Lithium secondary battery

Abstract

The present application relates to a lithium secondary battery.

Description

lithium secondary battery

[0001] This application claims the benefit of the filing dates 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] This application relates to a lithium secondary battery.

[0003] Due to the rapid increase in fossil fuel use, the demand for alternative or clean energy is increasing, and as part of this, the most actively researched field is power generation and storage using electrochemical reactions.

[0004] A representative example of an electrochemical device that currently utilizes this electrochemical energy is the secondary battery, and its application area is gradually expanding.

[0005] With the increasing technological development and demand for mobile devices, the demand for secondary batteries as an energy source is rapidly increasing. Among these secondary batteries, lithium secondary batteries, which boast high energy density and voltage, long cycle life, and low self-discharge rates, are commercialized and widely used. Furthermore, active research is being conducted on methods for manufacturing high-density electrodes with even higher energy density per unit volume as electrodes for these high-capacity lithium secondary batteries.

[0006] Typically, secondary batteries consist of a cathode, anode, electrolyte, and a separator. The cathode contains an anode active material that inserts and deintercalates lithium ions from the anode. Carbon-based materials such as graphite, while excellent in stability and reversibility, have limited capacity. Therefore, in areas targeting high capacity, there is increasing interest in using Si-based materials with high theoretical capacity as anode materials.

[0007] That is, lithium secondary batteries are typically manufactured using lithium-intercalated compounds such as LiCoO2 and LiMn2O4 for the positive electrode, and non-lithium-intercalated materials such as carbon-based or Si-based for the negative electrode. When charging, lithium ions inserted into the positive electrode move to the negative electrode through the electrolyte, and when discharging, lithium ions move again from the negative electrode to the positive electrode. During the charging reaction, the lithium that moves from the positive electrode to the negative electrode reacts with the electrolyte to form a passivation film, SEI (solid electrolyte interface), on the surface of the negative electrode. This SEI can stabilize the structure of the negative electrode by suppressing the movement of electrons required for the reaction between the negative electrode and the electrolyte, thereby preventing the decomposition reaction of the electrolyte. However, because it is an irreversible reaction, it causes the consumption of lithium ions. In other words, the lithium consumed by the formation of SEI does not return to the positive electrode during the subsequent discharge process, reducing the capacity of the battery.

[0008] Meanwhile, efforts to improve the performance of lithium secondary batteries are demanding the development of technologies that accelerate charging speeds. Rapid charging of lithium secondary batteries requires rapid lithium ion migration during the insertion process into the anode. Therefore, battery designs are being developed that reduce the lithium diffusion distance by forming a thin-film-like anode active material layer, and increase conductivity by forming a carbon coating on the surface, thereby reducing internal resistance and achieving higher output.

[0009] However, it is difficult to achieve high capacity with an anode having such a thin-film anode active material layer. In addition, it is difficult to achieve rapid charging with an anode that has achieved high capacity. In addition, when using a silicon-based anode, the Si content of the anode is increased due to the increase in energy density, and accordingly, in order to reduce the efficiency of the high-Ni anode material, which is the anode material, an excessive amount of sacrificial anode material is applied to balance with the anode. However, in such a case, the gas content of the battery cell itself increases and the stability problem that comes with it arises.

[0010] In particular, negative electrodes using silicon-based active materials have a problem in that the entry and exit of Li within the particles is uneven during the charge / discharge process, and the degradation of the silicon-based active material itself in the Li-rich phase is accelerated, resulting in increased cell swelling and performance degradation. In addition, since pulverization occurs due to volume expansion during the charge / discharge process, new particle surfaces are continuously exposed as the cycle progresses, so an SEI layer containing an inorganic LiF layer on the surface is required, and an electrolyte with an excess of FEC is used together to form LiF. However, an electrolyte with an excess of FEC worsens gas generation at high temperatures, which is detrimental to high-temperature life and thus causes safety issues.

[0011] Therefore, research is needed to improve the lifespan characteristics while maintaining cell resistance and reducing cell swelling while using a negative electrode containing a silicon-based active material for high capacity.

[0012] <Prior Art Literature>

[0013] (Patent Document 1) Japanese Patent Publication No. 2009-080971

[0014] As a result of research on the aforementioned problems, it was found that by using a silicon-based negative electrode to secure capacity characteristics and rapid charging performance, and at the same time controlling the crystal grain size and average particle diameter as a silicon-based active material within a certain range, FEC consumption in the electrolyte can be reduced, the problem of high-temperature gas generation can be controlled, and the reaction in the negative electrode can be uniformly controlled, thereby improving cell swelling and life characteristics.

[0015] That is, when the crystal grain size and average particle diameter of the silicon-based active material are controlled within a certain range, Li ions can be uniformly diffused, and accordingly, a lithium secondary battery with optimized cell swelling and cell bleeding ranges was derived.

[0016] Accordingly, the present application relates to a lithium secondary battery including a silicon-based negative electrode and having controlled specific cell swelling and cell bleeding values.

[0017] One embodiment of the present specification provides a lithium secondary battery comprising 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 comprises 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 wherein the lithium secondary battery satisfies the following formulas 1 and 2.

[0018] [Formula 1]

[0019] X1 ≤ 20%

[0020] [Formula 2]

[0021] Y1 ≤ 10%

[0022] In the above equations 1 and 2,

[0023] X1 is the cell swelling value, which means the change rate in the thickness of the negative active material layer of the silicon-based negative electrode in the fully charged state after 300 charge / discharge cycles in the range of 4.2 V to 3.2 V of the lithium secondary battery as a reference after activation.

[0024] Y1 is a cell breeding value, which means the rate of change in the thickness of the negative 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.

[0025] The lithium secondary battery according to the present application can secure a high-capacity and high-energy-density battery by using a silicon-based negative electrode, and can also secure rapid charging performance by forming the negative electrode thickness at the level of a thin film by using a silicon-based active material with excellent capacity characteristics.

[0026] In particular, the lithium secondary battery according to the present application is characterized by solving the problems of cell swelling and cell bleeding due to charge and discharge, which are problems of the existing silicon-based negative electrode. That is, by changing the type of silicon-based active material included in the silicon-based negative electrode to an active material with small crystal grains, the silicon-based active material with small crystal grains can be applied even in the Li-rich phase in relation to the positive electrode to make the reaction uniform, and accordingly, the range of cell swelling and cell bleeding is optimized, so that a lithium secondary battery with high capacity and high energy density and improved life characteristics can be secured.

[0027] Unlike the existing graphite-based negative electrode, the silicon-based negative electrode has a large capacity and therefore has a very large volume expansion, which limits the use of the entire negative electrode capacity. The cell is designed in a way that only a portion is used by controlling the NP of the cell, but as described above, the composition of the negative electrode active material layer is changed with a focus on the type of silicon-based active material, thereby optimizing the cell swelling and cell bleeding values.

[0028] That is, by including a silicon-based active material having a crystal grain size of 100 nm or less and an average particle diameter (D50) of 1 μm or more and 7 μm or less, that is, when using a silicon-based active material whose crystal grains satisfy the above range, it has the characteristic of being able to reduce the consumption of FEC in the electrolyte, and in the case of a silicon-based negative electrode battery, sudden death occurs along with the consumption of FEC, and the life performance deteriorates rapidly, but by applying a silicon-based active material having the above characteristics, the consumption of FEC is reduced, and the cell swelling and cell bleeding values ​​described above are optimized.

[0029] Figure 1 is a diagram showing a lithium secondary battery according to the present application.

[0030] Figure 2 is a diagram showing a method for calculating crystal grain size.

[0031] Before explaining the present invention, some terms are first defined.

[0032] When a part of this specification is said to "include" a certain component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0033] In this specification, ‘p to q’ means a range of ‘p or more and q or less.’

[0034] In this specification, "specific surface area" is measured by the BET method, and specifically, is calculated from the nitrogen gas adsorption amount at liquid nitrogen temperature (77K) using BELSORP-mino II of BEL Japan. That is, in this application, the BET specific surface area may mean the specific surface area measured by the above measurement method.

[0035] In this specification, "Dn" means particle size distribution, and means the particle size at the n% point of the cumulative distribution of particle numbers according to particle size. That is, D50 is the particle size at the 50% point of the cumulative distribution of particle numbers according to particle size, D90 is the particle size at the 90% point of the cumulative distribution of particle numbers according to particle size, and D10 is the particle size at the 10% point of the cumulative distribution of particle numbers according to particle size. Meanwhile, the particle size distribution can be measured using the laser diffraction method. Specifically, after the powder to be measured is dispersed in a dispersion medium, it is introduced into a commercially available laser diffraction particle size measuring device (e.g., Microtrac S3500) and the difference in diffraction pattern according to particle size is measured when the particles pass through the laser beam, thereby calculating the particle size distribution.

[0036] As used herein, the term "a polymer comprises a monomer as a monomer unit" means that the monomer participates in a polymerization reaction and is included as a repeating unit within the polymer. As used herein, "a polymer comprises a monomer" is interpreted to mean that the polymer comprises the monomer as a monomer unit.

[0037] In this specification, the term 'polymer' is understood to be used in a broad sense including copolymers unless 'homopolymer' is specified.

[0038] In this specification, the weight average molecular weight (Mw) and number average molecular weight (Mn) are polystyrene-converted 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 standard materials. In this specification, molecular weight means weight average molecular weight unless otherwise specified.

[0039] Hereinafter, the present invention will be described in detail with reference to the drawings so that those skilled in the art can easily practice it. However, the present invention can be implemented in various different forms and is not limited to the following description.

[0040] One embodiment of the present specification provides a lithium secondary battery comprising 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 comprises 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 wherein the lithium secondary battery satisfies Equations 1 and 2.

[0041] By changing the type of silicon-based active material included in the silicon-based negative electrode to an active material with small crystal grains, the silicon-based active material with small crystal grains can be applied evenly in the Li-rich phase in relation to the positive electrode, thereby optimizing the range of cell swelling and cell bleeding, and securing a lithium secondary battery with improved life characteristics while securing high capacity and high energy density.

[0042] FIG. 1 is a diagram showing a laminated structure of a lithium secondary battery according to one embodiment of the present application. Specifically, an anode (100) including a negative electrode active material layer (20) on one surface of an anode current collector layer (10) can be confirmed, and a cathode (200) including a positive electrode active material layer (40) on one surface of an anode current collector layer (50) can be confirmed, and it is shown that the anode (100) and a cathode (200) for a lithium secondary battery are formed in a laminated structure with a separator (30) interposed therebetween.

[0043] In the present application, X1 is a cell swelling value, which means the thickness change rate of the negative active material layer of the silicon-based negative electrode in a fully charged state after 300 charge / discharge cycles in the range of 4.2 V to 3.2 V of the lithium secondary battery after activation. In other words, cell swelling represents the thickness change rate of the negative electrode after a certain cycle based on the initial lithium secondary battery (after activation). In the case of existing graphite-based negative electrodes, the problem of cell swelling due to charge / discharge did not arise significantly, but in the case of silicon-based negative electrodes, volume expansion due to charge / discharge caused problems related to life characteristics. Accordingly, if the range of Equation 1 according to the present application is satisfied, it can have a feature that can secure life characteristics.

[0044] In one embodiment of the present application, Y1 is a cell bleeding value, which means the rate of change in the 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. That is, the cell bleeding value may mean the rate of change in the thickness of the negative electrode active material layer when the lithium secondary battery is operated for one cycle.

[0045] Ultimately, cell swelling can mean the rate of change in thickness after a cycle, and cell bleeding means the rate of change in thickness after one cycle. The cell bleeding value is a value reflected in the design of the cell and is an important factor in defining the shipping thickness, and cell swelling is a factor representing the deterioration of the cell and is a factor related to the lifespan.

[0046] Here, the term “lithium secondary battery after activation” generally refers to the activation process included, and refers to a lithium secondary battery in an initial charge state or an initial discharge state after one or two charging and discharging cycles, corresponding to a lithium secondary battery in an initial state.

[0047] In one embodiment of the present application, the above formula 1 can satisfy X1 ≤ 20%, X1 ≤ 17%, X1 ≤ 15%, and can satisfy 1% ≤ X1, 3% ≤ X1, and 5% ≤ X1.

[0048] In one embodiment of the present application, the above formula 2 can satisfy Y1 ≤ 10%, Y1 ≤ 8%, Y1 ≤ 7&, and can satisfy the ranges of 1% ≤ Y1, 3% ≤ Y1.

[0049] In one embodiment of the present application, a lithium secondary battery is provided, wherein the above formula 1 is 5% ≤ X1 ≤ 15%, and the above formula 2 is 3% ≤ Y1 ≤ 7%.

[0050] The lithium secondary battery according to the present application is characterized by simultaneously having the above formulas 1 and 2 within the above ranges. That is, the ranges of formulas 1 and 2 must be satisfied to prevent performance degradation and also to have the characteristic of improving the life characteristics of the lithium secondary battery.

[0051] Below, the positive electrode, negative electrode, electrolyte, and separator included in the lithium secondary battery are each described.

[0052] 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.

[0053] The negative electrode current collector layer generally has a thickness of 1 μm to 100 μm. The negative electrode current collector is not particularly limited as long as it has high conductivity and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel surface-treated with carbon, nickel, titanium, silver, etc., aluminum-cadmium alloy, etc. can be used. In addition, the bonding strength of the negative electrode active material can be strengthened by forming fine unevenness on the surface, and it can be used in various forms such as a film, sheet, foil, net, porous body, foam, non-woven fabric, etc.

[0054] However, the thickness can be varied depending on the type and purpose of the cathode used and is not limited thereto.

[0055] In one embodiment of the present application, the negative electrode active material layer includes a negative electrode active material layer composition, and the negative electrode active material layer composition includes a silicon-based active material, and the silicon-based active material is SiOx (x=0), SiOx (0 <x<2), SiC, 및 Si 합금으로 이루어진 군에서 선택되는 1 이상을 포함할 수 있다.

[0056] In one embodiment of the present application, the negative electrode active material layer composition may include at least one selected from the group consisting of a negative electrode conductive material and a negative electrode binder.

[0057] In one embodiment of the present application, the negative electrode active material layer includes a negative electrode active material layer composition, and the negative electrode active material layer composition may include at least one selected from the group consisting of a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0058] In one embodiment of the present application, the negative electrode active material layer includes a negative electrode active material layer composition, and the negative electrode active material layer composition may include a silicon-based active material; a negative electrode conductive material; and a negative electrode binder.

[0059] In one embodiment of the present application, the silicon-based active material is SiOx (x=0), SiOx (0 <x<2), SiC, 및 Si 합금으로 이루어진 군에서 선택되는 1 이상을 포함할 수 있다.

[0060] In one embodiment of the present application, the silicon-based active material is SiOx (x=0), SiOx (0 <x<2) 및 금속 불순물로 이루어진 군에서 선택되는 1 이상을 포함하며, 상기 실리콘계 활물질 100 중량부 기준 상기 SiOx (x=0)를 70 중량부 이상 포함할 수 있다.

[0061] In another embodiment, the silicon-based active material may contain 70 parts by weight or more, preferably 80 parts by weight or more, more preferably 90 parts by weight or more, of the SiOx (x=0) based on 100 parts by weight of the silicon-based active material, and may contain 100 parts by weight or less, preferably 99 parts by weight or less, more preferably 95 parts by weight or less.

[0062] In one embodiment of the present application, the silicon-based active material may contain a metal impurity, and at this time, the impurity may be a metal that may be generally contained in the silicon-based active material, and specifically, may contain 0.1 part by weight or less based on 100 parts by weight of the silicon-based active material.

[0063] In one embodiment of the present application, the silicon-based active material may be, in particular, pure silicon (Si). Using pure silicon (Si) as the silicon-based active material may mean that, based on 100 parts by weight of the total silicon-based active material, pure Si particles (SiOx (x=0)) not combined with other particles or elements are included within the above range.

[0064] According to one embodiment of the present application, a lithium secondary battery is provided in which the crystal grain size of the silicon-based active material is 100 nm or less.

[0065] In one embodiment of the present application, the crystal grain size of the SiOx (x=0) may be 100 nm or less.

[0066] 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 even more specifically 70 nm or less. The crystal grain size of the silicon-based active material may have a range of 10 nm or more, preferably 15 nm or more.

[0067] The above silicon-based active material has the above crystal grain size, and at this time, by satisfying the above range, the grain boundaries are widely distributed, so that when lithium ions are inserted, they are inserted uniformly, thereby reducing the stress applied when lithium ions are inserted into silicon particles, and thus alleviating particle breakage. As a result, it has a characteristic that can improve the life stability of the negative electrode. When the crystal grain size exceeds the above range, the grain boundaries within the particles become narrowly distributed, and in this case, lithium ions within the particles are inserted unevenly, so that the stress due to ion insertion is large, resulting in particle breakage.

[0068] In particular, by controlling the crystal grain size of the silicon-based active material, the reaction at the negative electrode can be made uniform when operating a lithium secondary battery, and thus, it has the characteristic of satisfying the ranges of Equations 1 and 2 described above.

[0069] That is, although there are methods for controlling volume expansion, such as increasing the binder or reducing the cell operating range, it was difficult to improve the cell design in a favorable direction unless the intrinsic characteristics were overcome, such as increasing the binder, increasing the resistance, and not securing the ED when reducing the cell operating range. However, it was found that the range of Equations 1 and 2 could be optimized by controlling the crystal grain size of the silicon-based active material as described above.

[0070] In the present application, a crystal grain means a crystal particle that is a collection of irregularly shaped microscopic particles in a metal or material, and the crystal grain size may refer to the diameter of an observed crystal grain. That is, in the present application, the crystal grain size refers to the size of a domain that shares the same crystal direction within a particle, and is a different concept from the particle size or particle diameter that expresses the size of a material.

[0071] In one embodiment of the present application, the crystal grain size can be calculated as the FWHM (Full Width at Half Maximum) value through XRD analysis. Specifically, the method for calculating the crystal grain size can be seen in Fig. 2. In Fig. 2, the remaining values ​​excluding L are measured through XRD analysis of a silicon-based active material, and the crystal grain size can be measured through the Debey-Scherrer equation, which shows that the FWHM and the crystal grain size are inversely proportional. In this case, the Debey-Scherrer equation is as shown in Equation 1-1 below.

[0072] [Formula 1-1]

[0073] FWHM=Kλ / LCosθ

[0074] In the above formula 1-1,

[0075] L represents the grain size, K is a constant, θ is the bragg angle, and λ is the wavelength of the X-ray.

[0076] In addition, the shape of the crystal grains is diverse and can be measured three-dimensionally, and the size of the crystal grains can generally be measured using the commonly used circle method and diameter measurement method, but is not limited thereto.

[0077] The above diameter measurement method can be measured by drawing 5-10 equilibrium lines, each of which is L mm long, on a microscopic photograph of the target particle, counting the number of grains z on the lines, and averaging them. At this time, only those that are completely included are counted, and those that cross are excluded. If the number of lines is P and the magnification is V, the average grain diameter can be calculated using the following equations 1-2.

[0078] [Formula 1-2]

[0079] Dm = (L*P*10 3 ) / (zV) (um)

[0080] In addition, the above circle method is a method of calculating the average area of ​​crystal grains by drawing a circle of a set diameter on a microscopic photograph of the target particle and calculating the number of crystal grains within the circle and the number of crystal grains that fall on the boundary line using the following equation 1-3.

[0081] [Formula 1-3]

[0082] Fm = (Fk * 10 6 ) / ((0.67n + z) V 2 ) (um 2 )

[0083] In the above equation 1-3, Fm represents the average particle area, Fk represents the measured area on the photograph, z represents the number of particles inside the circle, n represents the number of particles in the circular arc, and V represents the magnification of the microscope.

[0084] The average particle diameter (D50) of the silicon-based active material of the present invention may be 1 μm or more and 7 μm or less, and specifically, 2 μm or more and 5 μm or less. If the average particle diameter is less than the above range, the specific surface area of ​​the particles increases excessively, and the viscosity of the negative electrode slurry increases excessively. Accordingly, the particles constituting the negative electrode slurry are not smoothly dispersed. In addition, if the size of the silicon-based active material is excessively small, the contact area between the silicon particles and the conductive material is reduced by the complex composed of the conductive material and the binder in the negative electrode slurry, so that the possibility of the conductive network being disconnected increases, and the capacity retention rate is lowered. On the other hand, if the average particle diameter exceeds the above range, excessively large silicon particles exist, making the surface of the negative electrode unsmooth, and thus causing current density unevenness during charge and discharge. In addition, if the silicon particles are excessively large, the phase stability of the negative electrode slurry becomes unstable, thereby deteriorating the processability. Accordingly, the capacity retention rate of the battery decreases.

[0085] As described above, the silicon-based active material is characterized by simultaneously satisfying the above ranges for both the crystal grain size and the average particle diameter (D50). By satisfying the above ranges, the consumption of FEC in the electrolyte can be reduced, thereby reducing the initial FEC content to 10 parts by weight or less.

[0086] In one embodiment of the present application, the silicon-based active material generally has a characteristic BET surface area. The BET surface area of ​​the silicon-based active material is preferably 0.01 m 2 / g to 150.0 m 2 / g, more preferably 0.1 m 2 / g to 100.0 m 2 / g, especially preferably 0.2 m 2 / g to 80.0 m 2 / g, most preferably 0.2 m 2 / g to 18.0 m 2 / g. BET surface area is measured according to DIN 66131 (using nitrogen).

[0087] 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.

[0088] 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, 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, based on 100 parts by weight of the negative electrode active material layer composition.

[0089] The negative electrode active material layer composition according to the present application uses a specific negative electrode conductive material and negative electrode binder that can control the volume expansion rate during the charge and discharge process even when a silicon-based active material with a significantly high capacity is used within the above range, so that the performance of the negative electrode is not deteriorated even within the above range, and the negative electrode has excellent output characteristics during charge and discharge.

[0090] In one embodiment of the present application, the silicon-based active material may have a spherical or amorphous shape.

[0091] In one embodiment of the present application, the silicon-based active material may have a non-spherical shape and its 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.

[0092] In the present application, the circularity is determined by the following equation 1-1, where A is the area and P is the boundary line.

[0093] [Formula 1-1]

[0094] 4πA / P 2

[0095] Traditionally, graphite compounds were used exclusively as anode active materials. However, with the increasing demand for high-capacity batteries, attempts to blend silicon-based compounds to increase capacity are increasing. However, silicon-based compounds have limitations: their rapid volume expansion during charge / discharge processes damages the conductive paths formed within the anode active material layer, thereby degrading battery performance.

[0096] Accordingly, 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 hold the negative electrode conductive material during charging and discharging.

[0097] In one embodiment of the present application, the negative electrode conductive material may include at least one selected from the group consisting of a dot-shaped conductive material; a planar conductive material; and a linear conductive material.

[0098] In one embodiment of the present application, a lithium secondary battery is provided in which the negative electrode conductive material includes at least a linear conductive material.

[0099] In one embodiment of the present application, the dot-shaped conductive material can be used to improve conductivity of the negative electrode, and refers to a conductive material having a circular or dot-shaped shape that forms conductivity without causing chemical change. Specifically, the dot-shaped 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, paneth black, lamp black, thermal black, conductive fiber, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene derivatives, and preferably may include carbon black in terms of implementing high conductivity and excellent dispersibility.

[0100] In one embodiment of the present application, the dot-shaped conductive material has a BET specific surface area of ​​40 m 2 / g or more than 70m 2 / g or less, preferably 45m 2 / g or more than 65m 2 / g or less, more preferably 50m 2 / g or more than 60m 2 / g can be less.

[0101] In one embodiment of the present application, the particle size of the dot-shaped conductive material may be 10 nm to 100 nm, preferably 20 nm to 90 nm, and more preferably 40 nm to 60 nm.

[0102] In one embodiment of the present application, the conductive material may include a planar conductive material.

[0103] The above-mentioned planar conductive material refers to a conductive material that improves conductivity by increasing planar contact between silicon particles within the cathode, while simultaneously suppressing the disconnection of conductive paths due to volume expansion. The above-mentioned planar conductive material may be expressed as a plate-shaped conductive material or a bulk-shaped conductive material.

[0104] In one embodiment of the present application, the planar conductive material may include at least one selected from the group consisting of plate-shaped graphite, graphene, graphene oxide, and graphite flakes, and may preferably be plate-shaped graphite.

[0105] In one embodiment of the present application, the average particle diameter (D50) of the surface-shaped 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 above range is satisfied, the sufficient particle size facilitates dispersion without causing excessive viscosity increase in the negative electrode slurry. Therefore, the dispersion effect is excellent when dispersion is performed using the same equipment and time.

[0106] In one embodiment of the present application, the surface-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.

[0107] In one embodiment of the present application, the planar conductive material may be a high-specific surface area planar conductive material having a high BET surface area; or a low-specific surface area planar conductive material.

[0108] In one embodiment of the present application, a high surface area surface conductive material or a low surface area surface conductive material can be used without limitation as the surface conductive material, but in particular, since the surface conductive material according to the present application may be affected by dispersion to some extent in electrode performance, it may be particularly preferable to use a low surface area surface conductive material that does not cause dispersion problems.

[0109] In one embodiment of the present application, the surface-shaped conductive material has a BET specific surface area of ​​1 m 2 / g can be more than that.

[0110] In another embodiment, the surface-shaped conductive material has a BET surface area of ​​1 m 2 / g or more than 500m 2 / g or less, preferably 5m 2 / g or more than 300m 2 / g or less, more preferably 5m 2 / g or more than 250m 2 / g can be less.

[0111] In another embodiment, the surface-shaped conductive material is a high surface area surface-shaped conductive material, and has a BET surface area of ​​50 m 2 / g or more than 500m 2 / g or less, preferably 80m 2 / g or more than 300m 2 / g or less, more preferably 100m 2 / g or more than 250m 2 / g can satisfy the range below.

[0112] In another embodiment, the surface-shaped conductive material is a low surface area surface-shaped conductive material, and has a BET surface area of ​​1 m 2 / g or more than 40m 2 / g or less, preferably 5m 2 / g or more than 30m 2 / g or less, more preferably 5m 2 / g or more than 25m 2 / g can satisfy the range below.

[0113] Other conductive materials may include linear conductive materials such as carbon nanotubes. The carbon nanotubes may be bundle-type carbon nanotubes. The bundle-type carbon nanotubes may include a plurality of carbon nanotube units. Specifically, the term "bundle type" herein refers to a secondary shape in the form of a bundle or rope, in which a plurality of carbon nanotube units are arranged in a substantially identical orientation in parallel or entangled with their longitudinal axes in the carbon nanotube unit direction, unless otherwise specified. The carbon nanotube units have a cylindrical shape of a graphite sheet with a nano-sized diameter and an sp2 bonding structure. At this time, the graphite sheets may exhibit conductor or semiconductor properties depending on the curling angle and structure. The above bundled carbon nanotubes can be uniformly dispersed during the manufacture of a cathode compared to entangled type carbon nanotubes, and can smoothly form a conductive network within the cathode, thereby improving the conductivity of the cathode.

[0114] In one embodiment of the present application, the negative conductive material includes a linear conductive material, and the linear conductive material may be a carbon nanotube.

[0115] In one embodiment of the present application, the carbon nanotube may be a SWCNT or / and a MWCNT. When the linear conductive agent is a SWCNT, the length of the SWCNT may be 0.5 μm to 100 μm, preferably 1 μm to 80 μm.

[0116] In one embodiment of the present application, the negative electrode conductive material may include 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.

[0117] In another embodiment, the negative electrode conductive material may include 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.

[0118] In one embodiment of the present application, the negative electrode conductive material includes a planar conductive material and a linear conductive material, and the ratio of the planar conductive material:linear conductive material can satisfy 1:0.001 to 1:0.3.

[0119] In one embodiment of the present application, since the negative electrode conductive material includes a planar conductive material and a linear conductive material and satisfies the above composition and ratio, respectively, the life characteristics of an existing lithium secondary battery are not significantly affected, and the number of points at which charging and discharging are possible increases, resulting in excellent output characteristics at a high C-rate.

[0120] The negative electrode conductive material according to the present application has a completely separate composition from the conductive material applied to the positive electrode. That is, the negative electrode conductive material according to the present application serves to secure the contact between silicon-based active materials, which undergo a significant volume expansion of the electrode due to charging and discharging, and the positive electrode conductive material serves to provide some conductivity while acting as a buffer during rolling, and is completely different in composition and role from the negative electrode conductive material of the present invention.

[0121] In addition, the negative electrode conductive material according to the present application is applied to a silicon-based active material and has a completely different composition from the conductive material applied to a graphite-based active material. That is, the conductive material used in an electrode having a graphite-based active material simply has smaller particles than the active material, and thus has the characteristics of improving output characteristics and imparting some conductivity, and is completely different in composition and role from the negative electrode conductive material applied together with a silicon-based active material as in the present invention.

[0122] In one embodiment of the present application, the plate-shaped conductive material used as the aforementioned negative electrode conductive material has a structure and function different from those of carbon-based active materials generally used as negative electrode active materials. 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 processed into a spherical or dot-shaped form to facilitate the storage and release of lithium ions.

[0123] On the other hand, the plate-shaped conductive material used as the negative electrode conductive material is a material having a planar or plate-like shape, and can be expressed as plate-shaped graphite. In other words, it refers to a material included to maintain a conductive path within the negative electrode active material layer, and is not a material that plays a role in storing and releasing lithium, but rather a material that secures a conductive path in a planar shape within the negative electrode active material layer.

[0124] That is, in the present application, the use of plate-shaped graphite as a conductive material means that it is processed into a planar or plate-shaped form and used as a material that secures a conductive path rather than a role for storing or releasing lithium. In this case, the included negative electrode active material has high capacity characteristics for lithium storage and release, and serves to store and release all lithium ions delivered from the positive electrode.

[0125] On the other hand, in the present application, the use of a carbon-based active material as an active material means that it is processed into a dot or spherical shape and used as a material that plays a role in storing or releasing lithium.

[0126] 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), fluoroelastomer, polyacrylic acid, and a material in which hydrogens thereof are substituted with Li, Na, Ca, or the like, and may also include various copolymers thereof.

[0127] In the present application, the negative electrode binder may include an aqueous binder.

[0128] 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 in order to prevent distortion and structural deformation of the negative electrode structure when the volume of the silicon-based active material expands and relaxes. If the above-mentioned role is satisfied, all general negative electrode binders can be applied, and specifically, an aqueous binder can be used, and more specifically, a PAM binder can be used.

[0129] 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, and more preferably 20 parts by weight or less, based on 100 parts by weight of the negative electrode active material layer composition, and may be included in an amount of 5 parts by weight or more, or 8 parts by weight or more.

[0130] Compared to the existing carbon-based cathode, when using a Si-based cathode, a water-based binder is applied in the above weight portion, so that a dot-shaped conductive material with a low functional group content can be used, and according to the above characteristics, the dot-shaped conductive material has hydrophobicity, resulting in excellent bonding strength with the conductive material / binder.

[0131] In one embodiment of the present application, the silicon-based negative electrode can be formed by coating a negative electrode slurry containing the negative electrode active material layer composition on one or both sides of a negative electrode current collector layer.

[0132] In one embodiment of the present application, the negative electrode slurry may include a negative electrode active material layer composition; and a slurry solvent.

[0133] In one embodiment of the present application, the solid content of the cathode slurry can satisfy 5% or more and 40% or less.

[0134] In another embodiment, the solid content of the cathode slurry can satisfy a range of 5% or more and 40% or less, preferably 7% or more and 35% or less, and more preferably 10% or more and 30% or less.

[0135] The solid content of the above cathode slurry may mean the content of the cathode composition included in the cathode slurry, and may mean the content of the cathode composition based on 100 parts by weight of the cathode slurry.

[0136] When the solid content of the above negative electrode slurry satisfies the above range, the viscosity is appropriate when forming the negative electrode active material layer, thereby minimizing particle agglomeration of the negative electrode composition, and thus has the characteristic of efficiently forming the negative electrode active material layer.

[0137] In one embodiment of the present application, the slurry solvent may be used without limitation as long as it can disperse the above-described negative electrode composition, but specifically, water or NMP may be used.

[0138] In one embodiment of the present application, the porosity of the negative electrode active material layer can satisfy a range of 10% or more and 60% or less.

[0139] In another embodiment, the porosity of the negative electrode active material layer can satisfy a 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.

[0140] The above porosity varies depending on the composition and content of the silicon-based active material; conductive material; and binder included in the negative electrode active material layer, and in particular, the silicon-based active material and conductive material according to the present application are included in a specific composition and content portion to satisfy the above range, and accordingly, the electrode is characterized by having an appropriate range of electrical conductivity and resistance.

[0141] In one embodiment of the present application, the thickness of the negative electrode current collector layer may be 1 μm or more and 100 μm or less, and the thickness of the negative electrode active material layer may be 20 μm or more and 500 μm or less.

[0142] 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.

[0143] 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 at least one selected from the group consisting of a positive electrode active material; a positive electrode conductive material; and a positive electrode binder.

[0144] In the above positive electrode, the positive electrode current collector layer is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. may be used. In addition, the positive electrode current collector layer may typically have a thickness of 1 to 500 ㎛, and fine unevenness may be formed on the surface of the positive electrode current collector layer to increase adhesion to the positive electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0145] In one embodiment of the present application, a lithium secondary battery is provided, 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.

[0146] The above 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 It may include one or more selected from the group consisting of PO4 (M: Transition metal, x+y=1). At this time, the positive electrode active material is not limited to these.

[0147] Specifically, the cathode active material may be NCM or NCMA, which are commonly used.

[0148] In general, 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; lithium iron oxide such as LiFe3O4; or a compound having the chemical formula Li 1+c1 Mn 2-c1 Lithium manganese oxides such as O4(0≤c1≤0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-c2 M c2 Ni-site type lithium nickel oxide represented by O2 (wherein, M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01≤c2≤0.6); chemical formula LiMn 2-c3 M c3 Lithium manganese composite oxide represented by O2 (wherein 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 (wherein M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., in which a part of Li in the chemical formula is replaced with an alkaline earth metal ion, but is not limited thereto. The positive electrode may be Li-metal.

[0149] In one embodiment of the present application, the positive electrode active material includes a lithium composite transition metal compound including nickel (Ni), cobalt (Co), and manganese (Mn), and the lithium composite transition metal compound includes single particles or secondary particles, and the average particle diameter (D50) of the single particles may be 1 μm or more.

[0150] For example, the average particle diameter (D50) of the single particle may be 1 ㎛ or more and 12 ㎛ or less, 1 ㎛ or more and 8 ㎛ or less, 1 ㎛ or more and 6 ㎛ or less, more than 1 ㎛ and 12 ㎛ or less, more than 1 ㎛ and 8 ㎛ or less, or more than 1 ㎛ and 6 ㎛ or less.

[0151] Even if the above single particles are formed with a small particle size of 1 ㎛ or more and 12 ㎛ or less in average particle diameter (D50), the particle strength can be excellent. For example, the above single particles can have a particle strength of 650 kgf / cm. 2 When rolled with a force of 100 to 300 MPa, the particle strength can be achieved. Accordingly, the single particle can be rolled with a force of 650 kgf / cm. 2 Even when rolled with a strong force, the phenomenon of fine particle increase in the electrode due to particle breakage is alleviated, thereby improving the life characteristics of the battery.

[0152] The above single particles can be manufactured by mixing and calcining a transition metal precursor and a lithium source material. The secondary particles can be manufactured using a different method from the above single particles, and their composition may be the same as or different from that of the single particles.

[0153] The method for forming the above single particles is not particularly limited, but can generally be formed by increasing the firing temperature to cause underfiring, using additives such as grain growth accelerators that aid underfiring, or by changing the starting material.

[0154] For example, the sintering is performed at a temperature capable of forming single particles. To form these, the sintering should be performed at a higher temperature than that during secondary particle production. For example, when the precursor composition is the same, the sintering should be performed at a temperature that is about 30°C to 100°C higher than that during secondary particle production. The sintering temperature for forming the single particles may vary depending on the metal composition in the precursor. For example, when a high-content nickel (High-Ni) NCM-based lithium composite transition metal oxide having a nickel (Ni) content of 80 mol% or more is intended to be formed into single particles, the sintering temperature may be about 700°C to 1000°C, preferably about 800°C to 950°C. When the sintering temperature satisfies the above range, a cathode active material including single particles having excellent electrochemical properties can be produced. When the sintering temperature is lower than 790°C, a cathode active material including a lithium complex transition metal compound in the form of secondary particles can be manufactured, and when it exceeds 950°C, excessive sintering occurs and a layered crystal structure is not properly formed, which may result in a deterioration in electrochemical properties.

[0155] In this specification, the term "single particle" is used to distinguish it from a secondary particle formed by the agglomeration of tens to hundreds of primary particles, and is a concept that includes a single particle formed of one primary particle and a pseudo-single particle form that is an agglomeration of 30 or fewer primary particles.

[0156] Specifically, in the present invention, the single particle may be in the form of a single particle composed of one primary particle or a quasi-single particle that is an aggregate of 30 or fewer primary particles, and the secondary particle may be in the form of an aggregate of hundreds of primary particles.

[0157] In one embodiment of the present application, the lithium composite transition metal compound, which is the positive electrode active material, further includes secondary particles, and the average particle diameter (D50) of the single particles is smaller than the average particle diameter (D50) of the secondary particles.

[0158] In the present invention, the single particle may be in the form of a single particle composed of one primary particle or a quasi-single particle that is an aggregate of 30 or fewer primary particles, and the secondary particle may be in the form of an aggregate of hundreds of primary particles.

[0159] The lithium complex transition metal compound described above may further include secondary particles. The term "secondary particle" refers to a form formed by the aggregation of primary particles, and can be distinguished from the concept of a single particle, which includes a quasi-single particle form that is a single primary particle, a single particle, or an aggregation of 30 or fewer primary particles.

[0160] The particle diameter (D50) of the secondary particles may be 1 ㎛ to 20 ㎛, 2 ㎛ to 17 ㎛, and preferably 3 ㎛ to 15 ㎛. The specific surface area (BET) of the secondary particles may be 0.05 m 2 / g to 10 m 2 / g can be, preferably 0.1 m 2 / g to 1 m 2 / g may be, more preferably 0.3 m 2 / g to 0.8 m 2 / g could be.

[0161] In a further embodiment of the present application, the secondary particles are aggregates of primary particles, and the average particle diameter (D50) of the primary particles is 0.5 µm to 3 µm. Specifically, the secondary particles may be in the form of aggregates of hundreds of primary particles, and the average particle diameter (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.

[0162] When the average particle diameter (D50) of the primary particles satisfies the above range, a single-particle positive electrode active material having excellent electrochemical properties can be formed. If the average particle diameter (D50) of the primary particles is too small, the number of agglomerates of the primary particles forming the lithium nickel-based oxide particles increases, which reduces the effect of suppressing particle breakage during rolling. In addition, if the average particle diameter (D50) of the primary particles is too large, the lithium diffusion path within the primary particles becomes longer, which may increase resistance and deteriorate output characteristics.

[0163] According to an additional embodiment of the present application, the average particle diameter (D50) of the single particle is smaller than the average particle diameter (D50) of the secondary particle. Accordingly, even if the single particle is formed with a small particle diameter, the particle strength can be excellent, and as a result, the phenomenon of fine particles increasing in the electrode due to particle breakage can be alleviated, thereby improving the life characteristics of the battery.

[0164] In one embodiment of the present application, the average particle diameter (D50) of the single particles is 1 ㎛ to 18 ㎛ smaller than the average particle diameter (D50) of the secondary particles.

[0165] For example, the average particle diameter (D50) of the single particles may be 1 ㎛ to 16 ㎛ smaller than the average particle diameter (D50) of the secondary particles, may be 1.5 ㎛ to 15 ㎛ smaller, or may be 2 ㎛ to 14 ㎛ smaller.

[0166] When the average particle diameter (D50) of the single particle is smaller than the average particle diameter (D50) of the secondary particle, for example, when the above range is satisfied, the single particle can have excellent particle strength even if it is formed into a small particle diameter, and as a result, the phenomenon of an increase in fine particles in the electrode due to particle breakage is alleviated, thereby improving the life characteristics of the battery and improving the energy density.

[0167] According to an additional embodiment of the present application, the single particles are included in an amount of 15 to 100 parts by weight relative to 100 parts by weight of the positive electrode active material. The single particles may be included 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.

[0168] 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.

[0169] When the single particles in the above range are included, excellent battery characteristics can be exhibited in combination with the aforementioned negative electrode material. In particular, when the single particles are 15 parts by weight or more, the phenomenon of fine particles increasing in the electrode due to particle breakage during the rolling process after electrode production can be alleviated, thereby improving the life characteristics of the battery.

[0170] 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.

[0171] When the above range is satisfied, the aforementioned effect due to the presence of a single-particle positive electrode active material can be maximized. When including a secondary particle positive electrode active material, the component may be the same as the component exemplified by the single-particle positive electrode active material described above, or may be a different component, and may refer to a form in which the single-particle form is aggregated.

[0172] In one embodiment of the present application, the positive electrode active material may be included 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 still 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.

[0173] The above-described positive electrode active material layer may include a positive electrode conductive material and a positive electrode binder together with the positive electrode active material described above.

[0174] At this time, the positive electrode conductive material is used to provide conductivity to the electrode, and in the battery to be constructed, as long as it does not cause a chemical change and has electronic conductivity, it can be used without any special restrictions. Specific examples include graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, summer black, and carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, and silver; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and the like, and one of these may be used alone or a mixture of two or more may be used.

[0175] In addition, the positive electrode binder plays a role of improving the adhesion between positive electrode active material particles and the adhesiveness between the positive electrode active material and the positive electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluoroelastomer, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof.

[0176] The separator is used to separate the negative electrode and the positive electrode and to provide a passage for lithium ions. Any separator commonly used in secondary batteries can be used without any particular restrictions, and in particular, one having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof, may be used. In addition, a conventional porous non-woven fabric, for example, a non-woven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may be used. In addition, a coated separator containing a ceramic component or a polymer material may be used to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure.

[0177] 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 the manufacture of lithium secondary batteries.

[0178] Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0179] As the above non-aqueous organic solvent, 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-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolan, formamide, dimethylformamide, dioxolan, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxy methane, dioxolan derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyrropionate, ethyl propionate, etc. can be used.

[0180] In particular, among the above carbonate-based organic solvents, ethylene carbonate and propylene carbonate, which are cyclic carbonates, are high-viscosity organic solvents with high dielectric constants and thus can be preferably used because they easily dissociate lithium salts. In addition, when low-viscosity, low-dielectric constant linear carbonates such as dimethyl carbonate and diethyl carbonate are mixed and used in an appropriate ratio with these cyclic carbonates, an electrolyte with high electrical conductivity can be produced, so that they can be used even more preferably.

[0181] The metal salt may be a lithium salt, and the lithium salt is a substance that is easily dissolved in the non-aqueous electrolyte, for example, the anion of the lithium salt is 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 may be used.

[0182] In addition to the electrolyte components, the electrolyte may further include one or more additives, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ethers, ethylene diamine, n-glyme, hexaphosphoric acid triamide, nitrobenzene derivatives, sulfur, quinone imine 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 battery capacity decrease, and improving the discharge capacity of the battery.

[0183] The electrolyte according to the present application may contain an FEC content of 10 parts by weight or less based on 100 parts by weight of non-aqueous organic solvent.

[0184] In another embodiment, the electrolyte may contain an FEC content of 10 parts by weight or less, preferably 9 parts by weight or less, more preferably 8 parts by weight or less, and may contain 3 parts by weight or more, or 4 parts by weight or more, based on 100 parts by weight of the non-aqueous organic solvent.

[0185] A negative electrode using a silicon-based active material requires an SEI layer containing a LiF inorganic layer on the surface because new particle surfaces are continuously exposed as the cycle progresses due to pulverization caused by volume expansion during the charge / discharge process, and an electrolyte containing 10 parts by weight or more of FEC was used together to form LiF. However, an electrolyte with an excessive amount of FEC causes severe gas generation at high temperatures, which is detrimental to high-temperature lifespan and thus causes safety issues. Therefore, in the case of a lithium secondary battery according to the present application, a silicon-based active material having the characteristics described above can be applied to reduce the consumption of FEC, and it is characterized in that there is no problem in operation even if it includes the above-mentioned weight range. Accordingly, the amount of high-temperature gas generation is reduced, so that lifespan characteristics can be secured, and it has the characteristic of satisfying the ranges of Equations 1 and 2 described above.

[0186] In one embodiment of the present application, a lithium secondary battery is provided in which FEC is included in an amount of 10 parts by weight or less based on 100 parts by weight of a non-aqueous organic solvent included in the electrolyte, and after activating the lithium secondary battery and performing 300 charge / discharge cycles in a range of 4.2 V to 3.2 V, FEC is included in an amount of 5 parts by weight or more based on 100 parts by weight of a non-aqueous organic solvent included in the electrolyte.

[0187] At this time, the lithium secondary battery after the above activation is the same as described above.

[0188] The voltage and current of the above charging and discharging may refer to general voltages and currents used in the art.

[0189] That is, when the above-mentioned silicon-based active material is applied, the consumption of FEC can be reduced due to the properties of the silicon-based active material, and even if the initial FEC content is included in the above-mentioned range, the FEC in the lithium secondary battery that has passed the above-mentioned conditions remains above a certain range so that there is no problem in operation, and the life characteristics can be secured by satisfying the ranges of Equations 1 and 2.

[0190] One embodiment of the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same. The battery module and battery pack include the secondary battery having high capacity, high rate characteristics, and cycle characteristics, and thus 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.

[0191] Hereinafter, preferred embodiments are presented to help understand the present invention, but the above embodiments are only illustrative of the present disclosure, and it is obvious to those skilled in the art that various changes and modifications are possible within the scope and technical idea of ​​the present disclosure, and it is natural that such changes and modifications fall within the scope of the appended patent claims.

[0192] <Manufacturing Example>

[0193] Manufacturing of cathode

[0194] A negative electrode active material layer composition was prepared using Si (average particle diameter (D50): 5 μm, crystal grain size 40 nm) as a silicon-based active material, polyacrylamide as a first conductive agent, a second conductive agent, and 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 prepare a negative electrode slurry. (Solid content concentration: 28 wt%)

[0195] The first challenge material is a plate-shaped graphite (specific surface area: 17 m 2 / g, average particle diameter (D50): 3.5 μm), and the second conductive material is a carbon nanotube.

[0196] As a mixing method, the first conductive agent, the second conductive agent, the binder, and the water were dispersed using a homo mixer 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.

[0197] The above negative electrode slurry was applied at 3.00 mg / cm on both sides of a copper current collector (thickness: 15 μm) as a negative electrode current collector. 2 The coating was carried out with a loading amount of , rolled (roll pressed), and dried in a vacuum oven at 130°C for 10 hours to form a negative electrode active material layer (thickness: 23 μm).

[0198] Manufacturing of anodes

[0199] LiNi as a cathode active material 0.6 Co 0.2 Mn 0.2 O2 (average particle size (D50): 15㎛), carbon black (product name: Super C65, manufacturer: Timcal) as a conductive agent, and polyvinylidene fluoride (PVdF) as a binder were added to N-methyl-2-pyrrolidone (NMP) as a solvent for forming a cathode slurry at a weight ratio of 97:1.5:1.5 to prepare a cathode slurry (solid content concentration: 78 wt%).

[0200] The positive electrode slurry was applied to both sides of an aluminum current collector (thickness: 12㎛) as a positive electrode collector at a density of 537mg / 25cm. 2 A positive electrode was manufactured by coating with a loading amount, rolling, and drying in a vacuum oven at 130°C for 10 hours to form a positive electrode active material layer (thickness: 65 μm) (positive electrode thickness: 77 μm, porosity 26%).

[0201] A lithium secondary battery was manufactured by interposing a polyethylene separator between the positive electrode and the negative electrode of the above examples and comparative examples and injecting an electrolyte.

[0202] At this time, the electrolyte was used as a non-aqueous organic solvent, FEC:DMC, mixed at a weight ratio of 10:90, and a lithium secondary battery was manufactured using a FEC:DMC (10:90) LiPF61M solution.

[0203] A lithium secondary battery was manufactured as described above, except that the lithium secondary battery was manufactured as shown in Table 1 below.

[0204] Silicon-based active material average particle size (μm) Silicon-based active material crystal grain size (nm) Electrolyte Formula 1 Formula 2 Example 1540FEC:DMC(10:90)8.63.2 Example 2550FEC:DMC(10:90)9.33.8 Example 3560FEC:DMC(10:90)10.24.2 Example 4570FEC:DMC(10:90)11.35.2 Example 5540FEC:DMC(5:95)153.2 Comparative Example 15100FEC:DMC(10:90)2312.2 Comparative Example 25150FEC:DMC(10:90)2613.1 Comparative Example 35200FEC:DMC(10:90)2915 Comparative Example 410200FEC:DMC(10:90)4415Comparison Example 50.540FEC:DMC(10:90)5019Comparison Example 61040FEC:DMC(10:90)273.5Comparison Example 7540DMC7710

[0205] In the above Table 1, Equation 1 is a cell swelling evaluation. The lifespan of lithium secondary batteries including the negative electrodes manufactured 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 the initial (after activation) and fully charged state after 300 cycles.

[0206] Equation 2 measured the change rate in the thickness of the negative electrode active material layer in the charged state compared to the after-activation state when the lithium secondary battery including the negative electrode manufactured in the above examples and comparative examples was charged to 4.2 V at 0.33 C after the activation process as a cell breeding evaluation.

[0207] As can be seen in Table 1 above, when the content of FEC decreases, less LiF layer is formed, and accordingly, the value of cell swelling according to the progress of the cycle may increase, which can be confirmed through a comparison between Example 1 and Example 5.

[0208] As the crystal grain size of the silicon-based active material increases, the cell bleeding value increases, and as the cycle progresses, the degree of this increases and eventually cell swelling may become a problem. This was confirmed through the results of Comparative Examples 1 to 4.

[0209] In addition, when the average particle size of the silicon-based active material increases, cell degradation occurs due to pulverization as the cycle progresses, and when it is excessively small, side reactions accumulate, resulting in increased cell swelling, as confirmed through comparative examples 5 and 6.

[0210] Lastly, it was confirmed through Comparative Example 7 that when an electrolyte that does not include FEC is used, the LiF layer is not formed smoothly, and as a result, the lithium secondary battery deteriorates rapidly.

[0211] Room temperature life performance evaluation

[0212] The lithium secondary batteries containing the negative electrodes manufactured in the above examples and comparative examples were subjected to a life evaluation using an electrochemical charge / discharge test, and the capacity retention rate was evaluated at room temperature. The secondary batteries were subjected to a cycle test at 4.2-3.2 V 1C / 0.5C, and the number of cycles at which sudden drops occurred was measured, and the results are shown in Table 2 below.

[0213] N (times, time of sudden drop occurrence) Example 1620 Example 2611 Example 3598 Example 4587 Example 5589 Comparative Example 1563 Comparative Example 2552 Comparative Example 3550 Comparative Example 4522 Comparative Example 5490 Comparative Example 6540 Comparative Example 7250

[0214] High temperature life performance evaluation

[0215] The lithium secondary batteries containing the negative electrodes and electrolytes manufactured in the above examples and comparative examples were subjected to a life evaluation using an electrochemical charge / discharge tester, and the capacity retention rate was evaluated at 45°C. 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, and the results are shown in Table 3 below.

[0216] N (times, time of sudden drop occurrence) Example 1607 Example 2601 Example 3587 Example 4575 Example 5596 Comparative Example 1550 Comparative Example 2541 Comparative Example 3523 Comparative Example 4501 Comparative Example 5475 Comparative Example 6537 Comparative Example 7250

[0217] As can be seen from Tables 1 to 3 above, by changing the type of silicon-based active material included in the silicon-based negative electrode to an active material with small crystal grains, the silicon-based active material with small crystal grains can be applied evenly in the Li-rich phase in relation to the positive electrode, thereby optimizing the range of cell swelling and cell bleeding, and securing a lithium secondary battery with improved life characteristics while securing high capacity and high energy density.

[0218] Unlike the existing graphite-based negative electrode, the silicon-based negative electrode has a large capacity and therefore has a very large volume expansion, which limits the use of the entire negative electrode capacity. The cell is designed in a way that only a portion is used by controlling the NP of the cell, but the main purpose is to optimize the cell swelling and cell bleeding values ​​by changing the composition of the negative electrode active material layer centered on the type of silicon-based active material as described above.

Claims

1. A lithium secondary battery comprising a positive electrode; a silicon-based negative electrode; a separator provided between the positive electrode and the negative electrode; and an electrolyte. The above 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 above lithium secondary battery is a lithium secondary battery that satisfies the following equations 1 and 2: [Formula 1] X1 ≤ 20% [Formula 2] Y1 ≤ 10% In the above equations 1 and 2, X1 is the cell swelling value, which means the change rate in the thickness of the negative active material layer of the silicon-based negative electrode in the fully charged state after 300 charge / discharge cycles in the range of 4.2 V to 3.2 V of the lithium secondary battery as a reference after activation. Y1 is a cell breeding value, which means the rate of change in the thickness of the negative 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. In claim 1, The above equation 1 is 5% ≤ X1 ≤ 15%, A lithium secondary battery in which the above formula 2 is 3% ≤ Y1 ≤ 7%.

3. In claim 1, FEC is included in an amount of 10 parts by weight or less based on 100 parts by weight of non-aqueous organic solvent included in the above electrolyte. After activating the above lithium secondary battery and charging / discharging it 300 times in the 4.2V-3.2V range, A lithium secondary battery, wherein FEC is included in an amount of 5 parts by weight or more based on 100 parts by weight of non-aqueous organic solvent included in the electrolyte.

4. In claim 1, The above negative active material layer includes a negative active material layer composition, The above negative electrode active material layer composition includes a silicon-based active material, The above silicon-based active material is SiOx (x=0), SiOx (0 <x<2), SiC, 및 Si 합금으로 이루어진 군에서 선택되는 1 이상을 포함하는 것인 리튬 이차 전지.

5. In claim 4, The above silicon-based active material is SiOx (x=0), SiOx (0 <x<2) 및 금속 불순물로 이루어진 군에서 선택되는 1 이상을 포함하며, 상기 실리콘계 활물질 100 중량부 기준 상기 SiOx (x=0)를 70 중량부 이상 포함하는 것인 리튬 이차 전지.

6. In claim 4, A lithium secondary battery wherein the crystal grain size of the above silicon-based active material is 100 nm or less.

7. In claim 4, A lithium secondary battery wherein the average particle diameter (D50) of the above silicon-based active material is 1 μm or more and 7 μm or less.

8. A lithium secondary battery according to claim 4, wherein the silicon-based active material is included in an amount of 60 parts by weight or more based on 100 parts by weight of the negative active material layer composition.

9. In claim 4, the negative electrode active material layer composition includes at least one selected from the group consisting of a negative electrode conductive material; and a negative electrode binder. A lithium secondary battery wherein the negative electrode conductive material includes at least a linear conductive material.

10. A lithium secondary battery according to claim 9, wherein the negative electrode binder includes an aqueous binder.

11. In claim 1, the positive electrode comprises 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 above positive electrode active material layer includes a positive electrode active material layer composition, The above positive electrode active material layer composition includes a positive electrode active material, The above 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 A lithium secondary battery comprising at least one selected from the group consisting of PO4 (M: Transition metal, x+y=1).

12. In claim 1, The thickness of the above negative electrode current collector layer is 1 μm or more and 100 μm or less, A lithium secondary battery, wherein the thickness of the negative electrode active material layer is 20 μm or more and 500 μm or less.

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