Method for manufacturing positive electrode for secondary battery and secondary battery comprising the same

Abstract

The present invention provides a manufacturing method for a positive electrode of a secondary battery, which comprises: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature lower than the boiling point (T b ) of the solvent; (b) coating the heated positive electrode slurry composition onto a current collector; and (c) cooling the coated positive electrode slurry composition.

Description

[0001] Cross-reference to related applications

[0002] This application claims priority to Korean Patent Application No. 10-2020-0097885, filed with the Korean Intellectual Property Office on August 5, 2020, the entire disclosure of which is incorporated herein by reference. Technical Field

[0003] The present invention relates to a method for manufacturing a positive electrode for a secondary battery and a secondary battery comprising the positive electrode. Background Technology

[0004] Recently, with the increasing demand for electronic devices such as mobile devices, the development of technologies for weight reduction and miniaturization of electrochemical batteries (secondary batteries) to increase the portability of electronic devices has expanded. In addition to this trend, the growth of the electric vehicle (EV) market has accelerated due to increasingly stringent global regulations on fuel efficiency and emissions from automobiles, thus requiring the development of high-output and high-capacity batteries for such electric vehicles.

[0005] In such electrochemical cells (secondary cells), electrodes for secondary cells are typically manufactured by mixing electrode active materials, conductive materials, and binders in a solvent, dispersing the mixture using strong shear force, coating the resulting electrode slurry onto a current collector, and then drying the slurry. In this case, if the solvent used to dissolve the binder and disperse the electrode active materials and conductive materials remains after the drying process, this solvent can cause electrochemical side reactions in the secondary cell, adversely affecting its performance. Simultaneously, during drying, capillary forces generated by the exposed particles of the electrode active materials cause the solvent to flow from the interior to the surface of the electrode slurry layer. As a result, conductive materials or binders with relatively small particle sizes compared to the electrode active materials migrate, resulting in a large distribution of conductive materials or binders on the electrode surface. This uneven distribution of conductive materials and binders leads to increased resistance in the secondary cell or reduced adhesion between the substrate (current collector) and the electrode active material layer. Therefore, to address the aforementioned issues, a method has been investigated to suppress the migration of small particles (binders and conductive materials) in the electrode slurry while simultaneously increasing the viscosity of the electrode slurry by reducing its solvent content, thereby reducing the residual solvent content in the secondary battery. However, when reducing the solvent content to increase the viscosity of the electrode slurry, the slurry's fluidity decreases rapidly, leading to reduced processability, the fabrication of electrodes with uneven thickness, and a decrease in the adhesion between the current collector and the electrode active material layer. Therefore, research and development are needed to address the problem of reduced substrate adhesion due to the fluidity of high-viscosity solvents while simultaneously reducing solvent content. Summary of the Invention

[0006] One object of the present invention is to solve the problem of migration of conductive material and binder due to the flow of low viscosity solvent when drying a positive electrode slurry containing positive electrode active material, conductive material, binder and solvent, that is, the problem of reduced adhesion between the current collector and the positive electrode active material layer during the process of forming a positive electrode active material layer by coating the positive electrode slurry onto the current collector and drying the positive electrode slurry.

[0007] In one general aspect, a method for manufacturing a positive electrode for a secondary battery includes: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b (a) heating the positive electrode slurry composition to the current collector; (b) applying the heated positive electrode slurry composition to the current collector; and (c) cooling the applied positive electrode slurry composition.

[0008] Based on 100 parts by weight of the positive electrode slurry composition, the solid content in the positive electrode slurry composition is greater than 74 parts by weight and less than 90 parts by weight.

[0009] The viscosity (A1) of the positive electrode slurry composition before heating can be from 13,000 cP to 80,000 cP.

[0010] The heating temperature (T) of the positive electrode slurry composition can satisfy the following relationship 1:

[0011] [Relation 1]

[0012] 0.3T b <T<0.9T b

[0013] Where T b It is the boiling point of the solvent.

[0014] Cooling can be performed for 1 to 60 seconds at temperatures ranging from 10°C to 30°C.

[0015] The positive electrode slurry composition can satisfy the following relations 2 and 3:

[0016] [Relationship 2]

[0017] 1.3≤A1 / A2≤12

[0018] [Relationship 3]

[0019] 1.1≤A3 / A2≤10

[0020] Where A1 is the viscosity of the positive electrode slurry composition before heating, A2 is the viscosity of the positive electrode slurry composition after heating, and A3 is the viscosity of the positive electrode slurry composition after cooling.

[0021] The method for manufacturing the positive electrode for a secondary battery may further include (d) drying the cooled positive electrode slurry composition. The drying may be carried out at a temperature above 110°C and below 145°C for 20 to 150 seconds.

[0022] In another general aspect, the positive electrode for a secondary battery is manufactured by a method for manufacturing a positive electrode for a secondary battery, the method comprising the steps of: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b (a) heating the positive electrode slurry composition to the current collector; and (b) cooling the coated positive electrode slurry composition, wherein the positive electrode comprises: the current collector; and a layer of positive electrode active material formed on the current collector and containing positive electrode active material, conductive material and binder.

[0023] The adhesion force of the positive electrode active material layer to the current collector can be above 0.35 N / cm.

[0024] In the positive electrode, the difference between the maximum and minimum load values ​​at five or more locations with fixed intervals along the length of the positive electrode active material layer can be less than 10% of the total average load.

[0025] In another general aspect, a secondary battery includes: a positive electrode; a negative electrode; a separator located between the positive and negative electrodes; and an electrolyte, wherein the positive electrode is manufactured by a method for manufacturing a positive electrode for a secondary battery, the method comprising: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to below the boiling point (T0) of the solvent. b (a) heating the positive electrode slurry composition to the current collector; and (b) cooling the coated positive electrode slurry composition, wherein the positive electrode comprises: the current collector; and a layer of positive electrode active material formed on the current collector and containing positive electrode active material, conductive material and binder. Attached Figure Description

[0026] Figure 1 The results obtained by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis of the cross-sections of the positive electrodes according to Example 1 and Comparative Example 1 are shown in a graph illustrating the distribution of the adhesive in the positive electrode active material layer according to the thickness direction.

[0027] Figure 2 This is a schematic diagram of a sample obtained by selecting five points with fixed intervals in the width direction of the positive electrode active material layer and punching the selected five points to form a circular shape with a diameter of 38 mm in order to evaluate the uniformity of the positive electrode active material layer.

[0028] Figure 3 The graph illustrates the results obtained by normalizing the weight of the positive electrode active material layer (load level of the positive electrode active material layer composition) measured at each location of the positive electrode according to Example 1 and Comparative Example 2 in the width direction by dividing the total weight of the positive electrode active material layer. Detailed Implementation

[0029] Various advantages and features of the invention, as well as methods of implementing them, will become apparent from the following detailed description of embodiments with reference to the accompanying drawings. However, the invention is not limited to the embodiments described below, but can be implemented in various different forms. These embodiments are provided merely to complete the invention and to fully recognize the scope of the invention by those skilled in the art, and the invention will be defined by the scope of the claims. Hereinafter, detailed description of the methods for implementing the invention will be given with reference to the accompanying drawings. Throughout the drawings, the same reference numerals denote the same components, and "and / or" includes each and all of one or more combinations of the mentioned items.

[0030] Unless otherwise defined, all terms used in this specification (including technical and scientific terms) shall have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Throughout this specification, unless otherwise stated, the word "including" is to be construed as implying the inclusion of other elements, rather than excluding them. Furthermore, unless specifically stated herein, the singular form includes the plural form.

[0031] In this specification, when an element such as a layer, film, region, or plate is referred to as being "on" another element, it may be directly on the other element, or it may be located on the other element with other elements interposed between them.

[0032] In this specification, "viscosity" is measured using a Brookfield rotational viscometer at 5 s / second. -1 The shear rate is the value measured at the slurry temperature corresponding to each process, and its tolerance is ±5cP.

[0033] An embodiment of the present invention provides a method for manufacturing a positive electrode for a secondary battery, comprising: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b (a) heating the positive electrode slurry composition to the current collector; (b) applying the heated positive electrode slurry composition to the current collector; and (c) cooling the applied positive electrode slurry composition.

[0034] First, a positive electrode slurry composition containing a positive electrode active material, a binder, a conductive material, and a solvent is prepared.

[0035] Based on 100 parts by weight of the positive electrode slurry composition, the content of solids (positive electrode active material, binder and conductive material) in the positive electrode slurry composition can be greater than 74 parts by weight, specifically, greater than 74 parts by weight and less than 90 parts by weight, more specifically, greater than 74 parts by weight and less than 85 parts by weight.

[0036] Typically, in the manufacturing process of the positive electrode for secondary batteries, the slurry should have a viscosity of approximately 1000 cp to 10000 cp at 25°C to facilitate slurry coating and produce a positive electrode with uniform thickness. However, when preparing the positive electrode slurry composition by reducing the solvent content in the composition using conventional methods, resulting in a solids content of 80% by weight or more, the viscosity of the slurry becomes 53000 cP or more. This causes a rapid decrease in slurry fluidity, leading to lower processability, the production of a positive electrode with uneven thickness, and reduced adhesion between the current collector and the positive electrode active material layer. Therefore, conventional positive electrode slurry compositions contain approximately 74% by weight of solids.

[0037] On the other hand, in this invention, the flowability problem can be solved by heating the high-viscosity positive electrode slurry. However, when the solids exceed the above-mentioned weight range, the slurry composition does not contain the minimum amount of solvent required to disperse the solid particles when preparing the positive electrode slurry composition. This means that the shear force required to disperse the solid particles cannot be fully utilized, and the solid particles in the slurry composition may be coated onto the current collector in an unevenly dispersed state. When the solids are less than the above-mentioned weight range, the flowability of the slurry composition is sufficient, and therefore the technology proposed in this invention is not required.

[0038] At 25°C, the viscosity (A1) of the positive electrode slurry composition before heating can be from 13,000 cP to 80,000 cP, preferably from 15,000 cP to 75,000 cP, 20,000 cP to 75,000 cP, or 25,000 cP to 75,000 cP, more preferably from 35,000 cP to 70,000 cP, 45,000 cP to 70,000 cP, 50,000 cP to 70,000 cP, or 55,000 cP to 70,000 cP.

[0039] When the viscosity (A1) of the positive electrode slurry composition is less than 13000 cP at 25°C due to the increase in solvent content in the positive electrode slurry composition, the amount of solvent contained in the slurry composition is relatively large. This causes the adhesion between the positive electrode active material layer and the current collector to be weakened due to the migration of the adhesive, which may reduce the long-term stability.

[0040] When the viscosity (A1) of the cathode slurry composition exceeds 80,000 cP at 25°C, the increase in adhesion between the cathode active material layer and the current collector is not significant even with an increase in the content of solids (particularly the cathode active material and the binder).

[0041] Furthermore, as described below, the positive electrode slurry composition is coated onto the current collector in a state where the viscosity of the positive electrode slurry composition is reduced by heating, so that the binder in the coated film (the coated positive electrode slurry composition) does not migrate to the electrode surface. Therefore, even if the positive electrode slurry composition contains a small amount of binder, it can still have excellent adhesion to the substrate (current collector). Therefore, this positive electrode slurry composition can contain less binder than conventional positive electrode slurry compositions. Therefore, this positive electrode slurry composition can contain a larger amount of positive electrode active material than conventional positive electrode slurry compositions, to provide a positive electrode with improved electrode capacity.

[0042] Specifically, the weight ratio (content ratio) of the positive electrode active material to the binder in the solid can be 1:(0.01-0.1), specifically, 1:(0.01-0.05), more specifically, 1:(0.01-0.03).

[0043] The positive electrode active material can be used without restriction, as long as it is a positive electrode active material commonly used in secondary batteries. For example, the positive electrode active material can be a composite oxide of lithium and metals selected from cobalt, manganese, nickel and combinations thereof, but is not limited to this.

[0044] There are no particular limitations on the adhesive, as long as it is a conventional adhesive that can effectively bind the positive electrode active material particles together and also effectively bind the positive electrode active material to the current collector. For example, the adhesive can be polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, diacetyl cellulose, polyvinyl chloride, carboxylated polyvinyl chloride, polyvinyl fluoride, polymers including ethylene oxide, polyvinylpyrrolidone, polyurethane, polytetrafluoroethylene, polyvinylidene fluoride, polyethylene, polypropylene, styrene-butadiene rubber, acrylic styrene-butadiene rubber, epoxy resin, nylon, etc., but is not limited to these.

[0045] The solvent may be selected from at least one of the following: amine solvents such as N,N-dimethylaminopropylamine and diethylenetriamine; ether solvents such as ethylene oxide and tetrahydrofuran; ketone solvents such as methyl ethyl ketone; ester solvents such as methyl acetate; and polar aprotic solvents such as dimethylacetamide and N-methyl-2-pyrrolidone, but not limited thereto.

[0046] Conductive materials are used to impart conductivity to the positive electrode, and there are no particular restrictions, as long as they are conventional conductive materials that do not cause chemical changes in the secondary battery. For example, conductive materials can be natural graphite, artificial graphite, carbon black, acetylene black, Ketjen black, carbon fiber, carbon nanotubes, and combinations thereof, but are not limited to these.

[0047] The current collector can be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or stainless steel with a surface treated with carbon, nickel, titanium, silver, etc., but is not limited to these.

[0048] Then, the obtained high-viscosity positive electrode slurry composition is heated to below the boiling point (T0) of the solvent. b (temperature).

[0049] Therefore, the high-viscosity positive electrode slurry composition is heated at a temperature below its boiling point, which reduces the viscosity of the positive electrode slurry composition to a workable viscosity and allows the formation of a positive electrode with a uniform thickness. Thus, in the method for manufacturing a positive electrode for a secondary battery according to the present invention, a high-viscosity positive electrode slurry composition containing a large amount of solids is used, but the positive electrode slurry composition can be coated onto the current collector while its viscosity is reduced by heating.

[0050] However, when the heating temperature of the cathode slurry composition is higher than or equal to the boiling point of the solvent, strong vapor pressure is generated in the pipelines or storage containers used for transporting the slurry composition when the heated cathode slurry composition is transported and stored, which may lead to risks such as slurry leakage or explosion.

[0051] Therefore, heating can be carried out at a temperature below the boiling point of the solvent contained in the positive electrode slurry composition.

[0052] Specifically, the heating temperature (T) can satisfy the following relationship 1.

[0053] [Relation 1]

[0054] 0.3T b <T<0.9T b

[0055] In relation 1 above, T b It is the boiling point of the solvent.

[0056] In relation 1, 0.35T b <T<0.9T b 0.4T is preferred b <T<0.85T b 0.5T is preferred. b <T<0.8T b .

[0057] At the respective temperatures of each slurry composition, the viscosity (A2) of the heated slurry composition can be from 6000 cP to 10000 cP, preferably from 7000 cP to 9800 cP, and more preferably from 7500 cP to 9600 cP.

[0058] In step (b), the heated positive electrode slurry composition is coated onto the current collector. In this case, the coating can be performed without heating the slurry composition and the substrate (current collector) (without raising their temperature).

[0059] As a non-limiting example, any coating method known for forming films by conventionally coating a liquid phase can be used. For example, coating can be performed using spraying, dip coating, spin coating, gravure coating, slot die coating, doctor blade coating, roll coating, inkjet printing, flexographic printing, screen printing, electrodynamic inkjet printing, micro-contact printing, imprinting, reverse offset printing, bar coating, gravure offset printing, etc., but is not limited thereto.

[0060] In step (c), the coated positive electrode slurry composition is cooled.

[0061] After the positive electrode slurry composition is heated and coated onto the current collector, it is immediately cooled at a temperature of 10°C to 30°C, particularly 10°C to 25°C, and even more particularly 15°C to 25°C for 1 second to 60 seconds, particularly 1 second to 30 seconds, and even more particularly 3 seconds to 7 seconds.

[0062] The positive electrode slurry composition is cooled after heating, allowing its viscosity to return to a high level. Therefore, in the method for manufacturing a positive electrode for a secondary battery according to the present invention, a high-viscosity positive electrode slurry composition containing a large amount of solids is used. However, the positive electrode slurry composition is coated onto the current collector in a state where its viscosity is reduced by heating, and is immediately cooled after coating, allowing its viscosity to return to a high level. This suppresses the migration of solids (binders or conductive materials) that occurs during subsequent drying. Therefore, the present invention provides a positive electrode for a secondary battery with improved adhesion between the current collector and the positive electrode active material layer, and prevents problems such as reduced flowability and uneven electrode thickness that may be caused by high viscosity.

[0063] At the respective temperatures of each slurry composition, the viscosity (A3) of the cooled slurry composition can be from 8,000 cP to 60,000 cP, preferably from 10,000 cP to 55,000 cP, more preferably from 13,000 cP to 50,000 cP, and even more preferably from 35,000 cP to 45,000 cP.

[0064] That is, the positive electrode slurry composition can satisfy the following relational expression 2' and relational expression 3'.

[0065] [Relational expression 2']

[0066] A2<A1

[0067] [Relational expression 3']

[0068] A2<A3

[0069] In the above relational expression 2' and relational expression 3', A1 is the viscosity of the positive electrode slurry composition before heating, A2 is the viscosity of the positive electrode slurry composition after heating, and A3 is the viscosity of the coated film (positive electrode slurry composition) after cooling.

[0070] However, even if the heated positive electrode slurry composition is cooled within a limited time, the heated positive electrode slurry composition may not be cooled to the temperature of the positive electrode slurry composition before heating, and thus the viscosity of the positive electrode slurry composition recovered by cooling the heated positive electrode slurry composition may be lower than the viscosity of the positive electrode slurry composition before heating. Therefore, the above relational expressions 2' and 3' can be structured as A2<A3≤A1, but this is not limited thereto.

[0071] In one embodiment, after the positive electrode slurry composition is heated, the viscosity of the positive electrode slurry composition is reduced to 1 / 3 or less, and when the heated positive electrode slurry composition is cooled, the viscosity can be restored again to a viscosity similar to the original viscosity.

[0072] That is, the above relational expressions 2' and 3' can be restructured into the following relational expression 2 and relational expression 3.

[0073] [Relational expression 2]

[0074] 1.3≤A1 / A2≤12

[0075] In the above relational expression 2, 1.8≤A1 / A2≤11, specifically 3≤A1 / A2≤10, more specifically 6≤A1 / A2≤7.

[0076] [Relational expression 3]

[0077] 1.1≤A3 / A2≤10

[0078] In the above relational expression 3, 1.3≤A3 / A2≤9, specifically 3≤A3 / A2≤7, more specifically 4≤A3 / A2≤5.

[0079] Furthermore, the method for manufacturing the positive electrode for a secondary battery according to the present invention may further include (d) drying the cooled positive electrode slurry composition. In this case, the drying may be carried out at a temperature above 100°C and below 180°C, preferably 110°C to 145°C, more preferably 110°C to 135°C, and more specifically 110°C to 125°C, for 20 seconds to 150 seconds, preferably 50 seconds to 130 seconds, more preferably 70 seconds to 120 seconds, and most preferably 90 seconds to 110 seconds.

[0080] Another embodiment of the present invention provides a positive electrode for a secondary battery, the positive electrode being manufactured according to the above-described manufacturing method and 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.

[0081] Typically, when the positive electrode slurry composition is simply heated to reduce its viscosity, coated onto a current collector, and then dried at a high viscosity, the lighter conductive materials and binders, whose particle size is relatively smaller than that of the positive electrode active material, can easily migrate to the surface of the positive electrode slurry layer through the flow of solvent. In this case, the adhesion of the positive electrode active material layer to the substrate (current collector) may be significantly reduced.

[0082] However, the positive electrode for secondary batteries manufactured according to the present invention is produced by coating a positive electrode slurry composition onto a current collector while the viscosity of the positive electrode slurry composition is reduced by heating a high-viscosity positive electrode slurry composition. After coating the positive electrode slurry composition, the coated positive electrode slurry composition is immediately cooled to restore the viscosity of the positive electrode slurry composition to a high viscosity again, and then the cooled positive electrode slurry composition is dried. Therefore, the migration of solids (binders or conductive materials) can be suppressed.

[0083] Therefore, the binder and conductive material may not have a concentration gradient in the thickness direction of the positive electrode active material layer. 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 central part of the positive electrode active material layer, and a third region adjacent to the surface of the positive electrode active material layer, and is divided such that each region has the same thickness, the concentration deviation of the solids (binder or conductive material) contained in each region may be less than 0.4% by weight, specifically 0.01 to 0.4% by weight, more specifically 0.05 to 0.35% by weight.

[0084] Meanwhile, the first region, the second region, and the third region may have the same thickness, but the present invention is not limited thereto.

[0085] Furthermore, the present invention aims to solve problems such as reduced slurry fluidity and uneven cathode thickness caused by the use of high-viscosity slurries, and compared with related technologies, although a high-viscosity cathode slurry is used, the fluidity of the slurry can be improved to form a cathode with uniform thickness (uniform slurry loading) when coating the slurry.

[0086] In the positive electrode according to an embodiment of the present invention, the positive electrode active material layer can be uniformly formed in the length direction (width direction), specifically, the loading level of the positive electrode active material layer can be uniform.

[0087] Specifically, in the positive electrode, the difference between the maximum loading value and the minimum loading value of the positive electrode active material layer at five or more locations with fixed intervals along the length direction can be less than 10% of the total loading average value, for example, less than 9%, less than 8%, or less than 6%. Accordingly, the above effect can be further improved.

[0088] In this context, loading can refer to the weight of the solids in the positive electrode active material slurry or the weight of the positive electrode active material layer that is finally formed after the positive electrode slurry coated onto the positive electrode current collector has dried. As a non-limiting example, after obtaining a sample by punching at least five points with fixed intervals along the length (width) of the positive electrode to form a circular shape, the weight of the solids in the positive electrode active material slurry or the positive electrode active material layer in the sample can be measured.

[0089] Therefore, since the solid is uniformly dispersed in the positive electrode active material layer, the positive electrode for secondary batteries manufactured according to the present invention can have improved adhesion to the substrate (current collector).

[0090] In a preferred embodiment, the adhesion force of the positive electrode active material layer to the current collector can be above 0.35 N / cm, specifically, from 0.35 N / cm to 1.5 N / cm, and more specifically, from 0.45 N / cm to 0.8 N / cm.

[0091] 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.

[0092] Specifically, the positive electrode manufactured according to the present invention can have improved adhesion to the substrate and uniform thickness. Therefore, the secondary battery including the positive electrode can further improve long-term stability.

[0093] The negative electrode may include a current collector and a negative electrode active material layer disposed on the current collector. The material of the current collector may be copper or nickel, but is not limited thereto.

[0094] The negative active material is not particularly limited as long as it is a negative active material generally used for secondary batteries. For example, the negative active material may be a carbon-based negative active material, a silicon-based negative active material, or a mixture thereof, but is not limited thereto. The carbon-based negative active material may be one or more selected from the group consisting of artificial graphite, natural graphite and hard carbon. The silicon-based negative active material may be Si, SiO x (0 < x < 2), Si-Q alloy (herein, Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group XIII elements, Group XIV elements, Group XV elements, Group XVI elements, transition metals, rare earth elements and combinations thereof, and Q is not silicon), Si-carbon composites, or a mixture of at least one of the foregoing and SiO2.

[0095] The separator is not particularly limited as long as it is a separator known in the art. For example, the separator may be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene or combinations thereof, may be in the form of a non-woven fabric or a woven fabric, and may optionally be of a single-layer structure or a multilayer structure.

[0096] The electrolyte includes a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent may be 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 ester (DEE), methyl formate (MF), methyl propionate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN) or mixtures thereof, but is not limited thereto. The electrolytic salt is a substance dissolved in the non-aqueous organic solvent, which serves as a supply source of electrolyte metal ions in the secondary battery, enables the basic operation of the secondary battery, and promotes the migration of electrolyte metal ions between the positive electrode and the negative electrode. By way of non-limiting example, when the electrolytic metal is lithium, the electrolytic salt may be 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+1SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof, but not limited thereto. Furthermore, the electrolyte salt may be a known substance used at a concentration suitable for this purpose, and, if desired, may further include known solvents or additives to improve charge / discharge characteristics, flame retardant properties, etc.

[0097] Example

[0098] (Example 1)

[0099] Step 1: Preparation of positive electrode slurry

[0100] 79.5 wt% of nickel cobalt manganese oxide (NCM) (specifically, CSG131, Ni:Co:Mn = 8:1:1), 1.2 wt% of polyvinylidene fluoride, 0.8 wt% of carbon black, and 18.5 wt% of N-methylpyrrolidone (NMP) were mixed to prepare a cathode slurry (based on 81.5 wt% of the total slurry solids). The prepared cathode slurry was stirred in a stirring tank heated to 80°C for 30 minutes to maintain the temperature of the cathode slurry at 80°C.

[0101] Step 2: Manufacturing the positive electrode

[0102] The positive electrode slurry prepared in step 1 (heated to 80°C) was coated onto the aluminum current collector (aluminum foil with a thickness of 12 μm) using a slot die coater.

[0103] Then, the coated positive electrode slurry is placed at 25°C for 5 seconds, cooled to 50°C, and dried in a drying furnace heated with hot air at 140°C for 1 minute to obtain the positive electrode active material layer.

[0104] In this case, the thickness of the positive electrode active material layer is set to 50 μm.

[0105] Evaluation example

[0106] [Evaluation Example 1]: Measurement of cross-sectional images of the positive electrode active material layer obtained by SEM and evaluation of the adhesion between the active material layer and the current collector.

[0107] (Comparative Example 1)

[0108] The positive electrode was manufactured in the same manner as in Example 1, except that the heated positive electrode slurry was dried immediately after coating, instead of the cooling process of the coated positive electrode slurry in step 2 of Example 1.

[0109] (Comparative Example 2)

[0110] The same method as in Example 1 was used, except that the coated positive electrode slurry was dried and used as is, without performing the heating and cooling process of the positive electrode slurry in step 2 of Example 1.

[0111] (Evaluation Method)

[0112] * Measurement of cross-sectional images of the positive electrode active material layer captured by SEM-EDS

[0113] The cross-sections of the positive electrodes prepared according to Example 1 and Comparative Example 1 were cut using an ion-milling device. The fluorine distribution in the binder of the positive electrodes was then measured by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and the measurement results were compared and presented. Figure 1 middle.

[0114] * Evaluation of the homogeneity of the positive electrode active material layer

[0115] To analyze the uniformity of the positive electrode active material layers manufactured in Example 1 and Comparative Example 1, such as Figure 2 As shown, five points with fixed intervals were selected along the length of the positive electrode active material layer, and these points were punched into circular shapes with a diameter of 38 mm. The weight of the positive electrode active material layer in the punched specimen was then measured to analyze the loading level of the positive electrode active material layer. The weight of the positive electrode active material layer measured at each location (the loading level of the positive electrode active material layer composition) was divided by the total weight of the positive electrode active material layer for normalization. The normalization results are shown below. Figure 3 middle.

[0116] like Figure 3 As shown, when manufacturing the positive electrode active material layer, in the case where the positive electrode slurry is coated with a solution whose viscosity is reduced to approximately 9500 cp by heating the positive electrode slurry (Example 1), the difference in the loading value (maximum value - minimum value) of the positive electrode active material layer is 6% of the total average loading value measured at five points, which is very low. However, in the case where the positive electrode slurry is coated with a solution with a high viscosity of 64000 cp (Comparative Example 2), the difference in the loading value of the positive electrode active material layer is 15% of the total average loading value measured at five points, indicating an uneven distribution in which the loading level is high in the central part and low in the outer part.

[0117] *Viscosity measurement of positive electrode active material slurry

[0118] After preparing the positive electrode slurry composition at the corresponding temperature, the viscosity was measured using a rotational viscometer at the corresponding temperature of each slurry composition at a rate of 5 / s. -1The shear rate was used to measure the viscosity of each positive electrode slurry composition before and after heating and cooling.

[0119] Evaluation of the interfacial adhesive force between the active material layer and the current collector.

[0120] The positive electrodes prepared in Example 1 and Comparative Example 1 were cut into pieces 18 mm wide and 150 mm long. An 18 mm wide strip of tape was attached to the foil layer of the positive electrode, and then a roller with a load of 2 kg was used to ensure thorough adhesion. Double-sided tape was used to attach the positive electrode active material layer to one side of a tensile testing machine. The tape attached to the foil layer was then fixed to the other side of the tensile testing machine, and the adhesive force was measured. The measurement results are shown in Table 1.

[0121] [Table 1]

[0122]

[0123] Referring to Table 1, when observing the change in viscosity of the positive electrode slurry composition with temperature, it can be seen that the viscosity of the positive electrode slurry composition decreases immediately after the slurry composition is heated, and then recovers to a high viscosity after the slurry composition is cooled.

[0124] It can be seen that even though the positive electrode according to Example 1 was manufactured using a positive electrode slurry composition with the highest viscosity due to its high solid content (81.5% by weight), the positive electrode according to Example 1 exhibits high adhesion to the substrate. It can be seen that the positive electrode according to Comparative Example 1 used the same positive electrode slurry composition with a high solid content (81.5% by weight) as in Example 1, but unlike Example 1, it was manufactured by immediately drying the positive electrode slurry composition after coating, which was heated to a reduced viscosity without a cooling process. This resulted in lower adhesion to the substrate than in Example 1. This is because when the positive electrode slurry composition is dried at a low viscosity, the adhesive and conductive material migrate to the electrode surface layer due to solvent flow, increasing the adhesive concentration in the region adjacent to the surface of the positive electrode active material layer compared to the region adjacent to the substrate.

[0125] Meanwhile, the slurry of Comparative Example 2 contained the same amount of solids as in Example 1, resulting in high viscosity. However, the process of reducing the viscosity of the slurry by heating was not performed, making the viscosity of the slurry too high to manufacture a positive electrode. Therefore, the adhesion to the substrate in Comparative Example 2 may not be evaluated.

[0126] In addition, refer to Figure 1It can be confirmed that there is essentially no concentration gradient of binder and conductive material in the thickness direction of the positive electrode active material layer in Example 1. Analysis suggests this is due to the suppression of the migration of binder and conductive material during the slurry drying process. On the other hand, in Comparative Example 1, the slurry was dried immediately at a low viscosity, resulting in an increase in the concentration of binder and conductive material in the region adjacent to the surface of the positive electrode active material layer compared to the region adjacent to the substrate. [Evaluation Example 2]: The adhesion between the active material layer and the current collector was evaluated based on the change in the solid content of the positive electrode active material slurry (Examples 2 to 4 and Comparative Example 3).

[0127] The positive electrode was manufactured in the same manner as in Example 1, except that, in step 1 of Example 1, the viscosity of the positive electrode slurry was changed by altering the solid content in the positive electrode slurry and the heating temperature of the positive electrode slurry during preparation, as shown in Table 2. In this case, the heating temperatures of Examples 2 to 4 were adjusted so that the viscosity of each slurry was between 7800 cP and 8700 cP, and in Comparative Example 3, a slurry at room temperature (25°C) without heating was used.

[0128] (Evaluation Method)

[0129] The evaluation of the interfacial adhesion and viscosity between the active material layer and the current collector was carried out in the same manner as in Evaluation Example 1, and the evaluation results are shown in Table 2.

[0130] [Table 2]

[0131]

[0132] Referring to Table 2, it can be seen that when Examples 1 and 2 have a preferred solid content range, the fluidity of the solvent is reduced due to the high viscosity of the coated slurry, which suppresses the migration of the adhesive during the drying process. As a result, the adhesion between the substrate and the active material layer increases.

[0133] On the other hand, it can be confirmed that in Comparative Example 3, the adhesion was very low, and the analysis showed that the positive electrode formed using a slurry without a heating step at room temperature had an insufficient solid content, resulting in a relatively large amount of solvent in the slurry. Due to the migration of the adhesive under the same drying conditions, a higher concentration gradient of adhesive content was formed on the surface of the positive electrode.

[0134] Meanwhile, it can be confirmed that, due to the relatively small solid content in the slurry, Examples 3 and 4 show a trend of slightly lower adhesion than Examples 1 and 2, and based on 100 parts by weight of the positive electrode slurry composition, the solid content in the positive electrode slurry composition is greater than 74 parts by weight and less than 90 parts by weight.

[0135] [Evaluation Example 3]: Evaluate the adhesion between the active material layer and the current collector based on changes in the drying conditions of the positive electrode active material slurry.

[0136] (Examples 5 to 8)

[0137] 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 of Example 1 was changed as described in Table 3.

[0138] (Comparative Example 4)

[0139] Except for the change in the drying process of Comparative Example 3 as shown in Table 3, the positive electrode was manufactured in the same manner as Comparative Example 3.

[0140] (Evaluation Method)

[0141] 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 evaluation results are shown in Table 3.

[0142] [Table 3]

[0143]

[0144] Referring to Table 3, it can be confirmed that at low drying temperatures below 140°C (Examples 1, 5, and 6), increasing the drying time to evaporate the solvent suppresses adhesive migration, thus increasing the adhesion between the substrate and the active material layer. On the other hand, it can be confirmed that at excessively high drying temperatures (Examples 7 and 8), due to rapid drying, the active material particles are easily exposed to the solvent surface during the drying process. As a result, migration due to capillary action intensifies, reducing the adhesion between the substrate and the active material layer. Furthermore, it can be confirmed that in the case of a conventional positive electrode slurry with a solid content of 74% by weight (Comparative Example 4), despite undergoing the same drying process as in Example 6 (which is the condition with the best adhesion strength), the adhesion between the substrate and the active material layer is the lowest.

[0145] In the method for manufacturing the positive electrode for a secondary battery according to the present invention, in order to solve the problem of conventional manufacturing methods using a low-viscosity positive electrode slurry composition, namely, the problem that the adhesion between the current collector and the positive electrode active material layer is reduced due to the migration of conductive material and binder to the surface of the positive electrode active material layer caused by the flow of low-viscosity solvent during drying of the slurry composition, a high-viscosity positive electrode slurry composition is used. However, the positive electrode is manufactured by the following method: while the viscosity of the high-viscosity positive electrode slurry composition is reduced by heating, the high-viscosity positive electrode slurry composition is coated onto the current collector, the coated positive electrode slurry composition is cooled at room temperature, and then the cooled positive electrode slurry composition is dried. Therefore, while providing a positive electrode with improved adhesion between the current collector and the positive electrode active material layer for the secondary battery, problems such as reduced flowability and uneven thickness of the positive electrode that may be caused by high viscosity can be prevented.

[0146] The present invention has been described above, but it is not limited to the embodiments described above. It can be implemented in various different forms, and those skilled in the art will understand that it can be implemented in other specific forms without altering its technical spirit or essential characteristics. Therefore, it should be understood that the above embodiments are illustrative in all respects and not restrictive.

Claims

1. A method for manufacturing a positive electrode for a secondary battery, comprising: (a) Heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature below the boiling point T of the solvent. b Temperature; (b) Apply the heated positive electrode slurry composition onto the current collector; (c) Cooling the coated positive electrode slurry composition; and (d) The positive electrode slurry composition after drying and cooling. The cathode slurry composition is based on 100 parts by weight, wherein the solid content of the cathode slurry composition is greater than 74 parts by weight and less than 90 parts by weight. The cooling is performed at a temperature between 10°C and 30°C. The drying process is carried out at a temperature above 110°C and below 145°C. The positive electrode slurry composition thereon satisfies the following relations 2 and 3: [Relation 2] 1.3≤A1 / A2≤12 [Relationship 3] 1.1≤A3 / A2≤10 Wherein A1 is the viscosity of the positive electrode slurry composition before heating, A2 is the viscosity of the positive electrode slurry composition after heating, and A3 is the viscosity of the positive electrode slurry composition after cooling.

2. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the viscosity A1 of the positive electrode slurry composition before heating is 13000 cP to 80000 cP.

3. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the heating temperature T of the positive electrode slurry composition satisfies the following relationship 1: [Relation 1] 0.3T b <T<0.9T b Where T b The boiling point of the solvent is denoted as .

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

5. The method for manufacturing a positive electrode for a secondary battery according to claim 1, wherein the drying is performed for 20 to 150 seconds.

6. A positive electrode for a secondary battery, manufactured by a method for manufacturing a positive electrode for a secondary battery, the method comprising: (a) Heating a positive electrode slurry composition containing a positive electrode active material, a binder, and a solvent to a temperature below the boiling point T of the solvent. b (a) heating the positive electrode slurry composition to the current collector; (b) cooling the coated positive electrode slurry composition; and (d) drying the cooled positive electrode slurry composition. The cathode slurry composition is based on 100 parts by weight, wherein the solid content of the cathode slurry composition is greater than 74 parts by weight and less than 90 parts by weight. The cooling is performed at a temperature between 10°C and 30°C. The drying process is carried out at a temperature above 110°C and below 145°C. The positive electrode slurry composition thereon satisfies the following relations 2 and 3: [Relation 2] 1.3≤A1 / A2≤12 [Relationship 3] 1.1≤A3 / A2≤10 Wherein A1 is the viscosity of the positive electrode slurry composition before heating, A2 is the viscosity of the positive electrode slurry composition after heating, and A3 is the viscosity of the positive electrode slurry composition after cooling.

7. The positive electrode for a secondary battery according to claim 6, wherein the adhesion force of the positive electrode active material layer to the current collector is 0.35 N / cm or more.

8. The positive electrode for a secondary battery according to claim 6, wherein in the positive electrode, the difference between the maximum load value and the minimum load value at five or more locations with fixed intervals in the length direction of the positive electrode active material layer is less than 10% of the total average load.

9. A secondary battery, comprising: positive electrode; negative electrode; The membrane located between the positive electrode and the negative electrode; and Electrolyte The positive electrode is manufactured by a method for manufacturing a positive electrode for a secondary battery, the method comprising: (a) heating a positive electrode slurry composition containing a positive electrode active material, a binder and a solvent to a temperature below the boiling point Tb of the solvent; (b) applying the heated positive electrode slurry composition onto the current collector; (c) cooling the applied positive electrode slurry composition; and (d) drying the cooled positive electrode slurry composition. The cathode slurry composition is based on 100 parts by weight, wherein the solid content of the cathode slurry composition is greater than 74 parts by weight and less than 90 parts by weight. The cooling is performed at a temperature between 10°C and 30°C. The drying process is carried out at a temperature above 110°C and below 145°C. The positive electrode slurry composition thereon satisfies the following relations 2 and 3: [Relationship 2] 1.3≤A1 / A2≤12 [Relationship 3] 1.1≤A3 / A2≤10 Wherein A1 is the viscosity of the positive electrode slurry composition before heating, A2 is the viscosity of the positive electrode slurry composition after heating, and A3 is the viscosity of the positive electrode slurry composition after cooling.

Images