Lithium secondary battery

Abstract

Provided is a lithium secondary battery including: an electrolyte including a nonaqueous organic solvent, a lithium salt, and an additive represented by Chemical Formula 1; a positive electrode including a positive electrode active material including a Si-carbon composite; and a negative electrode including a negative electrode active material.

Description

Technical Field

[0001] The present invention relates to a lithium secondary battery. Background Art

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

[0003] As a positive electrode active material for lithium secondary batteries, lithium-transition metal oxides having a structure capable of intercalating lithium ions, such as LiCoO2, LiMn2O4, LiNi 1-x Co x O2 (0<x<1), etc., have been used.

[0004] As a negative electrode active material, various carbon-based materials (such as artificial graphite, natural graphite, and hard carbon capable of intercalating and deintercalating lithium ions) have been used. As an electrolyte for lithium secondary batteries, an organic solvent in which a lithium salt is dissolved has been used. Summary of the Invention

[0005] One embodiment provides a lithium secondary battery exhibiting improved high capacity and improved cycle life characteristics.

[0006] According to one embodiment, a lithium secondary battery includes: an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive represented by Chemical Formula 1; a negative electrode including a negative electrode active material containing a Si-carbon composite; and a positive electrode including a positive electrode active material.

[0007] [Chemical Formula 1]

[0008]

[0009] (In Chemical Formula 1,

[0010] R 1 ~R 8 are each independently a hydrogen atom, a substituted or unsubstituted C1~C30 alkyl group, a substituted or unsubstituted C2~C30 alkenyl group, a substituted or unsubstituted C2~C30 alkynyl group, a substituted or unsubstituted C3~C30 cycloalkyl group, a substituted or unsubstituted C3~C30 cycloalkenyl group, a substituted or unsubstituted C3~C30 cycloalkynyl group, or a substituted or unsubstituted C6~C30 aryl group.)

[0011] In Chemical Formula 1, R 1 ~R 8Each of them can be independently a hydrogen atom, a substituted or unsubstituted C1-C10 alkyl group, a substituted or unsubstituted C2-C10 alkenyl group, a substituted or unsubstituted C2-C10 alkynyl group, a substituted or unsubstituted C3-C10 cycloalkyl group, a substituted or unsubstituted C3-C10 cycloalkenyl group, a substituted or unsubstituted C3-C10 cycloalkynyl group, or a substituted or unsubstituted C6-C10 aryl group.

[0012] In one embodiment, the additive represented by Formula 1 may be sulfolane, methylsulfolane, dimethylsulfolane, or a combination thereof.

[0013] When the amount of non-aqueous organic solvent and lithium salt is 100 wt%, the amount of additive represented by chemical formula 1 can be 0.1 wt% to 10 wt%.

[0014] The amount of Si-C carbon composite can be 0.1 wt% to 5 wt% based on the total weight of the negative electrode active material. Furthermore, the negative electrode active material may further include crystalline carbon.

[0015] Non-aqueous organic solvents may include propionate ester solvents. Propionate ester solvents may be methyl propionate, ethyl propionate, propyl propionate, or combinations thereof. Furthermore, the amount of propionate ester solvent may be 5% to 80% by volume, based on the total volume of the non-aqueous organic solvents.

[0016] Si-carbon composites may include Si nanoparticles and amorphous carbon. According to one embodiment, the Si-carbon composite may include a core and a coating surrounding the core, wherein the core may include amorphous or crystalline carbon and Si nanoparticles, and the coating may include amorphous carbon.

[0017] In one embodiment, the coating may have a thickness of 1 nm to 100 nm. In one embodiment, the amount of Si nanoparticles may be 1 wt% to 60 wt% based on the total weight of the Si-carbon composite.

[0018] Other implementations are described in detail below.

[0019] The lithium secondary battery according to one embodiment of the present invention includes an electrolyte with good resistivity-oxidation stability, thereby improving high voltage characteristics and reducing resistance, thus exhibiting high capacity and excellent cycle life characteristics. Attached Figure Description

[0020] Figure 1 This is a schematic diagram of a lithium secondary battery according to an embodiment.

[0021] Figure 2 The graph shows the initial DC resistance, DC resistance under high temperature storage, and resistance increase rate of the lithium secondary battery cells according to Example 2, Example 5, and Comparative Example 3.

[0022] Figure 3 The graph shows the initial DC resistance, DC resistance under high temperature storage, and resistance increase rate of the lithium secondary battery cells according to Examples 1 to 6, Reference Examples 1 to 2, and Comparative Examples 1 to 7.

[0023] Figure 4 The graph shows the initial DC resistance, DC resistance under high temperature storage, and resistance increase rate of the lithium secondary battery cells according to Examples 1 to 3, Reference Example 1, and Comparative Example 5. Detailed Implementation

[0024] Embodiments of the invention are described in detail below. However, these embodiments are exemplary, and the invention is not limited thereto; rather, the invention is defined by the scope of the claims.

[0025] In this specification, unless otherwise defined, the term 'substitution' means that the hydrogen atoms of the compound are substituted by a substituent selected from the following: halogen atom (F, Br, Cl, or I), hydroxyl, alkoxy, nitro, cyano, amino, azide, formamidinyl, hydrazine, hydrazine, carbonyl, carbamoyl, thiol, ester, carboxyl or a salt thereof, sulfonic acid or a salt thereof, phosphoric acid or a salt thereof, C1–C20 alkyl, C2–C20 alkenyl, C2–C20 alkynyl, C6–C30 aryl, C7–C30 aralkyl, C1–C4 alkoxy, C1–C20 heteroalkyl, C3–C20 heteroarylalkyl, C3–C30 cycloalkyl, C3–C15 cycloalkenyl, C6–C15 cycloalkynyl, C2–C20 heterocycloalkyl, or combinations thereof.

[0026] One embodiment provides a lithium secondary battery comprising: an electrolyte including a non-aqueous organic solvent, a lithium salt, and an additive represented by chemical formula 1; a negative electrode including a negative electrode active material; and a positive electrode including a positive electrode active material.

[0027] [Chemical Formula 1]

[0028]

[0029] In chemical formula 1,

[0030] R 1 ~R 8 Each of the following is independently a hydrogen atom, a substituted or unsubstituted C1-C30 alkyl group, a substituted or unsubstituted C2-C30 alkenyl group, a substituted or unsubstituted C2-C30 alkynyl group, a substituted or unsubstituted C3-C30 cycloalkyl group, a substituted or unsubstituted C3-C30 cycloalkenyl group, a substituted or unsubstituted C3-C30 cycloalkynyl group, or a substituted or unsubstituted C6-C30 aryl group.

[0031] In one implementation, R 1 ~R 8 Each of them can be independently a hydrogen atom, a substituted or unsubstituted C1-C10 alkyl group, a substituted or unsubstituted C2-C10 alkenyl group, a substituted or unsubstituted C2-C10 alkynyl group, a substituted or unsubstituted C3-C10 cycloalkyl group, a substituted or unsubstituted C3-C10 cycloalkenyl group, a substituted or unsubstituted C3-C10 cycloalkynyl group, or a substituted or unsubstituted C6-C10 aryl group.

[0032] For example, the additive represented by Formula 1 may be sulfolane, methyl sulfolane (e.g., 3-methyl sulfolane), dimethyl sulfolane (e.g., 2,4-dimethyl sulfolane) or a combination thereof.

[0033] In this document, based on the weight of the non-aqueous organic solvent and lithium salt, i.e., the amount of the non-aqueous organic solvent and lithium salt is 100 wt% (based on the total amount of the non-aqueous organic solvent and lithium salt being 100 wt%), the amount of the additive represented by Formula 1 can be 0.1 wt% to 10 wt%, and according to one embodiment, it can be 0.5 wt% to 7.5 wt%, and according to another embodiment, it can be 2.5 wt% to 7.5 wt%. When the amount of the additive represented by Formula 1 is within the range, high-temperature reliability characteristics, such as a reduction in high-temperature resistance, can be achieved.

[0034] The negative electrode active material may further include crystalline carbon together with the Si-C composite. In this paper, the amount of Si-C composite may be 0.1 wt% to 5 wt% based on the total weight of the negative electrode active material (i.e., total weight 100 wt%).

[0035] When a negative electrode active material including a Si-C composite is used in a battery with an electrolyte including an additive of Formula 1, the increase in resistance at high temperatures can be effectively suppressed. This effect is particularly pronounced when the Si-C composite is used at 0.1 wt% to 5 wt%, and according to one embodiment, the Si-C composite is used at 1 wt% to 5 wt%, or according to another embodiment, the Si-C composite is used at 2.5 wt% to 5 wt%. When the Si-C composite is included at 0.1 wt% to 5 wt% as the negative electrode active material, the desired high capacity and volume expansion suppression effect can be obtained more effectively.

[0036] Si-carbon composites may include Si nanoparticles and amorphous carbon. According to one embodiment, the Si-carbon composite may include a core and a coating surrounding the core, wherein the core may include amorphous or crystalline carbon and Si nanoparticles, and the coating may include amorphous carbon.

[0037] Amorphous carbon can be soft carbon, hard carbon, mesophase pitch carbonization products, calcined coke, or mixtures thereof. Crystalline carbon can be natural graphite, artificial graphite, or combinations thereof.

[0038] When the Si-carbon composite includes Si nanoparticles and amorphous carbon, the mixing ratio of Si nanoparticles and amorphous carbon can be 2:1 to 1.5:1 by weight. Furthermore, if the Si-carbon composite includes a core and a coating, the amount of coating can be 0.08:1 to 0.2:1 based on 100 wt% of the total Si-carbon composite, and the amount of Si nanoparticles can be 1 wt% to 60 wt%, and according to one embodiment, it can be 3 wt% to 60 wt%. Additionally, based on 100 wt% of the total Si-carbon composite, the amount of amorphous carbon or crystalline carbon included in the core can be 20 wt% to 60 wt%.

[0039] In addition, the coating may have a thickness of 1 nm to 100 nm (e.g., 5 nm to 100 nm).

[0040] Furthermore, regardless of the shape of the Si-carbon composite, the Si nanoparticles can have a particle size of 5 nm to 150 nm. For example, it can be 10 nm to 150 nm, specifically 30 nm to 150 nm, more specifically 50 nm to 150 nm, narrowerly 60 nm to 100 nm, and even narrowerly 80 nm to 100 nm. In the specification, size can be particle size, and can be the average particle size. In this case, the average particle size can mean the particle size as a cumulative volume measurement (D50). When not otherwise defined, the average particle size indicates the average particle size (D50) when the cumulative volume in the particle distribution is about 50% by volume. D50 can be measured by methods well known to those skilled in the art (e.g., by a particle size analyzer, or by a transmission electron microscope image, or a scanning electron microscope image). Alternatively, data analysis can be performed using a dynamic light scattering measurement device, and the number of particles in each particle size range can be counted. Therefore, the average particle size (D50) value can be easily obtained through calculation.

[0041] In the electrolyte according to one embodiment, the non-aqueous organic solvent may include carbonate solvents, and may further include propionate solvents.

[0042] In non-aqueous organic solvents, the amount of propionate solvent can be 5% to 80% of the total volume of the non-aqueous organic solvent. When the non-aqueous organic solvent includes propionate solvent, especially within the above amounts, gas generation can be more effectively suppressed during high-temperature storage or use at high temperatures, particularly in bag-type applications.

[0043] Propionate solvents may be methyl propionate, ethyl propionate, propyl propionate, or combinations thereof. When propionate solvents are used in mixtures, the mixing ratio can be appropriately controlled. For example, propionate solvents can be used by mixing ethyl propionate and propyl propionate. In this document, non-aqueous organic solvents may include 5 vol% to 40 vol% ethyl propionate, 55 vol% to 75 vol% propyl propionate, and carbonate solvents as a residue. The mixing ratio of ethyl propionate and propyl propionate, by volume, may be 25:75 to 30:70. When propionate solvents (ethyl propionate and propyl propionate) are used, particularly in the aforementioned amounts, gas generation can be more effectively suppressed, and low-temperature cycle life characteristics can be further improved.

[0044] Carbonate solvents 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), butyl carbonate (BC), or combinations thereof. When carbonate solvents are used in mixtures, the mixing ratio can be appropriately controlled. Furthermore, carbonate solvents may preferably include mixtures having cyclic and chain carbonates. In this document, cyclic and chain carbonates are mixed together in a volume ratio of about 1:1 to about 1:9, and when the mixture is used as an electrolyte, it can have enhanced properties.

[0045] In one embodiment, the non-aqueous organic solvent may further include ester solvents, ether solvents, ketone solvents, alcohol solvents, or aprotic solvents.

[0046] Ester solvents may include methyl acetate, ethyl acetate, n-propyl acetate, tert-butyl acetate, methyl propionate, ethyl propionate, propyl propionate, γ-butyrolactone, decanoic acid lactone, valerate lactone, mevalonate lactone, caprolactone, etc.

[0047] Ether solvents can be dibutyl ether, tetraethylene glycol dimethyl ether, diethylene glycol dimethyl ether, dimethoxyethane, 2-methyltetrahydrofuran, tetrahydrofuran, etc., while ketone solvents can be cyclohexanone, etc.

[0048] Alcohol solvents may include ethanol, isopropanol, etc., and aprotic solvents may include nitriles such as R-CN (where R is a C2-C20 straight-chain, branched or cyclic hydrocarbon, and may include double bonds, aromatic rings or ether bonds), amides such as dimethylformamide, and dioxolane such as 1,3-dioxolane.

[0049] In addition, organic solvents may further include aromatic hydrocarbon solvents. Aromatic hydrocarbon organic solvents may be aromatic hydrocarbon compounds represented by chemical formula 2.

[0050] [Chemical Formula 2]

[0051]

[0052] (In chemical formula 2, R) 10 ~R 15 (The same or different, and selected from hydrogen, halogens, C1-C10 alkyl groups, haloalkyl groups, and combinations thereof.)

[0053] Specific examples of aromatic hydrocarbon organic solvents can be selected from 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, and fluoromethylbenzene. Benzene, 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, or combinations thereof.

[0054] The electrolyte may further include ethylene ethyl carbonate, vinylene carbonate, or ethylene carbonate compounds represented by Formula 3 as additives to improve cycle life.

[0055] [Chemical Formula 3]

[0056]

[0057] (In chemical formula 3, R) 16 and R 17 They may be the same or different and may each be independently hydrogen, halogen, cyano (CN), nitro (NO2), or C1-C5 fluoroalkyl, provided that at least one of R7 and R8 is halogen, cyano (CN), nitro (NO2), or C1-C5 fluoroalkyl, and R7 and R8 are not both hydrogen.

[0058] Examples of ethylene carbonate compounds include difluoroethylene carbonate, chloroethylene carbonate, dichloroethylene carbonate, brominated ethylene carbonate, dibromoethylene carbonate, nitroethylene carbonate, cyanoethylene carbonate, and fluoroethylene carbonate. The amount of additives used to improve cycle life characteristics can be used within appropriate limits.

[0059] Lithium salts dissolved in organic solvents provide lithium ions to the battery, essentially enabling the operation of a rechargeable lithium battery, and improving lithium ion transport between the positive and negative electrodes. Examples of lithium salts include at least one or two carrier salts selected from the following: 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+ 1SO2)(C y F 2y+1 Lithium difluoro(bis(oxalate)phosphate) phosphate (SO2) (where x and y are natural numbers, for example, integers from 1 to 20), lithium difluoro(bis(oxalate)phosphate) phosphate, LiCl, LiI, LiB(C2O4)2 (lithium bis(oxalate)borate: LiBOB), and lithium difluoro(oxalate)borate (LiDFOB). The concentration of lithium salts can range from about 0.1 M to about 2.0 M. When lithium salts are included within this concentration range, the electrolyte exhibits excellent performance and lithium-ion mobility due to optimal electrolyte conductivity and viscosity.

[0060] In one embodiment, the negative electrode including the negative electrode active material includes a negative electrode active material layer containing the negative electrode active material and a current collector supporting the negative electrode active material layer.

[0061] The negative electrode active material layer may include a negative electrode active material and a binder, and further include a conductive material.

[0062] In the negative electrode active material layer, the amount of negative electrode active material can be approximately 95 wt% to approximately 98 wt%. In the negative electrode active material layer, based on a total amount of 100 wt% of the negative electrode active material, the amount of binder can be approximately 1 wt% to approximately 5 wt%. When the negative electrode active material layer also includes conductive material, the negative electrode active material layer comprises approximately 90 wt% to approximately 98 wt% of negative electrode active material, approximately 1 wt% to approximately 5 wt% of binder, and approximately 1 wt% to approximately 5 wt% of conductive material.

[0063] The binder improves the adhesion between the negative electrode active material particles and the adhesion between the negative electrode active material particles and the current collector.

[0064] Adhesives include non-water-based adhesives, water-based adhesives, or combinations thereof.

[0065] The non-aqueous adhesive may be ethylene-propylene copolymer, polyacrylonitrile, polystyrene, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, polyamide-imide, polyimide, or a combination thereof.

[0066] Waterborne adhesives may include styrene-butadiene rubber, acrylated styrene-butadiene rubber, acrylonitrile-butadiene rubber, acrylic rubber, butyl rubber, fluororubber, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyepoxychloropropane, polyphosphazene, ethylene propylene diene copolymer, polyvinylpyridine, chlorosulfonated polyethylene, latex, polyester resin, acrylic resin, phenolic resin, epoxy resin, polyvinyl alcohol, or combinations thereof.

[0067] When an aqueous binder is used as a negative electrode binder, a cellulose compound can be further used as a thickener to provide viscosity. Cellulose compounds include one or more of carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or their alkali metal salts. The alkali metal can be Na, K, or Li. Based on 100 parts by weight of the negative electrode active material, the thickener can be included in an amount of about 0.1 parts by weight to about 3 parts by weight.

[0068] Conductive materials are included to provide electrode conductivity, and any conductive material can be used as a conductive material unless it causes a chemical change. Examples of conductive materials include: carbon-based materials (such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.); metallic materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers (such as polyphenylene derivatives); or mixtures thereof.

[0069] The current collector may include, but is not limited to, one selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof.

[0070] In one embodiment, the positive electrode comprising a positive electrode active material includes a positive electrode active material layer containing the positive electrode active material and a current collector supporting the positive electrode active material layer. The positive electrode active material may include a lithiation intercalation compound that reversibly inserts and deintercalates lithium ions, and specifically, one or more composite oxides of lithium and metals selected from cobalt, manganese, nickel, and combinations thereof may be used. More specifically, a compound represented by 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 HAVE BEEN 1-b X b O 2- c D c (0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.05);Li a HAVE BEEN 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-α T2(0.90≤a≤1.8,0≤b≤0.5,0≤c≤0.5,0<α<2);Li a Ni 1-b-c Mr 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 Mr 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 Mr 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 Ni b HAVE BEEN c G dO2(0.90≤a≤1.8, 0≤b≤0.9, 0≤c≤0.5, 0.001≤d≤0.1); Li a Ni 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; QS2; LiQS2; V2O5; LiV2O5; LiZO2; LiNiVO4; Li (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).

[0071] In the above chemical formulas, A is selected from Ni, Co, M and combinations thereof; X is selected from Al, Ni, Co, Mn, Cr, Fe, Mg, Sr, V, rare earth elements and combinations thereof; D is selected from O, F, S, P and combinations thereof; E is selected from Co, Mn and combinations thereof; T is selected from F, S, P and combinations thereof; G is selected from Al, Cr, Mn, Fe, Mg, La, Ce, Sr, V and combinations thereof; Q is selected from Ti, Mo, Mn and combinations thereof; Z is selected from Cr, V, Fe, Sc, Y and combinations thereof; and J is selected from V, Cr, Mn, Co, Ni, Cu and combinations thereof.

[0072] Furthermore, the compound may have a coating on the surface, or may be mixed with another compound having a coating. The coating may include at least one coating element compound selected from the group consisting of: oxides of coating elements, hydroxides of coating elements, hydroxy oxides of coating elements, oxycarbonates of coating elements, and hydroxycarbonates of coating elements. The compound used for the coating may be amorphous or crystalline. Coating elements included in the coating may include Mg, Al, Co, K, Na, Ca, Si, Ti, V, Sn, Ge, Ga, B, As, Zr, or mixtures thereof. By using these elements in the compound, the coating can be formed in a manner that does not adversely affect the properties of the positive electrode active material. For example, this method may include any coating method such as spraying or dipping, but since it is well known in the relevant art, it will not be described in further detail.

[0073] According to one embodiment, the positive electrode active material may appropriately be Li a Co 1-b X b D2(0.90≤a≤1.8, 0≤b≤0.5), Li a Co 1-b X b O 2-c D c (0.90≤a≤1.8, 0≤b≤0.5, 0≤c≤0.05), Li a Co 1-b X b O 2-c D c (0≤b≤0.5, 0≤c≤0.05), or combinations thereof.

[0074] In the positive electrode, the amount of positive active material can be approximately 90 wt% to approximately 98 wt%, based on the total weight of the positive active material layer.

[0075] In one embodiment, the positive electrode active material layer may further include a binder and a conductive material. In this document, based on the total amount of the positive electrode active material layer, the amounts of binder and conductive material may be 1 wt% to 5 wt%, respectively.

[0076] The binder improves the adhesion properties between the positive electrode active material particles and the adhesion properties between the positive electrode active material particles and the current collector. Examples of binders include polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, ethylene oxide-containing polymers, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylated styrene-butadiene rubber, epoxy resin, nylon, etc., but are not limited to these.

[0077] Conductive materials are included to provide electrode conductivity, and any conductive material can be used as a conductive material unless it causes a chemical change. Examples of conductive materials include carbon-based materials (such as natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, etc.); metallic materials including metal powders or metal fibers of copper, nickel, aluminum, silver, etc.; conductive polymers (such as polyphenylene derivatives); or mixtures thereof.

[0078] The current collector may be made of aluminum foil, nickel foil, or a combination thereof, but is not limited to these.

[0079] The positive and negative active material layers can be formed by mixing active materials, binders, and optionally conductive materials in a solvent to prepare an active material composition and then coating the active material composition onto a current collector. This method of preparing the active material layer is well known and therefore will not be described in detail in this specification. Solvents include, but are not limited to, N-methylpyrrolidone. Additionally, when the binder in the negative active material layer is a water-soluble binder, the solvent used to prepare the negative active material composition can be water.

[0080] Furthermore, a separator can be disposed between the positive and negative electrodes, depending on the type of rechargeable lithium battery. The separator can be made of polyethylene, polypropylene, polyvinylidene fluoride, or multilayers with two or more layers, and can be a hybrid multilayer (such as polyethylene / polypropylene double-layer separators, polyethylene / polypropylene / polyethylene triple-layer separators, polypropylene / polyethylene / polypropylene triple-layer separators, etc.).

[0081] According to one embodiment, the diaphragm may also be a composite porous diaphragm comprising a porous substrate and a functional layer located on the porous substrate. The functional layer may have additional functions, for example, it may be at least one of a heat-resistant layer and an adhesive layer. The heat-resistant layer may include a heat-resistant resin and optional fillers. Additionally, the adhesive layer may include an adhesive resin and optional fillers. The fillers may be organic fillers, inorganic fillers, or combinations thereof. The heat-resistant resin and the adhesive resin may be any material that can be used in diaphragms in the related art.

[0082] Figure 1 This is an exploded perspective view of a rechargeable lithium battery according to an embodiment of the present invention. The lithium secondary battery according to the embodiment is illustrated as a pouch battery, but is not limited thereto, and may include batteries of various shapes (such as cylindrical batteries and prismatic pouch batteries).

[0083] refer to Figure 1The lithium-ion pouch cell 100 according to an embodiment includes: an electrode assembly 40 manufactured by winding a positive electrode 10, a negative electrode 20, and a separator 30 disposed therebetween; a housing 50 including the electrode assembly 40; and electrode tabs 130 providing an electrical path for guiding current generated in the electrode assembly 40 to the outside. The housing 120 is sealed by overlapping its two sides facing each other. Additionally, an electrolyte is injected into the housing 120 including the electrode assembly 40, and the positive electrode 10, negative electrode 20, and separator 30 are immersed in an electrolyte solution (not shown).

[0084] Embodiments and comparative examples of the invention are described below. However, these examples are in no way intended to limit the scope of the invention.

[0085] (Example 1)

[0086] 1.3 M LiPF6 was dissolved in a non-aqueous organic solvent in which ethylene carbonate, propylene carbonate, ethyl propionate, and propyl propionate were mixed in a volume percentage (V%) of 10:15:30:45, and sulfolane of Formula 1a was added to prepare an electrolyte for lithium secondary battery cells. In this paper, based on a total amount of 100 wt% for the non-aqueous organic solvent and lithium salt, the amount of sulfolane of Formula 1a as the first additive was set to 2.5 wt%.

[0087] [Chemical Formula 1a]

[0088]

[0089] A negative electrode active material slurry was prepared by mixing 96 wt% of a negative electrode active material (in which natural graphite and Si-carbon composite were mixed in a weight ratio of 95:5), 2 wt% of styrene-butadiene rubber binder, and 2 wt% of carboxymethyl cellulose tackifier in an aqueous solvent. The negative electrode active material slurry was coated onto copper foil and dried, followed by pressure to prepare the negative electrode. In this paper, the Si-carbon composite comprises a core containing artificial graphite and silicon particles, and soft carbon coated on the surface of the core. Based on the total weight of the Si-carbon composite, the amount of artificial graphite is 40 wt%, the amount of silicon particles is 40 wt%, and the amount of amorphous carbon is 20 wt%. The soft carbon coating has a thickness of 20 nm, and the silicon particles have an average particle size D50 of 100 nm.

[0090] A positive electrode active material slurry was prepared by mixing 96 wt% LiCoO2 positive electrode active material, 2 wt% Ketjen Black conductive material, and 2 wt% polyvinylidene fluoride in N-methylpyrrolidone solvent. The positive electrode active material slurry was coated onto aluminum foil, dried, and then pressurized to prepare the positive electrode.

[0091] Using an electrolyte, a positive electrode, and a negative electrode, a 4.4V-class pouch-type lithium secondary battery cell is manufactured according to a standard procedure.

[0092] (Example 2)

[0093] The electrolyte was prepared using the same procedure as in Example 1, except that the total amount of non-aqueous organic solvent and lithium salt was 100 wt%, and the amount of additive of chemical formula 1a was changed to 5 wt%. The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 1, except that the electrolyte and the negative and positive electrodes of Example 1 were used.

[0094] (Example 3)

[0095] The electrolyte was prepared using the same procedure as in Example 1, except that the total amount of non-aqueous organic solvent and lithium salt was 100 wt%, the amount of additive of chemical formula 1a was changed to 10 wt%, and a pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 1, except that the electrolyte and the negative and positive electrodes of Example 1 were used.

[0096] (Refer to Example 1)

[0097] The negative electrode was prepared using the same procedure as in Example 1, except that the mixing ratio of natural graphite and Si-carbon composite was changed to 95:5 by weight. The electrolyte was prepared using the same procedure as in Example 1, except that the total amount of non-aqueous organic solvent and lithium salt was 100 wt%, and the amount of additive of chemical formula 1a was changed to 12.5 wt%. The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 1, except that the electrolyte and the negative and positive electrodes of Example 1 were used.

[0098] (Example 4)

[0099] The negative electrode was prepared using the same procedure as in Example 1, except that the mixing ratio of natural graphite and Si-carbon composite was changed to 97.5:2.5 by weight. The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 4, except that the negative electrode, electrolyte and positive electrode of Example 1 were used.

[0100] (Example 5)

[0101] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 4, except that the negative electrode of Example 4, the electrolyte of Example 2, and the positive electrode of Example 1 were used.

[0102] (Example 6)

[0103] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 4, except that the negative electrode of Example 4, the electrolyte of Example 3, and the positive electrode of Example 4 were used.

[0104] (See Example 2 for reference)

[0105] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 4, except that the negative electrode of Example 4, the electrolyte of Reference Example 1, and the positive electrode of Example 4 were used.

[0106] (Comparative Example 1)

[0107] 1.3 M LiPF6 was dissolved in a non-aqueous organic solvent in which ethylene carbonate, propylene carbonate, ethyl propionate and propyl propionate were mixed in a volume percentage of 10:15:30:45 to prepare an electrolyte for lithium secondary battery cells.

[0108] A negative electrode active material slurry was prepared by mixing 96 wt% natural graphite negative electrode active material, 2 wt% styrene-butadiene rubber binder, and 2 wt% carboxymethyl cellulose tackifier in an aqueous solvent. The negative electrode active material slurry was coated onto copper foil and dried, followed by pressure to prepare the negative electrode.

[0109] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 1, except that the electrolyte and the negative and positive electrodes of Example 1 were used.

[0110] (Comparative Example 2)

[0111] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Example 1, except that the electrolyte of Example 1, the negative electrode of Comparative Example 1, and the positive electrode of Comparative Example 1 were used.

[0112] (Comparative Example 3)

[0113] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Comparative Example 2, except that the electrolyte was prepared by changing the amount of sulfolane of Formula 1a to 5 wt% based on a total amount of 100 wt% of non-aqueous organic solvent and lithium salt.

[0114] (Comparative Example 4)

[0115] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Comparative Example 2, except that the electrolyte was prepared by changing the amount of sulfolane of Formula 1a to 10 wt% based on a total amount of 100 wt% of non-aqueous organic solvent and lithium salt.

[0116] (Comparative Example 5)

[0117] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Comparative Example 1, except that the electrolyte of Comparative Example 1, the negative electrode of Example 1, and the positive electrode of Comparative Example 1 were used.

[0118] (Comparative Example 6)

[0119] The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Comparative Example 1, except that the electrolyte of Comparative Example 1, the negative electrode of Example 5, and the positive electrode of Comparative Example 1 were used.

[0120] (Comparative Example 7)

[0121] The negative electrode was prepared using the same procedure as in Example 1, except that the mixing ratio of natural graphite and Si-carbon composite was changed to 92.5:7.5 by weight. The pouch-type lithium secondary battery cell was manufactured using the same procedure as in Comparative Example 1, except that the negative electrode, the electrolyte of Comparative Example 3, and the positive electrode of Comparative Example 1 were used.

[0122] The mixing ratios of Examples 1 to 6, Reference Examples 1 to 2, and Comparative Examples 1 to 7, and the amounts of sulfolane represented by chemical formula 1a are summarized in Table 1.

[0123] Table 1

[0124] Comparative Example 1 100 - 0 Comparative Example 2 100 - 2.5 Comparative Example 3 100 - 5 Comparative Example 4 100 - 10 Comparative Example 5 95 5 0 Comparative Example 6 97.5 2.5 0 Comparative Example 7 92.5 7.5 5 Example 1 95 5 2.5 Example 2 95 5 5 Example 3 95 5 10 Reference Example 1 95 5 12.5 Example 4 97.5 2.5 2.5 Example 5 97.5 2.5 5 Example 6 97.5 2.5 10 See Example 2 97.5 2.5 12.5

[0125] Evaluation of DC internal resistance (DC-IR: DC internal resistance)

[0126] The lithium secondary battery cells of Examples 1 to 6, Reference Examples 1 to 2, and Comparative Examples 1 to 7 were discharged at 60°C and at SOC 100 (state of charge, fully charged state, based on 100% of the total battery charge capacity, charged to 100% charge capacity) for 10 seconds at a constant current of 10A, 10 seconds at a constant current of 1A, 10 seconds at a constant current of 1A, and 4 seconds at a constant current of 10A. The voltage and current values ​​were measured immediately before storage. In addition, the battery cells were stored at 60°C for 30 days, and then the voltage and current values ​​were measured.

[0127] The DC resistance (DC-IR) is calculated using the equation ΔR = ΔV / ΔI from data at 18 seconds and 23 seconds. That is, it is obtained from: (Voltage measured after discharging at 10A for 10 seconds, at 1A for 10 seconds, and after discharging at 10A for 4 seconds - Voltage measured after discharging at 10A for 10 seconds and after discharging at 1A for 8 seconds) / Current after discharging at 10A for 10 seconds and 8 seconds.

[0128] The DCIR resistance increase rate is calculated using Equation 1 from the DC resistance immediately before storage and the DC resistance 30 days later.

[0129] As a result, the initial DC-IR and the rate of increase in resistance, as well as the DC-IR at 60°C after 3 days, are shown in Table 1. Additionally, to clearly confirm the effect dependent on the amount of Si-carbon composite, the results of Examples 2, 5, and Comparative Example 3 are shown in... Figure 2The results of Examples 1 to 6, Reference Examples 1 to 2, and Comparative Examples 1 to 7 are shown in the table. Figure 3 In addition, to clearly identify the effect of sulfolane in accordance with the amount of chemical formula 1a, the results are shown in... Figure 4 middle.

[0130] [Equation 1]

[0131] DCIR increase rate = [DCIR 30 days] / DCIR (0 days) × 100%

[0132] In Equation 1, DCIR 30 days indicates the DCIR 30 days later, and DCIR(0 days) indicates the DCIR immediately preceding the storage.

[0133] Table 2

[0134]

[0135] As shown in Table 2 and Figure 3 As shown, the lithium secondary battery cells according to Examples 1 to 6 (where artificial graphite and Si-carbon composites are used as negative electrode active materials and sulfolane of Formula 1a (especially sulfolane of Formula 1a is used in an amount of 0.1 wt% to 10 wt%)) exhibit a low rate of increase in resistance after storage at high temperature, while maintaining an appropriate initial resistance.

[0136] Despite using artificial graphite and Si-carbon composite as the negative electrode and sulfolane of chemical formula 1a, Reference Examples 1 to 2, which used a large amount of 12.5 wt% sulfolane, showed a much higher rate of increase in resistance after storage at high temperature.

[0137] As shown in Table 2 and Figure 3 The results of Comparative Examples 1 to 7 shown in the figure demonstrate that when the Si-carbon composite is not used as the negative electrode active material, even when sulfolane of Formula 1a is included, the rate of increase in resistance at high temperature is still relatively high. Figure 2 The results shown clearly demonstrate that when the Si-carbon composite is included as the negative electrode active material, the effect of reducing the rate of increase in resistance during high-temperature storage and DC-IR after storage at 60°C for 30 days is obtained by adding sulfolane of formula 1a.

[0138] In addition, from Figure 4 The results clearly show that electrolytes using sulfolane of formula 1a with amounts of 2.5 wt%, 5 wt%, and 10 wt%, respectively, exhibited an increase in resistivity during high-temperature storage and a decrease in DC-IR after storage at 60°C for 30 days.

[0139] While the invention has been described in conjunction with exemplary embodiments now considered practical, it should be understood that the invention is not limited to the disclosed embodiments, but rather is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the claims. Therefore, the foregoing embodiments should be understood as illustrative and not as limiting the invention in any way.

Claims

1. A lithium secondary battery, comprising: Electrolytes, including non-aqueous organic solvents, lithium salts, and additives represented by chemical formula 1; The negative electrode includes a negative electrode active material comprising a Si-carbon composite. as well as The positive electrode includes the positive electrode active material: [Chemical Formula 1] In chemical formula 1, R 1 ~R 8 Each of the following is independently a hydrogen atom, a substituted or unsubstituted C1-C30 alkyl group, a substituted or unsubstituted C2-C30 alkenyl group, a substituted or unsubstituted C2-C30 alkynyl group, a substituted or unsubstituted C3-C30 cycloalkyl group, a substituted or unsubstituted C3-C30 cycloalkenyl group, a substituted or unsubstituted C3-C30 cycloalkynyl group, or a substituted or unsubstituted C6-C30 aryl group. When the amounts of the non-aqueous organic solvent and the lithium salt are 100 wt%, the amount of the additive represented by Formula 1 is 0.1 wt% to 10 wt%. Based on the total weight of the negative electrode active material, the amount of the Si-C carbon composite is 0.1wt%~5wt%.

2. The lithium secondary battery according to claim 1, wherein R 1 ~R 8 Each of the following is independently a hydrogen atom, a substituted or unsubstituted C1-C10 alkyl group, a substituted or unsubstituted C2-C10 alkenyl group, a substituted or unsubstituted C2-C10 alkynyl group, a substituted or unsubstituted C3-C10 cycloalkyl group, a substituted or unsubstituted C3-C10 cycloalkenyl group, a substituted or unsubstituted C3-C10 cycloalkynyl group, or a substituted or unsubstituted C6-C10 aryl group.

3. The lithium secondary battery according to claim 1, wherein the additive represented by chemical formula 1 includes sulfolane, methylsulfolane, dimethylsulfolane, or a combination thereof.

4. The lithium secondary battery according to claim 1, wherein the negative electrode active material further comprises crystalline carbon.

5. The lithium secondary battery according to claim 1, wherein the non-aqueous organic solvent includes propionate ester solvents.

6. The lithium secondary battery according to claim 5, wherein the propionate solvent is methyl propionate, ethyl propionate, propyl propionate, or a combination thereof.

7. The lithium secondary battery according to claim 5, wherein the amount of the propionate solvent is 5% to 80% by volume, based on the total volume of the non-aqueous organic solvent.

8. The lithium secondary battery according to claim 1, wherein the Si-carbon composite comprises Si nanoparticles and amorphous carbon.

9. The lithium secondary battery according to claim 1, wherein the Si-carbon composite comprises a core and a coating surrounding the core. The core comprises amorphous or crystalline carbon and Si nanoparticles, and The coating comprises amorphous carbon.

10. The lithium secondary battery according to claim 9, wherein the coating has a thickness of 1 nm to 100 nm.

11. The lithium secondary battery according to claim 9, wherein the total amount of the Si-carbon composite is 100 wt%, and the amount of the Si nanoparticles is 1 wt% to 60 wt%.

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