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. 10-2023-0119535 filed with the Korean Intellectual Property Office on September 8, 2023, and Korean Patent Application No. 10-2023-0119486 filed with the Korean Intellectual Property Office on September 8, 2023, 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 fossil fuel use has led to an increasing demand for alternative and clean energy, and one of the most actively researched areas as part of this is the field of power generation and storage using electrochemical reactions.

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

[0005] As technological development and demand for mobile devices increase, the demand for secondary batteries as energy sources is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which have high energy density and voltage, long cycle life, and low self-discharge rate, have been commercialized and widely used. In addition, active research is being conducted on methods for manufacturing high-density electrodes with higher energy density per unit volume as electrodes for such high-capacity lithium secondary batteries.

[0006] Generally, secondary batteries are composed 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 excellent in stability and reversibility, but they have limitations in terms of capacity. Therefore, in fields where high capacity is the goal, there are increasing attempts to use Si-based materials, which have a high theoretical capacity, as the negative electrode material.

[0007] Lithium secondary batteries are typically manufactured using lithium-intercalated compounds such as LiCoO2 and LiMn2O4 as the positive electrode and non-lithium-intercalated materials such as carbonaceous and silicon-based materials as the negative electrode. During charging, lithium ions intercalated in the positive electrode migrate to the negative electrode via the electrolyte, and during discharge, the lithium ions migrate again from the negative electrode to the positive electrode. During charging, the lithium ions migrate from the positive electrode to the negative electrode react with the electrolyte, forming a solid electrolyte interface (SEI), a type of passivation film, 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 this reaction is irreversible, it results in the consumption of lithium ions. The lithium consumed by the formation of the SEI does not 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 called for the development of technology to increase the charging speed. To enable rapid charging of lithium secondary batteries, the lithium ions must move quickly during the process of insertion into the anode. Therefore, batteries are being designed to achieve high output by forming the anode active material layer to a thin film thickness to reduce the lithium diffusion distance, and then forming a carbon coating layer on the surface to increase conductivity, thereby reducing internal resistance.

[0009] However, it is difficult to achieve high capacity with anodes that have such thin anode active material layers. Furthermore, it is difficult to achieve fast charging with anodes that achieve high capacity. Furthermore, when using silicon-based anodes, the Si content of the anode must be increased to increase energy density. This reduces the efficiency of the high-Ni cathode material. To counterbalance this, an excess of sacrificial cathode material must be used. However, this increases the amount of gas in the battery cell itself, raising stability issues.

[0010] In particular, anodes using silicon-based active materials suffer from uneven Li transport within the particles during charge / discharge cycles, accelerating degradation of the silicon-based active material itself in the Li-rich phase, leading to increased cell swelling and performance degradation. Furthermore, volume expansion during charge / discharge cycles causes pulverization, continuously exposing new particle surfaces as cycles progress. This requires the formation of an SEI layer containing an inorganic LiF layer on the surface, and an electrolyte with excessive FEC is used to form LiF. However, electrolytes with excessive FEC generate excessive gas at high temperatures, which is detrimental to high-temperature life and poses safety concerns.

[0011] Therefore, research is needed to use a negative electrode containing a silicon-based active material for high capacity, while maintaining cell resistance, reducing cell swelling, and increasing life characteristics. [Prior art documents] [Patent documents]

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

[0013] As a result of research into the above issues, it was discovered that while using a silicon-based anode ensures capacity characteristics and fast charging performance, if the crystal grain size and average grain size of the silicon-based active material are adjusted within a certain range, it is possible to reduce the consumption of FEC in the electrolyte, control the problem of high-temperature gas generation, and uniformly adjust the reaction at the anode, thereby improving cell swelling and lifespan characteristics.

[0014] In other words, when the crystal grain size and average particle size of the silicon-based active material are adjusted within a certain range, Li ions can be uniformly diffused, resulting in a lithium secondary battery with optimized cell swelling and cell bleeding ranges.

[0015] Thus, the present application relates to a lithium secondary battery that includes a silicon-based negative electrode and is adjusted to specific cell swelling and cell breathing values. [Means for solving the problem]

[0016] One embodiment of the present specification provides 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 sides of the negative electrode current collector layer, and the lithium secondary battery satisfies the following formulas 1 and 2:

[0017] [Formula 1] X1≦20% [Formula 2] Y1≦10% In the formula 1 and formula 2, X1 is a cell swelling value, which means a thickness change rate of the negative electrode active material layer of the silicon-based negative electrode in a fully charged state after the lithium secondary battery is charged and discharged 300 times in a 4.2 V-3.2 V range, based on the cell swelling value after activation; Y1 is a cell breathing value, which means a change in thickness of the negative electrode active material layer of the silicon-based negative electrode when the lithium secondary battery is charged once in the range of 4.2 V to 3.2 V after activation. [Effects of the Invention]

[0018] 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 can also ensure rapid charging performance by using a silicon-based active material with excellent capacity characteristics, which allows the thickness of the negative electrode to be thin.

[0019] In particular, the lithium secondary battery according to the present application is characterized by having solved the problems of cell swelling and cell breathing that occur with charge and discharge, which are problems with conventional silicon-based negative electrodes. That is, by changing the type of silicon-based active material contained in the silicon-based negative electrode to an active material with small crystal grains, and by using a silicon-based active material with small crystal grains even in the Li-rich phase in relation to the positive electrode, it is possible to make the reaction uniform. As a result, the range of cell swelling and cell breathing is optimized, ensuring high capacity and high energy density, and achieving a lithium secondary battery with improved life characteristics.

[0020] Unlike conventional graphite-based anodes, silicon-based anodes have a large capacity, but their volume expansion is very large. This means that there is a limit to how much of the anode's capacity can be used. Therefore, cells are designed to use only a portion of the anode's capacity by adjusting the NP of the cell. However, as mentioned above, the cell swelling and cell breathing values ​​have been optimized by changing the composition of the anode active material layer, focusing on the type of silicon-based active material.

[0021] That is, when a silicon-based active material having a crystal grain size of 100 nm or less and an average particle size (D50) of 1 μm or more and 7 μm or less is used, that is, when a silicon-based active material having crystal grains that satisfy the above range is used, it has the characteristic of being able to reduce the consumption of FEC in the electrolyte. In the case of silicon-based negative electrode batteries, sudden death occurs as FEC is consumed, and the life performance deteriorates rapidly. However, by applying a silicon-based active material having the above characteristics, the consumption of FEC is reduced, and the above-mentioned cell swelling and cell breathing values ​​are optimized. [Brief explanation of the drawings]

[0022] [Figure 1] 1 is a diagram showing a lithium secondary battery according to the present application. [Figure 2] FIG. 1 is a diagram showing a method for calculating grain size. DETAILED DESCRIPTION OF THE INVENTION

[0023] Before describing the present invention, some terms will first be defined.

[0024] In this specification, unless otherwise specified, when a part "comprises" a certain component, it does not mean that it may further include other components, but does not exclude other components.

[0025] In this specification, "p to q" means a range of "not less than p and not more than q."

[0026] 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-mino II manufactured by BEL Japan Co., Ltd. That is, in the present application, the BET specific surface area may mean the specific surface area measured by the above-mentioned measurement method.

[0027] 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 according to particle size. That is, D50 is the particle size at the 50% point in the cumulative particle number distribution according to particle size, D90 is the particle size at the 90% point in the cumulative particle number distribution according to particle size, and D10 is the particle size at the 10% point in the cumulative particle number distribution according to particle size. Meanwhile, particle size distribution can 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). The particle size distribution is calculated by measuring the difference in diffraction patterns according to particle size when the particles pass through a laser beam.

[0028] In this specification, when a polymer contains a certain monomer as a monomer unit, it means that the monomer participates in a polymerization reaction and is included in the polymer as a repeating unit. In this specification, when a polymer contains a monomer, it is interpreted in the same way as when a polymer contains a monomer as a monomer unit.

[0029] In this specification, the term "polymer" is understood to be used in a broad sense, including copolymers, unless otherwise specified as a "homopolymer."

[0030] In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are polystyrene-equivalent molecular weights measured by gel permeation chromatography (GPC) using commercially available monodisperse polystyrene polymers (standard samples) of various degrees of polymerization as standard substances for molecular weight measurement. In this specification, molecular weight means weight average molecular weight unless otherwise specified.

[0031] The present invention will now be described in detail with reference to the accompanying drawings so that those skilled in the art can easily practice the present invention, although the present invention may be embodied in various different forms and is not limited to the following description.

[0032] One embodiment of the present specification provides 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 sides of the negative electrode current collector layer, and the lithium secondary battery satisfies Formula 1 and Formula 2.

[0033] The type of silicon-based active material contained in the silicon-based negative electrode has been changed to one with small crystal grains, and in relation to the positive electrode, a silicon-based active material with small crystal grains is used, even in the Li-rich phase, making it possible to make the reaction uniform.This optimizes the range of cell swelling and cell breathing, ensuring high capacity and high energy density, as well as a lithium secondary battery with improved life characteristics.

[0034] 1 is a diagram showing 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 interposed therebetween.

[0035] In this application, X1 is the cell swelling value, which refers to the rate of change in thickness of the negative electrode active material layer of the silicon-based negative electrode in a fully charged state after 300 charge-discharge cycles between 4.2 V and 3.2 V of the lithium secondary battery, relative to the value after activation. That is, cell swelling indicates the rate of change in thickness of the negative electrode after a specific cycle, relative to the initial lithium secondary battery (after activation). While conventional graphite-based negative electrodes did not pose a significant problem of cell swelling during charge and discharge, silicon-based negative electrodes have experienced problems related to life characteristics due to volume expansion during charge and discharge. Therefore, when the range of Equation 1 according to this application is satisfied, the battery can be characterized by ensuring life characteristics.

[0036] In one embodiment of the present application, Y1 is a cell breathing value, which means the rate of change in thickness of the negative electrode active material layer of the silicon-based negative electrode when the lithium secondary battery is charged once between 4.2 V and 3.2 V, based on the state after activation. That is, the cell breathing value may mean the rate of change in thickness of the negative electrode active material layer during one cycle of operation of the lithium secondary battery.

[0037] That is, cell swelling refers to the rate of change in thickness after cycling, and cell breathing refers to the rate of change in thickness after one cycle. The cell breathing value is a value reflected in the cell design and is an important factor in defining the shipping thickness, and cell swelling is a factor that indicates cell deterioration and is a factor related to the lifespan.

[0038] In this case, the term "activated lithium secondary battery" generally refers to the activation process that is included in the activated lithium secondary battery, and refers to a lithium secondary battery in an initial charged state or an initial discharged state after one or two charge and discharge cycles, and corresponds to a lithium secondary battery in an initial state.

[0039] In one embodiment of the present application, the formula 1 may satisfy X1≦20%, X1≦17%, or X1≦15%, or may satisfy 1%≦X1, 3%≦X1, or 5%≦X1.

[0040] In one embodiment of the present application, the formula 2 may satisfy Y1≦10%, Y1≦8%, Y1≦7%, or may satisfy the ranges of 1%≦Y1 and 3%≦Y1.

[0041] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the formula 1 satisfies 5%≦X1≦15%, and the formula 2 satisfies 3%≦Y1≦7%.

[0042] The lithium secondary battery according to the present application is characterized in that the formula 1 and the formula 2 simultaneously satisfy the above ranges. That is, when the ranges of the formula 1 and the formula 2 are satisfied, the battery is characterized in that it is possible to prevent performance degradation and improve the life characteristics of the lithium secondary battery.

[0043] The positive electrode, negative electrode, electrolyte, and separator contained in the lithium secondary battery will be described below.

[0044] 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.

[0045] The negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. Such a negative electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and has high conductivity. For example, copper, stainless steel, aluminum, nickel, titanium, fired carbon, those surface-treated with carbon, nickel, titanium, silver, etc. on the surface of copper or stainless steel, aluminum-cadmium alloy, etc. may be used. Also, fine irregularities may be formed on the surface to strengthen the binding force of the negative electrode active material, and it may be used in various forms such as films, sheets, foils, nets, porous bodies, foams, non-woven fabrics, etc.

[0046] However, the thickness can be variously deformed according to the type and use of the negative electrode used, and is not limited thereto.

[0047] In one embodiment of the present application, the negative electrode active material layer contains a negative electrode active material layer composition, the negative electrode active material layer composition contains a silicon-based active material, and the silicon-based active material may contain one or more selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloys.

[0048] In one embodiment of the present application, the negative electrode active material layer composition may contain one or more selected from the group consisting of a negative electrode conductive material; and a negative electrode binder.

[0049] 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.

[0050] 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.

[0051] In one embodiment of the present application, the silicon-based active material may include one or more selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), SiC, and Si alloys.

[0052] In one embodiment of the present application, the silicon-based active material includes one or more selected from the group consisting of SiOx (x = 0), SiOx (0 < x < 2), and metal impurities, and may include 70 parts by weight or more of the SiOx (x = 0) based on 100 parts by weight of the silicon-based active material.

[0053] In another embodiment, the SiOx (x = 0) may be included 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 included 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, based on 100 parts by weight of the silicon-based active material.

[0054] In one embodiment of the present application, the silicon-based active material may contain metal impurities. In this case, the impurities are metals that may generally be contained in the silicon-based active material, and specifically, may be contained in an amount of 0.1 part by weight or less based on 100 parts by weight of the silicon-based active material.

[0055] In one embodiment of the present application, particularly pure silicon (Si) may be used as the silicon-based active material. Using particularly pure silicon (Si) as the silicon-based active material may mean that, as described above, when based on 100 parts by weight of the silicon-based active material, it contains pure Si particles (SiOx (x = 0)) that are not combined with other particles or elements within the above range.

[0056] According to one embodiment of the present application, there is provided a lithium secondary battery in which the crystal grain size of the silicon-based active material is 100 nm or less.

[0057] In one embodiment of the present application, the crystal grain size of the SiOx (x = 0) may be 100 nm or less.

[0058] In another embodiment, the crystal grain size of the silicon-based active material may be 100 nm or less, preferably 90 nm or less, more preferably 85 nm or less, even more preferably 80 nm or less, specifically 75 nm or less, and more specifically 70 nm or less. The crystal grain size of the silicon-based active material may be 10 nm or more, preferably 15 nm or more.

[0059] The silicon-based active material has the above-mentioned crystal grain size, and by satisfying the above-mentioned range, the crystal grain boundaries are widely distributed, allowing lithium ions to be inserted uniformly during intercalation, reducing the stress exerted on the silicon particles during intercalation and thereby mitigating particle cracking. As a result, the negative electrode has the characteristic of improving its life stability. If the crystal grain size exceeds the above-mentioned range, the crystal grain boundaries within the particles are narrowly distributed, causing lithium ions to be inserted non-uniformly within the particles, resulting in large stress due to ion intercalation and particle cracking.

[0060] In particular, by adjusting the crystal grain size of the silicon-based active material, the reaction at the negative electrode during operation of the lithium secondary battery can be made uniform, thereby satisfying the ranges of the above-mentioned formulas 1 and 2.

[0061] That is, there are methods to control the volume expansion by increasing the amount of binder or reducing the cell operating range, but it is difficult to improve the cell design in a favorable direction unless the intrinsic characteristics such as increasing the resistance when increasing the amount of binder and not ensuring ED when reducing the cell operating range are overcome. However, it was found that the ranges of Equation 1 and Equation 2 can be optimized by adjusting the crystal grain size of the silicon-based active material as described above.

[0062] In this application, the term "crystal grain" refers to a collection of irregularly shaped microscopic crystal particles in a metal or material, and the term "crystal grain size" may refer to the diameter of the observed crystal grain. That is, in this application, the term "crystal grain size" refers to the size of a domain that shares the same crystal orientation within a particle, and is a different concept from the grain size or particle size, which represent the size of a substance.

[0063] In one embodiment of the present application, the crystal grain size can be calculated from the FWHM (Full Width at Half Maximum) value by XRD analysis. Specifically, a method for calculating the crystal grain size can be seen from FIG. 2. In FIG. 2, the remaining values ​​excluding L are measured by XRD analysis of the silicon-based active material, and the crystal grain size can be calculated according to the Debey-Scherrer equation, since FWHM and crystal grain size are inversely proportional to each other. In this case, the Debey-Scherrer equation is as shown in Equation 1-1 below.

[0064] [Formula 1-1] FWHM=Kλ / LCosθ

[0065] In the formula 1-1, L is the size of the crystal grain, K is a constant, θ is the Bragg angle, and λ is the wavelength of the X-ray.

[0066] Furthermore, the crystal grains have various shapes and can be measured three-dimensionally. Generally, the size of the crystal grains can be measured by commonly used circle method and diameter measurement method, but is not limited thereto.

[0067] The diameter measurement method involves drawing 5 to 10 equilibrium lines, each with a length of L mm, on a micrograph of the target particle, counting the number of crystal grains z on the lines, and averaging them. Only those grains that fit completely are counted, and those that do not fit are excluded. If the number of lines is P and the magnification is V, the average grain size can be calculated using the following formula 1-2.

[0068] [Formula 1-2] Dm=(L*P*10 3 ) / (zV)(um)

[0069] The circle method is a method in which a circle of a specified diameter is drawn on a micrograph of the target particle, and then the average area of ​​the crystal grains is calculated from the number of crystal grains that fit within the circle and the number of crystal grains that cross the boundary line, and can be calculated using the following formula 1-3.

[0070] [Formula 1-3] Fm=(Fk*10 6 ) / ((0.67n+z)V 2 )(um 2 )

[0071] In the above formulas 1-3, Fm is the average particle area, Fk is the measured area on the photograph, z is the number of particles that fit within the circle, n is the number of particles that span the arc, and V is the magnification of the microscope.

[0072] The average particle size (D50) of the silicon-based active material of the present invention may be 1 μm or more and 7 μm or less, specifically 2 μm or more and 5 μm or less. If the average particle size is below this range, the specific surface area of ​​the particles will increase excessively, resulting in an excessively high viscosity of the negative electrode slurry. As a result, the particles constituting the negative electrode slurry will not disperse smoothly. Furthermore, if the size of the silicon-based active material is too small, the contact area between the silicon particles and the conductive material will decrease due to the complex of the conductive material and the binder in the negative electrode slurry, increasing the possibility of the conductive network being broken, resulting in a decrease in capacity retention. On the other hand, if the average particle size exceeds this range, excessively large silicon particles will be present, resulting in an uneven negative electrode surface, which will cause non-uniform current density during charge and discharge. Furthermore, if the silicon particles are too large, the phase stability of the negative electrode slurry will become unstable, resulting in poor processability. This will result in a decrease in the capacity retention of the battery.

[0073] As described above, the silicon-based active material is characterized in that the crystal grain size and average particle size (D50) simultaneously satisfy the above ranges. By satisfying these ranges, the consumption of FEC in the electrolyte can be reduced, and the initial FEC content can be reduced to 10 parts by weight or less.

[0074] In one embodiment of the present application, the silicon-based active material generally 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 specific surface area is determined in accordance with DIN 66131 (using nitrogen).

[0075] 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.

[0076] In 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.

[0077] The negative electrode active material layer composition according to the present application is characterized in that, even when a silicon-based active material having an extremely high capacity is used within the above range, the performance of the negative electrode is not reduced and the output characteristics during charging and discharging are excellent, by using a specific negative electrode conductive material and a negative electrode binder that can suppress the volume expansion rate during charging and discharging, even when the silicon-based active material is contained within the above range.

[0078] In one embodiment of the present application, the silicon-based active material may have a spherical or amorphous shape.

[0079] In one embodiment of the present application, the silicon-based active material may have a non-spherical shape, and the sphericity 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 1-1, where A is the area and P is the perimeter.

[0081] [Formula 1-1] 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 been increasing in recent years as demand for high-capacity batteries has grown. However, silicon-based compounds have a limitation: their volume rapidly expands during the charge / discharge process, damaging the conductive pathways formed in the anode active material layer and 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 negative electrode binder serves to maintain 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-shaped conductive materials; sheet-shaped conductive materials; and linear conductive materials.

[0085] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the negative electrode conductive material includes at least a linear conductive material.

[0086] In one embodiment of the present application, the dot-like conductive material refers to a circular or dot-like conductive material that can be used to improve the conductivity of a negative electrode, does not cause a chemical change, is conductive, and has a conductive shape. 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, and preferably includes carbon black, which achieves high conductivity and excellent dispersibility.

[0087] 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 50m 2 / g or more 60m 2 / g or less.

[0088] 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.

[0089] In one embodiment of the present application, the conductive material may include a sheet-shaped conductive material.

[0090] The sheet-like conductive material refers to a conductive material that increases surface contact between silicon particles in the negative electrode to improve conductivity and prevents the conductive path from being broken due to volume expansion. The sheet-like conductive material may be referred to as a plate-like conductive material or a bulk-like conductive material.

[0091] In one embodiment of the present application, the sheet-like conductive material may include at least one selected from the group consisting of platelet graphite, graphene, graphene oxide, and graphite flakes, and may preferably be platelet graphite.

[0092] In one embodiment of the present application, the average particle size (D50) of the sheet-like 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 the above range, the particle size is sufficient to prevent an excessive increase in the viscosity of the negative electrode slurry and to facilitate dispersion. Therefore, when dispersion is performed using the same device and for the same time, the dispersion effect is excellent.

[0093] In one embodiment of the present application, the sheet-shaped conductive material provides a negative electrode composition having 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.

[0094] In one embodiment of the present application, the sheet-shaped conductive material may be a sheet-shaped conductive material having a high BET specific surface area; or a sheet-shaped conductive material having a low specific surface area.

[0095] In one embodiment of the present application, the sheet-like conductive material can be a sheet-like conductive material with a high specific surface area or a sheet-like conductive material with a low specific surface area, without any restrictions. However, since the dispersion of the sheet-like conductive material in the present application can have a certain degree of influence on electrode performance, it is particularly preferable to use a sheet-like conductive material with a low specific surface area in which dispersion does not cause problems.

[0096] In one embodiment of the present application, the sheet-shaped conductive material has a BET specific surface area of ​​1 m 2 / g or more.

[0097] In another embodiment, the sheet-shaped conductive material has a BET specific surface area of ​​1 m 2 / g or more 500m 2 / g or less, preferably 5m2 / g or more 300m 2 / g or less, more preferably 5m 2 / g or more 250m 2 / g or less.

[0098] In another embodiment, the sheet-shaped conductive material is a sheet-shaped conductive material having a high specific surface area, and 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.

[0099] In yet another embodiment, the sheet-shaped conductive material is a sheet-shaped conductive material having a low specific surface area, and 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 25m 2 / g or less.

[0100] 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 multiple carbon nanotube units. Specifically, unless otherwise specified, the term "bundle type" refers to a secondary shape in which multiple carbon nanotube units are arranged side by side or entangled with their longitudinal axes aligned in substantially the same direction, forming a bundle or rope-like structure. The carbon nanotube units each have a cylindrical graphite sheet with a nanometer-sized diameter and an sp2 bonding structure. Depending on the curved angle and structure of the graphite sheet, the carbon nanotube unit may exhibit conductive or semiconductive properties. Compared to entangled carbon nanotubes, the bundled carbon nanotubes can be more uniformly dispersed during negative electrode fabrication, smoothly forming a conductive network within the negative electrode, thereby improving the conductivity of the negative electrode.

[0101] 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.

[0102] 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.

[0103] In one embodiment of the present application, the negative electrode conductive material may be included in an amount of 5 parts by weight to 40 parts by weight based on 100 parts by weight of the negative electrode active material layer composition.

[0104] In another embodiment, the negative electrode conductive material may be included 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.

[0105] In one embodiment of the present application, the negative electrode conductive material may include a sheet-shaped conductive material and a linear conductive material, and the ratio of the sheet-shaped conductive material to the linear conductive material may satisfy 1:0.001 to 1:0.3.

[0106] In one embodiment of the present application, the negative electrode conductive material includes a sheet-like conductive material and a linear conductive material, and the aforementioned compositions and proportions are respectively satisfied, 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.

[0107] 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 form a contact between the silicon-based active materials, which undergo a very large volume expansion of the electrode upon charging and discharging, while the positive electrode conductive material serves to act as a buffer during rolling and to impart some conductivity, and therefore has a completely different structure and role from the negative electrode conductive material of the present invention.

[0108] 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 having 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.

[0109] In one embodiment of the present application, the plate-like 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 dotted shape to facilitate the storage and release of lithium ions.

[0110] In contrast, the plate-like conductive material used as the negative electrode conductive material is a material having a sheet or plate shape and can be expressed as plate-like graphite. That is, it is a material contained in the negative electrode active material layer to maintain a conductive path, and does not play a role in storing and releasing lithium, but refers to a material that ensures a conductive path in a sheet shape inside the negative electrode active material layer.

[0111] That is, in this application, the term "platy graphite is used as a conductive material" means that it is processed into a sheet or plate 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 contained therein has high capacity characteristics for storing and releasing lithium, and serves to store and release all lithium ions transferred from the positive electrode.

[0112] In contrast, 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.

[0113] 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.

[0114] In the present application, the negative electrode binder may include a water-based binder.

[0115] The negative electrode binder according to one embodiment of the present application serves to hold the silicon-based active material and the negative electrode conductive material together to prevent distortion and structural deformation of the negative electrode structure when the silicon-based active material expands and relaxes in volume. As long as the binder fulfills the above-mentioned role, any common negative electrode binder can be used. Specifically, a water-based binder may be used, and more specifically, a PAM-based binder may be used.

[0116] In one embodiment of the present application, the negative electrode binder may be included 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, based on 100 parts by weight of the negative electrode active material layer composition, or may be included in an amount of 5 parts by weight or more, or 8 parts by weight or more.

[0117] Compared to conventional carbon-based negative electrodes, when a Si-based negative electrode is used, a water-based binder is applied in the above-mentioned weight parts, which allows the use of a dot-like conductive material with a low content of functional groups. Due to the above characteristics, the dot-like conductive material is hydrophobic, resulting in excellent bonding strength between the conductive material and the binder.

[0118] In one embodiment of the present application, the silicon-based negative electrode may be formed by coating one or both sides of a negative electrode current collector layer with a negative electrode slurry containing the negative electrode active material layer composition.

[0119] In one embodiment of the present application, the negative electrode slurry may include a negative electrode active material layer composition; and a slurry solvent.

[0120] In one embodiment of the present application, the solid content of the negative electrode slurry may be 5% or more and 40% or less.

[0121] 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%.

[0122] 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.

[0123] When the solid content of the negative electrode slurry satisfies the above range, the viscosity during the formation of the negative electrode active material layer is suitable, and particle aggregation of the negative electrode composition is minimized, thereby enabling the negative electrode active material layer to be efficiently formed.

[0124] In one embodiment of the present application, the slurry solvent can be any solvent that can disperse the negative electrode composition described above, and specifically, water or NMP may be used.

[0125] 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.

[0126] In another embodiment, the porosity of the negative electrode active material layer may be in the range of 10% or more and 60% or less, preferably 20% or more and 50% or less, and more preferably 30% or more and 45% or less.

[0127] 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 a specific composition and content, thereby satisfying the above range, and the electrode is characterized by having an appropriate range of electrical conductivity and resistance.

[0128] In one embodiment of the present application, the negative electrode current collector layer may have a thickness of 1 μm or more and 100 μm or less, and the negative electrode active material layer may have a thickness of 20 μm or more and 500 μm or less.

[0129] 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.

[0130] 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.

[0131] The positive electrode current collector layer in the positive electrode is not particularly limited as long as it does not cause chemical changes in the battery and is conductive. For example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like 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 layer may be used in various forms, such as a film, sheet, foil, mesh, porous material, foam, or nonwoven fabric.

[0132] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the thickness of the positive electrode current collector layer is 1 μm or more and 100 μm or less, and the thickness of the positive electrode active material layer is 20 μm or more and 500 μm or less.

[0133] The positive electrode active material may be a commonly used positive electrode active material. Specifically, the positive electrode active material may be 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 The positive electrode active material may contain one or more selected from the group consisting of PO4 (M: transition metal, x+y=1). In this case, the positive electrode active material is not limited thereto.

[0134] Specifically, the positive electrode active material may be NCM or NCMA, which are commonly used.

[0135] Generally, the positive electrode active material includes layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or compounds substituted with one or more transition metals; 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 M c2 Ni-site type lithium nickel oxide represented by the chemical formula LiMnO2 (wherein M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and 0.01≦c2≦0.6 is satisfied); 2-c3 M c3 Examples of the 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); or 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.

[0136] In one embodiment of the present application, the positive electrode active material includes a lithium transition metal composite compound including nickel (Ni), cobalt (Co), and manganese (Mn), and the lithium transition metal composite compound may include single particles or secondary particles, and the single particles may have an average particle size (D50) of 1 μm or more.

[0137] 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 or less.

[0138] The single particles can have excellent particle strength even when formed into small particle sizes with an average particle size (D50) of 1 μm or more and 12 μm or less. For example, the single particles can have a strength of 650 kgf / cm 2 The particle strength may be 100 to 300 MPa when the particle is rolled with a force of 650 kgf / cm. 2 Even if the electrode is rolled with a strong force, the phenomenon of an increase in fine particles in the electrode due to particle cracking is alleviated, thereby improving the life characteristics of the battery.

[0139] The single particles can be prepared by mixing a transition metal precursor and a lithium source material and calcining the mixture. The secondary particles can be prepared by a method different from that for the single particles, and the composition of the secondary particles can be the same as or different from that of the single particles.

[0140] The method for forming the single particles is not particularly limited, but may generally be formed by over-firing at an elevated firing temperature, or may be prepared by using an additive such as a grain growth promoter that is useful for over-firing, or by changing the starting material.

[0141] For example, the calcination is performed at a temperature that allows the formation of single particles. To achieve this, the calcination must be performed at a temperature higher than that used for the production of secondary particles. For example, when the precursor composition is the same, the calcination must be performed at a temperature about 30°C to 100°C higher than that used for the production of 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 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 about 700°C to 1000°C, preferably about 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 produced. When the calcination temperature is lower than 790°C, a positive electrode active material containing a lithium transition metal compound in the form of secondary particles is produced. When the calcination temperature exceeds 950°C, excessive calcination may occur, resulting in an inadequate formation of a layered crystal structure and reduced electrochemical properties.

[0142] 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 that is an agglomeration of 30 or less primary particles.

[0143] Specifically, 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 in the form of an aggregate of several hundred primary particles.

[0144] In one embodiment of the present application, the lithium transition metal composite compound serving as 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.

[0145] 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 in the form of an aggregate of several hundred primary particles.

[0146] The lithium transition metal composite compound 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, which is an agglomeration of 30 or less primary particles.

[0147] The particle diameter (D50) of the secondary particles may be 1 μm to 20 μm, 2 μm to 17 μm, and 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.

[0148] In a further 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.

[0149] 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 long, increasing resistance and potentially reducing output characteristics.

[0150] According to a further embodiment of the present invention, 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 can have excellent particle strength even when formed to a small particle size, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, thereby improving the life characteristics of the battery.

[0151] 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.

[0152] 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.

[0153] 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 can have excellent particle strength even when formed to a small particle size, thereby mitigating the phenomenon of an increase in fine particles in the electrode due to particle cracking, and improving the life characteristics and energy density of the battery.

[0154] 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.

[0155] 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, relative to 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, relative to 100 parts by weight of the positive electrode active material.

[0156] 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 battery life characteristics.

[0157] In one embodiment of the present application, the lithium composite transition metal compound may further include secondary particles, and the secondary particles may be 85 parts by weight or less relative to 100 parts by weight of the positive electrode active material. 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 secondary particles may be 0 parts by weight or more relative to 100 parts by weight of the positive electrode active material.

[0158] When the above range is satisfied, the above-described effects due to the presence 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 as the single particle positive electrode active material, and may refer to an aggregated form of single particles.

[0159] 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, relative to 100 parts by weight of the positive electrode active material layer.

[0160] 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.

[0161] 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 may be used alone or in combination of two or more.

[0162] 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), vinylidene 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 binders may be used singly or in combination.

[0163] The separator separates the negative electrode and the positive electrode and provides a path for lithium ions to move. Any separator typically used in secondary batteries can be used without particular limitation. It is particularly preferable that the separator exhibits low resistance to electrolyte ion migration and excellent electrolyte humidification. Specifically, porous polymer films, such as those made from polyolefin polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or laminated structures of two or more layers thereof, may be used. Conventional porous nonwoven fabrics, such as nonwoven fabrics made from 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 polymeric material may be used, and may be selectively used as a single-layer or multi-layer structure.

[0164] In the present application, examples of the electrolyte include, but are not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, and molten inorganic electrolytes that can be used in manufacturing lithium secondary batteries.

[0165] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0166] Examples of the non-aqueous organic solvent that can be used include aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, γ-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, ethers, methyl propionate, and ethyl propionate.

[0167] In particular, among the carbonate organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, can be preferably used as high-viscosity organic solvents, because they have a high dielectric constant and dissociate lithium salts well. When such cyclic carbonates are mixed with linear carbonates having low viscosity and low dielectric constants, such as dimethyl carbonate and diethyl carbonate, in an appropriate ratio, an electrolyte having high electrical conductivity can be prepared, and therefore such cyclic carbonates can be more preferably used.

[0168] The metal salt may be a lithium salt, which is a substance that is easily dissolved in the non-aqueous electrolyte. For example, the anion of the lithium salt may be F - , Cl - , I - , NO3 - , N(CN)2 - , BF4 - , ClO4 - , PF6 - , (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 - One or more selected from the group consisting of:

[0169] In addition to the constituent components of the electrolyte, the electrolyte may further include one or more additives, such as a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivative, sulfur, quinoneimine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride, in order to improve the life characteristics of the battery, suppress a decrease in battery capacity, and improve the discharge capacity of the battery.

[0170] The electrolyte according to the present application may contain 10 parts by weight or less of FEC based on 100 parts by weight of the non-aqueous organic solvent.

[0171] In another embodiment, the electrolyte may contain 10 parts by weight or less, preferably 9 parts by weight or less, more preferably 8 parts by weight or less of FEC, based on 100 parts by weight of the non-aqueous organic solvent, or 3 parts by weight or more, or 4 parts by weight or more.

[0172] Anodes using silicon-based active materials undergo pulverization due to volume expansion during charge / discharge cycles, continuously exposing new particle surfaces as cycles progress. This requires an SEI layer containing an inorganic LiF layer on the surface. To achieve this, electrolytes containing 10 parts by weight or more of FEC have been used. However, electrolytes containing excessive FEC have safety concerns due to increased gas generation at high temperatures, which is detrimental to high-temperature life. Therefore, the lithium secondary battery of the present application is characterized by its ability to reduce FEC consumption by using a silicon-based active material with the aforementioned characteristics, resulting in satisfactory operation even when the FEC content is within the aforementioned weight range. This reduces high-temperature gas generation, ensures long-life characteristics, and satisfies the ranges of Equation 1 and Equation 2.

[0173] In one embodiment of the present application, there is provided a lithium secondary battery, wherein the electrolyte contains 10 parts by weight or less of FEC based on 100 parts by weight of the non-aqueous organic solvent, and the electrolyte contains 5 parts by weight or more of FEC based on 100 parts by weight of the non-aqueous organic solvent after the lithium secondary battery is charged and discharged for 300 cycles in a 4.2 V-3.2 V range, based on after activation.

[0174] In this case, the activated lithium secondary battery is the same as described above.

[0175] The charging and discharging voltages and currents may refer to common voltages and currents used in the art.

[0176] That is, when the above-described silicon-based active material is used, the consumption of FEC can be reduced due to the physical properties of the silicon-based active material. Even if the initial FEC content is within the above-described range, the FEC remains within a certain range or more in the lithium secondary battery after passing through the above conditions, so there is no problem in operation. Furthermore, by satisfying the ranges of Equation 1 and Equation 2, the battery has the characteristic of being able to ensure life characteristics.

[0177] According to one embodiment of the present invention, there is provided a battery module including the secondary battery as a unit cell, and a battery pack including the same. The battery module and the battery pack include the secondary battery having high capacity, high rate characteristics, and high cycle characteristics, and therefore can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0178] Below, preferred examples are presented to help understand the present invention. However, the following examples are merely for the purpose of illustrating the present description, and it will be apparent to those skilled in the art that various changes and modifications are possible within the scope and technical spirit of the present description, and it is natural that such changes and modifications fall within the scope of the appended claims.

[0179] [Example] <Production example> Anode manufacturing A negative electrode active material layer composition was prepared using Si (average particle size (D50): 5 μm, crystal grain size: 40 nm) as a silicon-based active material, a first conductive material, a second conductive material, and polyacrylamide as a binder in a weight ratio of 80:9.6:0.4:10. Distilled water was added as a solvent for forming a negative electrode slurry to produce a negative electrode slurry (solid concentration: 28 wt%).

[0180] 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.

[0181] As a mixing method, the first conductive material, the second conductive material, the binder, and the water were dispersed using a homomixer at 2500 rpm for 30 minutes, and then the silicon-based active material was added and dispersed at 2500 rpm for 30 minutes to prepare a negative electrode slurry.

[0182] 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. 2The coated layer was rolled and dried in a vacuum oven at 130° C. for 10 hours to form a negative electrode active material layer (thickness: 23 μm).

[0183] Cathode manufacturing LiNi as the positive electrode active material 0.6 Co 0.2 Mn 0.2 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 were added in a weight ratio of 97:1.5:1.5 to N-methyl-2-pyrrolidone (NMP) as a solvent for forming a positive electrode slurry to prepare a positive electrode slurry (solid concentration: 78 wt%).

[0184] The positive electrode slurry was applied to both sides of an aluminum current collector (thickness: 12 μm) at a rate of 537 mg / 25 cm. 2 The coated layer was rolled and dried in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer (thickness: 65 μm) to prepare a positive electrode (thickness: 77 μm, porosity: 26%).

[0185] A polyethylene separator was interposed between the positive electrode and the negative electrode of each of the examples and comparative examples, and an electrolyte was injected into the separator to prepare a lithium secondary battery.

[0186] In this case, the electrolyte was a non-aqueous organic solvent, FEC:DMC mixed in a weight ratio of 10:90, and a lithium secondary battery was fabricated using an FEC:DMC (10:90) LiPF61M solution.

[0187] Lithium secondary batteries were fabricated as described above, except that the lithium secondary batteries were fabricated as shown in Table 1 below.

[0188] [Table 1]

[0189] In Table 1, Equation 1 indicates the cell swelling evaluation. The lifespan of the lithium secondary batteries including the negative electrodes prepared in the Examples and Comparative Examples was evaluated using an electrochemical charger / discharger, and the thickness change rate of the negative electrode active material layer of the silicon-based negative electrode was measured in a fully charged state initially (after activation) and after 300 cycles.

[0190] Equation 2 was used to evaluate cell breathing. When the lithium secondary batteries including the negative electrodes prepared in the Examples and Comparative Examples were charged at 0.33 C to 4.2 V after the activation process, the change in thickness of the negative electrode active material layer in the charged state compared to after activation was measured.

[0191] As can be seen from Table 1, when the content of FEC is reduced, the LiF layer is formed less, which may increase the cell swelling value with cycling. This was confirmed by comparing Example 1 and Example 5.

[0192] When the crystal grain size of the silicon-based active material increases, the cell bleeding value increases, and this level accumulates as the cycle progresses, which can result in cell swelling becoming a problem. This result was confirmed by Comparative Examples 1 to 4.

[0193] In addition, it was confirmed in Comparative Examples 5 and 6 that if the average particle size of the silicon-based active material is large, cell deterioration occurs due to pulverization as the cycle progresses, and if the particle size is too small, side reactions accumulate and cell swelling increases.

[0194] Finally, it was confirmed from Comparative Example 7 that when an electrolyte not containing FEC is used, the LiF layer cannot be formed smoothly, which causes the lithium secondary battery to deteriorate rapidly.

[0195] Evaluation of room temperature life performance 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 rate at room temperature. The secondary batteries were subjected to a cycle test at 4.2-3.2V, 1C / 0.5C, and the number of cycles at which a sudden drop occurred was measured. The results are shown in Table 2 below.

[0196] [Table 2]

[0197] Evaluation of high temperature life performance The lithium secondary batteries including the negative electrodes and electrolytes prepared in the Examples and Comparative Examples were subjected to a lifespan evaluation using an electrochemical charger / discharger to evaluate the capacity retention rate at 45° C. The secondary batteries were subjected to a cycle test at 4.2-3.2 V, 1 C / 0.5 C, and the number of cycles at which a sudden drop occurred was measured and the results are shown in Table 3 below.

[0198] [Table 3]

[0199] As can be seen from Tables 1 to 3, the type of silicon-based active material contained in the silicon-based negative electrode is changed to an active material with small crystal grains, and a silicon-based active material with small crystal grains in the Li-rich phase is used in relation to the positive electrode, making the reaction uniform. As a result, it can be confirmed that the range of cell swelling and cell breathing is optimized, ensuring high capacity and high energy density, and also providing a lithium secondary battery with improved life characteristics.

[0200] Unlike conventional graphite-based anodes, silicon-based anodes have a large capacity, but their volume expansion is very large. This means that there is a limit to how much of the anode's capacity can be used. Therefore, cells are designed to use only a portion of the anode's capacity by adjusting the cell's NP. However, the main objective was to optimize cell swelling and cell breathing by changing the composition of the anode active material layer, focusing on the type of silicon-based active material, as mentioned above.

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 surfaces of the negative electrode current collector layer, The lithium secondary battery satisfies the following formulas 1 and 2: [Formula 1] X1≦20% [Formula 2] Y1≦10% In the formula 1 and formula 2, X1 is a cell swelling value, which means a thickness change rate of the negative electrode active material layer of the silicon-based negative electrode in a fully charged state after the lithium secondary battery is charged and discharged 300 times in a 4.2 V-3.2 V range, based on the cell swelling value after activation; Y1 is a cell breathing value, which means a change in thickness of the negative electrode active material layer of the silicon-based negative electrode when the lithium secondary battery is charged once in the range of 4.2 V to 3.2 V after activation.

2. The formula 1 satisfies 5%≦X1≦15%; 2. The lithium secondary battery according to claim 1, wherein the formula 2 satisfies 3%≦Y1≦7%.

3. The electrolyte contains 10 parts by weight or less of FEC based on 100 parts by weight of the non-aqueous organic solvent, After activation, the lithium secondary battery was charged and discharged 300 times in the 4.2V-3.2V range. The lithium secondary battery according to claim 1 , wherein the electrolyte contains 5 parts by weight or more of FEC based on 100 parts by weight of the non-aqueous organic solvent.

4. the negative electrode active material layer includes a negative electrode active material layer composition, the negative electrode active material layer composition contains a silicon-based active material, 4. 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), SiC, and a Si alloy.

5. 5. The lithium secondary battery according to claim 4, 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. 5. The lithium secondary battery according to claim 4, wherein the crystal grain size of the silicon-based active material is 100 nm or less.

7. 5. The lithium secondary battery according to claim 4, wherein the silicon-based active material has an average particle size (D50) of 1 μm or more and 7 μm or less.

8. The lithium secondary battery according to claim 4 , 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.

9. the negative electrode active material layer composition includes one or more selected from the group consisting of a negative electrode conductive material and a negative electrode binder, The lithium secondary battery according to claim 4 , wherein the negative electrode conductive material includes at least a linear conductive material.

10. The lithium secondary battery according to claim 9 , wherein the negative electrode binder comprises a water-based binder.

11. the positive electrode includes a positive electrode current collector layer; and a positive electrode active material layer provided on one or both surfaces 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).

12. the negative electrode current collector layer has a thickness of 1 μm or more and 100 μm or less, 2. The lithium secondary battery according to claim 1, wherein the thickness of the negative electrode active material layer is 20 μm or more and 500 μm or less.

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