Lithium secondary battery, battery module, and battery pack

Abstract

The present specification provides a lithium secondary battery, a battery module comprising same, and a battery pack comprising same, the lithium secondary battery comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode includes a negative electrode active material layer, the negative electrode active material layer includes a silicon-based active material, the electrolyte includes fluoroethylene carbonate (FEC), and a ratio (B / A) of parts by weight (B) of the silicon-based active material based on 100 parts by weight of the negative electrode active material included in the negative electrode active material layer to parts by weight (A) of the fluoroethylene carbonate (FEC) based on 100 parts by weight of the electrolyte is 4 to 7.

Description

Lithium secondary batteries, battery modules, and battery packs

[0001] The present invention claims the benefit of the filing date of Korean Patent Application No. 10-2025-0006303 filed with the Korean Intellectual Property Office on January 15, 2025, the entire contents of which are incorporated herein.

[0002] The present invention relates to a lithium secondary battery, a battery module including the same, and a battery pack including the same.

[0003] Recently, accompanied by the rapid proliferation of battery-powered electronic devices such as mobile phones, laptop computers, electric vehicles, power tools, and vacuum cleaners, the demand for rechargeable batteries that are small and lightweight yet possess relatively high capacity and / or high output is rapidly increasing. In particular, lithium-ion batteries are gaining prominence as power sources for electronic devices due to their lightweight nature and high energy density. Accordingly, active research and development efforts are underway to improve the performance of lithium-ion batteries.

[0004] In a lithium secondary battery, electrical energy is produced by oxidation and reduction reactions when lithium ions are inserted into or removed from the positive and negative electrodes, which are composed of active materials capable of lithium ion intercalation and deintercalation, with an organic or polymer electrolyte charged between them.

[0005] During the initial charging of a lithium secondary battery, lithium ions released from the positive electrode active material, such as lithium metal oxide, migrate to the negative electrode active material and are inserted between the layers of the negative electrode active material. At this time, due to the strong reactivity of lithium ions, electrolyte-derived films are formed on the surface of the electrode active material, such as a CEI (Cathode-Electrolyte Interface) film formed between the positive electrode and the electrolyte, or a SEI (Solid Electrolyte Interphase) film formed between the negative electrode and the electrolyte.

[0006] These electrolyte-derived films act as tunnels for lithium ions, preventing direct contact between the electrolyte and the electrode active material, thereby reducing adverse reactions during charging and discharging. However, there is a problem in which battery performance deteriorates because the electrolyte-derived films are not formed uniformly on the surface of the electrode active material, or they form unstably depending on the composition of the electrolyte.

[0007] The problem that the present invention aims to solve is to provide a lithium secondary battery comprising a silicon-based negative electrode active material, wherein the initial efficiency, charge / discharge characteristics, and / or lifespan are improved.

[0008] The problems that the present invention aims to solve are not limited to those mentioned above, and other unmentioned problems will be clearly understood by those skilled in the art from the description below.

[0009] One embodiment of the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode comprises a negative electrode active material layer, the negative electrode active material layer comprises a silicon-based active material, the electrolyte comprises fluoroethylene carbonate (FEC), and the ratio (B / A) of the weight part (A) of the silicon-based active material included in the negative electrode active material layer to the weight part (B) of the negative electrode active material included in the negative electrode active material layer relative to 100 weight parts of the electrolyte comprises 4 to 7.

[0010] One embodiment of the present invention provides a battery module including the lithium secondary battery.

[0011] One embodiment of the present invention provides a battery pack comprising the lithium secondary battery.

[0012] One embodiment of the present invention provides a battery pack including the battery module.

[0013] A lithium secondary battery according to one embodiment of the present invention comprises a silicon-based active material in the negative electrode and fluoroethylene carbonate (FEC) in the electrolyte, and by satisfying a specific ratio of the weight portions of the materials, the initial efficiency, fast charging characteristics, and lifespan characteristics can be improved.

[0014] The present specification will be described in more detail below.

[0015] In this specification, when a part is described as "comprising" a certain component, this means that, unless specifically stated otherwise, it does not exclude other components but may include additional components.

[0016] In this specification, when it is said that a member is located "on" another member, this includes not only cases where a member is in contact with another member, but also cases where another member exists between the two members.

[0017] Terms or words used in this specification should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted in a meaning and concept consistent with the technical spirit of the invention, based on the principle that the inventor can appropriately define the concept of the terms to best describe his invention.

[0018] In this specification, "p to q" means a range of "p or more and q or less".

[0019] The singular expressions of terms used in this specification include the plural expressions unless the context clearly indicates otherwise.

[0020] Preferred embodiments of the present invention are described in detail below. However, embodiments of the present invention may be modified in various forms, and the scope of the present invention is not limited to the embodiments described below.

[0021] One embodiment of the present invention provides a lithium secondary battery comprising: a positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte, wherein the negative electrode comprises a negative electrode active material layer, the negative electrode active material layer comprises a silicon-based active material, the electrolyte comprises fluoroethylene carbonate (FEC), and the ratio (B / A) of the weight part (A) of the silicon-based active material included in the negative electrode active material layer to the weight part (B) of the negative electrode active material included in the negative electrode active material layer relative to 100 weight parts of the electrolyte comprises 4 to 7.

[0022] In one embodiment of the present invention, the ratio (B / A) of the weight part (B) of the silicon-based active material included in the negative electrode active material layer to the weight part (A) of the fluoroethylene carbonate (FEC) based on 100 weight parts of the electrolyte may be 4 or more, 4.3 or more, or 4.5 or more, 7 or less, 6.5 or less, or 6, and may be 4 to 6.5, 4.3 to 6.5, or 4.3 to 6.

[0023] In the present specification, 100 parts by weight of the electrolyte includes the weight of the solvent, lithium salt, and additive included in the electrolyte.

[0024] In this specification, the electrolyte may refer to an electrolyte introduced during the manufacturing process, or may refer to an electrolyte that exists in a free state and is recoverable after disassembling a completed lithium secondary battery, without being impregnated in the electrodes and separators.

[0025] In a negative electrode using a silicon-based active material, a fluoroethylene carbonate (FEC) additive helps stabilize the negative electrode film and form a flexible polymer-based film by attaching LiF to the surface of the negative electrode active material. However, there is a problem in that the decomposition of fluoroethylene carbonate (FEC) is accelerated during high-temperature storage, which generates gas and accelerates the decomposition of the electrolyte.

[0026] The inventors of the present invention have discovered that initial efficiency, fast charging characteristics, and lifespan characteristics can be improved by determining an appropriate ratio of the content of silicon-based active material in the cathode and the content of fluoroethylene carbonate (FEC) added to the electrolyte.

[0027] If the ratio of the weight part (B) of the silicon-based active material included in the negative electrode active material layer to the weight part (A) of the fluoroethylene carbonate (FEC) based on 100 weight parts of the electrolyte exceeds the above range, a film is not properly formed on the surface of the negative electrode active material, and cracks occur in the negative electrode film due to volume expansion of the silicon-based active material, which increases contact between the negative electrode active material and the electrolyte and increases the rate of depletion of the electrolyte, thereby increasing the rate of degradation and side reactions within the cell. If it is below the above range, gas generation increases due to the excess amount of fluoroethylene carbonate (FEC), and in particular, HF, LiPO2F2, etc. are generated, which promotes the decomposition reaction of the electrolyte and causes performance degradation.

[0028] In one embodiment of the present invention, the anode may include an anode current collector and an anode active material layer stacked on the anode current collector.

[0029] The above-mentioned positive current collector is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the above-mentioned positive current collector may typically have a thickness of 1 to 500 μm, and fine irregularities may be formed on the surface of the current collector to increase the adhesion of the positive active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, nonwoven fabric, etc.

[0030] The above-mentioned positive electrode active material may include a lithium complex transition metal compound comprising nickel (Ni), cobalt (Co), and manganese (Mn). Additionally, the above-mentioned positive electrode active material may include nickel, cobalt, and manganese, and may further include aluminum, but is not limited thereto.

[0031] In addition, the positive electrode active material contains 80 mol% or more and less than 100 mol% of nickel among metals excluding lithium, and the lithium complex transition metal compound containing 80 mol% or more and less than 100 mol% of nickel among metals excluding lithium may include one or two or more mixtures represented by the following chemical formula 1.

[0032] [Chemical Formula 1]

[0033] Li a Ni 1-b-c-d Co b Mn c Q d O 2+δ

[0034] In the above formula, Q is one or more elements selected from the group consisting of Na, K, Mg, Ca, Sr, Ni, Co, Ti, Al, Si, Sn, Mn, Cr, Fe, V, and Zr, and 1≤a≤1.5, 0 <b≤0.5, 0<c≤0.5, 0≤d≤0.1, 0 <b+c+d≤20, -0.1≤δ≤1.0이다.

[0035] According to one embodiment of the present invention, the anode according to the above embodiment further comprises an anode binder and a conductive material.

[0036] The anode binder described above can serve to improve adhesion between anode active material particles and adhesion between anode active material particles and anode current collector. The anode binder may be one known in the art, and non-limiting examples include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one of these alone or a mixture of two or more may be used.

[0037] The conductive material included in the above positive active material layer is used to impart conductivity to the electrode, and can be used without special restrictions as long as it has electronic conductivity without causing chemical changes within the battery. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used.

[0038] In one embodiment of the present invention, the cathode may include a cathode current collector and a cathode active material layer laminated on the cathode current collector.

[0039] The above-mentioned negative current collector may be conductive without causing chemical changes in the battery, and is not particularly limited. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used as the current collector. Specifically, transition metals that adsorb carbon well, such as copper and nickel, may be used as the current collector. The thickness of the above-mentioned current collector may be 1 μm to 500 μm, but the thickness of the above-mentioned current collector is not limited thereto.

[0040] In one embodiment of the present invention, the negative electrode active material comprises a silicon-based active material.

[0041] In one embodiment of the present invention, the negative electrode active material may further include a carbon-based active material.

[0042] In one embodiment of the present invention, the silicon-based active material may be a silicon carbon composite.

[0043] According to one embodiment, the carbon-based active material may be graphite. The graphite may be natural graphite or artificial graphite, or a mixture of natural graphite and artificial graphite. In addition, when the graphite is a mixture of natural graphite and artificial graphite, the weight ratio of natural graphite to artificial graphite may be 50:50 to 90:10, and specifically 60:40 to 80:20 or 65:35 to 75:25.

[0044] In one embodiment of the present invention, the average particle size (D) of the graphite is 50 ) may be 10 μm to 20 μm. Specifically, the average particle size (D) of the graphite. 50The average particle size of the graphite may be 10㎛ or more, 13㎛ or more, or 15㎛ or more, 20㎛ or less, 18㎛ or less, or 16㎛ or less, or 13㎛ to 20㎛, 13㎛ to 18㎛, or 15㎛ to 18㎛. When the average particle size of the graphite satisfies the above range, the effect of particle aggregation is reduced, and slurry dispersibility can be improved.

[0045] According to one embodiment, the silicon carbon composite is a composite of Si and C, wherein Si and C (e.g., graphite) are present respectively. In this specification, the silicon carbon composite may be denoted as Si / C. The silicon carbon composite may consist of Si and C that are not bonded to each other, but may include additional components as needed. For example, the silicon carbon composite may or may not include silicon carbide denoted as SiC. If the silicon carbon composite includes silicon carbide, its content is 3 weight percent or less. The silicon carbon composite may exist in a crystalline, amorphous, or mixed state. According to one example, C in the silicon carbon composite may exist in an amorphous state.

[0046] According to one embodiment, the silicon carbon composite may be a porous carbon-based particle and a particle having silicon provided on at least a portion of the interior and surface of the porous carbon-based particle; or a porous silicon-based particle and a particle having carbon provided on at least a portion of the interior and surface of the porous silicon-based particle.

[0047] According to one embodiment, the silicon-carbon composite comprises porous carbon particles and silicon provided on at least a portion of the interior and surface of the porous carbon particles. The silicon may be formed by depositing silicon onto the porous carbon particles using a silane gas. If necessary, a carbon layer may be further formed on the surface of the silicon-carbon composite. Conductivity is imparted by the carbon layer, and the initial efficiency, lifespan characteristics, and battery capacity characteristics of the secondary battery may be improved. The total weight of the carbon layer may be included in an amount of 5% to 40% by weight based on 100% by weight of the total silicon-carbon composite particles. The carbon layer may comprise at least one of amorphous carbon and crystalline carbon.

[0048] According to one embodiment, the silicon-carbon composite may be a porous silicon particle and a particle having carbon provided on at least a portion of the interior and surface of the porous silicon particle. This can be formed by etching silicon oxide to form a silicon-based particle with a porous structure, such as a Si matrix, and then coating it with carbon. The carbon may be subject to the description of the carbon layer described above.

[0049] In one embodiment of the present invention, the average particle size (D) of the silicon-based active material is 50 ) may be 1 μm to 15 μm. For example, the average particle size (D) of the silicon-based active material. 50The average particle size of the silicon-based active material may be 1 μm or more, 1.2 μm or more, 1.3 μm or more, 1.5 μm or more, or 1.7 μm or more; 15 μm or less, 13 μm or less, 10 μm or less, 8 μm or less, or 5 μm or less; or 1.2 μm or more and 13 μm or less, 1.3 μm or more and 10 μm or less, or 1.5 μm or more and 8 μm or less. When the average particle size of the silicon-based active material satisfies the above range, particle cracking or collapse of the electrode structure due to volume expansion during the charging and discharging process is suppressed, thereby improving electrode stability.

[0050] In one embodiment of the present invention, the weight part (B) of the silicon-based active material included in the negative electrode active material layer may be 10 to 25 parts by weight based on 100 parts by weight of the negative electrode active material. Specifically, the weight part (B) of the silicon-based active material may be 10 parts by weight or more, 13 parts by weight or more, 15 parts by weight or more, or 16 parts by weight or more; may be 25 parts by weight or less, 23 parts by weight or less, 21 parts by weight or less, or 20 parts by weight or less; or may be 13 to 25 parts by weight, 13 to 20 parts by weight, or 16 to 20 parts by weight.

[0051] When the weight portion (B) of the silicon-based active material satisfies the above range, the electrode loading amount of the negative electrode is reduced, the thickness of the electrode is reduced, and thereby low resistance characteristics are achieved, improving rapid charge / discharge performance.

[0052] In one embodiment of the present invention, the negative electrode active material layer may additionally include a negative electrode binder in addition to the active material.

[0053] The above-mentioned cathode binder can serve to improve the adhesion between active material particles and the adhesion between the active material particles and the current collector. The above binder may be one known in the art, and non-limiting examples may include at least one selected from the group consisting of polyvinylidenefluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidenefluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogens thereof are substituted with Li, Na, or Ca, etc., and may also include various copolymers thereof.

[0054] The above-mentioned negative electrode active material layer may not include a conductive material, but may include a conductive material as needed. The conductive material included in the above-mentioned negative electrode active material layer is not particularly limited as long as it is conductive without causing chemical changes in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, ketjen black, channel black, Farnes black, lamp black, thermal black; conductive fibers such as carbon fibers or metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, aluminum, or nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; conductive materials such as polyphenylene derivatives may be used.

[0055] In one embodiment of the present invention, the thickness of the positive and negative active material layers is 10 μm or more and 500 μm or less, respectively. The thickness of the positive active material layer may be 90% to 110%, for example, 95% to 105%, of the thickness of the negative active material layer, and their thicknesses may be the same. Specifically, the thickness of the positive and negative active material layers may be 15 μm or more and 400 μm or less, 20 μm or more and 300 μm or less, 25 μm or more and 200 μm or less, or 30 μm or more and 100 μm or less, respectively.

[0056] In one embodiment of the present invention, the electrolyte may include fluoroethylene carbonate (FEC), and the weight part (A) of the fluoroethylene carbonate (FEC) may be included in an amount of 3 to 6.5 parts by weight based on 100 parts by weight of the electrolyte. Specifically, the weight part (A) of the fluoroethylene carbonate (FEC) may be 3 parts by weight or more, 3.5 parts by weight or more, or 4 parts by weight or more based on 100 parts by weight of the electrolyte, 6.5 parts by weight or less, 5.5 parts by weight or less, or 5 parts by weight or less, or 3 to 6.5 parts by weight, or 3 to 5.5 parts by weight, or 3.5 to 5 parts by weight.

[0057] In one embodiment of the present invention, the electrolyte comprises a solvent. The solvent may be used without limitation as long as it is commonly used in the art.

[0058] For example, the solvent may be a non-aqueous organic solvent. Specifically, the electrolyte may include one or more of a carbonate-based solvent and an ester-based solvent, and more specifically, may include one or more of a cyclic carbonate-based solvent, a linear carbonate-based solvent, and an ester-based solvent.

[0059] The above solvent may include a mixture comprising a cyclic carbonate-based solvent and a linear carbonate-based solvent, a mixture comprising a cyclic carbonate-based solvent and an ester-based solvent, or a cyclic carbonate-based solvent, a linear carbonate-based solvent, and an ester-based solvent.

[0060] The above-mentioned cyclic carbonate-based solvent may be included in an amount of 10 to 45 parts by weight per 100 parts by weight of the total solvent. For example, the above-mentioned cyclic carbonate-based solvent may be included in an amount of 10 parts by weight or more, 15 parts by weight or more, 20 parts by weight or more, or 30 parts by weight or more per 100 parts by weight of the total solvent, and may be included in an amount of 45 parts by weight or less, 40 parts by weight or less, 35 parts by weight or less, or 30 parts by weight or less, and may be 10 to 40 parts by weight, 15 to 35 parts by weight, or 20 to 30 parts by weight. When the above-mentioned cyclic carbonate-based solvent is included within the above range, the electrolyte can smoothly dissociate the lithium salt, thereby ensuring sufficient ion conductivity and cation transport capacity.

[0061] The above-mentioned cyclic carbonate-based solvent is a high-viscosity organic solvent, and when included in the solvent, the high dielectric constant of the cyclic carbonate-based solvent facilitates the dissociation of lithium salts within the electrolyte, thereby improving ion conductivity and cation transport capacity.

[0062] In one embodiment of the present invention, the cyclic carbonate-based solvent may 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.

[0063] The above linear carbonate-based solvent may be included in an amount of 55 to 90 parts by weight per 100 parts by weight of the total solvent. For example, the above linear carbonate-based solvent may be included in an amount of 55 parts by weight or more, 60 parts by weight or more, 65 parts by weight or more, or 70 parts by weight or more per 100 parts by weight of the total solvent; may be included in an amount of 90 parts by weight or less, 80 parts by weight or less, 70 parts by weight or less, or 60 parts by weight or less; or may be included in an amount of 60 to 90 parts by weight, 65 to 90 parts by weight, or 70 to 80 parts by weight. If the above linear carbonate-based solvent is not included within the above range per 100 parts by weight of the total solvent, the viscosity of the electrolyte may be excessively high, and the impregnation of the electrolyte may not be sufficiently ensured.

[0064] Accordingly, by including a cyclic carbonate-based solvent having a high dielectric constant and high viscosity and a linear carbonate-based solvent having a relatively low dielectric constant and relatively low viscosity in an appropriate ratio, the above electrolyte can sufficiently dissociate the lithium salt to provide an electrolyte having excellent ionic conductivity and excellent impregnation properties.

[0065] The above linear carbonate-based solvent may 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 ethyl propyl carbonate.

[0066] The above ester-based solvent may be included in an amount of 1 to 70 parts by weight per 100 parts by weight of the total solvent. For example, the above ester-based solvent may be included in an amount of 1 part by weight or more, 5 parts by weight or more, 10 parts by weight or more, or 20 parts by weight or more per 100 parts by weight of the total solvent; may be included in an amount of 70 parts by weight or less, 60 parts by weight or less, 50 parts by weight or less, or 40 parts by weight or less; or may be included in an amount of 5 to 60 parts by weight, 10 to 50 parts by weight, or 10 to 40 parts by weight. When the above ester-based solvent is included within the above ranges per 100 parts by weight of the total solvent, the impregnation properties of the electrolyte can be improved and low-temperature characteristics can be enhanced.

[0067] The above ester-based solvent is an organic solvent having low density, low viscosity, and a low melting point. When included in the solvent, it can prevent the viscosity of the electrolyte from becoming excessively high and improve low-temperature characteristics.

[0068] The above ester-based solvent may be at least one selected from the group consisting of methyl acetate (MA), ethyl acetate (EA), dimethyl acetate, methyl propionate (MP), ethyl propionate (EP), propyl propionate (PP), decanolide, valerolactone, mevalonolactone, and caprolactone.

[0069] The above ester-based solvent may be a linear ester-based solvent. Specifically, the above ester-based solvent may be at least one selected from the group consisting of methyl acetate, ethyl acetate (EA), dimethyl acetate, methyl propionate (MP), ethyl propionate (EP), and propyl propionate (PP).

[0070] In one embodiment of the present invention, the electrolyte may further include a lithium salt.

[0071] In one embodiment of the present invention, the lithium salt included in the electrolyte may be used without special limitations as long as it is a compound capable of providing lithium ions used in a lithium secondary battery. For example, the lithium salts may be independently the same or different, and may include LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, LiB(C2O4)2, LiFSI, LiTFSI, LiBF4, LiDFOB, or combinations thereof. Specifically, the lithium salt may be LiPF6 or LiFSI.

[0072] In one embodiment of the present invention, the concentration of the lithium salt included in the electrolyte may be 0.8 M to 2 M. For example, the concentration of the lithium salt may be 1 M or more, 1.1 M or more, or 1.2 M or more, 2 M or less, 1.8 M or less, 1.6 M or less, or 1.5 M or less, and may be 1 M to 2 M, 1.2 M to 1.8 M, or 1.2 M to 1.6 M.

[0073] If the concentration of the lithium salt is below the above range, the ionic conductivity in the electrolyte decreases, and it is difficult to form a stable solid electrolyte interface (SEI). On the other hand, if the concentration of the lithium salt exceeds the above range, the impregnation of the electrolyte decreases due to increased viscosity, and the decrease in ion mobility may cause an increase in resistance.

[0074] In one embodiment of the present invention, the electrolyte may further include an electrolyte additive. The electrolyte additive is a compound that facilitates the formation of an SEI film by electrochemical oxidation or reduction decomposition reaction, and may be at least one selected from the group consisting of cyclic carbonate compounds, halogen-substituted carbonate compounds, sulfone compounds, sulfate compounds, borate compounds, nitrile compounds, benzene compounds, amine compounds, silane compounds, and lithium salt compounds different from the lithium salt included in the electrolyte.

[0075] The above electrolyte additive can be selected depending on the type of lithium salt or solvent of the electrolyte. Specifically, the electrolyte additive comprises vinylene carbonate (VC), vinylethylene carbonate (VEC), methyl prop-2-in-1-yl carbonate, succinic anhydride (SA), propylphosphonic anhydride (T3P), 1,3-propane sulfate (PS), 1,4-butane sulfate, ethyl sulfate, 1,3-propene sulfate (PRS), 1,4-butene sulfate, 1-methyl-1,3-propene sulfate, ethylene sulfate, trimethylene sulfate (TMS), methyl trimethylene sulfate (MTMS), tetraphenyl borate, lithium oxalyl difluoroborate, succinonitrile, adiponitrile, acetonitrile, propionitrile, butyronitrile, and valeronitrile. It may be one or more selected from the group consisting of caprylonitrile, heptanitrile, cyclopentane carbonitrile, cyclohexane carbonitrile, 2-fluorobenzonitrile, 4-fluorobenzonitrile, difluorobenzonitrile, trifluorobenzonitrile, phenylacetonitrile, 2-fluorophenylacetonitrile, 4-fluoroacetonitrile, fluorobenzene, triethanolamine, ethylenediamine, tetravinylsilane, LiN(SO2F)2 (lithium bis(trifluoromethane sulfonyl)imide, LiTFSI), LiPO2F2, LiODFB, LiBOB, and LiBF4.

[0076] Specifically, the electrolyte additive may include one or more of vinylene carbonate, vinylethylene carbonate, methylprop-2-phosphorus-1-yl carbonate, succinic anhydride, 1-propene 1,3-sulfone, 1,3-propane sulfone, succinonitrile, and LiPO2F2.

[0077] The above electrolyte additive may be included in an amount of 3 to 6 parts by weight based on 100 parts by weight of the electrolyte. Specifically, it may be included in an amount of 3 parts by weight or more, 3.3 parts by weight or more, or 3.5 parts by weight or more; 6 parts by weight or less, 5.5 parts by weight or less, or 5 parts by weight or less; or 3 to 6 parts by weight, 3 to 5.5 parts by weight, or 3.3 parts by weight or more and 5 parts by weight. When the content of the additive satisfies the above range, the effects of improving low-temperature output, high-temperature storage characteristics, and high-temperature life characteristics of the battery are exhibited, and since the amount of SEI film-forming agent is not excessive and it decomposes sufficiently at high temperatures, the SEI film-forming agent does not exist as unreacted material or precipitate at room temperature, and accordingly, side reactions in the electrolyte during battery charging and discharging can be prevented.

[0078] In addition to the components of the above electrolyte, for the purpose of improving the lifespan characteristics of the battery, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, the above electrolyte may further include one or more additives such as, for example, a haloalkylene carbonate-based compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, triamide hexaphosphate, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, aluminum trichloride, lithium salt for corrosion prevention, or HF-scavenger.

[0079] In one embodiment of the present invention, the lithium secondary battery includes a positive electrode and a negative electrode, and may further include a separator.

[0080] The above separator separates the negative electrode and the positive electrode and provides a pathway for the movement of lithium ions. Any separator commonly used in secondary batteries can be used without special restrictions, and it is particularly desirable that it has low resistance to the movement of electrolyte ions and excellent electrolyte wettability. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer like ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber, may be used. Furthermore, a coated separator containing ceramic components or polymer materials may be used to ensure heat resistance or mechanical strength, and it may optionally be used in a single-layer or multi-layer structure.

[0081] According to another embodiment of the present invention, a battery module comprising the secondary battery as a unit cell and a battery pack comprising the same are provided. Since the battery module and the battery pack include the secondary battery having high capacity, high rate capability and cycle capability, they can be used as a power source for medium-to-large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles and power storage systems.

[0082] Hereinafter, preferred embodiments are presented to aid in understanding the present invention; however, the above embodiments are merely illustrative of the description, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and spirit of the description, and that such variations and modifications fall within the scope of the appended claims.

[0083]

[0084] <Preparation Example>

[0085] Example 1

[0086] 1) Preparation of electrolyte

[0087] LiPF6 and LiFSI were added as lithium salts at 1.0 M and 0.25 M, respectively, to an organic solvent mixed with ethylene carbonate (EC), ethylmethyl carbonate (EMC), dimethyl carbonate (DMC), and methyl acetate (MA) in a volume ratio of 20:5:65:10. In addition, fluoroethyl carbonate (FEC) was added to the electrolyte at 5 wt%, and vinylene carbonate (VC), methylprop-2-in-1-yl carbonate, succinonitrile (SN), LiPO2F2, 1-propene-1,3-sulfone (PRS), succinic anhydride (SA), and vinylethylene carbonate (VEC) were added as electrolyte additives at 3 wt%, 0.3 wt%, 0.2 wt%, 0.3 wt%, 0.3 wt%, 0.25 wt%, and 0.3 wt%, respectively.

[0088]

[0089] 2) Manufacturing of lithium secondary batteries

[0090] LiNi as a positive electrode active material 0.92 Co 0.06 Mn 0.02 An anode active material layer composition was prepared using O2 (average particle size (D50): 15 μm), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive material, and polyvinylidene fluoride (PVdF) as a binder in a weight ratio of 97:1.5:1.5. The above anode active material layer composition was added to N-methyl-2-pyrrolidone (NMP) as a solvent for forming the anode slurry to prepare an anode slurry with a solid content of 78 wt%. The above anode slurry was applied at a rate of 537 mg / 25 cm² to both sides of an aluminum current collector (thickness: 12 μm) used as the anode current collector. 2 An anode was manufactured by coating with a loading amount, rolling, and drying in a vacuum oven at 130°C for 10 hours to form an anode active material layer with a thickness of 65 μm (porosity 26%).

[0091] A negative electrode active material layer composition was prepared using a silicon carbon composite, artificial graphite, and natural graphite as negative electrode active materials (with a weight ratio of 20:40:40 for the silicon carbon composite, artificial graphite, and natural graphite), single-walled carbon nanotubes (SWCNTs) as conductive materials, and styrene butadiene rubber (SBR) and carboxymethylcellulose (CMC) as binders in a weight ratio of 98.07:1.8:0.13 (active material: binder: conductive material). The above negative electrode active material layer composition was added to distilled water as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry with a solid content of 25 wt%. The above negative electrode slurry was applied at a rate of 85 mg / 25 cm² to both sides of a copper current collector (thickness: 8 μm) as a negative electrode current collector. 2 A cathode was manufactured by coating with a loading amount, rolling, and drying in a vacuum oven at 130°C for 10 hours to form a cathode active material layer with a thickness of 33 μm (porosity 55%).

[0092] An electrode assembly was prepared by interposing a polyethylene (PE) separator between the anode and the cathode.

[0093]

[0094] Examples 2, 3 and Comparative Examples 1 to 4

[0095] A secondary battery was manufactured in the same manner as in Example 1, except that the composition of the electrolyte was changed as shown in Table 1 below.

[0096]

[0097] <Experimental Example>

[0098] The following items were evaluated for Examples 1 to 3 and Comparative Examples 1 to 4 prepared above.

[0099] Experimental Example 1: Evaluation of Initial Efficiency, Lifetime (Capacity Retention Rate), CC Capacity, and Discharge Capacity Characteristics

[0100] Charge and discharge were performed on the secondary batteries prepared in the examples and comparative examples, and the discharge capacity, initial efficiency, and capacity retention rate were evaluated and listed in Table 2 below.

[0101] The 1st and 2nd cycles were charged / discharged at 0.2C, and from the 3rd cycle to the 103rd cycle, the charging / discharging was performed at 1C.

[0102] Charging conditions: CC (constant current) / CV (constant voltage) (5mV / 0.005C current cut-off)

[0103] Discharge condition: CC (constant current) condition 1.5V

[0104] The 0.2C initial performance (%) was derived from the results of one charge / discharge cycle. Specifically, the 0.2C initial performance (%) was derived by the following calculation.

[0105] 0.2C Initial Performance (%) = 0.2C Discharge Capacity / Design Capacity X 100

[0106] The capacity retention rate and DCIR growth rate were derived by the following calculations after performing 1C / 1C charge / discharge cycles at 24℃ and 40℃, respectively.

[0107] Capacity Retention Rate (RT) (%) = (Discharge capacity after 100 cycles / Discharge capacity after 1 cycle) X 100

[0108] DCIR Growth Rate (%) = {(DC Internal Resistance after 100 cycles - DC Internal Resistance after 1 cycle) / DC Internal Resistance after 1 cycle} X 100 (Based on SOC 50%)

[0109] In addition, to measure performance at high rate speeds, charge and discharge cycles were performed at 3C, and the CC capacity and discharge capacity were measured.

[0110] CC Capacity (%) = Charging Capacity Value in 3C Constant Current Range / Design Capacity X 100

[0111] Discharge Capacity (%) = 3C Discharge Capacity / Design Capacity X 100

[0112] Si / C Content FEC Weight Part B / A 0.2C 1C / 1C @ 24℃ 1C / 1C @ 40℃ 3C-rate 3D-rate Initial Performance RTDC IR RTDC IRCC Capacity Discharge Capacity Example 1 205 49 4.1% 61.1% 384% 66.2% 180% 50.8% 66.3% Example 2 203 6.67 9 3.1% 52.1% 480% 63.4% 195% 50.5% 66.2% Example 3 235 4.69 5.0% 61.8% 370% 67.1% 181% 51.0% 66.2% Comparative Example 1 200 - 9 3.4% 42.3% 533% 57.6% 215% 45.8% 57.7% Comparative Example 2207.52.6793.8%58.7%423%47.1%192%50.0%64.2%Comparative Example 32010293.4%55.1%451%37.6%218%49.6%64.3%Comparative Example 4205.53.6493.8%58.6%422%48.0%193%49.8%64.5%

[0113]

[0114] According to Table 1 above, Examples 1 to 3, in which the ratio of the weight of the silicon-based active material of the present invention to the weight of fluoroethylene carbonate (FEC) is 4 to 7, were found to be superior in terms of capacity and capacity retention rate compared to Comparative Examples 1 to 4. On the other hand, Comparative Example 1 did not contain fluoroethylene carbonate (FEC) in the electrolyte, so a film was not properly formed on the surface of the silicon-based negative electrode active material, resulting in the detachment of organic matter and the failure of the SEI film to form smoothly, leading to the decomposition of the electrolyte and the generation of gas. Consequently, it exhibited inferior performance in terms of capacity retention rate, resistance increase rate, and capacity.

[0115] Comparative Examples 2 to 4 are cases where the electrolyte contains an excess amount of fluoroethylene carbonate (FEC). Although an SEI film is properly formed on the surface of the cathode, the excess FEC acts as a catalyst that decomposes the electrolyte and generates gas through thermal and electrochemical decomposition, resulting in inferior performance in terms of capacity retention rate, resistance increase rate, and capacity.

Claims

1. A positive electrode; a negative electrode; a separator between the positive electrode and the negative electrode; and an electrolyte, comprising The above cathode includes a cathode active material layer, and The above negative electrode active material layer includes a silicon-based active material, and The above electrolyte contains fluoroethylene carbonate (FEC), and A lithium secondary battery in which the ratio (B / A) of the weight part (B) of the silicon-based active material included in the negative electrode active material layer to the weight part (A) of the fluoroethylene carbonate (FEC) included in the negative electrode active material layer is 4 to 7.

2. In Claim 1, The above silicon-based active material is a silicon-carbon composite lithium secondary battery.

3. In Claim 1, A lithium secondary battery in which the weight part (B) of the silicon-based active material included in the negative electrode active material layer is 10 to 25 parts by weight based on 100 parts by weight of the negative electrode active material.

4. In Claim 1, A lithium secondary battery in which the weight part (A) of the fluoroethylene carbonate (FEC) is 3 to 6.5 parts by weight based on 100 parts by weight of the electrolyte.

5. In Claim 1, The above electrolyte is a lithium secondary battery comprising one or more of a carbonate-based solvent and an ester-based solvent.

6. In Claim 1, The above electrolyte is a lithium secondary battery further comprising a lithium salt.

7. In Claim 1, The above electrolyte further includes an electrolyte additive, and A lithium secondary battery comprising one or more of the above electrolyte additives, vinylethylene carbonate, methylprop-2-phosphorus-1-yl carbonate, succinic anhydride, 1-propene 1,3-sulfone, 1,3-propane sulfone, succinonitrile, and LiPO2F2.

8. In Claim 7, A lithium secondary battery comprising the above electrolyte additive in an amount of 3 to 6 parts by weight based on 100 parts by weight of the above electrolyte.

9. A battery module comprising a lithium secondary battery of any one of claims 1 to 8.

10. A battery pack comprising a lithium secondary battery of any one of claims 1 to 8.

11. A battery pack comprising a battery module according to claim 9.