Lithium secondary battery

Abstract

The present application relates to lithium secondary batteries.

Description

[Technical Field]

[0001] This application claims the benefit of the filing date of Korean Patent Application No. 10-2022-0187433, filed with the Korean Intellectual Property Office on December 28, 2022, the entire contents of which are incorporated herein by reference.

[0002] The present application relates to lithium secondary batteries. [Background technology]

[0003] The rapid increase in the use of fossil fuels has led to an increasing demand for alternative and clean energy, and one of the most actively researched fields in this area is power generation and storage using electrochemical reactions.

[0004] Currently, a typical example of an electrochemical element that uses electrochemical energy is a secondary battery, and the range of its use is expanding.

[0005] With the development of mobile device technologies and the increase in demand, the demand for secondary batteries as energy sources is rapidly increasing. Among such secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and are widely used. In addition, research into methods for manufacturing high-density electrodes with higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries is actively being conducted.

[0006] Generally, secondary batteries consist of a positive electrode, a negative electrode, an electrolyte, and a separator. The negative electrode contains an active material that inserts and extracts lithium ions from the positive electrode. Carbon-based materials such as graphite are stable and reversible, but they have limitations in terms of capacity. In areas where high capacity is desired, there are increasing attempts to use silicon-based materials, which have a high theoretical capacity, as the negative electrode material.

[0007] Lithium secondary batteries typically use lithium-intercalated compounds, such as LiCoO2 and LiMn2O4, in the positive electrode and non-lithium-intercalated materials, such as carbonaceous and silicon-based materials, in the negative electrode. During charging, lithium ions intercalated in the positive electrode migrate to the negative electrode via the electrolyte, and during discharge, lithium ions migrate from the negative electrode to the positive electrode again. During charging, the lithium ions migrate from the positive electrode to the negative electrode react with the electrolyte to form a passivation film, known as a solid electrolyte interface (SEI), on the surface of the negative electrode. This SEI stabilizes the structure of the negative electrode by inhibiting the transfer of electrons required for the reaction between the negative electrode and the electrolyte and preventing electrolyte decomposition. However, because it is an irreversible reaction, it results in the loss of lithium ions. The lithium consumed in the formation of the SEI cannot return to the positive electrode during the subsequent discharge process, reducing the battery capacity.

[0008] Meanwhile, efforts to improve the performance of lithium secondary batteries have led to the development of technologies to increase the charging speed. Rapid charging of lithium secondary batteries requires a high lithium ion migration rate during the process of lithium ion insertion into the anode. To address this issue, batteries are being designed to reduce internal resistance and achieve high output by forming a thin-film-like anode active material layer to reduce the lithium diffusion distance and forming a carbon coating layer on the surface to increase conductivity.

[0009] However, it is difficult to achieve high capacity with a negative electrode having such a thin film negative electrode active material layer. Furthermore, it is difficult to achieve fast charging with a high-capacity negative electrode. Furthermore, when using a silicon-based negative electrode, the Si content of the negative electrode is increased to increase energy density. This reduces the efficiency of the high-Ni positive electrode, so excessive sacrificial positive electrode material is used to balance the negative electrode. However, this increases the amount of gas in the battery cell itself, resulting in stability issues.

[0010] Generally, the use of fluorocarbon carbonate compounds as solvents in the electrolyte of lithium secondary batteries is limited. That is, fluorocarbon carbonate compounds are included to ensure the ionic conductivity of the electrolyte. However, if used in excess, problems arise, such as increased gas generation due to increased Lewis acid generation. Therefore, in the past, despite the advantages of fluorocarbon carbonate compounds, the content of these compounds was reduced, and to compensate for the resulting ionic conductivity, a material with a high dielectric constant, such as PC, was used together. However, even in this case, the problem of increased gas generation at high temperatures was not resolved.

[0011] Therefore, research is needed to maintain cell resistance, improve high-temperature cycle performance, and reduce gas generation during storage while using a negative electrode containing silicon-based active materials for high capacity. [Prior art documents] [Patent documents]

[0012] [Patent Document 1] Japanese Patent Application Laid-Open No. 2009-080971 Summary of the Invention [Problem to be solved by the invention]

[0013] As a result of research into the above-mentioned problems, we have found a method that can ensure capacity characteristics and fast charging performance by using a silicon-based anode, while maintaining cell resistance, improving high-temperature cycle performance, and reducing gas generation during storage by adjusting the electrolyte composition and content. That is, we have found that the electrolyte is the main factor in gas generation during long-term storage at high temperatures, and that by adjusting the electrolyte composition and content, it is possible to reduce gas generation without affecting the performance of the lithium secondary battery.

[0014] Therefore, the present application relates to a lithium secondary battery including a silicon-based negative electrode; and an electrolyte containing a specific additive.

Means for Solving the Problems

[0015] One embodiment of the present specification is a lithium secondary battery including a positive electrode; a silicon-based negative electrode; a separator provided between the positive electrode and the negative electrode; and an electrolyte, wherein the silicon-based negative electrode includes a negative electrode current collector layer; and a negative electrode active material layer provided on one or both surfaces of the negative electrode current collector layer, the negative electrode active material layer includes a negative electrode active material layer composition, the negative electrode active material layer composition includes a silicon-based active material, the silicon-based active material includes one or more selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloys, the electrolyte includes a solvent, a lithium salt, a fluorine group-containing additive, and vinylene carbonate (VC), based on 100 parts by weight of the electrolyte, vinylene carbonate is included in an amount of 0.3 parts by weight or less, based on 100 parts by weight of the solvent, a carbonate compound containing a fluorine group is included in an amount of 10 parts by weight or more, and the fluorine group-containing additive is included in an amount of 2 parts by weight or less based on 100 parts by weight of the electrolyte, and provides a lithium secondary battery.

Advantages of the Invention

[0016] The lithium secondary battery according to the present application can ensure a battery with high capacity and high energy density by using a silicon-based negative electrode, and by using a silicon-based active material with excellent capacity characteristics, the negative electrode thickness is formed at a thin film level, and rapid charging performance can be ensured.

[0017] In particular, the lithium secondary battery according to the present application is characterized by including a solvent, a lithium salt, a fluorine group-containing additive, and vinylene carbonate (VC) as the electrolyte.

[0018] More specifically, the composition contains 10 parts by weight or more of a carbonate compound containing a fluoro group per 100 parts by weight of the solvent, which ensures ionic conductivity. However, the use of such a solvent increases the amount of gas generated. Therefore, the composition contains the fluoro group-containing additive that forms an F-derived coating in the amount specified by the weight part, and the content of vinylene carbonate, which has a low thin film-forming effect on the Si-based active material layer, is reduced to the above-specified range, thereby controlling the amount of gas generated.

[0019] That is, vinylene carbonate remains on the silicon-based negative electrode without forming an effective coating, which causes a large amount of gas to be generated due to an oxidation reaction at the positive electrode. Therefore, the content of vinylene carbonate is adjusted and an additive that forms an effective coating on the silicon-based negative electrode is included.

[0020] As a result, the lithium secondary battery according to the present application has the characteristics that the amount of gas generated is reduced even when stored at high temperatures for a long period of time, thereby ensuring performance and ionic conductivity. [Brief explanation of the drawings]

[0021] [Figure 1] 1 is a diagram showing a lithium secondary battery according to the present application. DETAILED DESCRIPTION OF THE INVENTION

[0022] Before describing the present invention, some terms will first be defined.

[0023] In this specification, when a part is said to "comprise" a certain component, this does not mean that it excludes other components, but that it may further include other components, unless otherwise specified.

[0024] In this specification, "p to q" means "at least p and at most q."

[0025] In this specification, the "specific surface area" is measured by the BET method, and specifically, is calculated from the amount of nitrogen gas adsorption at liquid nitrogen temperature (77 K) using a BELSORP-mini II manufactured by BEL Japan Co., Ltd. That is, in the present application, the BET specific surface area can mean the specific surface area measured by the above-mentioned measurement method.

[0026] In this specification, "Dn" refers to particle size distribution, and refers to the particle size at the n% point in the cumulative particle number distribution by particle size. That is, D50 is the particle size (average particle size, median particle size) at the 50% point in the cumulative particle number distribution by particle size, D90 is the particle size at the 90% point in the cumulative particle number distribution by particle size, and D10 is the particle size at the 10% point in the cumulative particle number distribution by particle size. Meanwhile, particle size distribution may also be measured using the laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). When the particles pass through a laser beam, the difference in diffraction pattern due to particle size is measured to calculate the particle size distribution.

[0027] As used herein, when a polymer contains a certain monomer as a monomer unit, it means that the monomer participates in a polymerization reaction and is included as a repeating unit in the polymer. As used herein, when a polymer contains a monomer, this is interpreted as the same as when a polymer contains a monomer as a monomer unit.

[0028] In this specification, the term "polymer" is understood to be used in a broad sense, including copolymers, unless otherwise specified as a "homopolymer."

[0029] In this specification, the weight-average molecular weight (Mw) and the number-average molecular weight (Mn) are the polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using monodisperse polystyrene polymers (standard samples) of various degrees of polymerization commercially available for molecular weight measurement as the standard substance. In this specification, the molecular weight means the weight-average molecular weight unless otherwise specified.

[0030] [[ID=​​​​​​​​​​​​Therefore, the present invention can ensure ionic conductivity by including 10 parts by weight or more of a carbonate compound containing a fluoro group per 100 parts by weight of the solvent. However, using such a solvent increases the amount of gas generated. Therefore, the present invention is characterized in that the amount of gas generated is controlled by including the fluoro group-containing additive that forms an F-derived coating in the above-mentioned parts by weight and reducing the content of vinylene carbonate, which has a low thin film-forming effect on the Si-based active material layer, to the above-mentioned range.

[0034] 1 is a diagram illustrating a stacked structure of a lithium secondary battery according to one embodiment of the present application. Specifically, an anode 100 including an anode active material layer 20 on one side of an anode current collector layer 10 can be seen, and a cathode 200 including a cathode active material layer 40 on one side of a cathode current collector layer 50 can be seen, and the anode 100 and the lithium secondary battery cathode 200 are stacked with a separator 30 sandwiched between them.

[0035] The positive electrode, negative electrode, electrolyte, and separator contained in the lithium secondary battery will be described below.

[0036] In one embodiment of the present application, the electrolyte may include a solvent, a lithium salt, a fluoro-group-containing additive, and vinylene carbonate (VC).

[0037] The lithium secondary battery according to the present application has an electrolyte composition that can reduce the amount of gas generated even during high-temperature storage.

[0038] In one embodiment of the present application, the solvent may be, for example, an aprotic organic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, phosphoric acid triester, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ether, methyl propionate, or ethyl propionate.

[0039] In the present application, there is provided a lithium secondary battery, wherein the solvent is a mixed solvent containing two carbonate-based compounds.

[0040] In particular, among the carbonate-based mixed solvents, cyclic carbonates such as ethylene carbonate and propylene carbonate are preferred because they are high-viscosity organic solvents with high dielectric constants and good dissociation of lithium salts. Furthermore, when such cyclic carbonates are mixed with linear carbonates such as dimethyl carbonate and diethyl carbonate, which have low viscosity and low dielectric constants, in an appropriate ratio, an electrolyte having high electrical conductivity can be produced, and thus these cyclic carbonates are more preferred.

[0041] In particular, in one embodiment of the present application, the solvent contains 10 parts by weight or more of a carbonate compound containing a fluoro group, based on 100 parts by weight of the solvent.

[0042] In another embodiment, the solvent may contain 10 parts by weight or more, preferably 12 parts by weight or more, more preferably 15 parts by weight or more of a carbonate compound containing a fluoro group, based on 100 parts by weight of the solvent, and 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less.

[0043] Generally, fluorocarbon carbonate compounds are used as solvents in the electrolyte of lithium secondary batteries. Although fluorocarbon carbonate compounds are used to ensure ionic conductivity in the electrolyte, excessive use of these compounds can increase the amount of Lewis acid produced, resulting in increased gas generation. Therefore, despite the advantages of fluorocarbon carbonate compounds, their content has traditionally been reduced, and a material with a high dielectric constant, such as PC, has been used to compensate for the resulting ionic conductivity. However, this approach has not resolved the problem of gas generation at high temperatures.

[0044] In the case of the electrolyte according to the present application, the ionic conductivity can be ensured by including the carbonate compound containing a fluoro group as a solvent in the above-mentioned amount by weight, and the problem of gas generation caused by this can be solved by adjusting the content of vinylene carbonate described below and including a specific amount of a fluoro group-containing additive.

[0045] In one embodiment of the present application, there is provided a lithium secondary battery comprising the fluoro group-containing additive in an amount of 2 parts by weight or less based on 100 parts by weight of the electrolyte.

[0046] In another embodiment, the fluoro group-containing additive may be contained in an amount of 2 parts by weight or less, preferably 1.8 parts by weight or less, more preferably 1.5 parts by weight or less, or may be contained in an amount of 0.5 parts by weight or more, or 1 part by weight or more, based on 100 parts by weight of the electrolyte.

[0047] When the fluoro group-containing additive is included in the electrolyte, it functions to form an F-derived coating on the silicon-based anode, thereby solving the problem of gas generation even when the fluoro group-containing carbonate compound is included in the above-mentioned amount by weight.

[0048] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the fluoro group-containing additive is LiCF3COO or LiDFP.

[0049] The LiDFP may mean lithium difluorophosphate.

[0050] As mentioned above, the fluoro-group-containing additive undergoes reductive decomposition to form a stable inorganic coating component (LiF, LiO, etc.) on the silicon-based anode. Furthermore, the incorporation of the fluoro-group-containing additive allows the formation of an F-rich coating, which is beneficial for the silicon-based anode, even when the FEC content is limited (approximately 10%) as the electrolyte solvent, resulting in advantageous room-temperature performance.

[0051] The more FEC used as an electrolyte solvent (FEC 15% or more), the better its room temperature performance. However, excessive use is not possible due to gas generation at high temperatures. Therefore, even when FEC is limited to about 10%, adding a Li salt additive can form an F-rich coating, which replaces FEC and forms a coating that is advantageous for silicon-based anodes, maintaining room temperature performance.

[0052] In one embodiment of the present application, the electrolyte may contain vinylene carbonate in an amount of 0.3 parts by weight or less based on 100 parts by weight of the electrolyte.

[0053] In another embodiment, the electrolyte may contain 0.3 parts by weight or less, preferably 0.2 parts by weight or less, or 0 part by weight or more, or 0.1 part by weight or more, based on 100 parts by weight of the electrolyte.

[0054] When vinylene carbonate is applied to a carbon-based negative electrode, it has an excellent effect of forming a film on the negative electrode, and no unreacted vinylene carbonate remains in the electrolyte, resulting in no problems with gas generation. However, when applied to a silicon-based negative electrode, vinylene carbonate has only a small effect of forming a film on the negative electrode, and the unreacted vinylene carbonate oxidizes at the positive electrode during charging, causing a sudden increase in gas generation.

[0055] Therefore, the electrolyte of the lithium secondary battery including the silicon-based negative electrode according to the present application, which maximizes capacity characteristics, is characterized by including the vinylene carbonate in the above-mentioned amount by weight. When including the above-mentioned amount by weight, the electrolyte can appropriately form a coating on the negative electrode surface together with the additive containing a fluoro group, thereby reducing gas generation and preventing unreacted VC from remaining in the battery.

[0056] In the present application, the electrolyte has a viscosity of 1.1 g / cm at 25°C. 3 The present invention provides a lithium secondary battery having an ionic conductivity of 5.5 mS / cm or more.

[0057] In the present application, the electrolyte has a viscosity of 1.1 g / cm at 25°C. 3 Preferably, the viscosity is 1.11 g / cm or more at 25°C. 3 or more, and the viscosity at 25°C is 1.12 g / cm 3 or more, and the viscosity at 25°C is 2.0 g / cm 3 Preferably, the viscosity is 1.3 g / cm or less at 25°C. 3 More preferably, the viscosity is 1.25 g / cm or less at 25°C. 3 It may be the following:

[0058] In the present application, the electrolyte may have an ionic conductivity of 5.5 mS / cm or more, preferably 5.6 mS / cm or more, more preferably 5.7 mS / cm or more, at 25°C, or may have an ionic conductivity of 6.5 mS / cm or less, specifically 6.3 mS / cm or less, and more specifically 6.0 mS / cm or less.

[0059] In the case of the electrolyte according to the present application, by including a carbonate compound containing a fluoro group as a solvent in the aforementioned parts by weight, it is possible to ensure ionic conductivity within the aforementioned range, and the resulting problem of gas generation is resolved by adjusting the vinylene carbonate content and including a specific content of a fluoro group-containing additive, thereby resulting in an electrolyte with appropriate viscosity and ionic conductivity. As an example, from the perspective of optimizing the Li ion migration rate of High Ni NCM and silicon-based anodes, the electrolyte of the present invention itself is characterized by ensuring the aforementioned viscosity and ionic conductivity as physical properties that it should have.

[0060] In one embodiment of the present application, the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte, and the lithium salt is (CF3)2PF4 - , (CF3)3PF3 - , (CF3)4PF2 - , (CF3)5PF - , (CF3)6P - , CF3SO3 - , CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - , CF3CF2(CF3)2CO - , (CF3SO2)2CH - , (SF5)3C - , (CF3SO2)3C - , CF3(CF2)7SO3 - , CF3CO2 - , CH3CO2 - , SCN - and (CF3CF2SO2)2N - The anion comprises two or more anions selected from the group consisting of:

[0061] In one embodiment of the present application, the lithium salt may include LiPF6 and LiFSI.

[0062] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery.

[0063] The silicon-based negative electrode according to the present application may include a negative electrode current collector layer; and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer.

[0064] The negative electrode current collector layer typically has a thickness of 1 μm to 100 μm. Such a negative electrode current collector layer is not particularly limited as long as it has high conductivity without inducing chemical changes in the battery, and examples thereof include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like, and aluminum-cadmium alloys. Furthermore, the surface may be provided with fine irregularities to strengthen the binding force of the negative electrode active material, and the layer may be used in various forms such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0065] However, the thickness may vary depending on the type and application of the negative electrode used, and is not limited thereto.

[0066] In one embodiment of the present application, the negative electrode active material layer contains a negative electrode active material layer composition, and the negative electrode active material layer composition may contain a silicon-based active material.

[0067] In one embodiment of the present application, the negative electrode active material layer contains a negative electrode active material layer composition, and the negative electrode active material layer composition may contain one or more selected from the group consisting of a silicon-based active material, a negative electrode conductive material, and a negative electrode binder.

[0068] In one embodiment of the present application, the negative electrode active material layer contains a negative electrode active material layer composition, and the negative electrode active material layer composition may contain a silicon-based active material, a negative electrode conductive material, and a negative electrode binder.

[0069] In one embodiment of the present application, the silicon-based active material may contain one or more selected from the group consisting of SiOx (x = 0), SiOx (0 <x <), SiC, and Si alloys.

[0070] In one embodiment of the present application, the silicon-based active material contains one or more selected from the group consisting of SiOx (x = 0), SiOx (0 <x <2), and metal impurities, and based on 100 parts by weight of the silicon-based active material, the SiOx (x = 0) may be contained in an amount of 70 parts by weight or more.

[0071] In another embodiment, based on 100 parts by weight of the silicon-based active material, the SiOx (x = 0) may be contained in an amount of 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, and may be contained in an amount of 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0072] In one embodiment of the present application, particularly pure silicon (Si) may be used as the silicon-based active material. Using pure silicon (Si) as the silicon-based active material means that, as described above, when based on 100 parts by weight of the entire silicon-based active material, pure Si particles (SiOx (x = 0)) not bonded to other particles or elements are contained within the above range.

[0073] The average particle size (D50) of the silicon-based active material of the present invention may be 3 μm to 15 μm, specifically 4 μm to 13 μm, and more specifically 4.5 μm to 11 μm. If the average particle size is less than 3 μm, the specific surface area of ​​the particles increases excessively, resulting in an excessive increase in the viscosity of the negative electrode slurry. As a result, the particles constituting the negative electrode slurry are not dispersed smoothly. If the size of the silicon-based active material is too small, the contact area between the silicon particles and the conductive material is reduced by the composite of the conductive material and the binder in the negative electrode slurry, increasing the possibility of the conductive network being interrupted and resulting in a decrease in capacity retention. If the average particle size exceeds 10 μm, the presence of excessively large silicon particles results in an uneven negative electrode surface, resulting in non-uniform current density during charge and discharge. If the silicon particles are too large, the phase stability of the negative electrode slurry becomes unstable, degrading processability. This results in a decrease in the capacity retention of the battery.

[0074] In one embodiment of the present application, the silicon-based active material usually has a characteristic BET specific surface area. The BET specific surface area of ​​the silicon-based active material is preferably 0.01 m 2 / g~150.0m 2 / g, more preferably 0.1m 2 / g~100.0m 2 / g, particularly preferably 0.2m 2 / g~80.0m 2 / g, most preferably 0.2m 2 / g~18.0m 2 / g. The BET surface area is determined according to DIN 66131 (using nitrogen).

[0075] In one embodiment of the present application, the silicon-based active material may be, for example, in crystalline or amorphous form, and is preferably not porous. The silicon particles are preferably spherical or multi-platelet particles. Alternatively, the silicon particles may have a fibrous structure or may be in the form of a silicon-containing film or coating, but this is less preferred.

[0076] In one embodiment of the present application, the silicon-based active material may be included in an amount of 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

[0077] In yet another embodiment, the silicon-based active material may be included in an amount of 60 parts by weight or more, preferably 65 parts by weight or more, more preferably 70 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer composition, and may be included in an amount of 95 parts by weight or less, preferably 90 parts by weight or less, more preferably 80 parts by weight or less.

[0078] The negative electrode active material layer composition according to the present application uses a negative electrode conductor and a negative electrode binder that can control the volume expansion rate during charge and discharge, even when a silicon-based active material having a significantly high capacity is used within the above range, and is characterized by not deteriorating the performance of the negative electrode even when the silicon-based active material is contained within the above range, and by having excellent output characteristics during charge and discharge.

[0079] In one embodiment of the present application, the silicon-based active material may have a non-spherical morphology, and the sphericity thereof is, for example, 0.9 or less, for example, 0.7 to 0.9, for example, 0.8 to 0.9, for example, 0.85 to 0.9.

[0080] In this application, the circularity is determined by the following formula 1, where A is the area and P is the perimeter.

[0081] [Formula 1] 4πA / P 2

[0082] While graphite-based compounds have traditionally been used exclusively as anode active materials, attempts to incorporate silicon-based compounds into batteries to increase capacity have recently been increasing in response to growing demand for high-capacity batteries. However, silicon-based compounds have a limitation: their rapid volume expansion during charge / discharge processes can damage the conductive pathways formed within the anode active material layer, thereby reducing battery performance.

[0083] Therefore, in one embodiment of the present application, the negative electrode active material layer composition may include a negative electrode conductive material and a negative electrode binder. That is, the negative electrode conductive material serves to secure a conductive path, and the binder serves to control the negative electrode conductive material during charge and discharge.

[0084] In one embodiment of the present application, the negative electrode conductive material may include one or more selected from the group consisting of dot-like conductive materials, planar conductive materials, and linear conductive materials.

[0085] In one embodiment of the present application, the dot-like conductive material may be used to improve the conductivity of a negative electrode, and may be a circular or dot-like conductive material that provides conductivity without inducing a chemical change. Specifically, the dot-like conductive material may be at least one selected from the group consisting of natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives. Preferably, the dot-like conductive material may include carbon black, which provides high conductivity and excellent dispersibility.

[0086] In one embodiment of the present application, the point-like conductive material has a BET specific surface area of ​​40 m 2 / g or more 70m 2 / g or less, preferably 45m 2 / g or more 65m 2 / g or less, more preferably 50m2 / g or more 60m 2 / g or less.

[0087] In one embodiment of the present application, the particle size of the dotted conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 40 nm to 60 nm.

[0088] In one embodiment of the present application, the negative electrode conductive material may include a sheet conductive material.

[0089] The planar conductive material refers to a conductive material that increases the surface contact between silicon particles in the negative electrode to improve conductivity and simultaneously prevents the conductive path from being broken due to volume expansion. The planar conductive material may be expressed as a plate-type conductive material or a bulk-type conductive material.

[0090] In one embodiment of the present application, the sheet conductive material may include at least one selected from the group consisting of plate-like graphite, graphene, graphene oxide, and graphite flakes, and may preferably be plate-like graphite.

[0091] In one embodiment of the present application, the average particle size (D50) of the sheet conductive material may be 2 μm to 7 μm, specifically 3 μm to 6 μm, and more specifically 4 μm to 5 μm. When the average particle size satisfies this range, the particle size is sufficient, so the viscosity of the negative electrode slurry does not increase excessively, and dispersion is easy. Therefore, when dispersing using the same device and time, the dispersion effect is excellent.

[0092] In one embodiment of the present application, there is provided a negative electrode composition in which the planar conductive material has a D10 of 0.5 μm or more and 1.5 μm or less, a D50 of 4.0 μm or more and 5.0 μm or less, and a D90 of 7.0 μm or more and 15.0 μm or less.

[0093] In one embodiment of the present application, the sheet conductive material may be a high-specific surface area sheet conductive material having a high BET specific surface area; or a low-specific surface area sheet conductive material.

[0094] In one embodiment of the present application, the planar conductive material may be a high-specific surface area planar conductive material or a low-specific surface area planar conductive material without any restrictions. However, the planar conductive material of the present application is particularly preferably a low-specific surface area planar conductive material that does not cause dispersion problems, since the electrode performance may be affected to some extent by dispersion.

[0095] In one embodiment of the present application, the sheet conductive material has a BET specific surface area of ​​1 m 2 / g or more.

[0096] In another embodiment, the sheet conductive material has a BET specific surface area of ​​1 m 2 / g or more 500m 2 / g or less, and preferably 5m 2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 / g or less.

[0097] In another embodiment, the sheet conductive material is a high specific surface area sheet conductive material having a BET specific surface area of ​​50 m 2 / g or more 500m 2 / g or less, preferably 80m 2 / g or more 300m 2 / g or less, more preferably 100m 2 / g or more 250m 2 / g or less.

[0098] In another embodiment, the sheet conductive material is a sheet conductive material with a low specific surface area, and has a BET specific surface area of ​​1 m 2 / g or more 40m 2 / g or less, preferably 5m 2 / g or more 30m 2 / g or less, more preferably 5m 2 / g or more 25m2 / g or less.

[0099] Other conductive materials include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundled carbon nanotubes. The bundled carbon nanotubes may include a plurality of carbon nanotube units. Specifically, unless otherwise specified, the term "bundle type" refers to a secondary shape in which a plurality of carbon nanotube units are arranged parallel to each other or entangled with their longitudinal axes aligned in substantially the same direction, forming a bundle or rope. The carbon nanotube units have graphite sheets in the form of cylinders with nanosized diameters and an sp2 bonding structure. Depending on the winding angle and structure of the graphite sheets, they can exhibit conductive or semiconductive properties. Compared to entangled carbon nanotubes, the bundled carbon nanotubes can be more uniformly dispersed during negative electrode fabrication and can smoothly form a conductive network within the negative electrode, thereby improving the conductivity of the negative electrode.

[0100] In one embodiment of the present application, the negative electrode conductive material includes a linear conductive material, and the linear conductive material may be a carbon nanotube.

[0101] In one embodiment of the present application, the carbon nanotubes may be SWCNTs and / or MWCNTs. When the linear conductive material is SWCNTs, the length of the SWCNTs may be 0.5 μm to 100 μm, preferably 1 μm to 80 μm.

[0102] In one embodiment of the present application, the negative electrode conductive material may be contained in an amount of 5 parts by weight or more and 40 parts by weight or less based on 100 parts by weight of the negative electrode active material layer composition.

[0103] In yet another embodiment, the negative electrode conductive material may be contained in an amount of 5 parts by weight or more and 40 parts by weight or less, preferably 5 parts by weight or more and 30 parts by weight or less, and more preferably 5 parts by weight or more and 25 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition.

[0104] In one embodiment of the present application, the negative electrode conductive material includes a sheet conductive material and a linear conductive material, and the ratio of the sheet conductive material to the linear conductive material may satisfy 1:0.001 to 1:0.3.

[0105] In one embodiment of the present application, the negative electrode conductive material includes a planar conductive material and a linear conductive material, each of which satisfies the above-mentioned composition and ratio, thereby increasing the number of points at which charging and discharging are possible without significantly affecting the life characteristics of conventional lithium secondary batteries, and providing the characteristics of excellent output characteristics at high C-rates.

[0106] The negative electrode conductive material according to the present application has a completely different structure from the conductive material used in the positive electrode. That is, the negative electrode conductive material according to the present application serves to control the contact points between the silicon-based active material, which experiences a large volume expansion of the electrode during charging and discharging, while the positive electrode conductive material acts as a buffer during rolling and also provides some conductivity, and is completely different in structure and role from the negative electrode conductive material of the present invention.

[0107] Furthermore, the negative electrode conductive material according to the present application is applied to a silicon-based active material and has a completely different structure from conductive materials applied to graphite-based active materials. That is, conductive materials used in electrodes with graphite-based active materials simply have smaller particles than the active material, and therefore have the properties of improving output characteristics and imparting some conductivity, and are completely different in structure and role from negative electrode conductive materials applied together with silicon-based active materials as in the present invention.

[0108] In one embodiment of the present application, the plate-shaped conductive material used as the negative electrode conductive material has a structure and function different from that of a carbon-based active material generally used as a negative electrode active material. Specifically, the carbon-based active material used as the negative electrode active material may be artificial graphite or natural graphite, and refers to a material that is processed into a spherical or dot-like shape to facilitate the storage and release of lithium ions.

[0109] Meanwhile, the plate-type conductive material used as the negative electrode conductive material is a material having a planar or plate-like shape, which may be expressed as plate-type graphite. That is, it is a material contained in the negative electrode active material layer to maintain a conductive path within the layer, and does not play a role in storing or releasing lithium, but rather serves to ensure a planar conductive path within the negative electrode active material layer.

[0110] That is, in this application, the use of plate-shaped graphite as a conductive material means that it is processed into a planar or plate-like shape and used as a material to ensure a conductive path rather than to store or release lithium. In this case, the negative electrode active material included together has high capacity characteristics for storing and releasing lithium and plays a role in storing and releasing all lithium ions transferred from the positive electrode.

[0111] Meanwhile, in the present application, the term "carbon-based active material is used as an active material" means that the carbon-based active material is processed into a dotted or spherical shape and used as a material that stores or releases lithium.

[0112] In one embodiment of the present application, the negative electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which hydrogen is substituted with Li, Na, Ca, or the like, or may include various copolymers thereof.

[0113] The negative electrode binder according to one embodiment of the present application plays a role in controlling the silicon-based active material and the negative electrode conductive material to prevent twisting and deformation of the negative electrode structure during volume expansion and relaxation of the silicon-based active material. When the binder fulfills the above role, any conventional negative electrode binder may be used. Specifically, a water-based binder may be used, and more specifically, a PAM-based binder may be used.

[0114] In one embodiment of the present application, the negative electrode binder may be contained in an amount of 30 parts by weight or less, preferably 25 parts by weight or less, more preferably 20 parts by weight or less, or may be contained in an amount of 5 parts by weight or more, or 8 parts by weight or more, based on 100 parts by weight of the negative electrode active material layer composition.

[0115] Compared to conventional carbon-based negative electrodes, when a Si-based negative electrode is used, a water-based binder is applied in the above weight part, and a dot-like conductive material with a low content of functional groups may be used. Due to this characteristic, the dot-like conductive material has hydrophobicity and excellent bonding strength between the conductive material and the binder.

[0116] In one embodiment of the present application, the silicon-based negative electrode may be formed by coating one or both surfaces of a negative electrode current collector layer with a negative electrode slurry containing the negative electrode active material layer composition.

[0117] In one embodiment of the present application, the negative electrode slurry may include a negative electrode active material layer composition and a slurry solvent.

[0118] In one embodiment of the present application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0119] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, and more preferably 10% to 30%.

[0120] In another embodiment, the solid content of the negative electrode slurry may be in the range of 5% to 40%, preferably 7% to 35%, more preferably 10% to 30%.

[0121] The solid content of the negative electrode slurry refers to the content of the negative electrode composition contained in the negative electrode slurry, and may refer to the content of the negative electrode composition based on 100 parts by weight of the negative electrode slurry.

[0122] When the solid content of the negative electrode slurry satisfies the above range, the viscosity is appropriate during the formation of the negative electrode active material layer, and the agglomeration of particles of the negative electrode composition is minimized, thereby enabling the negative electrode active material layer to be efficiently formed.

[0123] In one embodiment of the present application, the slurry solvent may be any solvent that can disperse the negative electrode composition, and specifically, water or NMP may be used.

[0124] In one embodiment of the present application, the porosity of the negative electrode active material layer may be in the range of 10% or more and 60% or less.

[0125] In another embodiment, the porosity of the negative electrode active material layer may be in the range of 10% to 60%, preferably 20% to 50%, and more preferably 30% to 45%.

[0126] The porosity varies depending on the composition and content of the silicon-based active material, conductive material, and binder contained in the negative electrode active material layer. In particular, the silicon-based active material and conductive material according to the present application are contained in specific compositions and content portions to satisfy the above range, thereby providing the electrode with appropriate ranges of electrical conductivity and resistance.

[0127] In one embodiment of the present application, the positive electrode includes a positive electrode current collector layer; and a positive electrode active material layer provided on one or both sides of the positive electrode current collector layer.

[0128] In one embodiment of the present application, the positive electrode active material layer includes a positive electrode active material layer composition, and the positive electrode active material layer composition may include one or more selected from the group consisting of a positive electrode active material, a positive electrode conductive material, and a positive electrode binder.

[0129] The positive electrode current collector in the positive electrode is not particularly limited as long as it is conductive and does not induce chemical changes in the battery. 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. The positive electrode current collector layer may typically have a thickness of 1 to 500 μm, and fine irregularities may be formed on the surface of the positive electrode current collector layer to enhance adhesion to the positive electrode active material. The positive electrode current collector may be used in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0130] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the thickness of the positive electrode and negative electrode current collector layers is 1 μm or more and 100 μm or less, and the thickness of the positive electrode and negative electrode active material layers is 20 μm or more and 500 μm or less.

[0131] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material is LiNi x Co y Mn z O2(x+y+z=1);LiNi a Co b Mn c Al d O2(a+b+c+d=1);LiMn2O4;LiNi 0.5 Mn 1.5 O2; and LiM x Fe y PO4 (M: transition metal, x+y=1), but the positive electrode active material is not limited to these.

[0132] Specifically, the positive electrode active material may be NCM or NCMA, which are commonly used.

[0133] Generally, the positive electrode active material is a layered compound such as lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium iron oxide such as LiFe3O4; 1+c1 Mn 2-c1 O4 (0≦c1≦0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, and Cu2V2O7; chemical formula LiNi 1-c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMn 2-c3 M c3Examples of suitable lithium manganese composite oxides include, but are not limited to, lithium manganese composite oxides represented by Li2Mn3MO8 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≦c3≦0.6) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); and LiMn2O4, in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion. The positive electrode may be Li metal.

[0134] In one embodiment of the present application, the positive electrode active material includes a lithium transition metal composite including nickel (Ni), cobalt (Co), and manganese (Mn), and the lithium transition metal composite may include single particles or secondary particles, and the single particles may have an average particle size (D50) of 1 μm or more.

[0135] For example, the average particle size (D50) of the single particles may be 1 μm or more and 12 μm or less, 1 μm or more and 8 μm or less, 1 μm or more and 6 μm or less, more than 1 μm and 12 μm or less, more than 1 μm and 8 μm or less, or more than 1 μm and 6 μm.

[0136] The single particles may have excellent particle strength even when formed into a small particle size with an average particle size (D50) of 1 μm to 12 μm. For example, the single particles may have a strength of 650 kgf / cm 2 When the single particle is rolled at a pressure of 650 kgf / cm, it can have a particle strength of 100 MPa to 300 MPa. 2 Even if the electrode is rolled with a strong force, the phenomenon of an increase in fine particles in the electrode due to cracking of particles is alleviated, thereby improving the life characteristics of the battery.

[0137] The single particles may be prepared by mixing a transition metal precursor and a lithium source material and calcining the mixture. The secondary particles may be prepared by a method different from that for the single particles, and the composition thereof may be the same as or different from that of the single particles.

[0138] The method for forming the single particles is not particularly limited, but generally, the particles may be formed by over-firing at an elevated firing temperature, or by using an additive such as a particle growth promoter that is useful for over-firing, or by changing the starting material.

[0139] For example, the calcination is performed at a temperature sufficient to form single particles. To achieve this, the calcination temperature must be higher than that used to prepare secondary particles. For example, if the precursor composition is the same, the calcination temperature must be 30°C to 100°C higher than that used to prepare secondary particles. The calcination temperature for forming the single particles may vary depending on the metal composition of the precursor. For example, when a high-nickel (Ni) NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 80 mol% or more is to be formed into single particles, the calcination temperature may be 700°C to 1000°C, preferably 800°C to 950°C. When the calcination temperature satisfies the above range, a positive electrode active material containing single particles with excellent electrochemical properties can be prepared. If the calcination temperature is lower than 790°C, a positive electrode active material containing a lithium composite transition metal in the form of secondary particles can be prepared. However, if the calcination temperature exceeds 950°C, excessive calcination may result in insufficient formation of a layered crystal structure, resulting in reduced electrochemical properties.

[0140] In this specification, the term "single particle" is used to distinguish it from conventional secondary particles formed by agglomeration of tens to hundreds of primary particles, and is a concept that includes a single particle consisting of one primary particle and a quasi-single particle form that is an agglomeration of 30 or less primary particles.

[0141] Specifically, in the present invention, the single particle may be a single particle consisting of one primary particle or a quasi-single particle form which is an agglomerate of 30 or less primary particles, and the secondary particle may be a form in which several hundred primary particles are agglomerated.

[0142] In one embodiment of the present application, the lithium transition metal composite that is the positive electrode active material further includes secondary particles, and the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles.

[0143] In the present invention, the single particle may be a single particle consisting of one primary particle or a quasi-single particle which is an aggregate of 30 or less primary particles, and the secondary particle may be an aggregate of several hundred primary particles.

[0144] The lithium transition metal composite may further include secondary particles. The secondary particles refer to a form formed by agglomeration of primary particles, and can be distinguished from the concept of single particles, which includes one primary particle, one single particle, or a similar-single particle form that is an agglomeration of 30 or less primary particles.

[0145] The particle diameter (D50) of the secondary particles may be 1 μm to 20 μm, 2 μm to 17 μm, or preferably 3 μm to 15 μm. The specific surface area (BET) of the secondary particles may be 0.05 m 2 / g~10m 2 / g, preferably 0.1m 2 / g~1m 2 / g, more preferably 0.3m 2 / g~0.8m 2 / g.

[0146] In an additional embodiment of the present application, the secondary particles are aggregates of primary particles, and the average particle size (D50) of the primary particles is 0.5 μm to 3 μm. Specifically, the secondary particles may be in the form of aggregates of several hundred primary particles, and the average particle size (D50) of the primary particles may be 0.6 μm to 2.8 μm, 0.8 μm to 2.5 μm, or 0.8 μm to 1.5 μm.

[0147] When the average particle size (D50) of the primary particles satisfies the above range, a single-particle positive electrode active material with excellent electrochemical properties can be formed. If the average particle size (D50) of the primary particles is too small, the number of agglomerates of the primary particles forming the lithium nickel-based oxide particles increases, reducing the effect of suppressing particle cracking during rolling. If the average particle size (D50) of the primary particles is too large, the lithium diffusion path within the primary particles becomes longer, increasing resistance and potentially reducing output characteristics.

[0148] According to a further embodiment of the present application, the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles. As a result, the single particles may have excellent particle strength even when formed to a small diameter, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, thereby improving the lifespan of the battery.

[0149] In one embodiment of the present application, the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles by 1 μm to 18 μm.

[0150] For example, the average particle size (D50) of the single particles may be 1 μm to 16 μm smaller, 1.5 μm to 15 μm smaller, or 2 μm to 14 μm smaller than the average particle size (D50) of the secondary particles.

[0151] When the average particle size (D50) of the single particles is smaller than the average particle size (D50) of the secondary particles, for example, when the above range is satisfied, the single particles are formed to have a small diameter but have excellent particle strength, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, thereby improving the life characteristics and energy density of the battery.

[0152] According to a further embodiment of the present application, the single particles are contained in an amount of 15 to 100 parts by weight relative to 100 parts by weight of the positive electrode active material, or may be contained in an amount of 20 to 100 parts by weight, or 30 to 100 parts by weight relative to 100 parts by weight of the positive electrode active material.

[0153] For example, the single particles may be included in an amount of 15 parts by weight or more, 20 parts by weight or more, 25 parts by weight or more, 30 parts by weight or more, 35 parts by weight or more, 40 parts by weight or more, or 45 parts by weight or more, based on 100 parts by weight of the positive electrode active material. The single particles may be included in an amount of 100 parts by weight or less, based on 100 parts by weight of the positive electrode active material.

[0154] When the single particles are contained in the above range, excellent battery characteristics can be exhibited in combination with the above-mentioned negative electrode material. In particular, when the single particles are contained in an amount of 15 parts by weight or more, the phenomenon of an increase in fine particles in the electrode due to particle cracking during the rolling process after electrode fabrication can be mitigated, thereby improving the life characteristics of the battery.

[0155] In one embodiment of the present application, the lithium composite transition metal compound may further include secondary particles, and the amount of the secondary particles may be 85 parts by weight or less relative to 100 parts by weight of the positive electrode active material. The amount of the secondary particles may be 80 parts by weight or less, 75 parts by weight or less, or 70 parts by weight or less relative to 100 parts by weight of the positive electrode active material. The amount of the secondary particles may be 0 parts by weight or more relative to 100 parts by weight of the positive electrode active material.

[0156] When the above range is satisfied, the effects of the single particle positive electrode active material can be maximized. When the secondary particle positive electrode active material is included, the components thereof may be the same as or different from those exemplified for the single particle positive electrode active material, and may refer to an aggregated form of single particles.

[0157] In one embodiment of the present application, the positive electrode active material may be contained in an amount of 80 parts by weight or more and 99.9 parts by weight or less, preferably 90 parts by weight or more and 99.9 parts by weight or less, more preferably 95 parts by weight or more and 99.9 parts by weight or less, and even more preferably 98 parts by weight or more and 99.9 parts by weight or less, in 100 parts by weight of the positive electrode active material layer.

[0158] The positive electrode active material layer may contain a positive electrode conductive material and a positive electrode binder in addition to the above-described positive electrode active material.

[0159] In this case, the positive electrode conductive material is used to impart conductivity to the electrode and can be any material that has electronic conductivity without causing chemical changes in the battery that is constructed. Specific examples include graphite such as natural graphite and 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 whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive polymers such as polyphenylene derivatives. These materials may be used alone or in combination.

[0160] The positive electrode binder improves adhesion between positive electrode active material particles and between the positive electrode active material and the positive electrode current collector. Specific examples of the positive electrode binder include polyvinylidene fluoride (PVDF), polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, and various copolymers thereof. These may be used alone or in combination.

[0161] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator commonly used in secondary batteries can be used without particular limitations. In particular, a separator that exhibits low resistance to electrolyte ion movement and excellent electrolyte humidification capability is preferred. Specifically, a porous polymer film, such as a porous polymer film made of a polyolefin-based polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, or an ethylene / methacrylate copolymer, or a laminate structure of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may also be used. To ensure heat resistance or mechanical strength, a separator coated with a ceramic component or a polymer material may be used, and may be selectively used in a single-layer or multi-layer structure.

[0162] In addition to the constituent components of the electrolyte, the electrolyte may further contain one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinoneimine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrrole, 2-methoxyethanol, or aluminum trichloride, for the purpose of improving the life characteristics of the battery, suppressing a decrease in battery capacity, and improving the discharge capacity of the battery. [Example]

[0163] In the following, preferred examples are presented to aid in understanding the present invention. However, the following examples are merely illustrative of the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope of the present description and technical idea. Naturally, such changes and modifications fall within the scope of the claims. (Manufacturing example)

[0164] (1) Manufacturing of the positive electrode As the positive electrode active material, Li(Ni a Co b Mn c )O2 was used.

[0165] The NCM, excluding lithium (Li) and oxygen (O), had a Ni:Co:Mn ratio of 93:5:2, with a ratio of a:b:c of 93:5:2 (a:b:c = 0.93:0.05:0.02). The D50 was 3.8 μm, the Dmin and Dmax were 1.2 μm and 13 μm, respectively, and the BET specific surface area was 0.64 m. 2 and the true density is 4.78 g / cm 3 ~4.80g / cm 3 is.

[0166] The positive electrode active material, positive electrode conductive material (LB.CNT), and binder (PVdF, KF9700) were mixed in a weight ratio of 97.96:0.8:1.24 in a solvent (N-methylpyrrolidone, NMP) to prepare a positive electrode slurry. The positive electrode slurry was applied to a 25 μm-thick aluminum (Al) thin film as a positive electrode current collector, dried, and then rolled using a roll press to prepare a positive electrode.

[0167] (2) Manufacturing of the negative electrode A negative electrode active material layer composition was prepared using a silicon-based active material, Si (average particle size (D50): 5 μm), a first conductive material, a second conductive material, and polyacrylamide as a binder in a weight ratio of 80:9.6:0.4:10. The composition was added to distilled water as a solvent for forming a negative electrode slurry to prepare a negative electrode slurry (solid concentration: 28 wt%).

[0168] The first conductive material is plate-shaped graphite (specific surface area: 17 m 2 / g, average particle size (D50): 3.5 μm), and the second conductive material is carbon nanotubes.

[0169] As for the mixing method, the first conductive material, the second conductive material, the binder, and the water were dispersed in a homomixer at 2500 rpm for 30 minutes, and then the active material was added, followed by dispersing at 2500 rpm for 30 minutes to prepare a slurry.

[0170] The negative electrode slurry was applied at 3.00 mg / cm to both sides of a copper current collector (thickness: 15 μm) as a negative electrode current collector. 2 The coated layer was rolled and then dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer (thickness: 23 μm).

[0171] (3) Secondary battery manufacturing A compression-resistant thin film separator (PE 2 μm) and a ceramic coating 3 μm / 3 μm were interposed between the positive electrode and the negative electrode to fabricate an electrode assembly. The electrode assembly was placed inside a case, and an electrolyte was injected into the case to fabricate a lithium secondary battery.

[0172] In this case, the electrolyte had the composition and content shown in Table 1 below.

[0173] [Table 1]

[0174] Experimental example 1: Life evaluation The lithium secondary batteries including the negative electrodes prepared in the Examples and Comparative Examples were subjected to a lifespan evaluation using an electrochemical charger / discharger to evaluate the capacity retention. The secondary batteries were subjected to an in-situ cycle test at 4.2-3.27 V, 1 C / 1 C, and the capacity retention was measured every 50 cycles during the test, with the secondary batteries being charged / discharged at 1 C / 1 C (4.2-3.2 V). The results are shown in Table 2. Lifetime retention rate (%) = {(discharge capacity at Nth cycle) / (discharge capacity at 1st cycle)} x 100

[0175] Experimental example 2: Resistance increase rate measurement evaluation In Experimental Example 1, the capacity retention rate was measured after 50 cycles of 0.33C / 0.33C charge / discharge (4.2-3.0V) during the test, and then the resistance was measured by discharging at 2.5C pulses at SOC50, and the resistance increase rate was compared and analyzed. The results are shown in Table 2 below.

[0176] Experimental Example 3: Volume change due to gas generation using a pouch test For the lithium secondary batteries including the negative electrodes prepared in the Examples and Comparative Examples, pouch cells having a capacity of 1 Ah were fully charged to 4.2 V at 0.33 C and then stored in an oven at 60°C for 8 weeks. After fully discharging to 2.5 V, the gas generated in the pouch cells was extracted by GC / MS and quantitatively analyzed. The results are shown in Table 2 below.

[0177] [Table 2]

[0178] In Examples 1 to 3 of the present application, the content of vinylene carbonate was adjusted to include an additive that forms an effective coating on the silicon-based negative electrode, thereby resolving the problem of vinylene carbonate remaining on the silicon-based negative electrode without forming an effective coating, resulting in the generation of a large amount of gas due to an oxidation reaction at the positive electrode.As a result, it was confirmed that the lithium secondary battery of the present application has the characteristics of being able to reduce the amount of gas generation and ensure performance even when stored at high temperatures for a long period of time, while also being able to ensure ionic conductivity.

[0179] Comparative Example 1 contains less than 10 parts by weight of a carbonate compound containing a fluoro group based on 100 parts by weight of solvent, and it was confirmed that an effective coating could not be formed on the negative electrode, the capacity retention rate decreased, and the resistance increase rate increased.

[0180] Comparative Example 2 corresponds to a case where the additive containing a fluoro group according to the present invention is not included. In this case, it was confirmed that the inclusion of an excessive amount of FEC caused a rapid increase in the amount of gas generated at high temperatures, leading to a large volume change rate and a decrease in life performance.

[0181] Comparative Example 3 corresponds to a case where the additive containing a fluoro group according to the present application was included, but the vinylene carbonate content exceeded the range according to the present application. In the case of Comparative Example 3 using a silicon-based negative electrode, vinylene carbonate was unable to form an effective coating on the negative electrode and remained, causing a large amount of gas to be generated by an oxidation reaction at the positive electrode, resulting in a larger amount of gas generation than in the Examples. As a result, it was confirmed that the capacity retention rate decreased and the resistance increase rate increased.

[0182] Comparative Examples 4 to 6 correspond to comparative examples in which the content of vinylene carbonate was adjusted without the fluoro group-containing additive according to the present application. In these cases, it was similarly confirmed that the amount of gas generated at high temperatures increased rapidly, the volume change rate increased, and the life performance decreased due to the inclusion of FEC. [Explanation of symbols]

[0183] 10 Negative electrode current collector layer 20 Negative electrode active material layer 30 Separation membrane 40 Cathode active material layer 50 Positive electrode current collector layer 100 negative electrode 200 positive electrode

Claims

1. A lithium secondary battery comprising: a positive electrode; a silicon-based negative electrode; a separator disposed between the positive electrode and the negative electrode; and an electrolyte, The silicon-based negative electrode includes a negative electrode current collector layer; and a negative electrode active material layer provided on one or both sides of the negative electrode current collector layer, the negative electrode active material layer includes a negative electrode active material layer composition, the negative electrode active material layer composition includes a silicon-based active material, and the silicon-based active material includes one or more selected from the group consisting of SiOx (x=0), SiOx (0<x<2), SiC, and a Si alloy; the electrolyte comprises a solvent, a lithium salt, a fluoro-group-containing additive, and vinylene carbonate (VC); The electrolyte contains 0.1 parts by weight or more and 0.3 parts by weight or less of vinylene carbonate based on 100 parts by weight of the electrolyte, The solvent contains 10 parts by weight or more of a carbonate compound containing a fluoro group, based on 100 parts by weight of the solvent, The fluoro group-containing additive is contained in an amount of 0.5 parts by weight or more and 2 parts by weight or less based on 100 parts by weight of the electrolyte, The lithium secondary battery, wherein the fluoro group-containing additive is LiCF 3 COO.

2. 2. The lithium secondary battery according to claim 1, wherein the amount of gas generated per capacity is 5 ml / Ah or less when the lithium secondary battery is stored at 60° C. for 8 weeks.

3. 2. The lithium secondary battery according to claim 1, wherein the solvent is a mixed solvent containing two carbonate-based compounds.

4. The lithium salt is (CF ３ ), ２ PF ４ － , (CF ３ ), ３ PF ３ － , (CF ３ ), ４ PF ２ － , (CF ３ ), ５ PF － , (CF ３ ), ６ P － , CF ３ SO ３ － , CF ３ CF ２ SO ３ － , (CF ３ SO ２ ), ２ N － , (FSO ２ ), ２ N － , CF ３ CF ２ (CF ３ ), ２ CO － , (CF ３ SO ２ ), ２ CH － , (SF ５ ), ３ C － , (CF ３ SO ２ ), ３ C － , CF ３ (CF ２ ), ７ SO ３ － , CF ３ CO ２ － , CH ３ CO ２ － , SCN － and (CF ３ CF ２ SO ２ ), ２ N － 2. The lithium secondary battery according to claim 1, comprising two or more anions selected from the group consisting of:

5. 2. The lithium secondary battery according to claim 1, wherein the silicon-based active material comprises one or more selected from the group consisting of SiOx (x=0), SiOx (0<x<2), and metal impurities, and the silicon-based active material comprises 70 parts by weight or more of the SiOx (x=0) based on 100 parts by weight of the silicon-based active material.

6. The lithium secondary battery according to claim 1 , wherein the silicon-based active material is contained in an amount of 60 parts by weight or more based on 100 parts by weight of the negative electrode active material layer composition.

7. the positive electrode includes a positive electrode current collector layer; and a positive electrode active material layer provided on one or both sides of the positive electrode current collector layer, the positive electrode active material layer includes a positive electrode active material layer composition, the positive electrode active material layer composition contains a positive electrode active material, The positive electrode active material is LiNi ｘ Co ｙ Mn ｚ O ２ (x+y+z=1);LiNi ａ Co ｂ Mn ｃ Al ｄ O ２ (a+b+c+d=1);LiMn ２ O ４ ;LiNi ０．５ Mn １．５ O ２ and LiM ｘ Fe ｙ P.O. ４ 2. The lithium secondary battery according to claim 1, comprising one or more selected from the group consisting of (M: transition metal, x+y=1).

8. the thickness of the positive electrode and negative electrode current collector layers is 1 μm or more and 100 μm or less; 8. The lithium secondary battery according to claim 7, wherein the thickness of the positive electrode and negative electrode active material layers is 20 μm or more and 500 μm or less.

9. The electrolyte has a viscosity of 1.1 g / cm at 25°C. ３ 2. The lithium secondary battery according to claim 1, wherein the ionic conductivity is 5.5 mS / cm or more.

Images