Negative electrode and lithium secondary battery comprising the same

Abstract

The negative electrode includes a negative electrode active material layer containing a negative electrode active material, a binder, a first conductive agent, and a second conductive agent. In the negative electrode, a number average length of the first conductive agent measured by atomic force microscopy (AFM) is 0.7 μm to 6.0 μm, an ACA is 0.5 to 5.0, and a CA is 0.1 to 8.0, the ACA being defined using a BET specific surface area and a weight % of each of the negative electrode active material, the first conductive agent, and the second conductive agent as factors, the CA being defined using a BET specific surface area and a weight % of each of the first conductive agent and the second conductive agent as factors.

Description

Technical Field

[0001] Cross-references to related applications

[0002] This application claims the benefit of Korean Patent Application No. 10-2023-0180064, filed on December 12, 2023, the disclosure of which is incorporated herein by reference. Technical Field

[0004] This disclosure relates to a negative electrode and a lithium secondary battery including the negative electrode. Background Technology

[0005] Lithium-ion batteries typically consist of a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes contain active materials that can insert and extract lithium ions.

[0006] Lithium-ion rechargeable batteries are typically manufactured as follows: a separator is inserted between a positive electrode containing a positive electrode active material made of a lithium-containing transition metal oxide and a negative electrode containing a negative electrode active material capable of storing lithium ions to form an electrode assembly; the electrode assembly is inserted into a battery casing; a non-aqueous electrolyte is injected as a medium for transferring lithium ions; and then the electrode assembly is sealed. The non-aqueous electrolyte is typically made of lithium salt and an organic solvent capable of dissolving the lithium salt.

[0007] Meanwhile, various studies have been conducted on silicon-based active materials with large discharge capacity as negative electrode active materials in order to improve the capacity of the negative electrode. However, silicon-based active materials with large discharge capacity suffer from the problem that the conductivity inside the electrode deteriorates due to the volume expansion caused by the charging and discharging of the secondary battery.

[0008] On the other hand, high-stress binder polymers have been studied to address the volume expansion problem during battery charging and discharging. However, the use of binder polymers alone cannot solve the problem of conductivity degradation caused by the expansion / contraction of the negative electrode active material.

[0009] To address this issue, carbon nanotubes (CNTs) have been used as conductive agents. However, problems arise from the application of carbon nanotubes without considering the specific surface area and weight of the negative electrode active material and other conductive agents. These problems include blockage during dispersion and transport processes, or short circuits in the conductive network due to the contraction / expansion of the active material in the electrode during charging and discharging. This degrades the battery's lifespan characteristics.

[0010] Therefore, it is necessary to study negative electrodes that have no problems in the electrode manufacturing process and have excellent battery life characteristics. Summary of the Invention

[0011] Technical issues

[0012] The present invention aims to solve the above problems and aims to provide a negative electrode with excellent processability and excellent lifetime characteristics by controlling the number-average length of the first conductive agent, the ACA represented by Equation 1 below and the CA represented by Equation 2 below to meet the scope of the present disclosure.

[0013] Furthermore, this disclosure provides a lithium secondary battery with improved lifetime characteristics by including the negative electrode.

[0014] Technical solution

[0015] [1] To address the above problems, one aspect of this disclosure provides a negative electrode comprising a negative electrode active material layer, the negative electrode active material layer comprising a negative electrode active material, a binder, a first conductive agent, and a second conductive agent. The first conductive agent has a number-average length of 0.7 μm to 6.0 μm as measured by atomic force microscopy (AFM), an ACA of 0.5 to 5.0 as expressed by Equation 1 below, and a CA of 0.1 to 8.0 as expressed by Equation 2 below.

[0016] [Equation 1]

[0017]

[0018] In equation 1 above, S A The BET specific surface area (m²) of the first conductive agent 2 / g), W A The first conductive agent is % by weight of the total weight of the negative electrode active material layer. B The BET specific surface area (m²) of the second conductive agent 2 / g), W B The second conductive agent is the weight percentage of the total weight of the negative electrode active material layer, S C The BET specific surface area (m²) of the negative electrode active material 2 / g), and W C The weight of the negative electrode active material is based on the total weight of the negative electrode active material layer.

[0019] [Equation 2]

[0020]

[0021] In equation 2 above, S A The BET specific surface area (m²) of the first conductive agent 2 / g), W A The first conductive agent is % by weight of the total weight of the negative electrode active material layer. B The BET specific surface area (m²) of the second conductive agent 2 / g), and WB The weight of the second conductive agent is based on the total weight of the negative electrode active material layer.

[0022] [2] In the present invention [1] above, the number-average length of the first conductive agent measured by atomic force microscopy (AFM) can be from 0.8 μm to 4.0 μm.

[0023] [3] In the present invention, in [1] or [2] above, the ACA can be 0.9 to 2.5.

[0024] [4] In at least one of [1] to [3] above, CA can be 0.1 to 6.0.

[0025] [5] In at least one of [1] to [4] above, the negative electrode active material may include a silicon-based active material.

[0026] [6] In at least one of [1] to [5] above, the BET specific surface area of ​​the negative electrode active material can be 0.2 m². 2 / g to 20 m 2 / g.

[0027] [7] In at least one of [1] to [6] above, the BET specific surface area of ​​the first conductive agent may be 800 m². 2 / g to 1,500 m 2 / g.

[0028] [8] In at least one of [1] to [7] above, the first conductive agent may include single-walled carbon nanotubes.

[0029] [9] In at least one of [1] to [8] above, the BET specific surface area of ​​the second conductive agent may be 25 m². 2 / g to 200 m 2 / g.

[0030]

[10] In at least one of [1] to [9] above, the second conductive agent may include a dot-type conductive agent.

[0031]

[11] In order to solve the above problems, another aspect of the present disclosure provides a lithium secondary battery including a negative electrode according to at least one of [1] to

[10] above.

[0032] Beneficial effects

[0033] The negative electrode according to this disclosure is characterized in that the number-average length of the first conductive agent, the ACA expressed by Equation 1 above, and the CA expressed by Equation 2 above satisfy the scope of this disclosure. The advantage of a negative electrode satisfying these conditions is that the negative electrode slurry prepared during its manufacture has appropriate viscosity and excellent dispersibility, which prevents clogging or coating defects during the electrode manufacturing process and enables the manufacture of a uniform electrode. Furthermore, even when the active material shrinks and expands during charging and discharging, the conductive network can be appropriately maintained, thereby improving the battery's lifespan characteristics. Detailed Implementation

[0034] The words and terms used in this detailed description and claims should not be construed as limited to their general or dictionary meanings, but should be interpreted as having meanings and concepts corresponding to the technical concept of this disclosure, in accordance with the principle that the inventors may appropriately define terms and concepts for the purpose of best describing this disclosure.

[0035] In the following description, terms such as “comprising,” “including,” and “having” are intended to indicate the presence of the features, figures, steps, components, or combinations thereof described herein, and should not be construed as excluding the presence or possible addition of one or more other features, figures, steps, components, or combinations thereof.

[0036] In this disclosure, "specific surface area (m²)" 2 The amount of nitrogen adsorbed ( / g) is measured by the BET method and can be calculated by using, for example, BELSORP-mino II from BELJapan at a liquid nitrogen temperature (77 K).

[0037] In this disclosure, "average particle size D" 50 "This represents the particle size representing 50% of the total particle size distribution based on particle volume. Average particle size D" 50 Laser diffraction can be used for measurement. Typically, laser diffraction can measure particle sizes from submicron to several millimeters and can obtain results with high reproducibility and high resolution.

[0038] In this disclosure, the number-average length (μm) of the first conductive agent is measured by measuring the length of 400 or more first conductive agents from images measured using an atomic force microscope (AFM) (Asylum Research, Cypher ES AFM System) in AC Air Topography mode (tapping mode) at a set point of 0.4 V and a scan rate of 1.5 Hz at magnifications of 20 μm × 20 μm, 15 μm × 15 and / or 10 μm × 10 μm to obtain a counted cumulative length distribution.

[0039] negative electrode

[0040] The negative electrode is described below according to this disclosure.

[0041] In the negative electrode of related technologies, carbon nanotubes (CNTs) are used as conductive agents. However, problems arise because the specific surface area and weight of the negative electrode active material and other conductive agents are not taken into account when applying carbon nanotubes. For example, blockage occurs during the dispersion and transport processes, or short circuits occur in the conductive network due to the contraction / expansion of the active material in the electrode during charging and discharging. This degrades the battery's lifespan characteristics.

[0042] The inventors of this invention have conducted continuous research to solve this problem and have thus discovered that when the area ratio between the negative electrode active material and the conductive agent, as well as the area ratio between the conductive agents, are within a specific range while controlling the number-average length of the first conductive agent as measured by atomic force microscopy (AFM), the electrode manufacturing process can be excellent and the battery life characteristics can be improved, thereby completing this invention.

[0043] According to this disclosure, the negative electrode includes a negative electrode active material layer, which comprises a negative electrode active material, a binder, a first conductive agent, and a second conductive agent. The number-average length of the first conductive agent, measured using atomic force microscopy (AFM), is from 0.7 μm to 6.0 μm. The ACA, expressed by Equation 1 below, is from 0.5 to 5.0. The CA, expressed by Equation 2 below, is from 0.1 to 8.0.

[0044] [Equation 1]

[0045]

[0046] In equation 1 above, S A The BET specific surface area (m²) of the first conductive agent 2 / g), W A The first conductive agent is % by weight of the total weight of the negative electrode active material layer. B The BET specific surface area (m²) of the second conductive agent 2 / g), W B The second conductive agent is the weight percentage of the total weight of the negative electrode active material layer, S C The BET specific surface area (m²) of the negative electrode active material 2 / g), and W C The weight of the negative electrode active material is based on the total weight of the negative electrode active material layer.

[0047] [Equation 2]

[0048]

[0049] In equation 2 above, S A The BET specific surface area (m²) of the first conductive agent 2 / g), W A The first conductive agent is % by weight of the total weight of the negative electrode active material layer. B The BET specific surface area (m²) of the second conductive agent 2 / g), and W B The weight of the second conductive agent is based on the total weight of the negative electrode active material layer.

[0050] The number-average length of the first conductive agent, measured using atomic force microscopy (AFM), ranges from 0.7 μm to 6.0 μm. Specifically, the number-average length of the first conductive agent, measured using AFM, can be 0.7 μm or greater, 0.75 μm or greater, or 0.8 μm or greater, and can be 6.0 μm or less, 5.5 μm or less, 5.0 μm or less, 4.5 μm or less, or 4.0 μm or less. The number-average length can be measured using the aforementioned atomic force microscopy, and in this case, the length characteristics can be accurately measured, enabling the excellent effects of this disclosure to be achieved.

[0051] Typically, when measuring the number-average length of a conductive agent, in many cases, the volume-cumulative average particle size (D) of the dispersion in which the conductive agent is added is measured and used. 50 and / or D 90 This allows for the prediction of the formation and uniform distribution of conductive paths in the electrode. However, the particle size distribution in the dispersed state cannot accurately reflect the actual length of the conductive agent, leading to the problem that electrode performance differs even when dispersions with the same particle size distribution are applied. According to one embodiment of this disclosure, the length characteristics obtained from the counted cumulative length distribution in atomic force microscopy images are identical between the dispersed and electrode states, and therefore can be more effective in improving electrode performance.

[0052] When the number-average length is less than 0.7 μm, maintaining the conductive network connection path may be difficult due to volume changes in the electrode caused by charging and discharging. Furthermore, when the number-average length exceeds 6.0 μm, the number of first conductive agents per unit weight decreases, but the distribution of the first conductive agent within the electrode may be uneven. Additionally, the viscosity of the slurry may increase, potentially leading to reduced dispersibility, clogging, or poor coating during slurry transport. Therefore, when the above ranges are met, the distribution of the first conductive agent within the electrode can be maintained uniformly while appropriately maintaining the conductive network connection path, thereby improving lifetime characteristics.

[0053] The ACA, as expressed in Equation 1 above, is the ratio of the area of ​​the negative electrode active material to the area of ​​the first conductive agent and the second conductive agent, and can be obtained by multiplying the specific surface area and weight percentage of each material. To achieve the superior effects anticipated by this disclosure, an appropriate combination of the specific surface area and weight percentage of each of the first conductive agent, the second conductive agent, and the negative electrode active material is required.

[0054] In detail, in the denominator, the total area of ​​the conductive agent can be calculated by adding the value obtained by multiplying the specific surface area of ​​the first conductive agent by its weight percentage and the value obtained by multiplying the specific surface area of ​​the second conductive agent by its weight percentage. Furthermore, in the molecule, the total area of ​​the negative electrode active material can be calculated by multiplying the specific surface area of ​​the negative electrode active material by its weight percentage. When the ACA (which is the ratio of the total area of ​​the negative electrode active material to the total area of ​​the conductive agent) is controlled within a specific range, excellent processability and excellent lifetime characteristics can be achieved.

[0055] For example, a large total area of ​​the negative electrode active material indicates a relatively large loading of the negative electrode active material, a relatively large specific surface area, or a large average particle size D of the negative electrode active material. 50 Small. When the loading is large, an improvement in capacity characteristics can be expected; conversely, it may be a counterexample where no conductive pathways are formed normally. Furthermore, a large total area of ​​the conductive agent can indicate that conductive pathways are formed normally and a network structure exists, but it can also indicate a low loading of active material. Therefore, it can be understood that the area ratio of active material to conductive agent has a balance relationship. Moreover, regarding the formation of conductive pathways, it may be difficult to conclude that the conductive pathways are formed normally simply because the total area occupied by the conductive agent is large. Therefore, it may not be easy to determine the formation of conductive pathways based on factors such as the content or specific surface area of ​​the conductive agent, the content of the negative electrode active material, and the average particle size D. 50 The relationship between specific surface area and its use in improving battery performance.

[0056] However, according to one embodiment of this disclosure, the first conductive agent and the second conductive agent are used together in the negative electrode active material layer to simultaneously form conductive paths between negative electrode active materials, conductive paths inside the negative electrode active material, and conductive paths in the voids of the negative electrode active material. This allows the area occupied by the conductive agent and the degree of formation of the conductive path to be controlled as proportionally as possible, and thus the ratio relative to the area of ​​the negative electrode active material can be controlled, which can improve battery performance.

[0057] The ACA represented by Equation 1 above ranges from 0.5 to 5.0. Specifically, the ACA represented by Equation 1 above can be 0.5 or greater, 0.6 or greater, 0.7 or greater, 0.8 or greater, or 0.9 or greater, and can be 5.0 or less, 4.5 or less, 4.0 or less, 3.5 or less, 3.0 or less, or 2.5 or less. When the ACA is less than 0.5, the initial conductive path of the electrode can be sufficiently formed, but the content of the negative electrode active material decreases, resulting in an increase in the viscosity of the slurry and potentially poor processability. Therefore, the battery's lifespan characteristics deteriorate. Furthermore, when the ACA exceeds 5.0, the initial conductive path of the electrode is not sufficiently formed, and the battery's lifespan characteristics may deteriorate during charging and discharging. Therefore, when this range is met, no problems can occur during the electrode manufacturing process, enabling excellent processability, sufficient formation of the initial conductive path of the electrode, and continuous maintenance of the conductive path during charging and discharging, which can achieve excellent battery life characteristics.

[0058] The CA expressed by Equation 2 above is the ratio of the area of ​​the second conductive agent to the area of ​​the first conductive agent, and can be obtained by multiplying the specific surface area and weight percentage of each material. To achieve the superior effects anticipated by this disclosure, an appropriate combination of the specific surface area and weight percentage of each of the first and second conductive agents is also required.

[0059] Specifically, in the denominator, the total area of ​​the first conductive agent can be obtained by multiplying the specific surface area of ​​the first conductive agent by its weight percentage. Furthermore, in the numerator, the total area of ​​the second conductive agent can be obtained by multiplying the specific surface area of ​​the second conductive agent by its weight percentage. CA is the ratio of the total area of ​​the second conductive agent to the total area of ​​the first conductive agent, and excellent processability and lifespan characteristics can be achieved when the ratio between the total area of ​​the first and second conductive agents is controlled within a specific range.

[0060] The CA expressed by Equation 2 above is from 0.1 to 8.0. Specifically, the CA expressed by Equation 2 above can be from 0.1 to 7.5, more specifically from 0.1 to 7.0, more specifically from 0.1 to 6.5, and more specifically from 0.1 to 6.0. If the CA expressed by Equation 2 above is less than 0.1 or greater than 8.0, the conductive path may not be able to form a network structure properly, or the processability during electrode manufacturing may be poor. Therefore, when this range is met, the ratio between the area of ​​the first conductive agent and the area of ​​the second conductive agent can be appropriately controlled to achieve excellent electrode processability and to continuously maintain the conductive path during charging and discharging, which can improve the battery's lifespan characteristics.

[0061] Meanwhile, the negative electrode may include a negative electrode current collector and a negative electrode active material layer disposed on at least one side of the negative electrode current collector.

[0062] There are no particular restrictions on the negative electrode current collector, as long as it has high conductivity and does not cause chemical changes in the battery. Specifically, the negative electrode current collector can be, for example, copper, stainless steel, aluminum, nickel, titanium, calcined carbon, copper or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc., or an aluminum-cadmium alloy.

[0063] The thickness of the negative electrode current collector can typically range from 3 μm to 500 μm.

[0064] The negative electrode current collector can have microscopic irregularities on its surface to improve the bonding strength with the negative electrode active material. For example, the negative electrode current collector can be used in a variety of forms such as membranes, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0065] The negative electrode active material layer is disposed on at least one side of the negative electrode current collector. Specifically, the negative electrode active material layer may be disposed on one or both sides of the negative electrode current collector.

[0066] The negative electrode active material is a material that allows reversible insertion / extraction of lithium ions, and may include at least one selected from carbon-based active materials, silicon-based active materials and lithium metal, specifically, at least one selected from carbon-based active materials and silicon-based active materials.

[0067] Carbon-based active materials may include at least one selected from graphite, hard carbon, soft carbon, carbon black, graphene, and fibrous carbon, and specifically, may include graphite. Specifically, graphite may include at least one selected from synthetic graphite and natural graphite.

[0068] From the perspective of maintaining structural stability during charging and discharging and reducing side reactions with the electrolyte, the average particle size D of carbon-based active materials... 50 It can be from 10 μm to 30 μm, specifically from 15 μm to 25 μm.

[0069] Silicon-based active materials can be selected from SiO2. x (0≤x<2), Si / C composite materials, metal-doped SiO x (0≤x<2) and one or more of Si alloys, specifically selected from SiO x(0≤x<2) and one or more of Si / C composite materials. In the case of SiO2, since it does not react with lithium ions and therefore may not store lithium, "x" is selected within the above range. Furthermore, Si / C composite materials refer to a form in which carbon is physically mixed with silicon or silicon oxide. For example, Si / C composite materials can be formed by incorporating silane gas into porous carbon and pyrolyzing the mixture. Alternatively, Si / C composite materials can be formed by physically mixing carbon and silicon particles, by heat-treating the surface of silicon particles and coating with carbon, or by etching a silicon-based material and coating the etched silicon-based material with carbon. However, this disclosure is not limited thereto.

[0070] The BET specific surface area of ​​the negative electrode active material can be 0.2 m². 2 / g to 20 m 2 / g, specifically 1 m 2 / g to 15 m 2 / g, more specifically 1 m 2 / g to 10 m 2 / g. When the BET specific surface area of ​​the negative electrode active material is within the range, the specific surface area can be transformed into an appropriate specific surface area that meets the above ACA range, making the conductive connectivity of the negative electrode active material particles more stable, and reducing particle breakage caused by volume expansion.

[0071] From the perspective of maintaining structural stability during charging and discharging and reducing side reactions with the electrolyte, the average particle size D of silicon-based active materials is... 50 It can be from 1 μm to 30 μm, specifically from 3 μm to 20 μm, and more specifically from 5 μm to 10 μm.

[0072] Based on the total weight of the negative electrode active material layer, the negative electrode active material can be included in a content of 60% to 99% by weight, specifically 75% to 95% by weight.

[0073] Binders are used to improve the bonding force between the negative electrode active material layer and the negative electrode current collector, thereby improving battery performance. For example, binders may include at least one of the following: polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-copolymer-HFP), polyvinylidene fluoride (PVDF), polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, polyacrylamide, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluorinated rubber, and substances obtained by replacing hydrogen in the above materials with, for example, Li, Na, or Ca, and may also include a wide variety of copolymers thereof.

[0074] The binder may be included in the negative electrode active material layer in an amount of 0.5% to 10% by weight, specifically 1% to 5% by weight.

[0075] The BET specific surface area of ​​the first conductive agent can be 800 m². 2 / g to 1,500 m 2 / g, specifically 900 m 2 / g to 1,200 m 2 / g, more specifically 1,000 m 2 / g to 1,200 m 2 / g. When this range is met, the specific surface area of ​​the first conductive agent can become an appropriate specific surface area that satisfies the above ACA and / or CA ranges, and can significantly contribute to the formation of a conductive network between or on the surface of the negative electrode active materials.

[0076] The first conductive agent can be carbon nanotubes. Carbon nanotube graphite sheets have a cylindrical shape with a nanometer-sized diameter and exhibit sp... 2 Based on the angle and structure of the graphite sheet, carbon nanotubes can exhibit conductor or semiconductor properties. According to the number of bonds forming the walls, carbon nanotubes can be classified into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs), and multi-walled carbon nanotubes (MWCNTs).

[0077] The first conductive agent mentioned above can be a single-walled carbon nanotube. When the first conductive agent is a single-walled carbon nanotube, even a small amount of the first conductive agent can be added to ensure sufficient conductivity, thereby reducing the electrode resistance.

[0078] The average diameter of single-walled carbon nanotubes can range from 0.5 nm to 10 nm, specifically 0.5 nm to 5.0 nm, more specifically 0.5 nm to 3 nm, and even more specifically 0.5 nm to 2 nm. When this range is met, single-walled carbon nanotubes can form a suitable conductive network with the negative electrode active material. The average diameter can be obtained by observing the diameters of 100 single-walled carbon nanotubes contained in the negative electrode active material layer using TEM or AFM, and then calculating their average value.

[0079] Based on the total weight of the negative electrode active material layer, the first conductive agent can be included in an amount of 0.01 wt% to 0.5 wt%, specifically 0.05 wt% to 0.5 wt%, more specifically 0.05 wt% to 0.2 wt%. When this range is met, conductivity of the negative electrode active material layer can be easily achieved even with a low amount of the first conductive agent.

[0080] The BET specific surface area of ​​the second conductive agent can be 25 m². 2 / g to 200 m 2 / g, specifically 35 m 2 / g to 200 m 2 / g, more specifically 35 m 2 / g to 150 m 2 / g. When this range is met, the specific surface area of ​​the second conductive agent can be transformed into an appropriate specific surface area that satisfies the above ACA and / or CA range, thereby enabling excellent processability during electrode manufacturing and easily ensuring conductivity on the surface of the negative electrode active material.

[0081] The second conductive agent can be a point-type conductive agent. The point-type conductive agent can be carbon black, specifically, one or more selected from acetylene black, Ketjen black, channel black, furnace black, lampblack, and summer black. More specifically, the point-type conductive agent can be one or more selected from acetylene black and Ketjen black. The point-type conductive agent can typically be disposed in the gaps between the negative electrode active materials in the negative electrode active material layer, and can contribute to the formation of conductive paths in a manner different from the linear first conductive agent. Therefore, when using the second conductive agent (which is a point-type conductive agent having a specific surface area within the above range), battery performance can be controlled more effectively through the above equation.

[0082] Based on the total weight of the negative electrode active material layer, the second conductive agent can be included in an amount of 0.1 wt% to 5.0 wt%, specifically 0.1 wt% to 4.0 wt%, more specifically 0.1 wt% to 3.0 wt%. When this range is met, the slurry can have a suitable viscosity, enabling excellent processability during electrode manufacturing, and resulting in excellent conductivity on the surface of the negative electrode active material and excellent conductivity between the negative electrode active materials.

[0083] The average particle size D of the second conductive agent 50 It can be from 5 μm to 70 μm, specifically from 10 μm to 60 μm, and more specifically from 15 μm to 50 μm.

[0084] The thickness of the negative electrode active material layer can be from 10 μm to 100 μm, specifically from 50 μm to 80 μm.

[0085] The negative electrode can be manufactured by coating at least one side of the negative electrode current collector with a negative electrode slurry composition comprising a negative electrode active material, a binder, a first conductive agent, a second conductive agent and / or a solvent for forming the negative electrode slurry, followed by drying and rolling.

[0086] From the perspective of promoting the dispersion of the negative electrode active material, binder, and / or conductive agent, the solvent used to form the negative electrode slurry may include, for example, at least one selected from distilled water, N-methyl-2-pyrrolidone (NMP), ethanol, methanol, and isopropanol, and may include, for example, distilled water. The solids content of the negative electrode slurry may be from 30% to 80% by weight, specifically from 40% to 70% by weight.

[0087] Lithium secondary batteries

[0088] Next, a lithium secondary battery according to this disclosure will be described. The lithium secondary battery according to this disclosure includes the negative electrode described above, and specifically, can be a lithium secondary battery including the negative electrode according to this disclosure.

[0089] The lithium secondary battery described above specifically includes a positive electrode, a negative electrode facing the positive electrode, a separator inserted between the positive and negative electrodes, and an electrolyte. Since the negative electrode is the same as described above, its specific description is omitted, and only the other components will be described in detail below.

[0090] The lithium secondary batteries described above may optionally include a battery container for housing an electrode assembly consisting of a positive electrode, a negative electrode, and a separator, as well as a sealing member for sealing the battery container.

[0091] In the above lithium secondary batteries, the positive electrode may include a positive electrode current collector and a positive electrode active material layer disposed on the positive electrode current collector.

[0092] There are no particular restrictions on the positive electrode current collector, as long as it is conductive and does not cause chemical changes in the battery. For example, the positive electrode current collector can be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel whose surface has been treated with carbon, nickel, titanium, silver, etc.

[0093] The thickness of the positive electrode current collector can range from 3 μm to 500 μm, and microscopic irregularities can be included on its surface to improve the bonding strength with the positive electrode active material layer. For example, the positive electrode current collector can be used in a variety of forms such as films, sheets, foils, meshes, porous materials, foams, and nonwoven fabrics.

[0094] The positive electrode active material layer may contain positive electrode active material, and may also contain, as needed, conductive agents and binders.

[0095] The positive electrode active material is a compound that allows for reversible insertion and extraction of lithium, and specifically, may include a lithium metal oxide containing lithium and one or more metals (such as cobalt, manganese, nickel, or aluminum). More specifically, the lithium metal oxide may be, for example, a lithium-manganese-based oxide (such as, LiMnO2 and LiMn2O4), a lithium-cobalt-based oxide (such as, LiCoO2), a lithium-nickel-based oxide (such as, LiNiO2), a lithium-nickel-manganese-based oxide (such as, LiNi 1-Y Mn Y O2 (where 0 < Y < 1) and LiMn 2-Z Ni Z O4 (where 0 < Z < 2)), a lithium-nickel-cobalt-based oxide (such as, LiNi 1-Y1 Co Y1 O2 (where 0 < Y1 < 1)), a lithium-manganese-cobalt-based oxide (such as, LiCo 1-Y2 Mn Y2 O2 (where 0 < Y2 < 1) and LiMn 2-Z1 Co Z1 O4 (where 0 < Z1 < 2)), a lithium-nickel-manganese-cobalt-based oxide (such as, Li(Ni p Co q Mn r )O2 (where 0 < p < 1, 0 < q < 1, 0 < r < 1, and p + q + r = 1) and Li(Ni p1 Co q1 Mn r1 )O4 (where 0 < p1 < 2, 0 < q1 < 2, 0 < r1 < 2, and p1 + q1 + r1 = 2)), a lithium-nickel-cobalt-transition metal (M) oxide (such as, Li(Ni p2 Co q2 Mn r2 M s2 )O2 (where M is selected from Al, Fe, V, Cr, Ti, Ta, Mg, and Mo, p2, q2, r2, and s2 are each the atomic fraction of an independent element, 0 < p2 < 1, 0 < q2 < 1, 0 < r2 < 1, 0 < s2 < 1, and p2 + q2 + r2 + s2 = 1)), or lithium iron phosphate (such as, Li 1+a Fe 1-x M x (PO 4-b )X b (where M is one or more selected from Al, Mg, and Ti, X is one or more selected from F, S, and N, -0.5 ≤ a ≤ 0.5, 0 ≤ x ≤ 0.5, and 0 ≤ b ≤ 0.1)), and may include a compound of any one or two or more of the above materials.

[0096] Among the above materials, from the perspective of improving battery capacity characteristics and stability, lithium metal oxides can be, for example, LiCoO2, LiMnO2, LiNiO2, and lithium nickel manganese cobalt oxides (e.g., Li(Ni)O2). 1 / 3 Mn 1 / 3 Co 1 / 3 O2, Li(Ni) 0.6 Mn 0.2 Co 0.2 O2, Li(Ni) 0.5 Mn 0.3 Co 0.2 O2, Li(Ni) 0.7 Mn 0.15 Co 0.15 )O2 and Li(Ni 0.8 Mn 0.1 Co 0.1 O2), lithium nickel cobalt aluminum oxide (e.g., Li(Ni) 0.8 Co 0.15 Al 0.05 O2), lithium nickel manganese cobalt aluminum oxide (e.g., Li(Ni) 0.86 Co 0.05 Mn 0.07 Al 0.02 (O2), or lithium iron phosphate (e.g., LiFePO4), and may include compounds of any one or more of the above materials.

[0097] Based on the total weight of the positive electrode active material layer, the positive electrode active material may be included in a content of 60% to 99% by weight, specifically 70% to 99% by weight, and more specifically 80% to 98% by weight.

[0098] The positive electrode conductive agent is a component that further improves the conductivity of the positive electrode active material. There are no particular limitations on the positive electrode conductive agent, as long as it is conductive without causing chemical changes in the battery. It can be, for example, conductive materials including: carbon powder, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lampblack, or thermally cracked black; graphite powder, such as natural graphite, artificial graphite, or graphite with a well-formed crystal structure; conductive fibers, such as carbon fibers or metal fibers; fluorinated carbon powder; conductive powder, such as aluminum powder or nickel powder; conductive whiskers, such as zinc oxide or potassium titanate; conductive metal oxides, such as titanium oxide; and polyphenylene derivatives.

[0099] Typically, based on the total weight of the positive electrode active material layer, the positive electrode conductive agent can be included in a content of 1% to 20% by weight, specifically 1% to 15% by weight, and more specifically 1% to 10% by weight.

[0100] Positive electrode binders are components that assist in, for example, the bonding between active materials and conductive agents, as well as the bonding with current collectors.

[0101] Examples of adhesives include polyvinylidene fluoride, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polyethylene (PE), polypropylene, ethylene propylene diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber, fluorinated rubber, and a wide variety of copolymers.

[0102] Typically, based on the total weight of the positive electrode active material layer, the positive electrode binder can be included in a content of 1% to 20% by weight, specifically 1% to 15% by weight, and more specifically 1% to 10% by weight.

[0103] In the above-mentioned lithium secondary batteries, the separator separates the negative and positive electrodes and provides a transport path for lithium ions. There are no particular limitations on the separator, as long as it serves as a separator in the secondary battery; in particular, separators with low resistance to ion transport in the electrolyte and excellent electrolyte impregnation capabilities are used. Specifically, the separator can be a porous polymer membrane made of polyolefin-based polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, or ethylene / methacrylate copolymer, or a stacked structure of two or more of these. Furthermore, the separator can be a common porous nonwoven fabric, such as a nonwoven fabric made of high-melting-point glass fiber or polyethylene terephthalate fiber. In addition, to ensure heat resistance or mechanical strength, coated separators containing ceramic components or polymer materials can be used, or they can be used in the form of a single-layer or multi-layer structure.

[0104] Furthermore, the electrolyte used in this disclosure may be, for example, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, or a melt inorganic electrolyte that can be used in the manufacture of lithium secondary batteries, but is not limited thereto.

[0105] Specifically, the electrolyte may contain organic solvents and lithium salts.

[0106] There are no particular limitations on organic solvents, as long as they serve as a medium for transporting ions involved in the electrochemical reactions of the battery. Specifically, organic solvents can be: ester-based solvents, such as methyl acetate, ethyl acetate, γ-butyrolactone, or ε-caprolactone; ether-based solvents, such as dibutyl ether or tetrahydrofuran; ketone-based solvents, such as cyclohexanone; aromatic hydrocarbon-based solvents, such as benzene or fluorobenzene; carbonate-based solvents, such as dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl carbonate (MEC), ethyl methyl carbonate (EMC), ethylene carbonate (EC), or propylene carbonate (PC); alcohol-based solvents, such as ethanol or isopropanol; nitriles, such as R-CN (where R is a straight-chain, branched, or cyclic hydrocarbon group having 2 to 20 carbon atoms, which may contain double bonds, aromatic rings, or ether bonds); amides, such as dimethylformamide; dioxolane, such as 1,3-dioxolane; or sulfolane. Among these materials, carbonate-based solvents are preferred, and mixtures of cyclic carbonates (e.g., ethylene carbonate or propylene carbonate) with high ionic conductivity and high dielectric constant that can improve the charging and discharging performance of the battery and linear carbonate-based compounds (e.g., ethyl methyl carbonate, dimethyl carbonate or diethyl carbonate) with low viscosity can be used.

[0107] There are no particular restrictions on lithium salts, as long as they are compounds that can provide lithium ions used in lithium secondary batteries. For example, the negative ion of a lithium salt can be selected from F... - Cl - ,Br - I - NO3 - N(CN)2 - BF4 - CF3CF2SO3 - (CF3SO2)2N - (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2)2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N -At least one of the following, and the lithium salt can be, for example, LiPF6, LiClO4, LiAsF6, LiBF4, LiSbF6, LiAlO4, LiAlCl4, LiCF3SO3, LiC4F9SO3, LiN(C2F5SO3)2, LiN(C2F5SO2)2, LiN(CF3SO2)2, LiCl, LiI, or LiB(C2O4)2. The concentration of the lithium salt is in the range of 0.1 M to 2.0 M. When the concentration of the lithium salt falls within this range, the electrolyte has suitable conductivity and viscosity, enabling excellent electrolyte performance and efficient transport of lithium ions.

[0108] In addition to the electrolyte components mentioned above, the electrolyte may also contain one or more additives, including, for example, compounds based on haloalkyl carbonates (e.g., ethylene difluorocarbonate), pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol dimethyl ether, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted... The additives include azole ketones, N,N-substituted imidazolidines, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, and aluminum trichloride. Based on the total weight of the electrolyte, the additives may be included in amounts ranging from 0.1% to 5% by weight.

[0109] Hereinafter, embodiments of the present disclosure are described in detail to enable those skilled in the art to readily practice the present disclosure. However, the present disclosure may be implemented in a variety of forms and is not limited to the embodiments described herein.

[0110] Example 1: Manufacturing of the negative electrode

[0111] The Si / C composite material (specific surface area: 2.53 m²) will be mixed at a weight ratio of 90:10. 2 / g) and natural graphite (specific surface area: 0.93 m²) 2 The negative electrode active material obtained by ( / g) has a specific surface area of ​​2.37 m². 2 / g), water-based binder, first conductive agent (SWCNT, specific surface area: 1,000 m²), 2 / g) and second conductive agent (carbon black, specific surface area: 65 m²) 2 (g) is mixed with water as a solvent to prepare a negative electrode slurry composition.

[0112] Regarding the solid content of each material, the content of the negative electrode active material is 90% by weight, the content of the binder is 8.96% by weight, the content of the first conductive agent is 0.04% by weight, and the content of the second conductive agent is 1% by weight.

[0113] At this point, the water-based binder is a polymer obtained by polymerizing acrylamide and acrylic acid in a molar ratio of 60:40 (using ammonium persulfate polymerization initiator, 80°C, polymerization reaction lasting 6 hours).

[0114] Then, the negative electrode slurry composition is applied to one side of a copper current collector with a thickness of 18 μm until a thickness of 50 μm is achieved. After that, it is dried and rolled at 120°C to manufacture the negative electrode.

[0115] Examples 2 and 3 and Comparative Examples 1 to 3: Manufacturing of the negative electrode

[0116] The negative electrodes of Examples 2 and 3 and Comparative Examples 1 to 3 were manufactured in the same manner as in Example 1, except that the specific surface area and weight ratio of the negative electrode active material, the first conductive agent and the second conductive agent were applied as described in Table 1 below.

[0117] Based on the solid content of each material, Table 1 below describes the weight percentage of each of the negative electrode active material, the first conductive agent, and the second conductive agent, and the content of the binder is appropriately adjusted so that the sum of the weight percentage of each of the negative electrode active material, the binder, the first conductive agent, and the second conductive agent becomes 100 by weight.

[0118] Table 1 summarizes the above manufacturing methods, the number-average length (μm) of the first conductive agent, the ACA expressed by Equation 1 above, and the CA expressed by Equation 2 above.

[0119] [Table 1]

[0120]

[0121] At this point, the number-average length (μm) of the first conductive agent is measured using atomic force microscopy (AFM).

[0122] Specifically, after diluting the first conductive agent of each of the examples and comparative examples in water, 50 μl of the diluted solution was dropped onto a freshly cut mica surface and then dried in a vacuum to prepare a sample. Images of the prepared samples were captured using atomic force microscopy (AFM) (Asylum Research, Cypher ES AFM System) under the following conditions.

[0123] - Measurement mode: AC Air Topography mode (tapping mode)

[0124] - Measurement conditions: Setpoint 0.4 V, scan rate 1.5 Hz

[0125] - Measured image sizes: measured at magnifications of 20 μm × 20 μm, 15 μm × 15 μm, and 10 μm × 10 μm.

[0126] - Probe: AC160TS (n-type doped Si, reflective Al coating, f0 300 kHz, k 26 N / m)

[0127] From the AFM image obtained by measurements under the above conditions, the lengths of 400 or more first conductive agents are measured (using an image processing program) to obtain the length distribution, and then the number-average length (μm) is measured.

[0128] Experimental Example 1: Evaluation of Lifetime Characteristics and Coating Processability

[0129] (Lifetime characteristics)

[0130] A coin half-cell was prepared using the negative electrode of each of the examples and comparative examples.

[0131] Specifically, the negative electrode was stamped into a circle with a diameter of 14 mm to prepare the test negative electrode, lithium metal with a thickness of 0.3 mm was used as the positive electrode, porous polyethylene with a thickness of 0.1 mm was used as the separator, and an electrolyte obtained by dissolving 1 M LiPF6 in a mixed solution in which ethylene carbonate (EC) and ethyl methyl carbonate (EMC) were mixed in a volume ratio of 50:50 was injected to manufacture a coin half-cell with a diameter of 32 mm.

[0132] The above coin half-cell was charged at a constant current of 0.05 C until it reached 0.01 V, and then discharged at a constant current of 0.05 C until it reached 1.5 V. The charge and discharge process was set as one cycle, and a total of 30 cycles were performed to calculate the capacity retention.

[0133] Specifically, the capacity retention rate is calculated as follows.

[0134] Capacity retention rate (%) = (Discharge capacity at 30th cycle / Discharge capacity at 1st cycle)

[0135] (Coating processability)

[0136] During the manufacture of the negative electrode in the examples and comparative examples, when the negative electrode slurry composition was coated onto copper foil, the presence of blemishes on the electrode surface, the presence of pinholes, and the presence of filter blockage in the slurry circulation pump were inspected. When any problem occurred, the evaluation result was described as X, and when no problem occurred, the evaluation result was described as O.

[0137] [Table 2]

[0138]

[0139] Referring to Table 2 above, it can be seen that, compared with the comparative examples, no problems occurred in Examples 1 to 3 even during the coating process, and the capacity retention rate was also excellent.

[0140] In Comparative Example 1, it can be seen that CA exceeds the scope of this disclosure, and in this case, the conductive network is not sufficiently formed, resulting in a significant reduction in capacity retention.

[0141] In Comparative Examples 2 and 3, it can be seen that the ACA is less than or exceeds the scope of this disclosure. In this case, problems occur during the coating process, and the conductive network is not properly formed, resulting in a significant decrease in capacity retention.

Claims

1. A negative electrode, comprising: The negative electrode active material layer comprises a negative electrode active material, a binder, a first conductive agent, and a second conductive agent. The number-average length of the first conductive agent, measured using atomic force microscopy (AFM), ranges from 0.7 μm to 6.0 μm. The ACA, as expressed by Equation 1 below, ranges from 0.5 to 5.0, and The CA expressed by Equation 2 below ranges from 0.1 to 8.

0. [Equation 1] In Equation 1 above, the S A The BET specific surface area (m²) of the first conductive agent 2 / g), the W A The first conductive agent is a percentage of the total weight of the negative electrode active material layer, and the S B The BET specific surface area (m²) of the second conductive agent 2 / g), the W B The second conductive agent is a percentage of the total weight of the negative electrode active material layer, and the S C The BET specific surface area (m²) of the negative electrode active material 2 / g), and the W C The negative electrode active material is the weight percentage of the total weight of the negative electrode active material layer. [Equation 2] In Equation 2 above, the S A The BET specific surface area (m²) of the first conductive agent 2 / g), the W A The first conductive agent is a percentage of the total weight of the negative electrode active material layer, and the S B The BET specific surface area (m²) of the second conductive agent 2 / g), and the W B The second conductive agent is the weight of the total weight of the negative electrode active material layer.

2. The negative electrode according to claim 1, wherein the number-average length of the first conductive agent, as measured by atomic force microscopy (AFM), is 0.8 μm to 4.0 μm.

3. The negative electrode according to claim 1, wherein the ACA is 0.9 to 2.

5.

4. The negative electrode according to claim 1, wherein CA is from 0.1 to 6.

0.

5. The negative electrode according to claim 1, wherein the negative electrode active material comprises a silicon-based active material.

6. The negative electrode according to claim 1, wherein the BET specific surface area of ​​the negative electrode active material is 0.2 m². 2 / g to 20m 2 / g.

7. The negative electrode according to claim 1, wherein the BET specific surface area of ​​the first conductive agent is 800 m². 2 / g to 1,500m 2 / g.

8. The negative electrode according to claim 1, wherein the first conductive agent comprises single-walled carbon nanotubes.

9. The negative electrode according to claim 1, wherein the BET specific surface area of ​​the second conductive agent is 25 m². 2 / g to 200m 2 / g.

10. The negative electrode according to claim 1, wherein the second conductive agent comprises a dot-type conductive agent.

11. A lithium secondary battery, comprising the negative electrode according to claim 1.

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