Method for manufacturing a positive electrode for a secondary battery and a secondary battery containing a positive electrode

Abstract

To provide a manufacturing method of a positive electrode for a secondary battery, and the secondary battery including the positive electrode.SOLUTION: A manufacturing method of a positive electrode of a secondary battery according to the present invention includes a step (a) of heating a slurry composition for a positive electrode containing a positive electrode active material, a binder, and a solvent at temperature lower than a boiling point (Tb) of the solvent, a step (b) of applying the heated slurry composition for the positive electrode onto a current collector, and a step (c) of cooling the applied slurry composition for the positive electrode.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] This invention relates to a method for manufacturing a positive electrode for a secondary battery and to a secondary battery containing a positive electrode. [Background technology]

[0002] Recently, with the increasing demand for electronic devices such as mobile devices, development is expanding on the lightweight and miniaturization of electrochemical batteries (secondary batteries) to enhance the portability of these devices. In line with this trend, the growth of the electric vehicle (EV) market is accelerating due to the tightening of regulations related to automobile fuel efficiency and exhaust emissions worldwide, and there is a need for the development of high-power, high-capacity batteries for use in such electric vehicles.

[0003] In general, the electrodes for such electrochemical batteries (secondary batteries) are manufactured by mixing electrode active material, conductive material, and binder in a solvent, dispersing the mixture using a strong shear force, applying the resulting electrode slurry to a current collector, and then drying it. In this process, if the solvent used to disperse the electrode active material and conductive material and dissolve the binder remains after the drying process, it can cause electrochemical side reactions within the secondary battery, adversely affecting the battery's performance. On the other hand, capillary force generated when particles such as electrode active material are exposed to the solvent surface during the drying process causes the solvent to flow from the inside to the surface of the electrode slurry layer. This causes the conductive material and binder, which have relatively smaller particle sizes than the electrode active material, to migrate and become more abundant on the electrode surface. This uneven distribution of conductive material and binder can cause an increase in the resistance of the secondary battery or a decrease in the adhesion between the substrate (current collector) and the electrode active material layer. Therefore, in order to solve the above-mentioned problems, research has been conducted on methods such as reducing the solvent content in the electrode slurry to increase the viscosity of the slurry, thereby reducing the residual solvent content in the secondary battery and suppressing the migration phenomenon of small particles (binder and conductive material) in the electrode slurry. However, when the solvent content of the electrode slurry is reduced and the viscosity is increased, the fluidity of the electrode slurry decreases rapidly, reducing workability, resulting in the production of electrodes with uneven thickness, and causing problems such as a decrease in the adhesion strength between the current collector and the electrode active material layer within the electrode. Therefore, research and development are needed to reduce the solvent content while solving the problem of reduced substrate adhesion strength due to the fluidity of the high-viscosity solvent. [Overview of the project] [Problems that the invention aims to solve]

[0004] The object of the present invention is to solve the problem caused by the migration of the conductive material and binder due to the flow of the low-viscosity solvent during drying, in the process of applying a positive electrode slurry containing a positive electrode active material, a conductive material, a binder, and a solvent onto a current collector and drying it to form a positive electrode active material layer, namely, the problem of reduced adhesion between the current collector and the positive electrode active material layer. [Means for solving the problem]

[0005] One embodiment of the present invention is (a) a cathode slurry composition comprising a cathode active material, a binder, and a solvent, wherein the boiling point (T) of the solvent is b The present invention provides a method for manufacturing a positive electrode for a secondary battery, comprising the steps of (b) heating at a temperature lower than (a), (b) applying the heated positive electrode slurry composition onto a current collector, and (c) cooling the applied positive electrode slurry composition.

[0006] The solid content in the positive electrode slurry composition may be present in an amount of more than 55 and less than or equal to 90 parts by weight per 100 parts by weight of the positive electrode slurry composition.

[0007] The viscosity (A1) of the cathode slurry composition before heating can be 13,000 to 80,000 cp.

[0008] The temperature (T) of the aforementioned heating step can satisfy the following relational expression 1. [Relationship 1] 0.3T b <T<0.9T b In the above relational expression 1, T b is the boiling point of the solvent.

[0009] The cooling can be performed at a temperature of 10 to 30°C for 1 to 60 seconds.

[0010] The aforementioned slurry composition for the positive electrode may satisfy the following relational equations 2 and 3. [Relationship 2] 1.3 ≤ A1 / A2 ≤ 12 [Relationship 3] 1.1 ≤ A3 / A2 ≤ 10 In the relationships 2 and 3 described above, A1 is the viscosity of the cathode slurry composition before heating, A2 is the viscosity of the heated cathode slurry composition, and A3 is the viscosity of the cooled cathode slurry composition.

[0011] The process may further include step (d) of drying the cooled cathode slurry composition. In this case, the drying may be carried out at a temperature of 110°C or higher but less than 145°C for 20 to 150 seconds.

[0012] Another example is (a) a cathode slurry composition comprising a cathode active material, a binder, and a solvent, wherein the boiling point (T) of the solvent is b A positive electrode for a secondary battery is provided, manufactured by a method for manufacturing a positive electrode for a secondary battery, comprising the steps of (b) heating at a temperature lower than (a), (b) applying the heated positive electrode slurry composition onto a current collector, and (c) cooling the applied positive electrode slurry composition, the positive electrode comprising a current collector and a positive electrode active material layer formed on the current collector and containing a positive electrode active material, a conductive material, and a binder.

[0013] The adhesive strength of the positive electrode active material layer to the current collector may be 0.35 N / cm or more.

[0014] The positive electrode is such that the difference between the maximum and minimum loading values ​​of the positive electrode active material layer at at least five positions spaced at predetermined intervals in the longitudinal direction may be 10% or less of the overall average loading value.

[0015] Another embodiment includes a positive electrode, a negative electrode, a separator located between the positive and negative electrodes, and an electrolyte, wherein the positive electrode comprises (a) a slurry composition for the positive electrode containing a negative electrode active material, a binder, and a solvent, with the solvent having a boiling point (T bA method for manufacturing a positive electrode for a secondary battery, comprising: (a) a step of heating at a temperature lower than [a certain temperature]; (b) a step of applying the heated positive electrode slurry composition onto a current collector; and (c) a step of cooling the applied positive electrode slurry composition, and a secondary battery characterized by comprising a current collector and a positive electrode active material layer formed on the current collector and containing a positive electrode active material, a conductive material, and a binder is provided.

Advantages of the Invention

[0016] The method for manufacturing a positive electrode for a secondary battery according to the present invention solves the problems of the conventional method for manufacturing a positive electrode using a low-viscosity positive electrode slurry composition, that is, the problems that the conductive material and the binder migrate to the surface of the positive electrode active material layer due to the flow of the low-viscosity solvent during drying, resulting in a decrease in the adhesive force between the current collector and the positive electrode active material layer. To solve this problem, when using a high-viscosity positive electrode slurry composition, the high-viscosity positive electrode slurry composition is heated to reduce the viscosity and then applied onto the current collector, and after cooling at room temperature and then drying, a positive electrode for a secondary battery having an improved adhesive force between the current collector and the positive electrode active material layer can be provided, and there is an effect of preventing problems such as a decrease in fluidity and the formation of non-uniform positive electrode thickness caused by the high viscosity.

Brief Description of the Drawings

[0017] [Figure 1] The result of analyzing the cross-section of the positive electrodes of Example 1 and Comparative Example 1 by energy-dispersive X-ray spectroscopy (SEM-EDS) using a scanning electron microscope shows the distribution of the binder in the thickness direction of the positive electrode active material layer. [Figure 2] For the evaluation of the homogeneity of the positive electrode active material layer, a schematic diagram of a test piece punched out in a circular shape with a diameter of 38 mm by selecting five locations having a predetermined interval in the width direction of the positive electrode active material layer is shown. [Figure 3]This figure illustrates the normalized results obtained by dividing the weight of the positive electrode active material layer (loading amount of positive electrode active material layer composition), measured separately in the width direction of the positive electrode in Example 1 and Comparative Example 2, by the total weight of the positive electrode active material layer. [Modes for carrying out the invention]

[0018] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but can be embodied in a variety of different forms, although these embodiments are provided to complete the disclosure of the present invention and to fully inform those ordinary skill in the art to which the invention pertains, and the present invention is defined only by the scope of the claims. The specific details for carrying out the present invention will be described in detail with reference to the accompanying drawings below. Regardless of the drawings, the same part numbers refer to the same components, and "and / or" includes each of the items referred to and all combinations of one or more of them.

[0019] Unless otherwise defined, all terms used herein (including technical and scientific terms) are to be used in a sense that can be commonly understood by a person of ordinary skill in the art to which the invention pertains. Where the whole specification says that a part of it "includes" a component, this means, unless otherwise stated, that it may include other components rather than excluding them. Also, the singular form includes the plural form unless otherwise specified in the text.

[0020] In this specification, when a part such as a layer, film, region, or plate is said to be "on top of" or "on" another part, this includes not only the case where it is "directly on top of" the other part, but also the case where there is yet another part in between.

[0021] In this specification, "viscosity" is defined as the shear rate (5s) measured using a Brookfield rotary viscometer at the slurry temperature for each step. -1 The values ​​were measured using [a specific method / tool], and the tolerance is ±5cP.

[0022] One embodiment of the present invention is (a) a cathode slurry composition comprising a cathode active material, a binder, and a solvent, wherein the boiling point (T) of the solvent is b The present invention provides a method for manufacturing a positive electrode for a secondary battery, comprising the steps of (b) heating at a temperature lower than (a), (b) applying the heated positive electrode slurry composition onto a current collector, and (c) cooling the applied positive electrode slurry composition.

[0023] (a1) First, a slurry composition for the positive electrode is prepared, comprising a positive electrode active material, a binder, a conductive material, and a solvent.

[0024] The solid components (positive electrode active material, binder, and conductive material) in the positive electrode slurry composition may be present in an amount greater than 74 parts by weight, specifically greater than 74 and less than or equal to 90 parts by weight, and more specifically greater than 74 and less than or equal to 85 parts by weight, per 100 parts by weight of the positive electrode slurry composition.

[0025] Generally, in the manufacturing process of positive electrodes for secondary batteries, the slurry needs to have a viscosity of approximately 1,000 cp to 10,000 cp at 25°C to facilitate slurry application and produce a positive electrode with uniform thickness. However, when manufacturing a positive electrode slurry composition using conventional methods to reduce the solvent content so that the solid content is 80% by weight or more, the viscosity of the slurry exceeds 53,000 cp, resulting in a rapid decrease in slurry fluidity, poor workability, the production of a positive electrode with uneven thickness, and a decrease in the adhesion between the current collector and the positive electrode active material layer. Therefore, conventional positive electrode slurry compositions contain a solid content of approximately 74% by weight.

[0026] On the other hand, in the present invention, the fluidity problem can be solved by heating the high-viscosity positive electrode slurry. However, if the weight exceeds the above-mentioned range, the slurry composition for the positive electrode may not contain the minimum amount of solvent necessary to disperse the solid particles during production, and the shear force for particle dispersion may not act sufficiently. As a result, the solid particles in the slurry composition may be applied to the current collector in an unevenly dispersed state. If the weight is below the above-mentioned range, the fluidity of the slurry composition is already sufficient, and there is no need to apply the technique proposed in the present invention.

[0027] The viscosity (A1) of the cathode slurry composition before heating is 13,000 to 80,000 cp at 25°C, preferably 15,000 to 75,000 cp, 20,000 to 75,000 cp, or 25,000 to 75,000 cp, more preferably 35,000 to 70,000 cp, 45,000 to 70,000 cp, 50,000 to 70,000 cp, or 55,000 to 70,000 cp.

[0028] If the solvent content in the aforementioned cathode slurry composition increases and the viscosity (A1) of the slurry composition falls below 13,000 cp at 25°C, the large amount of solvent contained in the slurry composition may weaken the adhesion between the cathode active material layer and the current collector due to binder migration, potentially reducing long-term stability.

[0029] When the viscosity (A1) of the positive electrode slurry composition exceeds 80,000 cp at 25°C, the increase in adhesive strength between the positive electrode active material layer and the current collector is minimal even when the solid content (especially the positive electrode active material and binder) increases.

[0030] Furthermore, as will be described later, when the positive electrode slurry composition is heated and its viscosity is reduced, the binder within the coated film (the coated positive electrode slurry composition) does not migrate to the electrode surface. This allows for excellent adhesion to the substrate (current collector) even with a small amount of binder, and enables the composition to contain less binder than conventional positive electrode slurry compositions. As a result, it is possible to include more positive electrode active material than conventional positive electrode slurry compositions, providing a positive electrode with improved electrode dosage, which is advantageous.

[0031] Specifically, the weight ratio (content ratio) of positive electrode active material to binder in the solid content can be 1:0.01 to 0.1, more specifically 1:0.01 to 0.05, and more specifically 1:0.01 to 0.03.

[0032] The positive electrode active material can be any positive electrode active material commonly used in secondary batteries, without any limitations. Examples include, but are not limited to, composite oxides of lithium with metals selected from cobalt, manganese, nickel, and combinations thereof.

[0033] The binder is not particularly limited as long as it is a conventional binder that serves to ensure good adhesion between positive electrode active material particles and good adhesion between the positive electrode active material and the current collector. Examples include, but are not limited to, polyvinyl alcohol, carboxymethylcellulose, hydroxypropylcellulose, diacetylcellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers containing ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene-butadiene rubber, epoxy resin, nylon, etc.

[0034] The solvent can be at least one selected from the group consisting of amine-based solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether-based solvents such as ethylene oxide and tetrahydrofuran; ketone-based solvents such as methyl ethyl ketone; ester-based solvents such as methyl acetate; and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone, but is not limited thereto.

[0035] The conductive material is used to impart conductivity to the positive electrode and is not particularly limited as long as it is a conventional electron conductive material that does not cause chemical changes in the battery. As an example, it can be natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotube, and combinations thereof, but is not limited thereto.

[0036] The current collector can be stainless steel, aluminum, nickel, titanium, fired carbon, or a material obtained by surface-treating the surface of stainless steel with carbon, nickel, titanium, silver, etc., but is not limited thereto.

[0037] (a2) Next, the produced high-viscosity positive electrode slurry composition is heated at a temperature lower than the boiling point (T b ) of the solvent.

[0038] Thereby, the high-viscosity positive electrode slurry composition can have a reduced viscosity that makes the work easier and enables the formation of a positive electrode with a uniform thickness by being heated at a temperature below the boiling point. Thereby, the method for manufacturing a positive electrode for a secondary battery according to the present invention can apply the positive electrode slurry composition on the current collector in a state where the positive electrode slurry composition is heated to reduce the viscosity when using a high-viscosity positive electrode slurry composition containing a high content of solid matter.

[0039] However, if the heating temperature of the cathode slurry composition is above the boiling point of the solvent, strong vapor pressure may be generated inside the piping or storage container used for slurry transfer when the heated cathode slurry composition is transferred or stored, which could lead to risks such as water leakage or explosion.

[0040] Therefore, the heating can be performed at a temperature lower than the boiling point of the solvent contained in the cathode slurry composition.

[0041] Specifically, the heating temperature (T) can satisfy the following relational expression 1.

[0042] [Relationship 1] 0.3T b <T<0.9T b

[0043] In the above relational expression 1, T b is the boiling point of the solvent.

[0044] In the above relational equation 1, 0.35T b <T<0.9T b Preferably 0.4T b <T<0.85T b , more preferably 0.5T b <T<0.8T b It is possible.

[0045] The viscosity (A2) of the heated slurry composition may be 6,000 to 10,000 cp, preferably 7,000 to 9,800 cp, and more preferably 7,500 to 9,600 cp at the temperature of each slurry composition.

[0046] (b) In step (b), the heated positive electrode slurry composition is applied onto the current collector. This application can be performed without heating (increasing the temperature) of the slurry and the substrate (current collector).

[0047] As a non-limiting example, the coating may be any coating method generally known to be used to apply a liquid phase to form a film. For example, spray coating, dip coating, spin coating, gravure coating, slot die coating, doctor blade coating, roll coating, inkjet printing, flexography printing, screen printing, electrohydrodynamic printing, microcontact printing, imprinting, reverse offset printing, bar coating, gravure offset printing, etc., may be used, but are not limited to these.

[0048] (c) In step (c), the coated cathode slurry composition is cooled.

[0049] The cooling can be performed immediately after the positive electrode slurry composition is heated and applied to the current collector, at a temperature of 10-30°C, specifically 10-25°C, more specifically 15-25°C, for 1-60 seconds, specifically 1-30 seconds, more specifically 3-7 seconds.

[0050] The heated cathode slurry composition can regain its high viscosity upon cooling. Thus, the method for manufacturing a positive electrode for a secondary battery according to the present invention, when using a high-viscosity cathode slurry composition containing a high solid content, involves heating the cathode slurry composition to reduce its viscosity before applying it to the current collector, and then immediately after application, cooling the applied cathode slurry composition to restore its high viscosity. This suppresses the migration phenomenon of solids (binder or conductive material) that occurs during the subsequent drying process. Therefore, the present invention can provide a cathode for a secondary battery having improved adhesion between the current collector and the cathode active material layer, and has the effect of preventing problems such as reduced fluidity and the formation of uneven electrode thickness that can be caused by high viscosity.

[0051] The viscosity (A3) of the cooled slurry composition can be 8,000 cp to 60,000 cp, preferably 10,000 cp to 55,000 cp, more preferably 13,000 cp to 50,000 cp, and even more preferably 35,000 cp to 45,000 cp at the temperature of each slurry.

[0052] That is, the slurry composition for the positive electrode can be characterized by satisfying the following relational expressions 2' and 3'.

[0053] [Relational expression 2'] A2 < A1

[0054] [Relational expression 3'] A2 < A3

[0055] In the relational expressions 2' and 3', A1 is the viscosity of the slurry composition for the positive electrode before heating, A2 is the viscosity of the heated slurry composition for the positive electrode, and A3 is the viscosity of the cooled coating film (slurry composition for the positive electrode).

[0056] However, even when the slurry composition for the positive electrode heated within a limited time is cooled, it may not be cooled to the temperature of the slurry composition for the positive electrode before heating. Therefore, the viscosity of the recovered slurry composition for the positive electrode after cooling may be lower than the viscosity of the slurry composition for the positive electrode before heating. Accordingly, the relational expressions 2' and 3' can be summarized as A2 < A3 ≤ A1, but are not limited thereto.

[0057] In one embodiment, the viscosity of the slurry composition for the positive electrode can be reduced to 1 / 3 or less after heating and then can recover to a viscosity similar to the initial one during cooling.

[0058] That is, the relational expressions 2' and 3' can also be summarized as the following relational expressions 2 and 3.

[0059] [Relational expression 2] 1.3 ≤ A1 / A2 ≤ 12

[0060] In relational equation 2 above, 1.8 ≤ A1 / A2 ≤ 11, specifically 3 ≤ A1 / A2 ≤ 10, and more specifically 6 ≤ A1 / A2 ≤ 7.

[0061] [Relationship 3] 1.1 ≤ A3 / A2 ≤ 10

[0062] In relation 3 above, 1.3 ≤ A3 / A2 ≤ 9, specifically 3 ≤ A3 / A2 ≤ 7, and more specifically 4 ≤ A3 / A2 ≤ 5.

[0063] Furthermore, the method for manufacturing a positive electrode for a secondary battery according to the present invention may further include step (d) of drying the cooled positive electrode slurry composition. In this case, the drying may be carried out for 20 to 150 seconds, preferably 50 to 130 seconds, more preferably 70 to 120 seconds, most preferably 90 to 110 seconds, at a temperature of 100°C or higher and less than 180°C, preferably 110 to 145°C, preferably 110 to 135°C, more specifically 110 to 125°C.

[0064] Another embodiment of the present invention provides a positive electrode for a secondary battery manufactured by the above method, comprising a current collector and a positive electrode active material layer formed on the current collector and containing a positive electrode active material, a conductive material, and a binder.

[0065] Generally, if a positive electrode slurry composition is simply heated to reduce its viscosity, applied to a current collector, and dried while still highly viscous, conductive materials and binders, which have relatively smaller particle sizes and lighter weight than the positive electrode active material, may easily migrate toward the surface of the positive electrode slurry layer due to the flow of the solvent. In such cases, the adhesion strength of the positive electrode active material layer to the substrate (current collector) may be significantly reduced.

[0066] However, the positive electrode for secondary batteries manufactured according to the present invention is produced by heating a high-viscosity positive electrode slurry composition to reduce its viscosity and applying it to a current collector, then immediately after application, cooling the applied positive electrode slurry composition to restore its high viscosity, and then drying it. This method suppresses the migration phenomenon of solid components (binder or conductive material).

[0067] As a result, the binder and conductive material can be characterized in that they do not have a concentration gradient in the thickness direction of the positive electrode active material layer. More specifically, when the positive electrode active material layer is divided into a first region adjacent to the current collector, a second region located in the center of the positive electrode active material layer, and a third region adjacent to the surface of the positive electrode active material layer, if each region is divided to have the same thickness, the concentration deviation of the solid content (binder or conductive material) contained in each region may be 0.4% by weight or less, specifically 0.01 to 0.4% by weight, and more specifically 0.05 to 0.35% by weight.

[0068] On the other hand, the first, second, and third regions may have the same thickness, but the present invention is not limited thereto.

[0069] Furthermore, the present invention aims to solve problems such as reduced fluidity and the formation of uneven positive electrode thickness that can result from the use of high-viscosity slurries. Compared to conventional methods, it improves slurry fluidity and enables the formation of a uniform positive electrode thickness (uniform slurry loading) during slurry application, even when using a higher viscosity positive electrode slurry.

[0070] In one embodiment of the present invention, the positive electrode may have a positive electrode active material layer uniformly formed in the length direction (width direction), and specifically, the loading amount of the positive electrode active material layer may be uniform.

[0071] Specifically, the positive electrode may have a difference of 10% or less between the maximum and minimum loading values ​​of the positive electrode active material layer at at least five positions spaced at predetermined intervals in the longitudinal direction, which may be, for example, 9% or less, 8% or less, or 6% or less. This can further improve the effects described above.

[0072] In this case, the loading may refer to the weight of the positive electrode active material slurry solids or the weight of the positive electrode active material layer of the final positive electrode after the positive electrode slurry applied to the positive electrode current collector has dried. As a non-limiting example, a test specimen can be obtained by punching out at least five circular holes at predetermined intervals within the length (width) direction of the positive electrode, and then measuring the weight of the positive electrode active material slurry solids or the positive electrode active material layer within the test specimen.

[0073] Therefore, the positive electrode for secondary batteries manufactured according to the present invention is preferable because the solid components are homogeneously dispersed within the positive electrode active material layer, resulting in improved adhesion to the substrate (current collector).

[0074] In one preferred embodiment, the positive electrode active material layer may have an adhesive force to the current collector of 0.35 N / cm or more, specifically 0.35 to 1.5 N / cm, and more specifically 0.45 to 0.8 N / cm.

[0075] Furthermore, the present invention provides a secondary battery comprising a positive electrode, a negative electrode, a separator located between the positive electrode and the negative electrode, and an electrolyte.

[0076] In detail, the positive electrode manufactured according to the present invention can have improved substrate adhesion and thickness uniformity. As a result, the secondary battery containing the positive electrode can have improved long-term stability, which is preferable.

[0077] The negative electrode may include a current collector and a negative electrode active material layer located on the current collector. The current collector may be made of copper or nickel, but is not limited to these.

[0078] The negative electrode active material can be used without limitation as long as it is a negative electrode active material commonly used in secondary batteries. As an example, it can be a carbon-based negative electrode active material, a silicon-based negative electrode active material, or a mixture thereof, but is not limited thereto. The carbon-based negative electrode active material can be one or more selected from artificial graphite, natural graphite, and hard carbon. The silicon-based negative electrode active material is Si, SiO x (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Si-carbon composite, or a mixture of at least one of these and SiO2.

[0079] The separator is not particularly limited as long as it is a separator known in the art. For example, it can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or a combination thereof, can be in the form of a non-woven fabric or a woven fabric, and optionally can be used as a single-layer or multi-layer structure.

[0080] The electrolyte comprises a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent may be, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methyl carbonate (EMC), 1,2-dimethoxyethane (DME), γ-butyrolactone (BL), tetrahydrofuran (THF), 1,3-dioxolane (DOL), diethyl ether (DEE), methyl formate (MF), methyl propionate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN), or mixtures thereof. The electrolytic salt is dissolved in the non-aqueous organic solvent and acts as a source of electrolytic metal ions within the battery, enabling the operation of a basic secondary battery and promoting the movement of electrolytic metal ions between the positive and negative electrodes. As a non-limiting example, when the electrolytic metal is lithium, the electrolytic salts are LiPF6, LiBF4, LiTFSI, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiSbF6, LiAlO4, LiAlCl4, LiN(C x F 2x+1 SO2)(C y F 2y+1 The electrolytic salt may be, but is not limited to, SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof. Furthermore, known substances can be used in concentrations suitable for the purpose, and may further contain known solvents or additives as needed to improve charge-discharge characteristics, flame retardancy, etc.

[0081] Examples (Example 1) Step 1: Manufacturing of the positive electrode slurry A cathode slurry (81.5% by weight of total slurry solids) was prepared by mixing 79.5% by weight of nickel cobalt manganese oxide (NCM, specifically CSG131, Ni:Co:Mn=8:1:1), 1.2% by weight of polyvinylidene fluoride, 0.8% by weight of carbon black, and 18.5% by weight of N-methylpyrrolidone (NMP). The prepared cathode slurry was stirred for 30 minutes in a stirring tank heated to 80°C, and the slurry temperature was maintained at 80°C.

[0082] Step 2: Manufacturing the positive electrode The positive electrode slurry produced in step 1 (heated to 80°C) was applied to an aluminum current collector (aluminum foil with a thickness of 12 μm) using a slot die coater.

[0083] Next, the coated cathode slurry was left at 25°C for 5 seconds to cool to 50°C, and then dried for 1 minute in a drying oven heated with hot air at 140°C to complete the cathode active material layer.

[0084] In this case, the thickness of the positive electrode active material layer was set to 50 μm.

[0085] Evaluation example [Evaluation Example 1] Measurement of cross-sectional SEM image of the positive electrode active material layer and evaluation of the adhesion strength between the active material layer and the current collector.

[0086] (Comparative Example 1) The cathode was manufactured in the same manner as in Example 1, except that in Step 2 of Example 1, the step of cooling the coated cathode slurry was omitted, and the heated cathode slurry was dried immediately after application.

[0087] (Comparative Example 2) The same method as in Example 1 was used, except that the heating and cooling process of the positive electrode slurry was omitted in step 2 of Example 1, and the coated positive electrode slurry was used after drying.

[0088] (Evaluation method) *Measurement of cross-sectional SEM-EDS images of the positive electrode active material layer The cross-sections of the positive electrodes produced in Example 1 and Comparative Example 1 were cut using an ion-milling apparatus, and the fluorine distribution of the binder within the positive electrode was measured by energy-dispersive X-ray spectroscopy (SEM-EDS) using a scanning electron microscope. The results are shown in comparison with Figure 1.

[0089] *Evaluation of the homogeneity of the positive electrode active material layer To analyze the homogeneity of the positive electrode active material layers produced in Example 1 and Comparative Example 1, five locations with predetermined intervals along the length of the positive electrode active material layer were selected as shown in Figure 2. These locations were punched out in a circular shape with a diameter of 38 mm, and the weight of the positive electrode active material layer within each punched-out test piece was measured to analyze the loading level of the positive electrode active material layer composition within the positive electrode. The weights of the positive electrode active material layer measured at each location (loading level of the positive electrode active material layer composition) were normalized by dividing them by the total weight of the positive electrode active material layer, and the results are shown in Figure 3.

[0090] As illustrated in Figure 3, when the positive electrode active material layer is applied after the viscosity of the positive electrode slurry has been reduced to a level of 9,500 cp by heating during manufacturing (Example 1), the difference in loading values ​​of the positive electrode active material layer (maximum value - minimum value) is very low, at 6% of the average loading value measured at five locations. In contrast, when the positive electrode slurry is applied with a high viscosity of 64,000 cp (Comparative Example 2), the difference in loading values ​​of the positive electrode active material layer is 15% of the average loading value measured at five locations, indicating a non-uniform distribution with high loading in the center and low loading in the outer edges.

[0091] *Measurement of viscosity of positive electrode active material slurry The viscosity of the cathode slurry composition before heating, after heating, and after cooling was determined by measuring the shear rate (Shear Rate) of each slurry composition at the temperature after the cathode slurry was manufactured at that temperature using a rotational viscometer. -1 It was measured using [this method].

[0092] *Evaluation of interfacial adhesion between the active material layer and the current collector. The positive electrodes manufactured in Example 1 and Comparative Example 1 were cut to 18 mm wide and 150 mm long. An 18 mm wide tape was attached to the foil layer of the positive electrode, and then a roller with a 2 kg load was used to ensure that the tape adhered well to the foil layer. The active material layer of the positive electrode was attached to one side of a tensile testing machine using double-sided tape. The tape attached to the foil was fastened to the opposite side of the tensile testing machine, and the adhesive strength was measured. The results are shown in Table 1 below.

[0093] [Table 1]

[0094] Referring to Table 1, we can see that the viscosity of the cathode slurry composition changes with temperature, and that the viscosity of the cathode slurry composition decreases immediately after heating, and then recovers to a high viscosity after cooling.

[0095] The electrode according to Example 1 exhibits high substrate adhesion despite being manufactured using a cathode slurry composition with the highest viscosity due to its high solid content (81.5% by weight). The electrode according to Comparative Example 1 uses a high-viscosity cathode slurry composition with the same high solid content (81.5% by weight) as Example 1. However, unlike Example 1, it is manufactured by drying the cathode slurry composition immediately after application without a cooling process, after the viscosity has been reduced by heating. As a result, it exhibits lower substrate adhesion than Example 1. This is because, when drying is performed in the lowest viscosity state, the binder and conductive material migrate to the electrode surface layer due to solvent flow, increasing the binder concentration in the region adjacent to the cathode active material layer surface compared to the region adjacent to the substrate.

[0096] On the other hand, the slurry from Comparative Example 2 contained the same amount of solids as in Example 1 and had high viscosity. However, because the process of heating the slurry to reduce its viscosity was not performed, the viscosity of the slurry became excessively high, making it impossible to manufacture the positive electrode. Therefore, the substrate adhesion strength of Comparative Example 2 could not be evaluated.

[0097] Furthermore, referring to Figure 1, it was confirmed that in Example 1, the positive electrode active material layer did not show a substantially consistent concentration gradient of binder and conductive material in the thickness direction. This is analyzed to be due to the suppression of the binder and conductive material migration phenomenon that occurs during the slurry drying process. On the other hand, in Comparative Example 1, since the slurry dried immediately at its lowest viscosity, the concentrations of binder and conductive material increased in the region adjacent to the surface of the positive electrode active material layer compared to the region adjacent to the substrate.

[0098] [Evaluation Example 2]: Evaluation of the adhesion between the active material layer and the current collector due to changes in the solid content of the positive electrode active material slurry. (Examples 2-4, Comparative Example 3) In Example 1, the cathode was manufactured in the same manner as in Example 1, except that in Step 1, different solid content and heating temperatures were used during the production of the cathode slurry to achieve the viscosity of the cathode slurry as shown in Table 2 below. In this case, the heating temperatures for Examples 2 to 4 were adjusted so that the viscosity of each slurry was 7,800 to 8,700 cp, while Comparative Example 3 used a slurry at room temperature (25°C) without heating.

[0099] (Evaluation method) The interfacial adhesion strength and viscosity between the active material layer and the current collector were evaluated in the same manner as in Evaluation Example 1, and the results are shown in Table 2 below.

[0100] [Table 2]

[0101] Referring to Table 2, it was found that in Examples 1 and 2, which have a preferred solids content range, the viscosity of the applied slurry is high, reducing the fluidity of the solvent and suppressing the migration phenomenon of the binder during the drying process, thereby increasing the adhesion between the substrate and the active material layer.

[0102] On the other hand, while it was confirmed that Comparative Example 3 had very low adhesive strength, the positive electrode was formed in a room-temperature slurry without a heating step, and the solid content in the slurry was not sufficiently high, resulting in a relatively high concentration gradient of the binder on the surface side of the positive electrode due to the binder migration phenomenon under the same drying conditions.

[0103] On the other hand, Examples 3 and 4 showed a tendency for slightly lower adhesive strength compared to Examples 1-3 due to the relatively low solid content in the slurry. It was confirmed that the solid content in the positive electrode slurry composition was between 74 and 90 parts by weight per 100 parts by weight of the positive electrode slurry composition.

[0104] [Evaluation Example 3]: Evaluation of the adhesion strength between the active material layer and the current collector due to changes in the drying conditions of the positive electrode active material slurry. (Examples 5-8) The positive electrode was manufactured in the same manner as in Example 1, except that the drying process of the coated positive electrode slurry in step 2 was as described in Table 3 below.

[0105] (Comparative Example 4) The positive electrode was manufactured in the same manner as in Comparative Example 3, except that the drying process was as described in Table 3 below.

[0106] (Evaluation method) The interfacial adhesion between the active material layer and the current collector was evaluated in the same manner as in Evaluation Example 1, and the results are shown in Table 3 below.

[0107] [Table 3]

[0108] Referring to Table 3, when the drying temperature is low, below 140°C (Examples 1, 5, and 6), the drying time increases to evaporate all the solvent, but it can be confirmed that the migration phenomenon of the binder is suppressed, and the adhesive strength between the substrate and the active material layer increases. On the other hand, when the drying temperature is excessively high (Examples 7 and 8), the active material particles are more likely to be exposed to the surface of the solvent during the drying process due to rapid drying, which worsens the migration phenomenon due to capillary action, and it can be confirmed that the adhesive strength between the substrate and the active material layer decreases. Furthermore, in the case of a conventional cathode slurry with a solid content of 74% by weight (Comparative Example 4), it can be confirmed that it has the lowest adhesive strength despite performing the same drying process as Example 6, which has the best adhesive strength conditions.

[0109] Although embodiments of the present invention have been described above, the present invention is not limited to the embodiments described above and can be manufactured in a variety of different forms. Anyone with ordinary skill in the art to which the present invention belongs will understand that it can be implemented in other specific forms without changing the technical idea or essential features of the present invention. Therefore, the embodiments described above should be understood to be illustrative and not limiting in all respects.

Claims

1. (a) A slurry composition for a positive electrode comprising a positive electrode active material, a binder, and a solvent, with the boiling point (T) of the solvent. ｂ ) A step of heating at a lower temperature, (b) The step of applying the heated positive electrode slurry composition onto the current collector, (c) A step of cooling the coated cathode slurry composition, (d) The step of drying the cooled cathode slurry composition, A method for manufacturing a positive electrode for a secondary battery, wherein the cooling is performed at a temperature of 10 to 30°C, and the drying is performed at a temperature of 100°C or higher and less than 180°C.

2. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the solid content in the positive electrode slurry composition is contained in an amount of more than 74 and less than or equal to 90 parts by weight per 100 parts by weight of the positive electrode slurry composition.

3. The viscosity of the cathode slurry composition before heating (A １ The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the cp value is 13,000 to 80,000 cp.

4. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the temperature (T) of the heating step satisfies the following relational expression 1. [Relationship 1] 0.3T ｂ <T<0.9T ｂ In the above relational equation 1, T ｂ is the boiling point of the solvent.

5. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the cooling is performed for 1 to 60 seconds.

6. The method for manufacturing a positive electrode for a secondary battery according to claim 1, characterized in that the slurry composition for the positive electrode satisfies the following relational formulas 2 and 3. [Relationship Equation 2] 1.3≦A １ / A ２ ≦12 [Relationship Equation 3] 1.1≦A ３ / A ２ ≦10 In the above relational equations 2 and 3, A １ A is the viscosity of the cathode slurry composition before heating. ２ The viscosity of the heated cathode slurry composition, A ３ This is the viscosity of the cooled cathode slurry composition.

7. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the drying is carried out at a temperature of 110°C or higher and less than 145°C for 20 to 150 seconds.

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