Nonaqueous electrolyte for lithium secondary battery and lithium secondary battery containing same

Abstract

Provided are a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery including the same, wherein the non-aqueous electrolyte for a lithium secondary battery includes a non-aqueous organic solvent, a lithium salt, and a first additive represented by Chemical Formula 1 and a second additive represented by Chemical Formula 2 in a weight ratio of 0.5:1 to 10:1.

Description

Technical Field

[0001] The present invention relates to a non-aqueous electrolyte for a lithium secondary battery and a lithium secondary battery containing the same.

Background Art

[0002] Lithium secondary batteries have attracted attention as power sources for various electronic devices because of their high discharge voltage and high energy density.

[0003] As the positive electrode active material of a lithium secondary battery, oxides composed of lithium and transition metals having a structure capable of intercalating lithium ions, such as LiCoO2, LiMn2O4, LiNi 1-x Co x O2 (0 < x < 1), etc., are mainly used.

[0004] As the negative electrode active material, various forms of carbon-based materials including artificial and natural graphite and hard carbon capable of inserting / desorbing lithium are mainly used.

[0005] As the electrolyte of a lithium secondary battery, an organic solvent in which a lithium salt is dissolved is used.

Summary of the Invention

Problems to be Solved by the Invention

[0006] One embodiment is to provide a non-aqueous electrolyte for a lithium secondary battery that suppresses gas generation at high temperatures and exhibits improved high-temperature safety.

[0007] Another embodiment is to provide a lithium secondary battery containing the non-aqueous electrolyte.

Means for Solving the Problems

[0008] According to one embodiment, there is provided an electrolyte for a lithium secondary battery, comprising a non-aqueous organic solvent lithium salt, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2, wherein the mixing ratio of the first additive to the second additive is 0.5:1 to 10:1 by weight: [ka] (In the above Chemical Formula 1, R 1 ~R 8 are each independently a hydrogen atom, a substituted or unsubstituted C1 to C30 alkyl group, a substituted or unsubstituted C2 to C30 alkenyl group, a substituted or unsubstituted C2 to C30 alkynyl group, a substituted or unsubstituted C3 to C30 cycloalkyl group, a substituted or unsubstituted C3 to C30 cycloalkenyl group, a substituted or unsubstituted C3 to C30 cycloalkynyl group, or a substituted or unsubstituted C6 to C30 aryl group.

[0009] [ka] (In the above formula 2, A is a substituted or unsubstituted aliphatic chain or (—C2H4—O—C2H4—) n where n is an integer from 1 to 10.

[0010] In the above Chemical Formula 1, the R 1 ~R 8 may each independently be a hydrogen atom, a substituted or unsubstituted C1 to C10 alkyl group, a substituted or unsubstituted C2 to C10 alkenyl group, a substituted or unsubstituted C2 to C10 alkynyl group, a substituted or unsubstituted C3 to C10 cycloalkyl group, a substituted or unsubstituted C3 to C10 cycloalkenyl group, a substituted or unsubstituted C3 to C10 cycloalkynyl group, or a substituted or unsubstituted C6 to C10 aryl group.

[0011] The mixing ratio of the first additive and the second additive may be 0.5:1 to 5:1 by weight.

[0012] In one embodiment, the content of the first additive may be 0.25 wt % or more and less than 10 wt % when the content of the non-aqueous organic solvent and the lithium salt is 100 wt %, and the content of the first additive may be 0.5 wt % to 5 wt % when the content of the non-aqueous organic solvent and the lithium salt is 100 wt %.

[0013] The content of the second additive may be 0.1 wt % or more and less than 10 wt % when the content of the non-aqueous organic solvent and the lithium salt is taken as 100 wt %, and the content of the second additive may be 0.1 wt % to 5 wt % when the content of the non-aqueous organic solvent and the lithium salt is taken as 100 wt %.

[0014] The non-aqueous organic solvent may include a propionate solvent.

[0015] The propionate-based solvent may be methyl propionate, ethyl propionate, propyl propionate, or a combination thereof, and the content of the propionate-based solvent may be 30% to 80% by volume based on the total volume of the non-aqueous organic solvent.

[0016] Another embodiment provides a lithium secondary battery including the non-aqueous electrolyte, a positive electrode, and a negative electrode.

[0017] Other specific details of the embodiments are included in the detailed description below. [Effects of the Invention]

[0018] The electrolyte for a lithium secondary battery according to one embodiment of the present invention can suppress gas generation at high temperatures, and therefore can provide a lithium secondary battery with excellent safety at high temperatures. [Brief explanation of the drawings]

[0019] [Figure 1] 1 is a diagram illustrating a lithium secondary battery according to an embodiment; DETAILED DESCRIPTION OF THE INVENTION

[0020] DETAILED DESCRIPTION OF THE INVENTION The following detailed description of the present invention is given by way of example only and is not intended to limit the scope of the present invention, which is defined solely by the scope of the claims set forth below.

[0021] Unless otherwise defined in this specification, the term "substituted" means that a hydrogen atom in the compound is substituted with a substituent selected from a halogen atom (F, Br, Cl, or I), a hydroxy group, an alkoxy group, a nitro group, a cyano group, an amino group, an azido group, an amidino group, a hydrazino group, a hydrazono group, a carbonyl group, a carbamyl group, a thiol group, an ester group, a carboxyl group or a salt thereof, a sulfonic acid group or a salt thereof, phosphoric acid or a salt thereof, a C1-C20 alkyl group, a C2-C20 alkenyl group, a C2-C20 alkynyl group, a C6-C30 aryl group, a C7-C30 arylalkyl group, a C1-C4 alkoxy group, a C1-C20 heteroalkyl group, a C3-C20 heteroarylalkyl group, a C3-C30 cycloalkyl group, a C3-C15 cycloalkenyl group, a C6-C15 cycloalkynyl group, a C2-C20 heterocycloalkyl group, and combinations thereof.

[0022] According to one embodiment, a non-aqueous electrolyte for a lithium secondary battery includes a non-aqueous organic solvent, a lithium salt, a first additive represented by the following Chemical Formula 1, and a second additive represented by the following Chemical Formula 2: [ka] In the above Chemical Formula 1, R 1 ~R 8 are each independently a hydrogen atom, a substituted or unsubstituted C1 to C30 alkyl group, a substituted or unsubstituted C2 to C30 alkenyl group, a substituted or unsubstituted C2 to C30 alkynyl group, a substituted or unsubstituted C3 to C30 cycloalkyl group, a substituted or unsubstituted C3 to C30 cycloalkenyl group, a substituted or unsubstituted C3 to C30 cycloalkynyl group, or a substituted or unsubstituted C6 to C30 aryl group.

[0023] In one embodiment, the R 1 ~R 8 may each independently be a hydrogen atom, a substituted or unsubstituted C1 to C10 alkyl group, a substituted or unsubstituted C2 to C10 alkenyl group, a substituted or unsubstituted C2 to C10 alkynyl group, a substituted or unsubstituted C3 to C10 cycloalkyl group, a substituted or unsubstituted C3 to C10 cycloalkenyl group, a substituted or unsubstituted C3 to C10 cycloalkynyl group, or a substituted or unsubstituted C6 to C10 aryl group.

[0024] [ka] In the above formula 2, A is a substituted or unsubstituted aliphatic chain or (—C2H4—O—C2H4—) n and n is an integer from 1 to 10.

[0025] According to one embodiment, A is a hydrocarbon chain having 2 to 20 carbon atoms or (—C2H4—O—C2H4—) n and n may be an integer from 1 to 5.

[0026] At this time, the mixing ratio of the first additive and the second additive may be 0.5:1 to 10:1 by weight, 0.5:1 to 5:1 by weight, or 1:1 to 5:1 by weight.

[0027] As described above, an electrolyte for a lithium secondary battery according to one embodiment includes both a first additive and a second additive, particularly in a weight ratio of 0.5:1 to 10:1. The first additive forms a strong coating on the surface of the positive electrode to prevent deterioration of the positive electrode at high temperatures, and the second additive undergoes reductive decomposition to form a coating on the surface of the negative electrode to prevent collapse of the SEI layer due to side reactions of the electrolyte at high temperatures. In addition, the second additive stabilizes the lithium salt, thereby preventing metal elution from the positive electrode active material due to solvent decomposition and HF generation caused by decomposition of the lithium salt.

[0028] The use of a mixture of the first and second additives forms appropriate coatings on the positive and negative electrodes without significantly increasing the initial thickness, preventing decomposition of the negative electrode coating and deterioration of the positive electrode at high temperatures, and effectively reducing gas generation during high-temperature storage. Even if both the first and second additives are included, if the mixing ratio is outside the above range, i.e., if the first additive is used in an amount less than half that of the second additive, high-temperature characteristics may be reduced, and if the first additive is used in an amount more than 10 times that of the second additive, the viscosity of the electrolyte may increase, degrading the charging characteristics of the battery.

[0029] The effect of using the first additive and the second additive together is the same effect as that obtained when the compound of Formula 1 is used as the first additive.

[0030] This effect is obtained because the compound of Formula 1 contains sulfur but forms a stronger coating on the surface of the positive electrode than compounds such as sulfoxides that do not have the structure of Formula 1. Therefore, when a compound that contains sulfur but does not have the structure of Formula 1 is used as the first additive, the effect is weak and it is not appropriate.

[0031] In this case, the content of the first additive represented by Chemical Formula 1 may be 0.25 wt % or more and less than 10 wt % relative to the weight of the non-aqueous organic solvent and the lithium salt, i.e., when the content of the non-aqueous organic solvent and the lithium salt is 100 wt % (relative to the total content of the non-aqueous organic solvent and the lithium salt, 100 wt %), and according to one embodiment, may be 0.5 wt % to 5 wt %. When the content of the first additive represented by Chemical Formula 1 is within this range, it is appropriate because it can exhibit effects such as improved high-temperature reliability, for example, improved high-temperature capacity retention and reduced high-temperature resistance increase rate.

[0032] In addition, the content of the second additive represented by Chemical Formula 2 may be 0.1 wt % or more and less than 10 wt % relative to the weight of the non-aqueous organic solvent and the lithium salt, i.e., when the content of the non-aqueous organic solvent and the lithium salt is 100 wt % (relative to the total content of the non-aqueous organic solvent and the lithium salt, 100 wt %), and according to one embodiment, it may be 0.1 wt % to 5 wt %, or 0.1 wt % to 3 wt %. When the content of the second additive represented by Chemical Formula 2 is within the above range, it is appropriate because it can exhibit effects such as improved high-temperature reliability, for example, improved high-temperature capacity retention and reduced high-temperature resistance increase rate.

[0033] In one embodiment of the electrolyte, the non-aqueous organic solvent may include a carbonate-based solvent or a propionate-based solvent. The propionate-based solvent may be methyl propionate, ethyl propionate, propyl propionate, or a combination thereof. When one or more propionate-based solvents are used, the mixing ratio thereof may be appropriately adjusted.

[0034] In the non-aqueous organic solvent, the content of the propionate-based solvent may be 30 to 80% by volume, and in one embodiment, 40 to 80% by volume, based on the total volume of the non-aqueous organic solvent. When the content of the propionate-based solvent is within this range, the electrolyte impregnation property can be improved.

[0035] The carbonate-based solvent may be dimethyl carbonate (DMC), diethyl carbonate (DEC), dipropyl carbonate (DPC), methyl propyl carbonate (MPC), ethyl propyl carbonate (EPC), methyl ethyl carbonate (MEC), ethylene carbonate (EC), propylene carbonate (PC), butylene carbonate (BC), or a combination thereof. When using more than one carbonate-based solvent, the mixing ratio may be adjusted appropriately. Furthermore, when using the carbonate-based solvent, it is preferable to mix a cyclic carbonate with a chain carbonate. In this case, mixing the cyclic carbonate and the chain carbonate in a volume ratio of 1:1 to 1:9 can provide excellent electrolyte performance.

[0036] In one embodiment, the non-aqueous organic solvent may further include an ester, an ether, a ketone, an alcohol, or an aprotic solvent. Examples of the ester solvent include methyl acetate, ethyl acetate, n-propyl acetate, t-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanolide, valerolactone, mevalonolactone, and caprolactone.

[0037] Examples of the ether solvent include dibutyl ether, tetraglyme, diglyme, dimethoxyethane, 2-methyltetrahydrofuran, and tetrahydrofuran, and examples of the ketone solvent include cyclohexanone.

[0038] Examples of the alcohol solvent that can be used include ethyl alcohol and isopropyl alcohol, and examples of the aprotic solvent that can be used include nitriles such as T-CN (where T is a hydrocarbon group having 2 to 20 carbon atoms and having a linear, branched, or cyclic structure, and may contain a double-bonded aromatic ring or an ether bond), amides such as dimethylformamide, and dioxolanes such as 1,3-dioxolane.

[0039] The non-aqueous organic solvent may further include an aromatic hydrocarbon-based organic solvent, which may be an aromatic hydrocarbon-based compound represented by the following formula 3: [ka] (In the above Chemical Formula 3, R 9 ~R 14 are the same or different and are selected from the group consisting of hydrogen, halogen, alkyl groups having 1 to 10 carbon atoms, haloalkyl groups, and combinations thereof.

[0040] Specific examples of the aromatic hydrocarbon organic solvent include benzene, fluorobenzene, 1,2-difluorobenzene, 1,3-difluorobenzene, 1,4-difluorobenzene, 1,2,3-trifluorobenzene, 1,2,4-trifluorobenzene, chlorobenzene, 1,2-dichlorobenzene, 1,3-dichlorobenzene, 1,4-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, iodobenzene, 1,2-diiodobenzene, 1,3-diiodobenzene, 1,4-diiodobenzene, 1,2,3-triiodobenzene, 1,2,4-triiodobenzene, toluene, It is selected from the group consisting of fluorotoluene, 2,3-difluorotoluene, 2,4-difluorotoluene, 2,5-difluorotoluene, 2,3,4-trifluorotoluene, 2,3,5-trifluorotoluene, chlorotoluene, 2,3-dichlorotoluene, 2,4-dichlorotoluene, 2,5-dichlorotoluene, 2,3,4-trichlorotoluene, 2,3,5-trichlorotoluene, iodotoluene, 2,3-diiodotoluene, 2,4-diiodotoluene, 2,5-diiodotoluene, 2,3,4-triiodotoluene, 2,3,5-triiodotoluene, xylene, and combinations thereof.

[0041] The lithium secondary battery electrolyte may further include vinyl ethyl carbonate, vinylene carbonate, an ethylene carbonate-based compound represented by the following Chemical Formula 4, or a combination thereof, as a life-enhancing additive to improve battery life. [ka] (In the above-mentioned Chemical Formula 4, R 15 and R 16 are each independently selected from the group consisting of hydrogen, a halogen group, a cyano group (CN), a nitro group (NO), and a fluorinated alkyl group having 1 to 5 carbon atoms, 15 and R 16 at least one of R is selected from the group consisting of a halogen group, a cyano group (CN), a nitro group (NO2), and a fluorinated alkyl group having 1 to 5 carbon atoms, 15 and R16 are not all hydrogen.)

[0042] Representative examples of the ethylene carbonate-based compound include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, bromoethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, etc. When such a life-improving additive is further used, the amount used can be appropriately adjusted.

[0043] The lithium salt dissolves in an organic solvent and acts as a lithium ion source in the battery, enabling basic lithium secondary battery operation and promoting lithium ion migration between the positive and negative electrodes. Representative examples of such lithium salts include LiPF6, LiSbF6, LiAsF6, LiPO2F2, LiN(SO2C2F5)2, Li(CF3SO2)2N, LiN(SO3C2F5)2, Li(FSO2)2N (lithium bis(fluorosulfonyl)imide (LiFSI)), LiC4F9SO3, LiClO4, LiAlO2, LiAlCl4, LiPO2F2, LiN(C x F 2x+1 SO2)(C y F 2y+1 The electrolyte contains one or more supporting electrolyte salts selected from the group consisting of lithium bis(oxalato)borate (LiBOB), lithium difluoro(bisoxolato)phosphate (LiCl), lithium I, LiB(CO) (where x and y are natural numbers, e.g., integers of 1 to 20), lithium difluoro(bisoxolato)phosphate, LiCl, LiI, LiB(CO) (lithium bis(oxalato)borate (LiBOB)), and lithium difluoro(oxolato)borate (LiDFOB). The lithium salt concentration is preferably in the range of 0.1M to 2.0M. When the lithium salt concentration is within this range, the electrolyte has appropriate conductivity and viscosity, thereby exhibiting excellent electrolyte performance and allowing lithium ions to migrate effectively.

[0044] One embodiment provides a lithium secondary battery including the non-aqueous electrolyte.

[0045] The lithium secondary battery includes the non-aqueous electrolyte, a negative electrode, and a positive electrode.

[0046] In one embodiment, the negative electrode includes a negative electrode active material layer containing a negative electrode active material, and a current collector supporting the negative electrode active material layer.

[0047] The negative electrode active material may be a material capable of reversibly intercalating / deintercalating lithium ions, lithium metal, a lithium metal alloy, a material capable of doping and dedoping lithium, or a transition metal oxide.

[0048] Examples of materials capable of reversibly intercalating / deintercalating lithium ions include carbon materials, i.e., carbon-based negative electrode active materials commonly used in lithium secondary batteries. Representative examples of carbon-based negative electrode active materials 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 or artificial graphite. Examples of amorphous carbon include soft carbon, hard carbon, mesophase pitch carbide, and calcined coke.

[0049] As the lithium metal alloy, an alloy of lithium and a metal selected from the group consisting of Na, K, Rb, Cs, Fr, Be, Mg, Ca, Sr, Si, Sb, Pb, In, Zn, Ba, Ra, Ge, Al and Sn can be used.

[0050] The material capable of doping and dedoping lithium includes Si, SiO x(0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, and is not Si), Si-carbon composite, Sn, SnO2, Sn-R alloy (where R is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, and is not Sn), Sn-carbon composite, etc. can be mentioned, and it is also possible to mix and use at least one of these with SiO2. As the elements Q and R, 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 can be used.

[0051] As the transition metal oxide, lithium titanate can be used.

[0052] The negative electrode active material according to one embodiment can include a Si-C composite including a Si-based active material and a carbon-based active material.

[0053] The Si-based active material is Si, SiO x (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of an alkali metal, an alkaline earth metal, a Group 13 element, a Group 14 element, a Group 15 element, a Group 16 element, a transition metal, a rare earth element, and combinations thereof, and is not Si) or combinations thereof can be possible.

[0054] The average particle size of the Si-based active material can be 50 nm to 200 nm.

[0055] When the average particle size of the Si-based active material is within the above range, volume expansion that occurs during charge and discharge can be suppressed, and conductive path disconnection due to particle crushing during charge and discharge can be prevented.

[0056] The Si-based active material may be included in an amount of 1 wt % to 60 wt % of the total weight of the Si-C composite, for example, 3 wt % to 60 wt %.

[0057] According to another embodiment, the negative electrode active material may further include crystalline carbon in addition to the Si—C composite.

[0058] When the negative electrode active material includes both a Si-C composite and crystalline carbon, the Si-C composite and crystalline carbon may be included in the form of a mixture, and in this case, the Si-C composite and crystalline carbon may be included in a weight ratio of 1:99 to 50:50. More specifically, the Si-C composite and crystalline carbon may be included in a weight ratio of 5:95 to 20:80.

[0059] The crystalline carbon may include, for example, graphite, and more specifically, may include natural graphite, artificial graphite, or a mixture thereof.

[0060] The average particle size of the crystalline carbon may be 5 μm to 30 μm.

[0061] The average particle size herein may be the size of the particle at 50% volumetric ratio (D50) in the cumulative size-distribution curve.

[0062] The Si-C composite may further include a shell surrounding the surface of the Si-C composite, the shell including amorphous carbon, and the thickness of the shell may be 5 nm to 100 nm.

[0063] The amorphous carbon may include soft carbon, hard carbon, mesophase pitch carbonized material, calcined coke, or a mixture thereof.

[0064] The amorphous carbon may be included in an amount of 1 to 50 parts by weight, for example, 5 to 50 parts by weight, or 10 to 50 parts by weight, based on 100 parts by weight of the carbon-based active material.

[0065] The negative electrode active material layer includes a negative electrode active material and a binder, and may optionally further include a conductive material.

[0066] The negative electrode active material layer may contain 95 wt% to 99 wt% of the negative electrode active material based on the total weight of the negative electrode active material layer. The negative electrode active material layer may contain 1 wt% to 5 wt% of the binder based on the total weight of the negative electrode active material layer. When a conductive material is further included, the negative electrode active material may be contained in an amount of 90 wt% to 98 wt%, the binder in an amount of 1 wt% to 5 wt%, and the conductive material in an amount of 1 wt% to 5 wt%.

[0067] The binder serves to firmly adhere the negative active material particles to each other and to the current collector. The binder may be a non-aqueous binder, an aqueous binder, or a combination thereof.

[0068] Examples of the non-aqueous binder include ethylene propylene copolymer, polyacrylonitrile, polystyrene, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamideimide, polyimide, and combinations thereof.

[0069] Examples of the water-based binder include styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, polymers containing ethylene oxide, polyvinylpyrrolidone, polyepichlorohydrin, polyphosphazene, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenol resin, epoxy resin, polyvinyl alcohol, and combinations thereof.

[0070] When an aqueous binder is used as the negative electrode binder, a cellulose-based compound capable of imparting viscosity may be further included as a thickener. The cellulose-based compound may be a mixture of one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or alkali metal salts thereof. The alkali metal may be Na, K, or Li. The amount of the thickener used may be 0.1 to 3 parts by weight per 100 parts by weight of the negative electrode active material.

[0071] The conductive material is used to impart conductivity to the electrode, and any material that does not cause a chemical change in the constructed battery and is electron-conductive can be used. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, denka black, and carbon fiber; metal-based materials such as metal powder or metal fiber of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and conductive materials containing mixtures thereof.

[0072] The current collector may be selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0073] In one embodiment, the positive electrode comprising the positive electrode active material comprises a positive electrode active material layer comprising the positive electrode active material and a current collector supporting the positive electrode active material layer.

[0074] The positive electrode active material may be a compound capable of reversible intercalation and deintercalation of lithium (lithiated intercalation compound), and specifically may be one or more of composite oxides of lithium and a metal selected from cobalt, manganese, nickel, and combinations thereof. More specifically, a compound represented by any one of the following chemical formulas may be used: Li a A 1-b X b D2(0.90≦a≦1.8,0≦b≦0.5);Li a A 1-b X b O 2-c D c (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.05);Li a E 1-b X b O 2-c D c (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.05);Li a E 2-b X b O 4-c D c (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.05);Li a Ni 1-b-c Co b X c D α (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.5,0<α≦2);Li a Ni 1-b-c Co b X c O 2-α T α (0.90≦a≦1.8,0≦b≦0.5,0≦c≦0.5,0<α<2);Li a Ni 1-b-c Co b X c O 2-αT2(0.90≦a≦1.8.0≦b≦0.5.0≦c≦0.5.0<α<2);Li a Nor 1-b-c Mn b X c D α (0.90≦a≦1.8.0≦b≦0.5.0≦c≦0.5.0<α≦2);Li a Nor 1-b-c Mn b X c O 2-α T α (0.90≦a≦1.8.0≦b≦0.5.0≦c≦0.5.0<α<2);Li a Nor 1-b-c Mn b X c O 2-α T2(0.90≦a≦1.8.0≦b≦0.5.0≦c≦0.5.0<α<2);Li a Nor b E c G d O2(0.90≦a≦1.8.0≦b≦0.9.0≦c≦0.5.0.001≦d≦0.1);Li a Nor b Co c Mn d G e O2(0.90≦a≦1.8.0≦b≦0.9.0≦c≦0.5.0≦d≦0.5.0.001≦e≦0.1);Li a NiG b O2(0.90≦a≦1.8,0.001≦b≦0.1)Li a CoG b O2(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn 1-b G b O2(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn2G b O4(0.90≦a≦1.8,0.001≦b≦0.1);Li a Mn 1-g G g PO4(0.90≦a≦1.8.0≦g≦0.5);QO2;QC2;LiQC2;B2O5;LiV2O5;LiZO2;LiNiVO4Li (3-f) J2(PO4)3(0≦f≦2);Li (3-f)Fe2(PO4)3(0≦f≦2);Li a FePO4(0.90≦a≦1.8)

[0075] In the above chemical formula, A is selected from the group consisting of Ni, Co, Mn, and combinations thereof; X is selected from the group consisting of Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements, and combinations thereof; D is selected from the group consisting of O, F, S, P, and combinations thereof; E is selected from the group consisting of Co, Mn, and combinations thereof; T is selected from the group consisting of F, S, P, and combinations thereof; G is selected from the group consisting of Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V, and combinations thereof; Q is selected from the group consisting of Ti, Mo, Mn, and combinations thereof; Z is selected from the group consisting of Cr, V, Fe, Sc, Y, and combinations thereof; and J is selected from the group consisting of V, Cr, Mn, Co, Ni, Cu, and combinations thereof.

[0076] Of course, the compound may have a coating layer on its surface, or the compound may be mixed with a compound having a coating layer. The coating layer may include at least one coating element compound selected from the group consisting of oxides of the coating elements, hydroxides of the coating elements, oxyhydroxides of the coating elements, oxycarbonates of the coating elements, and hydroxycarbonates of the coating elements. The compounds forming the coating layer may be amorphous or crystalline. The coating element contained in the coating layer may be Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or a mixture thereof. The coating layer formation process may be performed using any coating method (e.g., spray coating, dipping, etc.) that does not adversely affect the physical properties of the positive electrode active material, as this is well understood by those skilled in the art and will not be described in detail here.

[0077] In the positive electrode, the content of the positive electrode active material may be 90 wt % to 98 wt % based on the total weight of the positive electrode active material layer.

[0078] In one embodiment, the positive electrode active material layer may further include a binder and a conductive material, each of which may be present in an amount of 1 wt % to 5 wt % based on the total weight of the positive electrode active material layer.

[0079] The binder serves to firmly adhere the positive electrode active material particles to each other and to firmly adhere the positive electrode active material to the current collector. Representative examples of the binder include, but are not limited to, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, and nylon.

[0080] The conductive material is used to impart conductivity to the electrode, and any material that does not cause a chemical change in the constructed battery and is electron-conductive can be used. Examples of conductive materials include carbon-based materials such as natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, and carbon fiber; metal-based materials such as metal powder or metal fiber of copper, nickel, aluminum, and silver; conductive polymers such as polyphenylene derivatives; and conductive materials containing mixtures thereof.

[0081] The current collector may be, but is not limited to, aluminum foil, nickel foil, or a combination thereof.

[0082] The positive and negative electrode active material layers are formed by mixing an active material, a binder, and optionally a conductive material in a solvent to prepare an active material composition, and then coating the active material composition on a current collector. Since the method for forming the active material layer is widely known in the art, a detailed description thereof will be omitted herein. Examples of the solvent include, but are not limited to, N-methylpyrrolidone. Furthermore, when a water-based binder is used in the negative electrode active material layer, water can be used as the solvent in preparing the negative electrode active material composition.

[0083] Depending on the type of lithium secondary battery, a separator may be present between the positive and negative electrodes. Such a separator may be made of polyethylene, polypropylene, polyvinylidene fluoride, or a multilayer film of two or more of these materials. Of course, mixed multilayer films such as a polyethylene / polypropylene two-layer separator, a polyethylene / polypropylene / polyethylene three-layer separator, or a polypropylene / polyethylene / polypropylene three-layer separator may also be used.

[0084] According to another embodiment, the separator may be a composite porous separator including a porous substrate and a functional layer disposed on the porous substrate. The functional layer may provide additional functionality, and may be, for example, at least one of a heat-resistant layer and an adhesive layer. The heat-resistant layer may include a heat-resistant resin and, optionally, a filler. The adhesive layer may include an adhesive resin and, optionally, a filler. The filler may be an organic filler, an inorganic filler, or a combination thereof. The heat-resistant resin, adhesive resin, and filler may be any material that can be used in separators in the art.

[0085] An exploded perspective view of a lithium secondary battery according to an embodiment of the present invention is shown in Fig. 1. Although the lithium secondary battery according to the embodiment is described as a pouch-type battery, the present invention is not limited thereto and can be applied to batteries of various shapes such as cylindrical and prismatic types.

[0086] Referring to FIG. 1, a lithium secondary battery 100 according to one embodiment includes a battery assembly including a positive electrode 114, a negative electrode 112 positioned opposite the positive electrode 114, a separator 113 disposed between the positive electrode 114 and the negative electrode 112, and an electrolyte (not shown) impregnating the positive electrode 114, the negative electrode 112, and the separator 113; a battery container 120 housing the battery assembly; and a sealing member 140 sealing the battery container 120. [Example]

[0087] Examples of the present invention and comparative examples are described below. However, the following examples are merely examples of the present invention, and the present invention is not limited to the following examples.

[0088] (Comparative Example 1) An electrolyte for a lithium secondary battery was prepared by dissolving 1.5M LiPF6 in a non-aqueous organic solvent in which ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate were mixed in a ratio of 20:10:30:40% by volume.

[0089] Example 1 A lithium secondary battery electrolyte was prepared by dissolving 1.5M LiPF6 in a non-aqueous organic solvent containing ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate in a 20:10:30:40 volume % mixture, and adding a first additive, sulfolane, represented by Formula 1a below, and a second additive, compound represented by Formula 2a below. The first additive, sulfolane, represented by Formula 1a below, was 5 wt % based on 100 wt % of the total non-aqueous organic solvent and lithium salt, and the second additive, compound represented by Formula 2a below, was 0.5 wt % based on 100 wt % of the total non-aqueous organic solvent and lithium salt, resulting in a mixing ratio of the first additive to the second additive of 10:1.

[0090] [ka]

[0091] [ka]

[0092] A negative electrode active material slurry was prepared by mixing 97 wt% of artificial graphite negative electrode active material, 1 wt% of Ketjenblack conductive material, 1 wt% of styrene-butadiene rubber binder, and 1 wt% of carboxymethyl cellulose thickener in distilled water solvent. The negative electrode active material slurry was coated on copper foil, dried, and rolled to prepare a negative electrode.

[0093] A positive electrode active material slurry was prepared by mixing 96 wt% of LiCoO2 positive electrode active material, 2 wt% of Ketjenblack conductive material, and 2 wt% of polyvinylidene fluoride in N-methylpyrrolidone solvent. The positive electrode active material slurry was coated on an aluminum foil, dried, and rolled to prepare a positive electrode.

[0094] A 4.4V pouch-type lithium secondary battery was manufactured by a conventional method using the electrolyte, the positive electrode, and the negative electrode.

[0095] (Examples 2 to 11 and Comparative Examples 2 to 5) An electrolyte was prepared in the same manner as in Example 1, except that the amounts of sulfolane of Chemical Formula 1a and the compound of Chemical Formula 2a were changed as shown in Table 1 below. A lithium secondary battery was prepared using the electrolyte and the anode and cathode of Example 1 in the same manner as in Example 1.

[0096] [Table 1]

[0097] The lithium secondary batteries of Examples 1 to 11 and Comparative Examples 1 to 5 were charged at a constant current and constant voltage at 25°C under conditions of 0.7C, 4.4V, and 0.05C cut-off, and then stored at 60°C for 30 days. The battery thickness before storage (initial thickness) was measured, and the battery thickness after 30 days of storage was also measured. From these results, the battery thickness increase rate was calculated using the following equation 1, and the results are shown in Table 2 below, along with the initial thickness and the thickness at 60°C after 30 days.

[0098] [Formula 1] Thickness increase rate [%] = [(battery thickness after 30 days at 60°C - battery thickness before storage) / battery thickness before storage] x 100

[0099] Experimental Example 2: Evaluation of capacity retention rate and capacity recovery rate According to Experimental Example 1, the lithium secondary battery was left at a high temperature (60° C.) for 30 days, and then discharged at a constant current of 0.2 C to 3.0 V at room temperature (25° C.), and the discharge capacity was measured.

[0100] Next, to evaluate the capacity recovery rate, the lithium secondary battery whose discharge capacity was confirmed was recharged under a constant current condition of 0.2 C up to 4.4 V and a constant voltage condition with a final current of 0.05 C, and then discharged under a constant current condition of 0.2 C down to 3.0 V to measure the discharge capacity. The capacity recovery rate was calculated using the following equation 2, and the results are shown in Table 2 below.

[0101] [Formula 2] Capacity recovery rate [%] = [discharge capacity of recharged lithium secondary battery after high temperature storage / initial discharge capacity before high temperature storage] x 100

[0102] [Table 2]

[0103] As shown in Table 2, in Examples 1 to 11, in which an electrolyte solution containing the first and second additives in a weight ratio of 0.5:1 to 10:1 was used, excellent capacity retention and capacity recovery rates were maintained, and the thickness increase rate during high-temperature storage was significantly low, which indicates that the amount of gas generated during high-temperature storage was effectively reduced.

[0104] On the other hand, Comparative Example 1, which used an electrolyte solution that did not contain the first and second additives, showed a very high thickness increase rate during high-temperature storage, while Comparative Examples 2 and 4, which used electrolyte solutions that contained the first and second additives but at a mixing ratio of 0.1:1 and therefore contained an excessively small amount of the first additive, showed only a small effect in reducing the thickness increase rate. Furthermore, Comparative Examples 3 and 5, which used electrolyte solutions that contained an excessively large amount of the first additive, also showed a lower effect in reducing the thickness increase rate during high-temperature storage than the Examples.

[0105] The present invention is not limited to the above-described embodiment, and can be manufactured in various different forms, and a person skilled in the art to which the present invention pertains can understand that the present invention can be embodied in other specific forms without changing the technical idea or essential features of the present invention. Therefore, the above-described embodiment should be understood to be illustrative in all respects and not limiting.

Claims

1. non-aqueous organic solvent; lithium salts; and The present invention includes a first additive represented by the following chemical formula 1a and a second additive represented by the following chemical formula 2a: The mixing ratio of the first additive and the second additive is 0.5:1 to 10:1 by weight; the content of the first additive is 1.5% by weight to 5% by weight, when the content of the non-aqueous organic solvent and the content of the lithium salt are taken as 100% by weight; The content of the second additive is 0.1% by weight to 5% by weight, when the content of the non-aqueous organic solvent and the content of the lithium salt are taken as 100% by weight. Non-aqueous electrolyte for lithium secondary batteries. 【Chemistry 1】 【Chemistry 2】

2. 2. The nonaqueous electrolyte for a lithium secondary battery according to claim 1, wherein the mixing ratio of the first additive to the second additive is 0.5:1 to 5:1 by weight.

3. 2. The non-aqueous electrolyte for a lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent comprises a propionate-based solvent.

4. 4. The non-aqueous electrolyte for a lithium secondary battery according to claim 3, wherein the propionate-based solvent is methyl propionate, ethyl propionate, propyl propionate, or a combination thereof.

5. 5. The non-aqueous electrolyte for a lithium secondary battery according to claim 4, wherein the content of the propionate-based solvent is 30% by volume to 80% by volume based on the total volume of the non-aqueous organic solvent.

6. A non-aqueous electrolyte according to any one of claims 1 to 5; a positive electrode; and negative electrode A lithium secondary battery comprising:

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