material content may be 80 wt % to 99 wt %, for example 90 wt % to 99 wt %, based on the total weight of the solids in the positive electrode slurry. When the amount of the positive electrode active material is less than 80 wt %, the capacity may be reduced due to reduced energy density.

[0110] The binder is a component that helps the bonding between the active material and the conductive agent and the bonding with the current collector, wherein the binder is generally added in an amount of 1% to 30% by weight based on the total weight of the solids in the positive electrode slurry. Examples of the binder may include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, styrene-butadiene rubber, fluororubber or various copolymers thereof.

[0111] In addition, the conductive agent is a material that provides conductivity without causing adverse chemical changes in the battery. The conductive agent may be added in an amount of 0.5 wt % to 20 wt % based on the total weight of the solids in the positive electrode slurry.

[0112] Examples of the conductive agent may include the following conductive materials, for example: 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 having a well-developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powder, such as fluorocarbon powder, aluminum powder or nickel powder; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0113] In addition, the solvent of the positive electrode slurry may include an organic solvent, such as N-methyl-2-pyrrolidone (NMP), and its amount may be such that a desired viscosity is obtained when the positive electrode active material, the binder, and the conductive agent are included. For example, the content of the solvent may be such that the solid concentration in the positive electrode slurry containing the positive electrode active material, the binder, and the conductive agent is 5 wt % to 90 wt %, preferably 5 wt % to 80 wt %.

[0114] (2) Negative electrode

[0115] The negative electrode of the present invention may include a negative electrode active material, and may be prepared by coating a negative electrode collector with a negative electrode slurry including the negative electrode active material, a binder, a conductive agent, a solvent, and the like, and then drying and roll-pressing the coated negative electrode collector.

[0116] The negative electrode current collector typically has a thickness of 3 to 500 μm. The negative electrode current collector is not particularly limited, as long as it has high conductivity and does not cause adverse chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with one of carbon, nickel, titanium, or silver, and aluminum-cadmium alloys can be used. Furthermore, similar to the positive electrode current collector, the negative electrode current collector can have a finely uneven surface to improve the bonding strength with the negative electrode active material. The negative electrode current collector can be used in various shapes, such as films, sheets, foils, meshes, porous bodies, foams, and non-woven fabrics.

[0117] In addition, the negative electrode active material may include at least one selected from a carbon material capable of reversibly intercalating / deintercalating lithium ions, a metal or an alloy of lithium and the metal, a metal composite oxide, a material dopable and dedopable with lithium, lithium metal, and a transition metal oxide.

[0118] As a carbon material capable of reversibly intercalating / deintercalating lithium ions, a carbon-based negative electrode active material commonly used in lithium ion secondary batteries can be used without particular limitation, and as a typical example, crystalline carbon and / or amorphous carbon can be used. Examples of crystalline carbon can include graphite, such as irregular, planar, flaky, spherical or fibrous natural graphite or artificial graphite, and examples of amorphous carbon can include soft carbon (low temperature sintered carbon) or hard carbon, mesophase pitch carbide and fired coke, etc.

[0119] As the metal or an alloy of lithium and the metal, a metal selected from the group consisting of copper (Cu), nickel (Ni), sodium (Na), potassium (K), rubidium (Rb), cesium (Cs), francium (Fr), beryllium (Be), magnesium (Mg), calcium (Ca), strontium (Sr), silicon (Si), antimony (Sb), lead (Pb), indium (In), zinc (Zn), barium (Ba), radium (Ra), germanium (Ge), aluminum (Al), and tin (Sn), or an alloy of lithium and the metal can be used.

[0120] As the metal composite oxide, one selected from the group consisting of 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 (where Me is Mn, Fe, Pb, or Ge; Me' is Al, B, P, Si, elements of Groups I, II, and III of the periodic table, or a halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8) can be used.

[0121] The materials that can be doped and de-doped with lithium may 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, and 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), and a mixture of SiO2 and at least one of them can also be used. The element Y can be selected from the group consisting of Mg, Ca, Sr, Ba, Ra, Sc, Y, Ti, Zr, Hf, Rf, V, Nb, Ta, Db, 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.

[0122] Examples of the transition metal oxide may include lithium-containing titanium composite oxide (LTO), vanadium oxide, and lithium vanadium oxide, etc.

[0123] Based on the total weight of the solids in the negative electrode paste, the content of the negative electrode active material can be 80% to 99% by weight.

[0124] The binder is a component that helps to bond the conductive agent, active material, and current collector. The binder is generally added in an amount of 1 to 30 weight percent based on the total weight of the solids in the negative electrode slurry. Examples of the binder include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer, styrene-butadiene rubber, fluororubber, or various copolymers thereof.

[0125] The conductive agent is a component that further improves the conductivity of the negative electrode active material. The conductive agent can be added in an amount of 0.5% to 20% by weight based on the total weight of the solids in the negative electrode slurry. Any conductive agent can be used without specific limitation, as long as it has conductivity and does not cause adverse chemical changes in the battery. For example, the following conductive materials can be used, such as: 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 with a well-developed crystal structure; conductive fibers, such as carbon fibers or metal fibers; conductive powders, such as fluorocarbon powders, aluminum powders, or nickel powders; conductive whiskers, such as zinc oxide whiskers or potassium titanate whiskers; conductive metal oxides, such as titanium oxide; or polyphenylene derivatives.

[0126] The solvent of the negative electrode slurry may include water or an organic solvent, such as NMP and alcohol, and the amount thereof may be such that a desired viscosity is obtained when the negative electrode active material, a binder, and a conductive agent are included. For example, the solvent content may be such that the solid content concentration in the negative electrode slurry containing the negative electrode active material, the binder, and the conductive agent is 30% to 80% by weight, preferably 40% to 70% by weight.

[0127] (3) Diaphragm

[0128] The lithium secondary battery of the present invention includes a separator located between a positive electrode and a negative electrode.

[0129] The separator separates the positive electrode and the negative electrode and provides a movement path for lithium ions. Any separator can be used as the separator without particular limitation as long as it is commonly used in lithium secondary batteries, and in particular, a separator having excellent wettability and excellent stability to the electrolyte and low resistance to the transmission of electrolyte ions is preferably used.

[0130] Specifically, as a separator, a porous polymer film can be used, for example, a porous polymer film prepared from a polyolefin polymer (such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer); or a laminated structure having two or more layers thereof. In addition, a typical porous non-woven fabric can be used, such as a non-woven fabric formed of a high melting point glass fiber or polyethylene terephthalate fiber, etc. In addition, a coated separator including a ceramic component or a polymer material can be used to ensure heat resistance or mechanical strength, and a single-layer or multi-layer structure can be used.

[0131] The lithium secondary battery of the present invention as described above can be used in portable devices such as mobile phones, notebook computers, and digital cameras; and electric vehicles such as hybrid electric vehicles (HEVs), and the like.

[0132] Therefore, according to another embodiment of the present invention, a battery module including the lithium secondary battery as a unit cell and a battery pack including the battery module are provided.

[0133] The battery module 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 (EVs), hybrid electric vehicles (HEVs) and plug-in hybrid electric vehicles (PHEVs); and power storage systems.

[0134] 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, or the like can be used.

[0135] The lithium secondary battery of the present invention can be used not only as a battery cell used as a power source for small devices but can also be preferably used as a unit cell in a medium or large-sized battery module including a plurality of battery cells.

[0136] Hereinafter, the present invention will be described with reference to specific examples.

[0137] Example

[0138] Example 1

[0139] (Preparation of non-aqueous electrolyte)

[0140] A non-aqueous organic solution was prepared by mixing ethylene carbonate (EC) and ethyl methyl carbonate (EMC) in a volume ratio of 30:70, and then dissolving LiPF6 in the mixture to 1.0 M. 1H-imidazole-1-carboxylic acid propargyl ester (HSO2, 0.3 wt%), lithium difluorophosphate (DFP, 1 wt%), 2,2,2-trifluoroethyl methyl carbonate (FEMC, 3 wt%), and 1-propylene-1,3-sultone (PRS, 0.3 wt%) were added to the non-aqueous organic solution (95.4 wt%) to prepare a non-aqueous electrolyte (100 wt%).

[0141] (Preparation of lithium secondary battery)

[0142] The positive electrode active material (LiNi 0.85 Co 0.05 Mn 0.08 Al 0.02 O2), a conductive agent (carbon black), and a binder (polyvinylidene fluoride) were added to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 98:0.7:1.3 to prepare a positive electrode slurry (solid content: 7 wt%). A 12 μm thick aluminum (Al) film as a positive electrode current collector was coated with the positive electrode slurry, dried, and then roll-pressed to prepare a positive electrode.

[0143] The negative electrode active material (graphite:SiO = 94.5:5.5 weight ratio), binder (SBR-CMC) and conductive agent (carbon black) were added to water as a solvent at a weight ratio of 96.7:2.3:1 to prepare a negative electrode slurry (solid content: 50 weight%). An 8 μm thick copper (Cu) film as a negative electrode current collector was coated with the negative electrode slurry, dried, and then roll-pressed to prepare a negative electrode.

[0144] The electrode assembly was prepared by sequentially stacking a positive electrode, a polyolefin-based porous separator coated with inorganic particles (Al2O3), and a negative electrode.

[0145] The electrode assembly was housed in a pouch-type battery case, and the prepared non-aqueous electrolyte was injected therein to prepare a lithium secondary battery.

[0146] Example 2

[0147] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that 1-propylene-1,3-sultone was not added and the content of the nonaqueous organic solution was changed to 95.7% by weight.

[0148] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0149] Example 3

[0150] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that 1-propylene-1,3-sultone was not added, vinylene carbonate (VC) was added in an amount of 0.5 wt %, and the content of the nonaqueous organic solution was changed to 95.2 wt %.

[0151] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0152] Example 4

[0153] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that 1-propylene-1,3-sultone was not added, 1,3-propanesultone (PS) was added in an amount of 0.5 wt %, and the content of the nonaqueous organic solution was changed to 95.2 wt %.

[0154] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0155] Example 5

[0156] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that 1-propylene-1,3-sultone was not added, the content of 1H-imidazole-1-carboxylate was changed to 0.1 wt %, and the content of the nonaqueous organic solution was changed to 95.9 wt %.

[0157] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0158] Comparative Example 1

[0159] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that lithium difluorophosphate was not added and the content of the nonaqueous organic solution was changed to 96.4% by weight.

[0160] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0161] Comparative Example 2

[0162] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that propargyl 1H-imidazole-1-carboxylate was not added and the content of the nonaqueous organic solution was changed to 95.7% by weight.

[0163] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0164] Comparative Example 3

[0165] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that methyl carbonate 2,2,2-trifluoroethyl was not added and the content of the nonaqueous organic solution was changed to 98.4% by weight.

[0166] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0167] Comparative Example 4

[0168] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that lithium difluorophosphate and 1-propylene-1,3-sultone were not added, and the content of the nonaqueous organic solution was changed to 96.7% by weight.

[0169] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0170] Comparative Example 5

[0171] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that 1H-imidazole-1-carboxylic acid propargyl ester and 1-propylene-1,3-sultone were not added, and the content of the nonaqueous organic solution was changed to 96% by weight.

[0172] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0173] Comparative Example 6

[0174] A nonaqueous electrolyte was prepared in the same manner as in Example 1, except that methyl carbonate 2,2,2-trifluoroethyl ester and 1-propylene-1,3-sultone were not added and the content of the nonaqueous organic solution was changed to 98.7% by weight.

[0175] A lithium secondary battery was prepared in the same manner as in Example 1, except that the nonaqueous electrolyte prepared above was injected.

[0176] <Experimental Example>

[0177] The volume change rate and resistance increase rate of the lithium secondary batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 6 after high-temperature storage were measured as follows.

[0178] [Volume change rate compared with Example 1 (%)]

[0179] Each of the lithium secondary batteries prepared in Examples 1 to 5 and Comparative Examples 1 to 6 was initially charged and discharged by fully charging at 0.33C / 4.2V under constant current-constant voltage conditions at 25°C and fully discharging at 0.33C / 2.5V under constant current, and each lithium secondary battery was discharged at 2.5C for 10 seconds at an SOC of 50% to determine the initial resistance.

[0180] Each lithium secondary battery that had undergone initial charge and discharge was charged at 4.2 V and stored at 60° C. for 4 weeks (SOC was 100%), and then the insulated lithium secondary battery was placed in a bowl filled with distilled water at room temperature, and the volume of the lithium secondary battery after storage at high temperature was measured with the aid of a TWD-150DM device manufactured by Two-pls, Co.

[0181] Table 1 below shows how much the volume of each of Examples 2 to 5 and Comparative Examples 1 to 6 increased compared to Example 1, based on the volume of the battery prepared in Example 1 after high-temperature storage.

[0182] [Resistance increase rate compared with Example 1 (%)]

[0183] Each lithium secondary battery prepared in Examples 1 to 5 and Comparative Examples 1 to 6 was fully charged at 0.33C / 4.2V under constant current-constant voltage conditions at 25°C and fully discharged at 0.33C / 2.5V under constant current to perform initial charge and discharge, and each lithium secondary battery was discharged at 2.5C for 10 seconds at 50% SOC to determine the initial resistance. The resistance value was calculated by dividing the drop voltage during discharge by the current. In this case, the voltage was measured using a PNE-0506 charge / discharge device (manufacturer: PNE SOLUTION Co., Ltd., 5V, 6A).

[0184] Each lithium secondary battery subjected to initial charge and discharge was stored at 60° C. for 4 weeks (SOC was 100%), and then the capacity was confirmed in the same manner as the initial charge and discharge, and similarly, the resistance increase rate was calculated by measuring the resistance in the same manner as above.

[0185] Table 1 below shows how much the resistance value of each of Examples 2 to 5 and Comparative Examples 1 to 6 increased compared to Example 1, based on the resistance value of the battery prepared in Example 1 after high-temperature storage.

[0186] Table 1

[0187]

[0188] The results in Table 1 confirm that the batteries prepared in Examples 1 to 5 (including batteries containing all the first to third additives) have smaller volume changes and resistance changes after high-temperature storage than the batteries in Comparative Examples 1 to 6. In particular, it can be confirmed that Example 1, which uses all the first to fourth additives, has the smallest volume change and resistance change.

[0189] In addition, it was confirmed that in Example 3, VC was further included as the fifth additive, and thus the resistance increase rate was improved due to the effect of strengthening the negative electrode, and it was confirmed that in Example 4, PS used as the fifth additive had an excellent effect of reducing the amount of gas generation, and thus the volume change rate was improved.

[0190] Meanwhile, it can be confirmed that, under the same condition as Example 1 in which PRS is included, Comparative Examples 1 to 3 not including one of the first to third additives have greater volume change rates and resistance change rates than Example 1.

[0191] In addition, it can be confirmed that under the same conditions as Example 2 in which PRS is not included, Comparative Examples 4 to 6, which do not include one of the first to third additives, also have a greater volume change rate and resistance change rate than Example 2. When the lithium secondary battery is stored at high temperature, oxidized gas (such as CO2) is generated on the positive electrode due to the solvent side reaction, and the amount of generation increases faster as the storage time becomes longer. If bubbles caused by gas production exist on the surface of the electrode, the reaction area is reduced. The bubbles generated are non-conductors and therefore act as resistors, and a higher current flows in a reduced area to cause a larger voltage drop, thereby increasing the resistance.

[0192] That is, it can be seen that the lithium secondary battery including the nonaqueous electrolyte of the present invention has an effect of improving durability by forming a positive electrode film, and therefore, as a result of the experiment, the volume change rate and the resistance increase rate are improved.

Claims

1. A non-aqueous electrolyte for a lithium secondary battery, comprising: a lithium salt; an organic solvent; a compound represented by Formula 1A as a first additive; lithium difluorophosphate as a second additive; and a compound represented by Formula 2 as a third additive: [Formula 1A] Wherein, in Formula 1A, L1 is a C1-C3 alkylene group, and R1 to R3 are each independently hydrogen or a C1-C3 alkyl group, [Formula 2] Wherein, in Formula 2, L2 is a direct bond or a C1-C3 alkylene group, R4 is a C1-C3 alkyl group substituted with at least one fluorine, and R5 is a C1-C3 alkyl group.

2. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The compound represented by Formula 1A is propargyl 1H-imidazole-1-carboxylate.

3. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein In Formula 2, L2 is a direct bond or a methylene group, R4 is a C1 or C2 alkyl group substituted with at least one fluorine, and R5 is a methyl group or an ethyl group.

4. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The compound represented by Formula 2 is 2,2,2-trifluoroethyl methyl carbonate.

5. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein Based on the total weight of the non-aqueous electrolyte, the content of the first additive is 0.1% by weight to 1% by weight.

6. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein Based on the total weight of the non-aqueous electrolyte, the content of the second additive is 0.1% by weight to 1.5% by weight.

7. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein Based on the total weight of the non-aqueous electrolyte, the content of the third additive is 1% by weight to 5% by weight.

8. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, further comprising 1-propene-1,3-sultone as a fourth additive.

9. The non-aqueous electrolyte for lithium secondary batteries according to claim 8, wherein Based on the total weight of the non-aqueous electrolyte, the content of the fourth additive is 0.1% by weight to 1% by weight.

10. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, further comprising at least one selected from vinylene carbonate (VC) and 1,3-propane sultone (PS) as a fifth additive.

11. The non-aqueous electrolyte for lithium secondary batteries according to claim 1, wherein The organic solvent includes a cyclic carbonate solvent and a linear carbonate solvent.

12. A lithium secondary battery, comprising: a positive electrode containing a positive electrode active material; a negative electrode containing a negative electrode active material; a separator disposed between the positive electrode and the negative electrode; and the non-aqueous electrolyte according to claim 1.

13. The lithium secondary battery according to claim 12, wherein The positive electrode active material contains a lithium composite transition metal oxide represented by Formula 3: [Formula 3] Li (Ni a Co b Mr c M d )O2 Wherein, in Formula 3, [[ID=3,5]]M is W, Cu, Fe, V, Cr, Ti, Zr, Zn, Al, In, Ta, Y, La, Sr, Ga, Sc, Gd, Sm, Ca, Ce, Nb, Mg, B or Mo, and a, b, c and d are atomic fractions of each independent element, and satisfy 0.5 ≤ a < 1, 0 < b ≤ 0.3, 0 < c ≤ 0.3, 0 ≤ d ≤ 0.05 and a + b + c + d = 1.

14. The lithium secondary battery according to claim 13, wherein In Formula 3, a, b, c and d are respectively 0.70 ≤ a ≤ 0.95, 0.025 ≤ b ≤ 0.20, 0.025 ≤ c ≤ 0.20 and 0 ≤ d ≤ 0.03.

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