Self-supporting film for dry electrode, method for manufacturing the self-supporting film, dry electrode including the self-supporting film, and secondary battery

Abstract

According to one embodiment of the present disclosure, there is provided a self-supporting film for a dry electrode, the self-supporting film including an active material, a binder including a fibrous polymer, and a carrier, wherein the carrier is an oxidized vapour grown carbon fibre (Ox-VGCF), a method of manufacturing the self-supporting film, a dry electrode including the self-supporting film, and a secondary battery are provided.

Description

Technical Field

[0001] Cross-reference to related applications

[0002] This application claims the benefits of Korean Patent Application No. 10-2021-0011774, filed on January 27, 2021, and Korean Patent Application No. 10-2022-0003420, filed on January 10, 2022, the contents of which are incorporated herein by reference in their entirety.

[0003] This disclosure relates to a self-supporting membrane for a dry electrode, a method for manufacturing the self-supporting membrane, a dry electrode including the self-supporting membrane, and a secondary battery. Background Technology

[0004] Due to the rapid increase in the use of fossil fuels, the demand for alternative or clean energy sources is constantly increasing, with the most active research areas being electrochemical power generation and electricity storage.

[0005] Currently, secondary batteries are a representative example of electrochemical devices that utilize this electrochemical energy, and their application range is gradually expanding.

[0006] Among these secondary batteries, typical lithium-ion batteries are used not only as energy sources for mobile devices but also as power sources for electric vehicles and hybrid electric vehicles, which can replace fossil fuel-powered cars such as gasoline and diesel vehicles, which are major contributors to air pollution. Their use is also expanding in applications such as auxiliary power supplies formed through grids.

[0007] The manufacturing process of this type of lithium secondary battery mainly consists of three processes: electrode process, assembly process, and molding process. The electrode process is further divided into active material mixing process, electrode coating process, drying process, rolling process, slitting process, winding process, and similar processes.

[0008] The active material mixing process is a process of mixing coating materials used to form the active layer of the electrode, wherein an actual electrochemical reaction occurs in the electrode. Specifically, an electrode active material, which is an essential component of the electrode, conductive materials and fillers as other additives, an adhesive for bonding between particles and adhering to the current collector, a solvent for imparting viscosity and dispersing particles, and the like are mixed to prepare a fluid slurry.

[0009] Compositions mixed in this way to form the electrode active layer are also referred to as electrode mixtures in a broad sense.

[0010] Next, an electrode coating process is performed to apply the electrode mixture to a conductive current collector, and a drying process is performed to remove the solvent contained in the electrode mixture. In addition, the electrode is rolled and manufactured to a predetermined thickness.

[0011] Meanwhile, as the solvent in the electrode mixture evaporates during the drying process, defects such as pinholes or cracks may appear in the pre-formed electrode active layer. Furthermore, due to uneven drying inside and outside the active layer, particle floating occurs due to differences in solvent evaporation rates; that is, particles from the earlier-dried portion float to the surface, creating gaps with the later-dried portions, thus reducing electrode quality. Especially in the case of thick-film coating, the longer solvent evaporation time during electrode drying exacerbates the floating phenomenon of relatively light conductive materials and binders, resulting in a fatal drawback: manufacturing high-quality electrodes is difficult.

[0012] Therefore, in order to solve the above problems, a drying device that can regulate the evaporation rate of the solvent while ensuring uniform drying inside and outside the active layer is being studied. However, such a drying device is very expensive and requires considerable cost and time to operate, which is disadvantageous in terms of manufacturing processability.

[0013] Therefore, there has been active research recently on the fabrication of dry electrodes that do not use solvents.

[0014] Dry electrodes are typically manufactured by laminating a self-supporting membrane, which is made in the form of a film containing active materials, binders, conductive materials, and the like, onto a current collector.

[0015] Therefore, it has attracted much attention as an innovative technology that can manufacture thick film electrodes without the use of organic solvents that are harmful to the human body, while solving the problem of high process costs caused by long drying time.

[0016] Meanwhile, polytetrafluoroethylene (PTFE) powder is primarily used as a binder in the fabrication of this dry electrode. In this PTFE, long fibrous structures with an atomic arrangement of (-CF2-CF2-)n are sequentially arranged within primary particles with diameters of several hundred nm or less. When shear force is applied under certain conditions, they are stretched into fibers. These PTFE nanofibers act like ropes, connecting the active and conductive materials to maintain the electrode's shape.

[0017] However, because PTFE is easily fibrous even with very small shear forces at room temperature (19°C or higher), even slight movement can lead to the fatal drawback of agglomeration with surrounding PTFE particles.

[0018] Therefore, in such dry electrodes, achieving uniform dispersion of primary PTFE particles under process conditions without applying shear force is a core technology. Thus, there is an urgent need to develop a technology that can achieve an electrode by binding only a small amount of PTFE to the active material through fiberization. Summary of the Invention

[0019] [Technical Issues]

[0020] This disclosure is designed to solve the above-mentioned problems. The purpose of this disclosure is to provide a self-supporting film that can uniformly disperse PTFE primary particles, improve the coating efficiency of PTFE adhesive, and firmly form a conductive network, thereby improving battery performance, and a method for manufacturing the self-supporting film.

[0021] Another object of this disclosure is to provide a dry electrode including the self-supporting membrane, and a secondary battery including the dry electrode.

[0022] [Technical Solution]

[0023] According to one embodiment of this disclosure, a self-supporting membrane for dry electrodes is provided.

[0024] The self-supporting membrane comprises an active material, an adhesive containing a fibrous polymer, and a carrier.

[0025] The carrier is surface-oxidized vapor-grown carbon fiber (Ox-VGCF).

[0026] At this point, the fibrous polymer can be fiberized to combine the active material and the carrier.

[0027] In one embodiment, the carrier may be a carbon fiber in which surface oxygen functional groups are increased by surface oxidation treatment, wherein the carrier may include surface oxygen functional groups in an amount of 5% to 15% by weight based on the total weight.

[0028] Furthermore, the carrier can have an average diameter of 50 nm to 500 nm, an average length of 1 μm to 30 μm, and an average length of 10 μm. 2 / g to 150m 2 Specific surface area per g.

[0029] Furthermore, the carrier may have a graphitization degree of 0.1 to 2.0.

[0030] Furthermore, in one specific embodiment, the carrier may be included in an amount from 0.1% to 10% by weight, based on the total weight of the self-supporting membrane.

[0031] In one specific embodiment, the fibrous polymer may be polytetrafluoroethylene (PTFE).

[0032] According to another embodiment of the present invention, a method for manufacturing a self-supporting membrane is provided, the method comprising the following steps:

[0033] (a) Mixing an adhesive and a carrier, including a fibrous polymer, to obtain a mixture;

[0034] (b) The mixture and the active material are combined, and the resulting mixture is subjected to high-shear mixing to obtain a fibrous composition; and

[0035] (c) The fibrous composition is formed into a film.

[0036] The carrier is surface-oxidized vapor-grown carbon fiber (Ox-VGCF).

[0037] The fibrous polymer may be polytetrafluoroethylene (PTFE).

[0038] In one embodiment, the mixing in step (a) may include simply mixing the adhesive and carrier comprising the fibrous polymer at a temperature of 19°C or lower, or loading the adhesive and carrier mixture comprising the fibrous polymer together into a mill after mixing.

[0039] Here, the grinding machine can be an air jet mill.

[0040] In this case, the mixture may have a structure in which the primary particles of the fibrous polymer are uniformly dispersed and loaded on the surface of a carrier.

[0041] In one specific embodiment, the high-shear mixing in step (b) can be carried out for 1 minute to 30 minutes in the range of 10 rpm to 500 rpm.

[0042] In one specific embodiment, the forming in step (c) can be carried out by hot rolling in a calender.

[0043] Meanwhile, according to another embodiment of the present disclosure, a dry electrode is provided, comprising: a current collector and a self-supporting film formed on the current collector, wherein the current collector has a structure in which a primer layer is coated on a metal foil.

[0044] Meanwhile, according to another embodiment of the present disclosure, a secondary battery including the dry electrode is provided, wherein an electrode assembly including the dry electrode, counter electrode and separator is installed together with a lithium-containing non-aqueous electrolyte in a battery casing. Attached Figure Description

[0045] Figure 1 These are SEM images based on the reference example.

[0046] Figure 2 This is a SEM image of Preparation Example 1 according to the present disclosure.

[0047] Figure 3 This is a SEM image of the fibrous composition according to Example 1. Detailed Implementation

[0048] In order to better understand this disclosure, it will be described in more detail below.

[0049] The terms or words used in this specification and claims should not be construed as limited to ordinary or dictionary terms, and should be interpreted using meanings and concepts consistent with the technical concept of this disclosure, based on the principle that the inventors may appropriately define the terms in order to best describe their own disclosure.

[0050] The technical terminology used herein is for the purpose of describing particular embodiments only and is not intended to limit the scope of this disclosure. The singular forms “a,” “an,” and “the” are also intended to include the plural forms unless the context clearly indicates otherwise.

[0051] Furthermore, throughout the specification, when a section is referred to as "including" a certain component, unless otherwise stated, it means that the section may further include other components, without excluding other components.

[0052] According to one embodiment of this disclosure, a self-supporting membrane for dry electrodes can be provided, the self-supporting membrane comprising an active material, an adhesive comprising a fibrous polymer, and a carrier.

[0053] The carrier is surface-oxidized vapor-grown carbon fiber (Ox-VGCF).

[0054] At this point, the fibrous polymer can be fiberized to combine the active material and the carrier. As described above, in the self-supporting film for the dry electrode, the fibrous polymer, while fiberizing, acts like a rope to bundle and connect other powders without solvent, and also serves to bind them together. Therefore, their dispersibility is very important. If not properly dispersed, a self-supporting film cannot be manufactured, or even if it is manufactured, it is difficult to ensure the physical properties of the self-supporting film. Therefore, desorption of the active material may occur during processes such as rolling. Subsequently, depending on the operation of the battery, its life characteristics may decline sharply.

[0055] In this regard, the inventors have discovered that when a binder comprising a fibrous polymer is added during mixing to manufacture a self-supporting membrane, such as Figure 1 As shown, the fibrous polymer nanoparticles exist in a dense state, and these primary particles are not easy to aggregate and disperse even under very small shear forces, thus failing to firmly form a self-supporting film, greatly reducing efficiency, ultimately affecting the physical properties of the electrode, and leading to the deterioration of battery characteristics.

[0056] Therefore, through in-depth research, the inventors discovered that when a carrier is first mixed with a binder comprising a fibrous polymer and used to manufacture a self-supporting membrane, the dispersion of the fibrous polymer is promoted. In particular, the inventors found that in surface-oxidized vapor-grown carbon fibers (Ox-VGCF), surface oxygen functional groups are introduced onto the surface of the Ox-VGCF, and due to the increased oxygen functional groups, the nano-sized fibrous polymer primary particles are readily adsorbed onto the surface of the carrier and are more easily dispersed. Furthermore, the inventors discovered that when the mixture is subsequently mixed with active materials to form fibers, the active materials can be integrally bonded, thereby improving the performance of the dry electrode and the battery using the dry electrode, thus completing this disclosure.

[0057] At this point, compared to VGCF without surface oxidation treatment, the support may be a support in which the surface oxygen functional groups are increased to 5% to 15% by weight, specifically, to 8% to 12% by weight.

[0058] The content of surface oxygen functional groups can be determined by elemental analysis. Specifically, the C, H, and N content of the support is detected by an elemental analyzer (CHN-coder MT-5, Yanako), and the oxygen difference is calculated by reflecting the amount of residual ash.

[0059] When the surface oxygen functional groups are sufficiently contained in this manner, the nano-sized fibrous polymer primary particles are readily adsorbed onto the surface of the support, which is preferred. However, excessively high concentrations can lead to problems such as electrochemical surface side reactions, which is not preferred.

[0060] Furthermore, the carrier may have an average diameter of 50 nm to 500 nm and an average length of 1 μm to 30 μm. More specifically, the average diameter may be 100 nm to 200 nm, and the average length may be 1 μm to 15 μm.

[0061] If the average diameter is too small and exceeds the aforementioned range, a small radius of curvature results in difficulty in supporting the material due to interference between the primary particles of the fibrous polymer. If it is too thick, the dispersion / loading efficiency is significantly reduced, which is undesirable. Furthermore, if the average length is too long, the powder flowability is significantly reduced, and the dispersion / loading efficiency also decreases. If the average length is too short, the aggregation rate between the carriers increases, and the formation of long conductive networks between the active materials is also undesirable. Therefore, to adequately disperse the binder comprising the fibrous polymer of this disclosure, it is preferable to use Ox-VGCF with a predetermined thickness within the aforementioned range, excellent linearity, and surface oxygen functional groups.

[0062] The average diameter and average length of the carrier can be measured by SEM. More specifically, a carrier solution diluted to 1% by weight in acetone is dropped onto a Si wafer and dried. The average diameter and length of 300 individuals are then measured using an SEM (Scanning Electron Microscopy, JEOL, JSM-7500F).

[0063] Furthermore, the specific surface area of ​​the carrier used for the uniform dispersion of the fibrous polymer can be 10 m². 2 / g to 150m 2 / g, specifically 10m 2 / g to 100m 2 / g.

[0064] Specific surface area can be measured using the Brunauer-Emmett-Teller (BET) method. For example, specific surface area can be measured using a specific surface area analyzer (Belsarp-II mini from Bell Japan Inc.) via the 6-point BET method based on nitrogen adsorption.

[0065] When the fibrous polymer has the above-mentioned specific surface area, it can be easily dispersed and loaded. If the specific surface area is too small and exceeds the above range, the fibrous polymer is not easily dispersed; if the specific surface area is too large, gases may be generated due to side reactions with the electrolyte solution, which is not preferred.

[0066] Furthermore, the carrier is made of a carbon material with excellent conductivity and has a cylindrical tube shape with excellent linearity, thus possessing a large specific surface area. Therefore, the formation of a conductive network between active materials in the self-supporting membrane can be further enhanced, and therefore, the carrier is more preferably included.

[0067] Furthermore, the graphitization degree (ID / IG ratio) of supports with increased surface oxygen functional groups can be from 0.1 to 2.0.

[0068] The degree of graphitization (ID / IG ratio) was obtained by Raman spectroscopy. In the Raman spectrum, 1590 cm⁻¹... -1 The nearby G peak originates from the E2g vibrational mode of the carbon sp2 bond, while when the carbon sp2 bond is defective, the peak at 1350 cm⁻¹... -1 A D peak appears nearby. The lower the ID / IG ratio (D / G peak intensity ratio), the higher the degree of graphitization. Therefore, when using carbon-based particles with high graphitization, the capacity and electrical performance of the battery can be improved due to the high conductivity of the carbon-based particles.

[0069] Meanwhile, based on the total weight of the self-supporting membrane, the carrier may be included in an amount of 0.1% to 10% by weight, specifically 0.5% to 10% by weight.

[0070] If the content exceeds the above range and is too high, the content of active material may be reduced compared to the limits of effectiveness, which is not preferable from an energy density perspective. At the same time, when the content is too low, sufficient fibrous polymer dispersion cannot be obtained.

[0071] Meanwhile, the fibrous polymer may be polytetrafluoroethylene (PTFE). Based on the total weight of the adhesive, the fibrous polymer may be included in an amount of 50% by weight or more, or it may be 100% by weight.

[0072] Furthermore, in addition to polytetrafluoroethylene (PTFE), the adhesive may further include polyethylene oxide (PEO), polyvinylidene fluoride (PVdF), polyvinylidene fluoride-co-hexafluoropropylene (PVdF-HFP), and the like. Moreover, the adhesive may further include other adhesives known in the art and may vary depending on whether the dry electrode is a positive or negative electrode.

[0073] Furthermore, the active material can vary depending on whether the dry electrode is a positive or negative electrode.

[0074] For fabricating a self-supporting film to form a dry cathode, the active material is not limited, as long as it is in the form of a lithium transition metal oxide, lithium iron phosphate, or a metal oxide. For example, the active material can be: a layered compound, such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; a lithium manganese oxide, such as Li... 1+x Mn2- x O4 (where x is 0 to 0.33), LiMnO3, Li2MnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, LiV3O4, V2O5, and Cu2V2O7; and LiNi 1-x M x Ni-position lithium nickel oxide represented by O2 (where M = Co, Mn, Al, Cu, Fe, Mg, Ca, Zr, Ti, B, P, W, Si, Na, K, Mo, V, Nb, Ru, or Ga, and x = 0.01 to 0.3); LiMn 2-x M x Lithium manganese complex oxides represented by O2 (where M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); LiMn2O4, in which Li is partially replaced by an alkaline earth metal ion; lithium metal phosphate LiMPO4 (where M = Fe, CO, Ni or Mn); disulfides; Fe2(MoO4)3; and the like, but not limited thereto.

[0075] To fabricate a self-supporting membrane for forming a dry negative electrode, the active material may include: carbon, such as non-graphitized carbon and graphite-based carbon; and metal composite oxides, such as Li. x Fe2O3 (0≤x≤1), Lix WO2(0≤x≤1), Sn x Me 1-x Me′ y O z (Me: Mn, Fe, Pb, Ge; Me′: Al, B, P, Si, elements in groups 1, 2, and 3 of the periodic table, halogens; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8); lithium metal; lithium alloys; silicon-based alloys; tin-based alloys; silicon-based oxides, such as SiO, SiO / C, SiO x (1 < x < 2), SiO2; metal oxides, such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4 and Bi2O5; conductive polymers, such as polyacetylene; Li-Co-Ni based materials; and the like.

[0076] However, specifically, the active material used to manufacture the self-supporting membrane can be a positive electrode active material, more specifically, lithium transition metal oxides, lithium nickel-manganese-cobalt oxides, oxides in which the lithium nickel-manganese-cobalt oxides are partially replaced by different transition metals, lithium iron phosphate, or similar.

[0077] Meanwhile, when using graphite as the main active material for the negative electrode, compared to the application of the positive electrode active material, it is not necessary to use a conductive material with a large specific surface area, or its amount can be greatly reduced. Therefore, compared to the positive electrode, it is easier to realize the dry electrode with the PTFE binder fiber. However, since the negative electrode material using the silicon-based active material usually has a higher capacity than the positive electrode material, the thickness of the negative electrode is usually made thinner than that of the positive electrode in the electrode design. Therefore, the process difficulty in manufacturing the self-supporting film may increase.

[0078] Simultaneously, the self-supporting membrane may further include a conductive material, wherein the conductive material is not particularly limited, as long as it has high conductivity and does not cause chemical changes in the corresponding battery. Examples include: graphite such as natural and artificial graphite; graphene; activated carbon; activated carbon fibers; carbon black such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; metal powders such as fluorinated carbon, aluminum, and nickel; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. Specifically, the conductive material may include at least one selected from the group consisting of activated carbon, graphite, carbon black, graphene, and single-walled or multi-walled carbon nanotubes, for uniform mixing and improving conductivity, and more specifically, the conductive material may include carbon black or activated carbon.

[0079] At this point, the active material, conductive material, adhesive containing fibrous polymer, and carrier may be included such that the weight ratio of active material: conductive material: adhesive containing fibrous polymer and carrier is 60 to 99.8 wt%: 0 to 20 wt%: 0.2 to 20 wt%, specifically, 80 to 99 wt%: 0.1 to 10 wt%: 0.9 to 10 wt%.

[0080] If the content of the binder and carrier exceeds the above range and is too high, the fibrous polymer of the binder becomes rigid and subsequently over-fibrillates, which may place a load on the process equipment and significantly increase the resistance of the electrode itself. Conversely, if the content of the binder and carrier is too low, sufficient fibrillation is not possible, which may lead to problems such as difficulty in manufacturing self-supporting films or degradation of electrode physical properties, such as electrode peeling.

[0081] If the content of the conductive material exceeds the above range and is too high, the content of the active material will be relatively reduced, which may lead to volume reduction and may hinder the fiberization of PTFE. Conversely, if the content of the conductive material is too low, sufficient conductivity may not be ensured, or the physical properties of the electrodes of the self-supporting film may deteriorate, which is not preferred.

[0082] In some cases, fillers, as components used to suppress electrode expansion, can be further added to the mixture. The fillers are not particularly limited, as long as they are fibrous materials and do not cause chemical changes in the corresponding battery. For example, olefin-based polymers, such as polyethylene and polypropylene, and fibrous materials, such as glass fibers or carbon fibers, can be used.

[0083] Furthermore, according to another embodiment of this disclosure, a method for manufacturing a self-supporting membrane is provided, the method comprising the following steps:

[0084] (a) Mixing an adhesive and a carrier, including a fibrous polymer, to obtain a mixture;

[0085] (b) The mixture and the active material are combined, and the resulting mixture is subjected to high-shear mixing to obtain a fibrous composition; and

[0086] (c) The fibrous composition is formed into a film.

[0087] The carrier is surface-oxidized vapor-grown carbon fiber.

[0088] The same applies to materials such as adhesives, carriers, and active materials that include basic fibrous polymers.

[0089] Furthermore, unlike existing technologies, in this disclosure, an adhesive comprising a fibrous polymer is first mixed with a carrier to obtain a mixture as in step (a). At this point, the mixing is performed without applying shear force.

[0090] When shear force is applied, the fibrillation of fibrous polymers can occur simultaneously with mixing. This significantly reduces their effectiveness for subsequent bonding of active and conductive materials.

[0091] Specifically, the mixing in step (a) can be performed by simply mixing the adhesive and carrier containing the fibrous polymer at 19°C or below, or by adding the adhesive and carrier mixture, including the fibrous polymer, together into the mill after mixing. Here, the mill can be, for example, an air jet mill.

[0092] The reason for mixing at a low temperature of 19°C or below is that shearing and fiberization can be prevented by mixing below the phase transition temperature of PTFE, which is mainly used as a fibrous polymer.

[0093] Furthermore, when using the air jet mill, the particles are ground by impact between particles mixed in a high-pressure / high-speed rotating air atmosphere, but the application of high shear force generated by a conventional impeller mixer is suppressed, thereby crushing the PTFE primary particles without impact fiberization and allowing them to be uniformly dispersed / loaded on the carrier surface.

[0094] At the same time, depending on the mixing method, the structure of the mixture may have predetermined differences.

[0095] Specifically, when a simple mixing of an adhesive comprising a fibrous polymer and a carrier is performed at 19°C or below, the adhesive comprising the fibrous polymer can be uniformly dispersed and mixed together with the carrier.

[0096] Simultaneously, when the binder and carrier mixture is added to a milling machine that applies only impact and not shear force, such as an air jet mill, and further mixed, the primary particles of the fibrous polymer can be uniformly dispersed and loaded along the carrier, as follows: Figure 2 As shown.

[0097] Specifically, compared to simple mixing, the dispersion effect is superior when the mixture of the carrier and the binder, including the fibrous polymer, is placed in a mill such as an air jet mill and mixed, thereby uniformly dispersing and loading the primary particles of the fibrous polymer onto the carrier surface. This further improves electrode characteristics and battery performance, leading to further enhancements in both. Specifically, this mixing can be performed by placing the primary particles of the fibrous polymer on the surface of the carrier to form a structure in which they are uniformly dispersed and loaded, for example, using an air jet mill.

[0098] In this case, the adhesive and carrier, including the fibrous polymer, can be mixed in a weight ratio of 20:80 to 50:50.

[0099] If the carrier content exceeds the above range, the degree of fibrosis of the fibrous polymer is unsatisfactory, leading to electrode quality degradation. When the content is too low, sufficient dispersion of the fibrous polymer cannot be achieved, which is not preferred.

[0100] At this point, simple mixing can be, for example, using a powder mixer (KM Tech) equipped with a cooler to cool to about 5°C to minimize heat generation in the mixer, the powder mixer running at 3000 rpm to 5000 rpm for 30 seconds, twice to five times per minute, specifically three times.

[0101] The mixing is carried out, and further mixing in the mill is performed without applying shear force. For example, when using a mill such as an air jet mill (Sturtevant, Micronizer), the feeding conditions for the mixture can be 4 kgf / cm³. 2 Up to 6 kgf / cm 2 Grinding conditions can be maintained at 2 kgf / cm³. 2 Up to 4 kgf / cm 2 conduct.

[0102] Then, the mixture obtained in step (a) is mixed together with the active material and the conductive material.

[0103] At this point, the mixing in step (b) can be carried out using a powder mixer (KM Tech) equipped with a cooler to cool to about 5°C in the same manner as premixing to ensure uniform dispersion. Specifically, the mixing can be carried out 2 to 5 times at 5000 rpm to 20000 rpm for 30 seconds to 1 minute, specifically 3 times.

[0104] Then, high-shear mixing is applied to obtain a fibrous composition, where high-shear mixing refers to the degree to which the fibrous polymer can be fibrous. Specifically, high-shear mixing can be performed for 1 to 30 minutes in the range of 10 to 500 rpm. This high-shear mixing can be performed, for example, by placing it in a twin-screw kneader (PBV-0.1L manufactured by Irei Shokai). If insufficient shear force is applied, adequate fibrous polymer formation may not be achieved, and if the shear force is too large, the fibrous polymer may break, which is undesirable.

[0105] In addition, the mixing temperature for preparing the high-shear mixing of the fibrous composition can be from 20°C to 120°C, specifically from 40°C to 120°C, and more specifically from 60°C to 120°C.

[0106] It is known in the art that when PTFE, as a polymer, is subjected to shear force through such high-shear mixing, the polymer is fiberized to bond active and conductive materials.

[0107] Then, when the fibrous composition is formed by high-shear mixing, the step of forming it into a film is carried out, wherein the forming in step (c) can be carried out by hot rolling using a calender.

[0108] At this point, calendering can be formed on the calendering rolls. The diameter of the rolls can be, for example, 50 to 1000 mm, specifically 100 to 1000 mm, and more specifically 100 to 500 mm.

[0109] In addition, the surface temperature of the calender roll can be from 20 to 200°C, specifically from 40 to 150°C, and more specifically from 60 to 150°C.

[0110] By applying shear pressure using these calendering rolls, self-supporting films can eventually be manufactured.

[0111] The thickness of the self-supporting membrane thus manufactured can be from 10 to 1000 μm, specifically from 50 to 500 μm, and more specifically from 100 to 500 μm.

[0112] That is, the self-supporting film for dry electrodes according to this disclosure can be manufactured to be thicker, and is therefore suitable for manufacturing thick film electrodes.

[0113] Meanwhile, according to another embodiment of the present disclosure, a dry electrode is provided, the dry electrode further comprising a current collector and a self-supporting film according to claims 1 to 6 formed on the current collector, wherein the current collector has a structure having a primer layer coated on a metal foil.

[0114] The PTFE polymer used to manufacture self-supporting membranes binds active and conductive materials together through fibrosis, and can be formed into a membrane. However, since it does not adhere to current collectors, adhesion between the current collector and the self-supporting membrane can be ensured by using a current collector coated with a primer layer.

[0115] There are no particular limitations on the current collector, as long as it has high conductivity without causing chemical changes in the battery. For example, stainless steel, aluminum, nickel, titanium, sintered carbon, and materials formed by surface treatment of aluminum or stainless steel with carbon, nickel, titanium, silver, or the like can be used. The current collector can have fine irregularities formed on its surface, or it can be processed into a mesh to improve the bonding strength of the positive electrode active material, and it can be used in various forms such as films, sheets, foils, meshes, porous bodies, foams, and nonwoven structures.

[0116] The primer layer can be applied to the current collector in whole or in part; more specifically, it can be applied to the entire current collector.

[0117] This primer layer may include a conductive material and an adhesive. There are no limitations on the conductive material; it can be a carbon-based material such as carbon black, carbon nanotubes, graphene, or graphite. The adhesive may include fluorine-based adhesives (including PVDF and PVDF copolymers), acrylic adhesives, and water-based adhesives that are soluble in solvents.

[0118] The bonding of the self-supporting membrane to the current collector coated with a primer layer can be achieved through lamination.

[0119] The lamination can also be performed by laminating rollers, wherein the laminating rollers can be maintained at a temperature of 80°C to 200°C.

[0120] Meanwhile, according to another embodiment of this disclosure, a secondary battery including the dry electrode is provided.

[0121] Specifically, the electrode assembly, including the dry electrode, separator, and counter electrode, can have a structure that integrates it together with the electrolyte into the battery casing. Since other configurations of secondary batteries are well known in the prior art, their description will be omitted.

[0122] The present disclosure will be described in detail below through examples, comparative examples and test examples so that those skilled in the art can easily understand it.

[0123] <Reference Example>

[0124] The sample was carefully positioned so that the polytetrafluoroethylene (PTFE) prevented the maximum shear force from being applied to the ribbon, and the sample was processed and its SEM image was taken and displayed. Figure 1 middle.

[0125] <Preparation of Oxidized Vapor Grown Carbon Fibers (Ox-VGCF) with Increased Surface Oxygen Functional Groups>

[0126] Commercially available VGCF (Showadenko's VGCF-H) was mixed with a 30 wt% HNO3 solution. The mixture was stirred at 60°C for 8 hours for oxidation treatment, thoroughly washed with distilled water, and then dried in a vacuum oven at 150°C for 24 hours. This yielded surface-oxidized vapor-grown carbon fibers (Ox-VGCF) with approximately 10.3 wt% oxygen functional group content, an average diameter of 150 nm, an average length of 6 μm, and a specific surface area of ​​13 m². 2 / g, degree of graphitization: 0.42).

[0127] The content of oxygen functional groups on the surface was determined by elemental analysis. Specifically, the C, H, and N content of the support was detected by an elemental analyzer (CHN-coder MT-5, Yanako) to calculate the oxygen differential by reflecting the amount of residual ash.

[0128] To measure the average diameter and average length of the carrier, a carrier solution diluted to 1% by weight in acetone was dropped onto a Si wafer and dried. The average diameter and length of 300 individuals were obtained by SEM (Scanning Electron Microscopy, JEOL, JSM-7500F).

[0129] In addition, the specific surface area was measured by the 6-point BET method based on nitrogen adsorption using a specific surface area analyzer (Belsorp-II mini from Bell Japan Inc).

[0130] The degree of graphitization (ID / IG ratio) was measured by analyzing the Raman spectrum using an Ar-ion laser with a wavelength of 514.5 nm using a Raman spectroscopy measurement device (NRS-2000B, Jasco).

[0131] <Preparation Example 1>

[0132] Polytetrafluoroethylene (PTFE) as a binder and surface-oxidized vapor-grown carbon fibers (Ox-VGCF) with increased surface oxygen functional groups were prepared at a 1:1 weight ratio. The mixture was premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler cooled to 5°C. The mixture was then ground / dispersed / loaded using an air jet mill (Sturtevant, Micronizer) with a feed condition of 5 kgf / cm³. 2 The grinding conditions were 3 kgf / cm³. 2 .

[0133] Take SEM images of the mixture prepared above, and display them below. Figure 2 middle.

[0134] <Preparation Example 2>

[0135] Polytetrafluoroethylene (PTFE) as a binder and surface-oxidized vapor-grown carbon fibers (Ox-VGCF) with increased surface oxygen functional groups were prepared in a 1:1 weight ratio and premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C.

[0136] <Preparation Example 3>

[0137] Polytetrafluoroethylene (PTFE) and commercially available unoxidized carbon fiber (Showadenko, VGCF-H) were prepared as a binder at a 1:1 weight ratio and premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C. The mixture was then ground / dispersed / loaded using an air jet mill (Sturtevant, Micronizer) with a feed condition of 5 kgf / cm³. 2 The grinding conditions were 3 kgf / cm³. 2 .

[0138] <Preparation Example 4>

[0139] A mixture was prepared by combining polytetrafluoroethylene (PTFE) as a binder and commercially available activated carbon (Kuraray, YP-80F) as a carrier in a 1:1 weight ratio. The mixture was premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that reaches 5°C. The mixture was then ground / dispersed / loaded using an air jet mill (Sturtevant, Micronizer) with a feed condition of 5 kgf / cm³. 2 The grinding conditions were 3 kgf / cm². 2 .

[0140] <Preparation Example 5>

[0141] Polytetrafluoroethylene (PTFE) as a binder and commercially available activated carbon (Kuraray, YP-80F) as a carrier were prepared in a 1:1 weight ratio and premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C.

[0142] <Preparation Example 6>

[0143] A mixture of polytetrafluoroethylene (PTFE) as a binder and commercially available carbon black (Imerys, Super C45) as a carrier was prepared at a 1:1 weight ratio and premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler to 5°C. The mixture was then ground / dispersed / loaded using an air jet mill (Sturtevant, Micronizer) with a feed condition of 5 kgf / cm³. 2 The grinding conditions were 3 kgf / cm³. 2 .

[0144] <Preparation Example 7>

[0145] A mixture was prepared by using polytetrafluoroethylene (PTFE) as a binder and commercially available activated carbon (Kuraray, YP-80F) as a carrier in a 1:1 weight ratio and premixing it three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C.

[0146] <Preparation Example 8>

[0147] Polytetrafluoroethylene (PTFE), commercially available activated carbon (Kuraray, YP-80F), and commercially available untreated carbon fiber (Showadenko, VGCF-H) were prepared as binders in a weight ratio of 2:1:1 and were premixed three times at 5000 rpm with 1-minute intervals using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C.

[0148] <Example 1>

[0149] Using a powder mixer (KM Tech) equipped with a cooler that cools to 5°C, 96g of LiMn2O4 as the positive electrode active material and 4g of the mixture prepared in Preparation Example 1 as the conductive material and binder were dispersed three times at 10,000 rpm with 1-minute intervals to obtain a mixture. The mixture was then placed in a high-shear mixer twin-screw kneader (Irie Shokai, PBV-0.1L) and mixed at 100 rpm for 3 minutes at 90°C to prepare a fibrous composition.

[0150] SEM images of the fibrotic composition are as follows: Figure 3 As shown.

[0151] refer to Figure 3 It can be seen that the PTFE adhesive is uniformly fibrous overall.

[0152] The fibrous composition was placed in a laboratory calender (roller diameter: 200 mm, roller temperature: 100 °C, 20 rpm) to adjust the distance between the rollers, thereby finally producing a self-supporting film with a thickness of 200 μm.

[0153] <Example 2>

[0154] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 2 was used as the conductive material and binder in Example 1.

[0155] <Comparative Example 1>

[0156] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 3 was used as the conductive material and binder in Example 1.

[0157] <Comparative Example 2>

[0158] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 4 was used as the conductive material and binder in Example 1.

[0159] <Comparative Example 3>

[0160] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 5 was used as the conductive material and binder in Example 1.

[0161] <Comparative Example 4>

[0162] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 6 was used as the conductive material and binder in Example 1.

[0163] <Comparative Example 5>

[0164] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 7 was used as the conductive material and binder in Example 1.

[0165] <Comparative Example 6>

[0166] A self-supporting film with a thickness of 200 μm was manufactured in the same manner as in Example 1, except that the mixture prepared in Preparation Example 8 was used as the conductive material and binder in Example 1.

[0167] <Experimental Example 1>

[0168] The self-supporting membrane was placed on one side of an aluminum foil (20 μm, Primer Coated Al Foil, Dongwon Systems) coated with a primer layer mixed with carbon black:PVDF binder, and the electrode was prepared by lamination using a lamination roller maintained at 120°C.

[0169] A coin-shaped half-cell was fabricated using an electrode and lithium metal as the counter electrode, and an electrolyte solution containing 1 M LiPF6 in a solvent with EC:DMC:DEC = 1:2:1.

[0170] The coin-shaped half-cell manufactured above was charged and discharged at 25°C, a voltage range of 3.0 to 4.30V, and a current rate of 0.2C. The discharge capacity ratio was measured relative to the discharge capacity when charged and discharged at a current rate of 2.0C. The results are shown in Table 1 below.

[0171] [Table 1]

[0172] 2.0C discharge capacity ratio (%, relative to 0.2C discharge capacity) Example 1 98.3 Example 2 97.1 Comparative Example 1 95.7 Comparative Example 2 91.4 Comparative Example 3 89.1 Comparative Example 4 92.6 Comparative Example 5 85.2 Comparative Example 6 92.3

[0173] Referring to Table 1, it can be confirmed that when used with the surface-oxidized vapor-grown carbon fibers with increased surface oxygen functional groups according to this disclosure, excellent battery characteristics can be exhibited. In particular, even better results can be obtained when further mixed using an air-jet mill.

[0174] Furthermore, upon examining Comparative Example 1, even when using unoxidized VGCF, an improved effect can be observed by exhibiting the predetermined dispersion effect, but the effect is insufficient and is very poor compared to the present disclosure. Not only when using activated carbon and carbon black, but also when activated carbon and VGCF are used together, the effect is significantly reduced compared to the present disclosure.

[0175] [Industrial Applicability]

[0176] According to this disclosure, a carrier for surface-oxidized vapor-grown carbon fibers (Ox-VGCF) is mixed with a binder comprising a fibrous polymer and used to manufacture a self-supporting membrane. Here, the primary particles of the fibrous polymer can be uniformly dispersed / loaded in the mixture for manufacturing the self-supporting membrane. Therefore, since the self-supporting membrane can form a uniform and robust bond through the fibrillation of the fibrous polymer, the physical properties of the electrode can be improved, and thus the performance of the battery including the self-supporting membrane can also be improved.

[0177] Furthermore, by using the carrier, the conductive network between the active materials can be made more robust, which is more beneficial for improving battery performance.

Claims

1. A self-supporting membrane for dry electrodes, The self-supporting membrane comprises an active material, an adhesive containing a fibrous polymer, and a carrier. The carrier is surface-oxidized vapor-grown carbon fiber; The carrier is a carbon fiber with increased surface oxygen functional groups through surface oxidation treatment; The carrier comprises surface oxygen functional groups in an amount of 5% to 15% by weight based on the total weight. The average diameter of the carrier is 50 nm to 500 nm, and the average length is 1 μm to 30 μm. The specific surface area of ​​the carrier is 10m². 2 / g to 150m 2 / g; and The carrier is contained in an amount of 0.1% to 10% by weight, based on the total weight of the self-supporting membrane.

2. The self-supporting membrane according to claim 1, wherein: The fibrous polymer is fiberized to bind the active material and the carrier.

3. The self-supporting membrane according to claim 1, wherein: The degree of graphitization of the carrier is 0.1 to 2.

0.

4. The self-supporting membrane according to claim 1, wherein: The fibrous polymer is polytetrafluoroethylene.

5. A method for manufacturing the self-supporting membrane as claimed in claim 1, the method comprising the following steps: (a) Mixing an adhesive and a carrier, including a fibrous polymer, to obtain a mixture; (b) The mixture and the active material are mixed together, and the resulting mixture is subjected to high-shear mixing to obtain a fibrous composition; and (c) The fibrous composition is formed into a film. The carrier is surface-oxidized vapor-grown carbon fiber.

6. The method according to claim 5, wherein: The fibrous polymer is polytetrafluoroethylene.

7. The method according to claim 5, wherein: The mixing in step (a) includes simply mixing the adhesive and carrier comprising the fibrous polymer at a temperature of 19°C or lower, or loading the adhesive and carrier mixture comprising the fibrous polymer together into a mill after mixing.

8. The method according to claim 7, wherein: The grinding machine is an air jet grinding machine.

9. The method according to claim 5, wherein: The mixture has a structure in which primary particles of the fibrous polymer are uniformly dispersed and loaded on the surface of the carrier.

10. The method according to claim 5, wherein: The high-shear mixing in step (b) is carried out for 1 to 30 minutes in the range of 10 to 500 rpm.

11. The method according to claim 5, wherein: The forming in step (c) is carried out by hot rolling in a calender.

12. A dry electrode, comprising: Current collector, and a self-supporting membrane formed on said current collector according to any one of claims 1 to 4, The current collector described therein has a structure in which a primer layer is coated on a metal foil.

13. A secondary battery, said secondary battery comprising the dry electrode of claim 12, This includes the electrode assembly consisting of the dry electrode, counter electrode, and separator, which is assembled together with a lithium-containing non-aqueous electrolyte into the battery casing.

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