Negative electrode for secondary battery and secondary battery including same

Abstract

A negative electrode for a secondary battery according to some embodiments comprises: a current collector; a first active material layer on the current collector; and a second active material layer on the first active material layer, wherein the first active material layer includes a first silicon-based active material and a first conductive material, the second active material layer includes a second silicon-based active material and a second conductive material, the content of the first silicon-based active material is greater than the content of the second silicon-based active material, and a ratio (Sct / Sst) of the sum of the surface areas (Sct) of the first and second conductive materials to the sum of the surface areas (Sst) of the first and second silicon-based active materials may be in the range of 0.8-1.2. Some embodiments of the present invention exhibit excellent capacity, cycle characteristics, low-temperature charge / discharge performance, and rapid charge / discharge performance.

Description

Negative electrode for secondary battery and secondary battery including same

[0001] The present invention relates to a negative electrode for a secondary battery and a secondary battery including the same.

[0002] Specifically, the present invention relates to a negative electrode for a secondary battery having a double-layer structure including a silicon-based active material and a secondary battery including the same.

[0003] This disclosure claims the benefit of priority to Republic of Korea Patent Application No. 10-2024-0095561, filed July 19, 2024, and Republic of Korea Patent Application No. 10-2025-0095796, filed July 16, 2025, the entire contents of which are incorporated by reference herein.

[0004]

[0005] To reduce dependence on fossil fuels and carbon emissions, the use of rechargeable secondary batteries, which can be recharged for extended periods of time, is increasing. Secondary batteries are used in a wide range of applications, including vehicles, portable electronic devices, and energy storage systems (ESS). In particular, lithium secondary batteries, which utilize lithium ions as their ions, boast superior energy density and lifespan, and are actively researching and developing them.

[0006] The basic performance of lithium secondary batteries, such as capacity, output, and lifespan, is greatly influenced by the anode active material contained in the lithium secondary battery. Carbon-based active materials are mainly used as anode active materials, but carbon-based active materials have a theoretical capacity of approximately 372 mAh / g, which limits their ability to increase the energy density of lithium secondary batteries. Therefore, attempts are being made to use silicon-based active materials as anode active materials, which have a theoretical capacity of approximately 4010 mAh / g, which is more than 10 times higher than that of carbon-based active materials. However, silicon-based active materials have a problem in that their volume change rate during charge and discharge is high, exceeding 300%, which causes the conductive path to be broken during continuous charge and discharge processes, preventing them from functioning as active materials.

[0007]

[0008] The technical idea of ​​the present invention aims to solve a problem by providing a negative electrode for a secondary battery having excellent capacity, cycle characteristics, low-temperature charge / discharge performance, and rapid charge / discharge performance, and a secondary battery including the negative electrode.

[0009]

[0010] Some embodiments of the present invention that can solve the above-described problems are as follows.

[0011] According to some embodiments, a negative electrode for a secondary battery includes a current collector; a first active material layer on the current collector; and a second active material layer on the first active material layer, wherein the first active material layer includes a first silicon-based active material and a first conductive material, and the second active material layer includes a second silicon-based active material and a second conductive material, and the content of the first silicon-based active material is greater than the content of the second silicon-based active material, and the sum of surface areas (S) of the first and second silicon-based active materials st ) of the sum of the surface areas of the first and second challenge materials (S ct ) ratio (S ct / S st ) can be in the range of 0.8 to 1.2.

[0012] In some embodiments, the content of the first conductive material may be greater than the content of the second conductive material.

[0013] In some embodiments, the sum of the surface areas of the first silicon-based active material (S s1 ) of the surface areas of the first challenge material (S c1 ) ratio (S c1 / S s1 ) is in the range of 0.8 to 1.2, and the sum of the surface areas of the second silicon-based active materials (S s2 ) of the surface areas of the second challenger (S c2 ) ratio (S c2 / S s2 ) can be in the range of 0.8 to 1.2.

[0014] In some embodiments, the sum of the surface areas of the first silicon-based active material (S s1 ) of the surface areas of the first challenge material (S c1 ) ratio (S c1 / S s1 ) is in the range of 0.9 to 1.1, and the sum of the surface areas of the second silicon-based active materials (S s2 ) of the surface areas of the second challenger (S c2 ) ratio (S c2 / S s2 ) can be in the range of 0.9 to 1.1.

[0015] In some embodiments, the first and second conductive materials may comprise linear conductive materials.

[0016] In some embodiments, the linear conductive material may include one or more of a carbon nanotube (CNT), a carbon nanofiber (CNF), and a vapor-grown carbon fiber (VGCF).

[0017] In some embodiments, the linear conductive material may have an average length of 5 μm or greater.

[0018] In some embodiments, the first active material layer includes a first binder, the second active material layer includes a second binder, and the viscosity of the first binder may be greater than the viscosity of the second binder.

[0019] In some embodiments, the viscosity of the first binder may be at least 10 times the viscosity of the second binder.

[0020] In some embodiments, the first active material layer includes a first carbon-based active material, the second active material layer includes a second carbon-based active material, the first and second carbon-based active materials include natural graphite and artificial graphite, and the content of natural graphite in the first carbon-based active material may be greater than the content of artificial graphite, and the content of artificial graphite in the second carbon-based active material may be greater than the content of natural graphite.

[0021] A secondary battery according to some embodiments includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, wherein the negative electrode may be the negative electrode for the secondary battery described above.

[0022] In some embodiments, the positive electrode may include a positive electrode active material represented by the following chemical formula 1.

[0023] [Chemical Formula 1]

[0024] Li p Ni 1-q-r-s Co q Mn r M 1 s O2

[0025] In the above chemical formula 1,

[0026] M 1 is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr,

[0027] 0.9≤p≤1.5, 0≤q≤1, 0≤r≤0.5, 0≤s≤0.1, 0≤q+r+s≤1.

[0028] In some embodiments, the positive electrode may include a positive electrode active material represented by the following chemical formula 2.

[0029] [Chemical Formula 2]

[0030] Li 1+a Fe 1-b M 2 b (PO 4-c )X c

[0031] In the above chemical formula 2,

[0032] M 2 contains one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y,

[0033] X contains one or more elements selected from the group consisting of F, S and N,

[0034] a, b, and c are in the ranges of -0.5≤a≤0.5, 0≤b≤0.9, and 0≤x≤0.8, respectively.

[0035] In some embodiments, the secondary battery may be cylindrical.

[0036] In some embodiments, the secondary battery may be for an automobile or an ESS (Energy Storage System).

[0037]

[0038] Some embodiments of the present invention have excellent capacity, cycle characteristics, low-temperature charge / discharge performance, and rapid charge / discharge performance.

[0039] The effects of the embodiments of the present invention are not limited to the effects mentioned above, and other effects not mentioned can be clearly derived and understood by those skilled in the art to which the embodiments of the present invention pertain from the following description. In other words, unintended effects resulting from practicing the embodiments of the present invention can also be clearly derived and understood by those skilled in the art to which the embodiments of the present invention pertain.

[0040]

[0041] Figure 1 is a drawing showing the results of Experimental Example 1.

[0042] Figure 2 is a drawing showing the results of Experimental Example 1.

[0043] Figure 3 is an image of the first active material layer (lower layer) and the second active material layer (upper layer) taken with SEM in Experimental Example 2.

[0044] Figure 4 is an image of the first active material layer (lower layer) taken by SEM in Experimental Example 4.

[0045]

[0046] The terms or words used in this specification should not be interpreted as limited to their usual or dictionary meanings, but should be interpreted in a meaning that conforms to the technical idea of ​​the present invention based on the principle that the inventor can appropriately define the meaning of the terms or words to explain his or her own invention in the best way.

[0047] In this specification, terms such as "include" or "have" are intended to specify the presence of a feature, number, step, operation, component, part or combination thereof described in the specification, but should be understood to not preclude the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts or combinations thereof. In addition, when it is said that a part such as a layer, film, region or plate is "on" another part, this includes not only the case where it is "directly above" the other part, but also the case where there is another part in between. Conversely, when it is said that a part such as a layer, film, region or plate is "under" another part, this includes not only the case where it is "directly below" the other part, but also the case where there is another part in between.

[0048] It should be understood that the examples and drawings are merely examples of the present invention and do not represent all of the technical ideas of the present invention, and that various equivalents and modifications may be substituted for them.

[0049] When describing the present invention, if it is determined that a detailed description of a known configuration or function may obscure the gist of the present invention, the detailed description will be omitted.

[0050]

[0051] Cathode for secondary batteries

[0052] A negative electrode for a secondary battery may include a current collector, a first active material layer, and a second active material layer. The first active material layer may be positioned on the current collector. The second active material layer may be positioned on the first active material layer. The first active material layer may be referred to as the "lower layer." The second active material layer may be referred to as the "upper layer."

[0053] The current collector may include a conductive material that does not cause a chemical change in the battery. As non-limiting examples, the current collector may include copper, stainless steel, aluminum, nickel, titanium, or calcined carbon. In some embodiments, the current collector may include copper. As a non-limiting example, the current collector may include a copper or stainless steel surface treated with carbon, nickel, titanium, silver, or the like. The surface of the current collector may have a micro-roughened structure formed to increase adhesion to the active material layer. The shape of the current collector may be a sheet, film, foil, net, porous body, foam, or non-woven fabric. The thickness of the current collector may range from about 3 μm to about 500 μm. The thickness of the current collector may be determined depending on the target output, target capacity, etc. of the battery.

[0054] The first active material layer may include a first silicon-based active material and a first conductive material. The second active material layer may include a second silicon-based active material and a second conductive material. Since both the first and second active material layers include silicon-based active materials, some embodiments may have high capacities.

[0055] The thickness of each of the first active material layer and the second active material layer may be in the range of about 8 μm to 240 μm. The thickness ratio of the first active material layer and the second active material layer may be in the range of about 80:20 to about 20:80. The thickness of each of the first active material layer and the second active material layer may be determined according to the target output, target capacity, etc. of the battery. In some embodiments, the thickness of the first active material layer and the thickness of the second active material layer may be substantially the same.

[0056] Each of the first silicon-based active material and the second silicon-based active material comprises Si, silicon oxide particles (SiO x, 0 <x≤1), Si-금속 합금 및 실리콘-탄소 복합체 중 하나 이상을 포함할 수 있다. 제1 실리콘계 활물질 및 제2 실리콘계 활물질 각각은 실리콘계 활물질의 외부 표면에 형성되어 있는 탄소 코팅층을 포함할 수 있다. 일부 실시예들에서, 제1 실리콘계 활물질 및 제2 실리콘계 활물질 각각은 실리콘 산화물 입자(SiO x , 0 <x≤1)를 포함할 수 있다. 일부 실시예들에서, 제1 실리콘계 활물질 및 제2 실리콘계 활물질 각각은 SiO를 포함할 수 있다.

[0057] In some embodiments, the content of the first silicon-based active material may be greater than that of the second silicon-based active material. That is, the silicon-based active material may be included in a larger amount in the lower layer than in the upper layer. As the diffusion of lithium ions decreases during low-temperature charge / discharge, lithium ions are concentrated in the upper layer. At this time, if the silicon-based active material is included in a larger amount in the upper layer than in the lower layer or if the silicon-based active material is included in the upper layer equally as in the lower layer, the silicon-based active material included in the upper layer may be excessively used. If the silicon-based active material is excessively used, crystals (Li) that cause degradation of the silicon-based active material x Si, etc.) may be formed and high-resistance SEI (solid electrolyte interphase) and by-products may increase.

[0058] In some embodiments, the weight ratio of the first silicon-based active material to the second silicon-based active material may be in the range of 51:49 to 99:1. In some embodiments, the weight ratio of the first silicon-based active material to the second silicon-based active material may be 52:48 or greater, 53:47 or greater, 54:46 or greater, 55:45 or greater, 56:44 or greater, 57:43 or greater, 58:42 or greater, 59:41 or greater, or 60:40 or greater. In some embodiments, the weight ratio of the first silicon-based active material to the second silicon-based active material may be 98:2 or less, 97:3 or less, 96:4 or less, 95:5 or less, 94:6 or less, 93:7 or less, 92:8 or less, 91:9 or less, 90:10 or less, 89:11 or less, 88:12 or less, 87:13 or less, 86:14 or less, or 85:15 or less.

[0059] In some embodiments, the sum of the content of the first silicon-based active material and the content of the second silicon-based active material may be in a range of 5 wt% to 30 wt%, based on the sum of the weight of the first active material layer and the weight of the second active material layer. In some embodiments, the sum of the content of the first silicon-based active material and the content of the second silicon-based active material may be 6 wt% or more, 7 wt% or more, 8 wt% or more, 9 wt% or more, or 10 wt% or more, based on the sum of the weight of the first active material layer and the weight of the second active material layer. In some embodiments, the sum of the content of the first silicon-based active material and the content of the second silicon-based active material may be 29 wt% or less, 28 wt% or less, 27 wt% or less, 26 wt% or less, 25 wt% or less, 24 wt% or less, 23 wt% or less, 22 wt% or less, 21 wt% or less, or 20 wt% or less, based on the sum of the weight of the first active material layer and the weight of the second active material layer. The sum of the content of the first silicon-based active material and the content of the second silicon-based active material may be determined according to a target output, a target capacity, a target lifespan, etc. of the battery.

[0060] Each of the first conductive material and the second conductive material may include a material having conductivity without causing a chemical change in the battery. As a non-limiting example, each of the first conductive material and the second conductive material may include at least one of a dot-shaped conductive material and a linear conductive material. The dot-shaped conductive material may include at least one of carbon black such as acetylene black; ketjen black; channel black; furnace black; lamp black; and thermal black, fluorocarbon, aluminum powder, nickel powder, zinc oxide, potassium titanate, titanium oxide, and polyphenylene. The linear conductive material may include at least one of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and vapor-grown carbon fibers (VGCFs). The carbon nanotube may include at least one of a single-walled carbon nanotube (SWCNT) and a multi-walled carbon nanotube (MWCNT). In some embodiments, each of the first conductive material and the second conductive material may include a single-walled carbon nanotube.

[0061] In some embodiments, each of the first and second conductive materials may include linear conductive materials. Linear conductive materials may be advantageous over dot-shaped conductive materials in maintaining a conductive network of a silicon-based active material, which exhibits a large volume change during charge and discharge. The average length of the linear conductive materials may be in the range of about 5 μm to about 20 μm. If the average length of the linear conductive materials is too short, the ability of the silicon-based active material to maintain a conductive network may be reduced.

[0062] The average length of a linear conductor can be calculated from an image captured by a scanning electron microscope (SEM), which is the arithmetic mean of the lengths of the linear conductors. That is, the average length of the linear conductors can be calculated by extracting several samples from the SEM image of the preceding conductor, measuring the lengths of the extracted samples, and then taking the arithmetic mean. As a specific example, the average length of SWCNTs can be calculated as follows. A solution (solid content 1 wt%) containing SWCNTs and CMC (carboxymethyl cellulose) in a weight ratio of 40:60 is diluted 1000-fold with water. 20 mL of the diluted solution is filtered, the filter containing the SWCNTs is dried, and then photographed using a SEM. Several samples are extracted from the SEM image, the lengths of the extracted samples are measured, and the arithmetic mean is taken to calculate the average length of the SWCNTs.

[0063] In some embodiments, the content of the first conductive material may be greater than that of the second conductive material. That is, the conductive material may be contained in greater amounts in the lower layer than in the upper layer. When the conductive material is contained in greater amounts in the lower layer than in the upper layer, the porosity of the upper layer may be greater than that of the lower layer, and the pore structure of the upper layer may be improved compared to that of the lower layer. Accordingly, the rapid charge / discharge performance of some embodiments may be enhanced.

[0064] In some embodiments, the weight ratio of the first conductive material to the second conductive material may be in the range of 51:49 to 99:1. In some embodiments, the weight ratio of the first conductive material to the second conductive material may be 52:48 or greater, 53:47 or greater, 54:46 or greater, 55:45 or greater, 56:44 or greater, 57:43 or greater, 58:42 or greater, 59:41 or greater, or 60:40 or greater. In some embodiments, the weight ratio of the first conductive material to the second conductive material may be 98:2 or less, 97:3 or less, 96:4 or less, 95:5 or less, 94:6 or less, 93:7 or less, 92:8 or less, 91:9 or less, or 90:10 or less.

[0065] In some embodiments, the sum of the content of the first conductive material and the content of the second conductive material, based on the sum of the weight of the first active material layer and the weight of the second active material layer, may be in the range of 0.005 wt% to 2 wt%. In some embodiments, the sum of the content of the first conductive material and the content of the second conductive material, based on the sum of the weight of the first active material layer and the weight of the second active material layer, may be 0.01 wt% or more, 0.02 wt% or more, 0.03 wt% or more, or 0.04 wt% or more. In some embodiments, the sum of the content of the first conductive material and the content of the second conductive material, based on the sum of the weight of the first active material layer and the weight of the second active material layer, may be 1.5 wt% or less, 1.0 wt% or less, 0.5 wt% or less, 0.4 wt% or less, 0.3 wt% or less, 0.2 wt% or less, or 0.1 wt% or less. The content of the first challenger and the content of the second challenger are S described below. ct / S st , S c1 / S s1 or S c2 / S s2 It can be appropriately selected by considering the value of .

[0066] In this specification, the sum of the surface areas of the first and second silicon-based active materials (S st ) is the specific surface area (m) of the first silicon-based active material. 2 / g) multiplied by the weight (g) of the first silicon-based active material and the specific surface area (m) of the second silicon-based active material 2 / g) means the sum of the values ​​obtained by multiplying the weight (g) of the second silicon-based active material by the sum of the surface areas of the first and second conductive materials (S ct ) is the specific surface area (m) of the first challenge material 2 / g) multiplied by the weight (g) of the first conductive material and the specific surface area (m) of the second conductive material 2 / g) means the sum of the values ​​obtained by multiplying the weight (g) of the second conductive material by the sum of the surface areas of the first silicon-based active material (S s1 ) is the specific surface area (m) of the first silicon-based active material. 2 / g) means the value obtained by multiplying the weight (g) of the first silicon-based active material by the sum of the surface areas (S) of the second silicon-based active material. In this specification, s2 ) is the specific surface area (m) of the first silicon-based active material. 2 / g) means the value obtained by multiplying the weight (g) of the second silicon-based active material. In this specification, the sum of the surface areas of the first conductive material (S c1 ) is the specific surface area (m) of the first challenge material 2 / g) means the value obtained by multiplying the weight (g) of the first conductive material. In this specification, the sum of the surface areas of the second conductive material (S c2 ) is the specific surface area (m) of the second challenger. 2 / g) multiplied by the weight (g) of the second conductive agent. The specific surface area can be measured by the BET (Brunauer-Emmett-Teller; BET) method. For example, it can be measured by the BET 6-point method by nitrogen gas adsorption flow method using a porosimetry analyzer (Bell Japan Inc, Belsorp-II mini).

[0067] In some embodiments, the specific surface area of ​​the first silicon-based active material is about 3 m 2 / g to about 25 m 2 / g or about 5 m 2 / g to about 20 m 2 / g may be in the range. In some embodiments, the specific surface area of ​​the second silicon-based active material is about 3 m 2 / g to about 25 m 2 / g or about 5 m 2 / g to about 20 m 2 / g may be in the range. In some embodiments, the specific surface area of ​​the first challenge material is about 200 m 2 / g to about 2000 m 2 / g may be in the range. In some embodiments, the specific surface area of ​​the first challenge material is about 1900 m 2 / g or less, about 1800 m 2 / g or less, about 1700 m 2 / g or less, about 1600 m 2 / g or less, about 1500 m 2 / g or less, about 1400 m 2 / g or less, about 1300 m 2 / g or less or about 1200 m 2 / g or less. In some embodiments, the specific surface area of ​​the first challenge material is 220 m 2 / g or more, 240 m 2 / g or more, 260 m 2 / g or more, 280 m 2 / g or more or 300 m 2 / g or more. In some embodiments, the specific surface area of ​​the second challenger is about 200 m 2 / g to about 2000 m 2 / g may be in the range. In some embodiments, the specific surface area of ​​the second challenger is about 1900 m 2 / g or less, about 1800 m 2 / g or less, about 1700 m 2 / g or less, about 1600 m 2 / g or less, about 1500 m 2 / g or less, about 1400 m 2 / g or less, about 1300 m 2 / g or less or about 1200 m 2 / g or less. In some embodiments, the specific surface area of ​​the second challenger is 220 m 2 / g or more, 240 m 2 / g or more, 260 m 2 / g or more, 280 m 2 / g or more or 300 m 2 / g can be more than that.

[0068] In some embodiments, the sum of the surface areas of the first and second silicon-based active materials (S st ) of the sum of the surface areas of the first and second challenge materials (S ct ) ratio (S ct / S st ) can be in the range of 0.8 to 1.2. The sum of the surface areas of the first and second silicon-based active materials (S st ) of the sum of the surface areas of the first and second challenge materials (S ct ) ratio (S ct / S st ) may be too low or too high, the cycle characteristics may deteriorate. In some embodiments, the sum of the surface areas of the first and second silicon-based active materials (S st ) of the sum of the surface areas of the first and second challenge materials (S ct ) ratio (S ct / S st ) can range from 0.9 to 1.1.

[0069] In some embodiments, the sum of the surface areas of the first silicon-based active material (S s1 ) for the sum of the surface areas of the first challenger (S c1 ) ratio (S c1 / S s1 ) can be in the range of 0.8 to 1.2. The sum of the surface areas of the first silicon-based active materials (S s1 ) for the sum of the surface areas of the first challenger (S c1 ) ratio (S c1 / S s1) may be too low or too high, the cycle characteristics may deteriorate. In some embodiments, the sum of the surface areas of the first silicon-based active material (S s1 ) for the sum of the surface areas of the first challenger (S c1 ) ratio (S c1 / S s1 ) can range from 0.9 to 1.1.

[0070] In some embodiments, the sum of the surface areas of the second silicon-based active materials (S s2 ) for the second challenge material, the sum of the surface areas (S c2 ) ratio (S c2 / S s2 ) can be in the range of 0.8 to 1.2. The sum of the surface areas of the second silicon-based active materials (S s2 ) for the second challenge material, the sum of the surface areas (S c2 ) ratio (S c2 / S s2 ) may be too low or too high, the cycle characteristics may deteriorate. In some embodiments, the sum of the surface areas of the second silicon-based active materials (S s2 ) for the second challenge material, the sum of the surface areas (S c2 ) ratio (S c2 / S s2 ) can range from 0.9 to 1.1.

[0071] In this specification, the viscosity of the binder means the viscosity at room temperature of a dispersion (solid content of about 30 wt% to about 40 wt%) in which the binder is dispersed in a solvent such as water or NMP.

[0072] In some embodiments, the first active material layer may include a first binder and the second active material layer may include a second binder. The viscosity of the first binder may be greater than the viscosity of the second binder. The adhesive strength of the first binder may be greater than the adhesive strength of the second binder. As described above, the content of the first silicon-based active material may be greater than the content of the second silicon-based active material. That is, when charge and discharge are repeated several times, the volume change of the first active material layer may be greater than the volume change of the second active material layer. In this case, if the first active material layer includes a binder with high viscosity and excellent adhesive strength, the volume change can be suppressed. Accordingly, the cycle characteristics of some embodiments may be improved.

[0073] In some embodiments, the viscosity of the first binder may be at least 10 times the viscosity of the second binder. In some embodiments, the viscosity of the first binder may be at least 20 times, at least 30 times, at least 40 times, at least 50 times, at least 60 times, at least 70 times, at least 80 times, at least 90 times the viscosity of the second binder. In some embodiments, the viscosity of the first binder may be no more than 200 times, no more than 190 times, no more than 180 times, no more than 170 times, no more than 160 times, no more than 150 times, no more than 140 times, no more than 130 times, no more than 120 times, or no more than 110 times the viscosity of the second binder. If the viscosity of the first binder is too high compared to the viscosity of the second binder, the electrical resistance of the first active material layer may increase due to the first binder. In some embodiments, the viscosity of the first binder can be from about 800 cps to about 1200 cps, from about 900 cps to about 1100 cps, from about 950 cps to about 1050 cps, or from about 990 cps to about 1010 cps. In some embodiments, the viscosity of the second binder can be from about 5 cps to about 15 cps, from about 7 cps to about 13 cps, or from about 9 cps to about 11 cps.

[0074] In some embodiments, the weight average molecular weight of the first binder may be greater than the weight average molecular weight of the second binder. In some embodiments, the weight average molecular weight of the first binder may be in the range of about 200 kDa to about 600 kDa. In some embodiments, the weight average molecular weight of the first binder may be at least about 210 kDa, at least about 220 kDa, at least about 230 kDa, at least about 240 kDa, at least about 250 kDa, at least about 260 kDa, at least about 270 kDa, at least about 280 kDa, at least about 290 kDa, or at least about 300 kDa. In some embodiments, the weight average molecular weight of the first binder can be about 550 kDa or less, about 500 kDa or less, about 450 kDa or less, about 400 kDa or less, or about 350 kDa or less. In some embodiments, the weight average molecular weight of the second binder can be in the range of about 50 kDa to about 300 kDa. In some embodiments, the weight average molecular weight of the second binder can be about 100 kDa or greater, about 110 kDa or greater, about 120 kDa or greater, or about 130 kDa or greater. In some embodiments, the weight average molecular weight of the second binder can be about 250 kDa or less, about 200 kDa or less, about 190 kDa or less, or about 180 kDa or less.

[0075] In some embodiments, each of the first binder and the second binder may include one or more of polyvinylidene fluoride, polyvinyl alcohol, carboxymethylcellulose, starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene terpolymer, sulfonated ethylene-propylene-diene terpolymer, acrylic copolymer, styrene-butadiene rubber, and styrene-butadiene rubber-carboxymethylcellulose fluoroelastomer. In some embodiments, each of the first binder and the second binder may include one or more of an acrylic copolymer and a styrene-butadiene rubber. In some embodiments, each of the first binder and the second binder may include a mixture of an acrylic copolymer and a styrene-butadiene rubber.

[0076] In some embodiments, the first active material layer may include a first thickener and the second active material layer may include a second thickener. As a non-limiting example, each of the first thickener and the second thickener may include carboxymethylcellulose (CMC). In some embodiments, the sum of the amount of the first thickener and the amount of the second thickener may be in a range of about 0.05 wt. % to about 2 wt. %, based on the sum of the weight of the first active material layer and the weight of the second active material layer. In some embodiments, the sum of the amount of the first thickener and the amount of the second thickener may be 0.06 wt. % or greater, 0.07 wt. % or greater, 0.08 wt. % or greater, 0.09 wt. % or greater, or 0.1 wt. % or greater, based on the sum of the weight of the first active material layer and the weight of the second active material layer. In some embodiments, the sum of the content of the first thickener and the content of the second thickener may be 1.8 wt% or less, 1.6 wt% or less, 1.4 wt% or less, or 1.2 wt% or less, based on the sum of the weight of the first active material layer and the weight of the second active material layer.

[0077] In some embodiments, the first active material layer may include a first carbon-based active material and the second active material layer may include a second carbon-based active material. Each of the first carbon-based active material and the second carbon-based active material may include natural graphite and artificial graphite. The content of natural graphite in the first carbon-based active material may be greater than the content of artificial graphite. The content of artificial graphite in the second carbon-based active material may be greater than the content of natural graphite. That is, the lower layer may include more natural graphite than artificial graphite, and the upper layer may include more artificial graphite than natural graphite. In general, artificial graphite has a lower degree of sphericity than natural graphite. Artificial graphite with a relatively low degree of sphericity is difficult to roll, which increases the porosity of the active material layer. On the other hand, natural graphite with a relatively high degree of sphericity is easy to roll, which decreases the porosity of the active material layer. In order to facilitate the movement of lithium ions into the electrode and increase the diffusion rate of lithium ions, it is preferable that the porosity of the upper layer be greater than that of the lower layer. Finally, some embodiments may have excellent cycle characteristics, low-temperature charge / discharge performance, and rapid charge / discharge performance.

[0078] In some embodiments, the weight ratio of natural graphite and artificial graphite in the first carbon-based active material may be in the range of 60:40 to 90:10. In some embodiments, the weight ratio of natural graphite and artificial graphite in the first carbon-based active material may be in the range of 60:40 to 80:20. In some embodiments, the weight ratio of natural graphite and artificial graphite in the first carbon-based active material may be in the range of 60:40 to 70:30. In some embodiments, the content of natural graphite and artificial graphite in the second carbon-based active material may be in the range of 10:90 to 40:60. In some embodiments, the content of natural graphite and artificial graphite in the second carbon-based active material may be in the range of 10:90 to 30:70. In some embodiments, the content of natural graphite and artificial graphite in the second carbon-based active material may be in the range of 10:90 to 20:80.

[0079] In some embodiments, the sphericity of the natural graphite may be 0.92 or greater, 0.93 or greater, or 0.94 or greater. In some embodiments, the sphericity of the natural graphite may be 0.97 or less, or 0.96 or less. In some embodiments, the sphericity of the artificial graphite may be 0.6 or greater, 0.65 or greater, or 0.7 or greater. In some embodiments, the sphericity of the artificial graphite may be 0.91 or less, or 0.9 or less. The sphericity may be calculated using a device such as a SEM. Specifically, the sphericity of the graphite particle is the ratio of the circumference of a circle having the same area as a graphite particle photographed by an SEM divided by the perimeter of the graphite particle photographed by the SEM. In this case, the arithmetic mean value for the sphericity for about 10,000 graphite particles may be referred to as the sphericity of the sample.

[0080]

[0081] secondary batteries

[0082] A secondary battery may include a positive electrode, a negative electrode, and a separator interposed between the positive and negative electrodes. Since the negative electrode has been described above, its description will be omitted.

[0083] The positive electrode may include a positive electrode current collector and a positive electrode active material layer.

[0084] The description of the positive electrode current collector is substantially the same as that of the negative electrode current collector described above, so further description thereof is omitted. However, in some embodiments, the positive electrode current collector may include aluminum.

[0085] The positive electrode active material layer may include a positive electrode active material, a binder, and a conductive agent. The description of the binder and the conductive agent is substantially the same as described above, and thus further description thereof will be omitted. However, in some embodiments, the positive electrode active material layer may include polyvinylidene fluoride as a binder. The positive electrode active material layer may include a thickener. The description of the thickener is substantially the same as described above, and thus further description thereof will be omitted.

[0086] In some embodiments, the positive electrode active material may be represented by the following chemical formula 1.

[0087] [Chemical Formula 1]

[0088] Li p Ni 1-q-r-s Co q Mn r M 1 s O2

[0089] In the above chemical formula 1,

[0090] M 1 is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr,

[0091] 0.9≤p≤1.5, 0≤q≤1, 0≤r≤0.5, 0≤s≤0.1, 0≤q+r+s≤1.

[0092]

[0093] In some embodiments, the positive electrode active material may be represented by the following chemical formula 2.

[0094] [Chemical Formula 2]

[0095] Li 1+a Fe 1-b M 2 b (PO 4-c )X c

[0096] In the above chemical formula 2,

[0097] M 2 contains one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y,

[0098] X contains one or more elements selected from the group consisting of F, S and N,

[0099] a, b, and c are in the ranges of -0.5≤a≤0.5, 0≤b≤0.9, and 0≤x≤0.8, respectively.

[0100]

[0101] In some embodiments, the secondary battery may be cylindrical. Here, cylindrical refers to the three-dimensional shape of the battery case. The cylindrical secondary battery may include a jelly roll-type electrode assembly wound with a separator interposed between the positive and negative electrodes.

[0102] In some embodiments, the secondary battery may be for an automobile or an ESS (Energy Storage System).

[0103]

[0104] Below, examples are described through comparison with comparative examples.

[0105] <Examples and Comparative Examples>

[0106] Example 1-1

[0107] A first negative electrode slurry was prepared by mixing (solid content 33.3 wt%) the first silicon-based active material, SiO, the first carbon-based active material, natural graphite and artificial graphite, the first binder with a viscosity of about 1000 cps, the first thickener, CMC, and the first conductive material, single-walled carbon nanotubes (average length 5 μm), in distilled water. The content of SiO was 15.7 wt% based on the solid content in the first negative electrode slurry. The sum of the surface areas of SiO included in the first negative electrode slurry (S s1 ) is the sum of the surface areas of single-walled carbon nanotubes contained in the first cathode slurry (S c1 ) ratio (S c1 / S s1 ) was about 1.0.

[0108] A second negative electrode slurry was prepared by mixing (solid content 33.3 wt%) the second silicon-based active material SiO, the second carbon-based active material natural graphite and artificial graphite, the second binder with a viscosity of about 10 cps, the second thickener CMC, and the second conductive material single-walled carbon nanotubes (average length 5 μm) in distilled water. The content of SiO was 9.9 wt% based on the solid content in the second negative electrode slurry. The sum of the surface areas of SiO included in the second negative electrode slurry (S s2 ) is the sum of the surface areas of single-walled carbon nanotubes contained in the second cathode slurry (S c2 ) ratio (S c2 / S s2 ) was about 1.0.

[0109] A first cathode slurry was applied onto a copper thin film, a second cathode slurry was applied onto the first cathode slurry, and then dried to manufacture a cathode including a first active material layer (lower layer) and a second active material layer (upper layer). The thickness ratio of the first active material layer and the second active material layer was 1:1.

[0110] Li(Ni) as a cathode active material 0.8 MN 0.1 CO 0.1)02, polyvinylidene fluoride as a binder and carbon black as a conductive material were mixed in N-methylpyrrolidone at a weight ratio of 96:2:2 (solid content 70 wt%) to prepare a cathode slurry. The cathode slurry was applied onto an aluminum thin film and then dried to prepare a cathode.

[0111] An electrode assembly was manufactured by sequentially stacking and winding a positive electrode, a separator, and a negative electrode. The electrode assembly was placed in a cylindrical case and an electrolyte was injected to manufacture a cylindrical secondary battery.

[0112]

[0113] Examples 1-2 to 1-5 and Comparative Examples 1-1 to 1-3

[0114] Examples 1-2 to 1-5 and Comparative Examples 1-2 to 1-3 are S c1 / S s1 Wow S c2 / S s2 A cylindrical secondary battery was manufactured in the same manner as in Example 1-1 except that the method was different (see Table 1).

[0115] Comparative Example 1-1 manufactured a cylindrical secondary battery in the same manner as Example 1-1, except that the content of SiO based on the solid content in the first negative electrode slurry and the content of SiO based on the solid content in the second negative electrode slurry were different (see Table 1).

[0116]

[0117] --SiO content S c1 / S s1 S c2 / S s2Example 1-1 First active material layer (lower layer) 15.7 1.0 - Second active material layer (upper layer) 9.9 - 1.0 Example 1-2 First active material layer (lower layer) 15.7 0.82 - Second active material layer (upper layer) 9.9 - 0.84 Example 1-3 First active material layer (lower layer) 15.7 0.92 - Second active material layer (upper layer) 9.9 - 0.90 Example 1-4 First active material layer (lower layer) 15.7 1.05 - Second active material layer (upper layer) 9.9 - 1.10 Example 1-5 First active material layer (lower layer) 15.7 1.18 - Second active material layer (upper layer) 9.9 - 1.16 Comparative example 1-1 First active material layer (lower layer) 12.8 1.0 - Second Active material layer (upper layer) 12.8-1.0 Comparative example 1-2 First active material layer (lower layer) 15.7 0.75 Second active material layer (upper layer) 9.9-0.72 Comparative example 1-3 First active material layer (lower layer) 15.7 1.26 Second active material layer (upper layer) 9.9 1.24

[0118] Examples 2-1 to 2-5 and Comparative Examples 2-1 to 2-2

[0119] The content of SiO is 15 wt% based on the solid content in the first cathode slurry, and the content of SiO is 5 wt% based on the solid content in the second cathode slurry. c1 / S s1 Wow S c2 / S s2 A cylindrical secondary battery was manufactured in the same manner as in Example 1-1, except that the components were changed as shown in Table 2 below.

[0120]

[0121] -S c1 / S s1 S c2 / S s2 Example 2-11.01.0 Example 2-20.800.83 Example 2-30.940.92 Example 2-41.081.09 Example 2-51.131.17 Comparative Example 2-10.710.75 Comparative Example 2-21.221.23

[0122] Examples 3-1 to 3-5 and Comparative Examples 3-1 to 3-2

[0123] The content of SiO is 20 wt% based on the solid content in the first cathode slurry, and the content of SiO is 10 wt% based on the solid content in the second cathode slurry, and S c1 / S s1 Wow S c2 / S s2 A cylindrical secondary battery was manufactured in the same manner as in Example 1-1, except that the components were changed as shown in Table 3 below.

[0124]

[0125] -S c1 / S s1 S c2 / S s2 Example 3-11.01.0 Example 3-20.840.81 Example 3-30.940.91 Example 3-41.081.07 Example 3-51.161.18 Comparative Example 3-10.720.77 Comparative Example 3-21.261.24

[0126] Examples 4-1 to 4-5 and Comparative Examples 4-1 to 4-2

[0127] The content of SiO is 32 wt% based on the solid content in the first cathode slurry, and the content of SiO is 8 wt% based on the solid content in the second cathode slurry. c1 / S s1 Wow S c2 / S s2 A cylindrical secondary battery was manufactured in the same manner as in Example 1-1, except that the components were changed as shown in Table 4 below.

[0128]

[0129] -S c1 / S s1 S c2 / S s2 Example 4-11.01.0 Example 4-20.810.83 Example 4-30.930.90 Example 4-41.091.07 Example 4-51.181.18 Comparative Example 4-10.770.73 Comparative Example 4-21.261.25

[0130] <Experimental Example>Experimental Example 1: Cycle characteristics, low-temperature charge / discharge performance, and rapid charge / discharge performance according to the content of the first silicon-based active material and the second silicon-based active material

[0131] The energy retention rate for the cycles of Example 1-1 and Comparative Example 1-1 was evaluated by performing rapid charge / discharge several times at 2.7 A at 10°C. The results are shown in Fig. 1.

[0132] The energy retention rate for the cycles of Example 1-1 and Comparative Example 1-1 was evaluated by performing rapid charge / discharge several times at 4.0 A at 10°C. The results are shown in Fig. 2.

[0133] Referring to FIGS. 1 and 2, it can be seen that Example 1-1, in which the content of the first silicon-based active material is greater than that of the second silicon-based active material, has a superior energy retention rate compared to Comparative Example 1-1, in which the content of the first silicon-based active material is the same as that of the second silicon-based active material.

[0134]

[0135] Experimental Example 2: Degree of degradation of SiO in the first active material layer (lower layer) and the second active material layer (upper layer) according to the content of the first silicon-based active material and the second silicon-based active material.

[0136] Comparative Example 1-1 was charged and discharged several times (approximately 100 times) at low temperature (approximately 10°C), and Comparative Example 1-1 was disassembled. The first active material layer (lower layer) and the second active material layer (upper layer) were photographed using SEM. The photographed images are as shown in Fig. 3.

[0137] Referring to Figure 3, the SiO of the second active material layer (upper layer) causes the degradation of SiO. x It can be seen that Si crystals are formed and that the second active material layer (upper layer) has more byproducts than the first active material layer (lower layer). That is, the second active material layer (upper layer) has a lot of electrochemically isolated SiO, so there are many unreacted sites, and it can be seen that lithium ions are lost from the second active material layer (upper layer).

[0138] This is because, when charging at low temperatures, the diffusion of lithium ions is reduced, making it difficult for lithium ions to move to the first active material layer (lower layer). In the case of Comparative Example 1-1, the content of the second silicon-based active material is the same as the content of the first silicon-based active material, so the second silicon-based active material in the second active material layer (upper layer) was overused. From this, it can be seen that Example 1-1, in which the content of the first silicon-based active material is greater than the content of the second silicon-based active material, will have superior cycle characteristics compared to Comparative Example 1-1, in which the content of the first silicon-based active material is the same as the content of the second silicon-based active material.

[0139]

[0140] Experimental Example 3: S c1 / S s1 and S c2 / S s2 Cycle characteristics according to

[0141] Examples 1-1 to 1-5, Comparative Examples 1-2 to 1-3, Examples 2-1 to 2-5, Comparative Examples 2-1 to 2-2, Examples 3-1 to 3-5, Comparative Examples 3-1 to 3-2, Examples 4-1 to 4-5, and Comparative Examples 4-1 to 4-2 were CC (constant current) charged at 0.5 C at 25 °C (cut-off voltage 4.2 V), CV (constant voltage) charged (cut-off current 0.0125 C), and then discharged at 0.5 C (cut-off voltage 2.5 V). After repeating this charge / discharge cycle 100 times, the capacity retention rate was evaluated. The results are shown in Table 5.

[0142] Examples 1-1 to 1-5, Comparative Examples 1-2 to 1-3, Examples 2-1 to 2-5, Comparative Examples 2-1 to 2-2, Examples 3-1 to 3-5, Comparative Examples 3-1 to 3-2, Examples 4-1 to 4-5, and Comparative Examples 4-1 to 4-2 were CC charged at 0.5 C (cut-off voltage 4.2 V) and CV charged (cut-off current 0.0125 C) at 25 °C, and then discharged at 2.0 C (cut-off voltage 2.5 V). After repeating this charge / discharge cycle 100 times, the capacity retention rate was evaluated. The results are shown in Table 6.

[0143]

[0144] S c1 / S s1 S c2 / S s2 Capacity retention rate (%) (0.5C / 0.5C) Comparative Example 1-20.75 0.7277 Exemplary Example 1-20.8 20.8482 Exemplary Example 1-30.9 20.9086 Exemplary Example 1-11.01.089 Exemplary Example 1-41.05 1.1084 Exemplary Example 1-51.18 1.1682 Comparative Example 1-31.26 1.2472 Comparative Example 2-10.7 10.7575 Exemplary Example 2-20.80 0.8381 Exemplary Example 2-30.94 0.9285 Exemplary Example 2-11.01.087 Exemplary Example 2-41.08 1.0983 Exemplary Example 2-51.13 1.1782 Comparative Example 2-21.221.2365Comparative Example 3-10.720.7786Embodiment 3-20.840.8188Embodiment 3-30.940.9189Embodiment 3-11.01.091Embodiment 3-41.081.0790Embodiment 3-51.161.1887Comparative Example 3-21.261.2479Comparative Example 4-10.770.7361Embodiment 4-20.810.8372Embodiment 4-30.930.9075Embodiment 4-11.01.087Embodiment 4-41.091.0786Embodiment 4-51.181.1884Comparative Example 4-21.261.2567

[0145] S c1 / S s1 S c2 / S s2Capacity retention rate (%) (0.5C / 2.0C) Comparative Example 1-20.75 0.7277 Exemplary Example 1-20.8 20.8483 Exemplary Example 1-30.9 20.9084 Exemplary Example 1-11.01.088 Exemplary Example 1-41.05 1.1085 Exemplary Example 1-51.18 1.1681 Comparative Example 1-31.26 1.2476 Comparative Example 2-10.7 10.7578 Exemplary Example 2-20.80 0.8382 Exemplary Example 2-30.94 0.9285 Exemplary Example 2-11.01.087 Exemplary Example 2-41.08 1.0984 Exemplary Example 2-51.13 1.1780 Comparative Example 2-21.221.2377Comparative Example 3-10.720.7779Embodiment 3-20.840.8181Embodiment 3-30.940.9183Embodiment 3-11.01.085Embodiment 3-41.081.0782Embodiment 3-51.161.1880Comparative Example 3-21.261.2475Comparative Example 4-10.770.7374Embodiment 4-20.810.8381Embodiment 4-30.930.9087Embodiment 4-11.01.092Embodiment 4-41.091.0790Embodiment 4-51.181.1889Comparative Example 4-21.261.2577

[0146] Referring to Table 5 and Table 6, S ct / S st These 0.8 to 1.2 examples are S ct / S st It can be seen that the capacity retention rate is superior compared to the comparative examples where it is less than 0.8 or exceeds 1.2. In addition, S ct / S st You can see that the closer it gets to 1, the better the capacity retention rate becomes.

[0147] Experimental Example 4: Cycle characteristics according to the length of linear conductive material

[0148] After Example 2-1 was charged and discharged 200 times at 1C and disassembled, the first active material layer (lower layer) was photographed using SEM. The photographed image is shown in Fig. 4.

[0149] Referring to Fig. 4, it can be seen that the size of the pores within the first active material layer (lower layer) is approximately 1.38 μm to approximately 2.84 μm. Accordingly, it can be seen that the length of the linear conductive material is preferably 5 μm or more.

[0150]

[0151] The above description is intended solely to illustrate the present invention. The scope of the present invention should be interpreted in accordance with the claims, and all technical ideas within the scope equivalent or equivalent thereto should be construed as being included within the scope of the present invention.

Claims

A current collector; a first active material layer on the current collector; and a second active material layer on the first active material layer, The first active material layer includes a first silicon-based active material and a first conductive material, The second active material layer includes a second silicon-based active material and a second conductive material, The content of the first silicon-based active material is greater than the content of the second silicon-based active material, The sum of the surface areas of the first and second silicon-based active materials (S st ) of the sum of the surface areas of the first and second challenge materials (S ct ) ratio (S ct / S st ) is a negative electrode for a secondary battery in the range of 0.8 to 1.

2. In claim 1, A negative electrode for a secondary battery, wherein the content of the first conductive material is greater than the content of the second conductive material. In claim 1, The sum of the surface areas of the first silicon-based active material (S s1 ) of the surface areas of the first challenge material (S c1 ) ratio (S c1 / S s1 ) is in the range of 0.8 to 1.2, The sum of the surface areas of the second silicon-based active materials (S s2 ) of the surface areas of the second challenger (S c2 ) ratio (S c2 / S s2 ) A negative electrode for a secondary battery having a value in the range of 0.8 to 1.

2. In claim 1, The sum of the surface areas of the first silicon-based active material (S s1 ) of the surface areas of the first challenge material (S c1 ) ratio (S c1 / S s1 ) is in the range of 0.9 to 1.1, The sum of the surface areas of the second silicon-based active materials (S s2 ) of the surface areas of the second challenger (S c2 ) ratio (S c2 / S s2 ) A negative electrode for a secondary battery having a value in the range of 0.9 to 1.

1. In claim 1, The first and second conductive materials are negative electrodes for secondary batteries including linear conductive materials. In claim 5, The above linear conductive material is a negative electrode for a secondary battery comprising at least one of carbon nanotubes (CNTs), carbon nanofibers (CNFs), and vapor-grown carbon fibers (VGCFs). In claim 5, The above linear conductive material is a negative electrode for a secondary battery having an average length of 5 ㎛ or more. In claim 1, The first active material layer includes a first binder, The second active material layer includes a second binder, A negative electrode for a secondary battery, wherein the viscosity of the first binder is greater than the viscosity of the second binder. In claim 8, A secondary battery negative electrode, wherein the viscosity of the first binder is at least 10 times that of the second binder. In claim 1, The first active material layer includes a first carbon-based active material, The second active material layer includes a second carbon-based active material, The above first and second carbon-based active materials include natural graphite and artificial graphite, The content of natural graphite in the above first carbon-based active material is greater than the content of artificial graphite, A negative electrode for a secondary battery, wherein the content of artificial graphite in the second carbon-based active material is greater than the content of natural graphite. It comprises a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode, A secondary battery wherein the above negative electrode is a negative electrode according to claim 1. In claim 11, The above positive electrode is a secondary battery including a positive electrode active material represented by the following chemical formula 1: [Chemical Formula 1] Li p Ni 1-q-r-s Co q Mr r M 1 s O2 In the above chemical formula 1, M 1 is at least one element selected from the group consisting of Al, Zr, Ti, Mg, Ta, Nb, Mo and Cr, 0.9≤p≤1.5, 0≤q≤1, 0≤r≤0.5, 0≤s≤0.1, 0≤q+r+s≤1. In claim 11, The above positive electrode is a secondary battery including a positive electrode active material represented by the following chemical formula 2: [Chemical Formula 2] Li 1+a Fe 1-b M 2 b (PO 4-c )X c In the above chemical formula 2, M 2 contains one or more elements selected from the group consisting of Al, Mg, Ni, Co, Mn, Ti, Ga, Cu, V, Nb, Zr, Ce, In, Zn and Y, X contains one or more elements selected from the group consisting of F, S and N, a, b, and c are in the ranges of -0.5≤a≤0.5, 0≤b≤0.9, and 0≤x≤0.8, respectively. In claim 11, The above secondary battery is a cylindrical secondary battery. In claim 11, The above secondary battery is a secondary battery for automobiles or ESS (Energy Storage System).

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