Method for manufacturing a negative electrode for a secondary battery and a secondary battery including a negative electrode.

Abstract

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

Description

Technical Field

[0001] The present invention relates to a method for manufacturing a negative electrode for a secondary battery and a secondary battery including the negative electrode. Background Technology

[0002] Recently, with the increasing demand for electronic devices such as mobile devices, the development of lightweight and miniaturized technologies for electrochemical batteries (secondary batteries) to increase the portability of electronic devices has expanded. In addition, the growth of the electric vehicle (EV) market has accelerated due to increasingly stringent global regulations on automotive fuel efficiency and emissions, thus necessitating the development of high-output and high-capacity batteries for such EVs.

[0003] In such electrochemical cells (secondary cells), the electrodes are typically prepared by coating an electrode slurry—made by mixing electrode active materials, conductive materials, and binders in a solvent and dispersing the mixture using strong shear force—onto a current collector, followed by drying. In this case, if the solvent used to dissolve the binder and disperse the electrode active materials and conductive materials remains after drying, it can cause electrochemical side reactions in the secondary cell, adversely affecting its performance. Simultaneously, capillary forces generated during drying, due to the exposure of electrode active material particles to the solvent surface, 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 smaller particle sizes than 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, methods have been investigated to suppress the migration of small particles (binders and conductive materials) in the electrode slurry by increasing its viscosity by reducing the solvent content, thereby simultaneously 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 the solvent content. Summary of the Invention

[0004] One object of the present invention is to solve the problem caused by the migration of conductive materials and adhesives due to the flow of low-viscosity solvent when drying a negative electrode slurry containing negative electrode active material, conductive material, binder and solvent, that is, the problem of reduced adhesion between the current collector and the negative electrode active material layer during the process of forming a negative electrode active material layer by coating the negative electrode slurry onto the current collector and drying the negative electrode slurry.

[0005] In one general aspect, a method for manufacturing a negative electrode for a secondary battery includes: (a) heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b (a) heating the slurry composition for the negative electrode to the temperature of the current collector; (b) applying the heated slurry composition for the negative electrode to the current collector; and (c) cooling the coated slurry composition for the negative electrode.

[0006] Based on 100 parts by weight of the slurry composition for the negative electrode, the solid content in the slurry composition for the negative electrode can be greater than 55 parts by weight and less than 85 parts by weight.

[0007] The viscosity (A1) of the slurry composition used for the negative electrode before heating can be 12,000-70,000 cp.

[0008] The heating temperature (T) of the slurry composition used for the negative electrode can satisfy the following relationship 1:

[0009] [Relation 1]

[0010] 0.3T b <T<0.9T b

[0011] Where T b is the boiling point of the solvent.

[0012] Cooling can be performed at a temperature of 10-30°C for 1-60 seconds.

[0013] The slurry composition used for the negative electrode can satisfy the following relations 2 and 3:

[0014] [Relation 2]

[0015] 1.3≤A1 / A2≤12

[0016] [Relationship 3]

[0017] 1.2≤A3 / A2≤10

[0018] Wherein, A1 is the viscosity of the slurry composition for the negative electrode before heating, A2 is the viscosity of the slurry composition for the negative electrode after heating, and A3 is the viscosity of the slurry composition for the negative electrode after cooling.

[0019] The method for manufacturing the negative electrode for a secondary battery may further include (d) drying and cooling the slurry composition for the negative electrode. Drying may be carried out at a temperature above 90°C and below 135°C for 20-150 seconds.

[0020] In another general aspect, a negative electrode for a secondary battery is manufactured by a method for manufacturing a negative electrode for a secondary battery, the method comprising: (a) heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b (a) heating the slurry composition for the negative electrode onto the current collector; and (b) cooling the coated slurry composition for the negative electrode, the negative electrode comprising: the current collector; and a layer of negative electrode active material formed on the current collector and comprising negative electrode active material, conductive material and binder.

[0021] The adhesion force of the negative electrode active material layer to the current collector can be above 0.20 N / cm.

[0022] In the negative electrode, the difference between the maximum and minimum load values ​​at five or more locations with fixed intervals along the length of the negative electrode active material layer can be less than 10% of the total average load.

[0023] In yet another general aspect, a secondary battery includes: a negative electrode; a positive electrode; a separator located between the negative electrode and the positive electrode; and an electrolyte, wherein the negative electrode is manufactured by a method for manufacturing a negative electrode for a secondary battery, the method comprising: (a) heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to below the boiling point (T0) of the solvent. b (a) heating the slurry composition for the negative electrode to the temperature; (b) applying the heated slurry composition for the negative electrode to the current collector; and (c) cooling the applied slurry composition for the negative electrode; and the negative electrode comprises: the current collector; and a layer of negative electrode active material formed on the current collector and comprising negative electrode active material, conductive material and binder.

[0024] Other features and aspects will become apparent from the following detailed description, drawings and claims. Attached Figure Description

[0025] Figure 1 The graph illustrates the distribution of the adhesive in the negative electrode active material layer according to the thickness direction, based on the results obtained by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) analysis of the cross-sections of the negative electrodes of Example 1 and Comparative Example 1.

[0026] Figure 2This is a schematic diagram of a sample obtained by selecting five points with fixed intervals along the length of the negative electrode active material layer and punching holes at the five selected points to form a circle with a diameter of 38 mm in order to evaluate the uniformity of the negative electrode active material layer.

[0027] Figure 3 This is a graph showing the results obtained by normalizing the weight by dividing the weight of the negative electrode active material layer (the loading level of the negative electrode active material layer composition) measured at each location of the negative electrode according to Example 1 and Comparative Example 2 along the length direction by the total weight of the negative electrode active material layer. Detailed Implementation

[0028] 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 only to complete the invention and to enable those skilled in the art to fully appreciate its scope, which is defined by the scope of the claims. The detailed description of the embodiments for implementing the invention will now be given in conjunction with the accompanying drawings. Throughout the drawings, the same reference numerals denote the same parts, and "and / or" includes each of the mentioned items and combinations of one or more of them.

[0029] 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. Throughout this specification, unless otherwise stated, the word "comprising" of any element shall 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.

[0030] 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 may have other elements inserted between them on top of the other element.

[0031] In this specification, "viscosity" is measured using a Brookfield rotational viscometer at 5 s / second. -1 The values ​​were measured at the slurry temperature corresponding to each process at the shear rate of ), and the tolerance was ±5cP.

[0032] One embodiment of the present invention provides a method for preparing a negative electrode for a secondary battery, comprising: (a) heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point (T0) of the solvent. b(a) heating the slurry composition for the negative electrode to the temperature of the current collector; (b) applying the heated slurry composition for the negative electrode to the current collector; and (c) cooling the coated slurry composition for the negative electrode.

[0033] First, a slurry composition for a negative electrode containing a negative electrode active material, a binder, a conductive material, and a solvent is prepared.

[0034] Based on 100 parts by weight of the slurry composition for the negative electrode, the content of solids (negative electrode active material, binder and conductive material) in the slurry composition for the negative electrode can be greater than 55 parts by weight, specifically greater than 55 parts by weight and less than 85 parts by weight, more specifically greater than 55 parts by weight and less than 65 parts by weight.

[0035] Typically, in the manufacturing process of negative electrodes for secondary batteries, the slurry needs to have a viscosity of approximately 1000 to 10000 cp at 25°C to facilitate slurry applying work and produce a negative electrode with uniform thickness. However, when preparing a slurry composition for the negative electrode by reducing the solvent content in the slurry composition using conventional methods to achieve a solids content of 60% by weight or more, the slurry viscosity reaches 50000 cP or more. This leads to a rapid decrease in slurry fluidity, resulting in reduced processability, the production of a negative electrode with uneven thickness, and reduced adhesion between the current collector and the negative electrode active material layer. Therefore, conventional slurry compositions for negative electrodes contain approximately 50% by weight of solids.

[0036] On the other hand, in this invention, the flowability problem can be solved by heating the high-viscosity slurry used for the negative electrode. However, when the solids exceed the above-mentioned weight range, the slurry composition for the negative electrode does not contain the minimum amount of solvent required to disperse the solid particles, causing the shear force of the dispersed solid particles to not be fully utilized. Therefore, the solid particles in the slurry composition may be coated onto the current collector in a non-uniformly dispersed state. And when the solids are less than the above-mentioned weight range, the flowability of the slurry composition is sufficient, and the technology proposed in this invention is not required.

[0037] The viscosity (A1) of the slurry composition for the negative electrode before heating can be 12,000-70,000 cP at 25°C, preferably 20,000-70,000 cP, and more preferably 30,000-60,000 cP.

[0038] When the viscosity (A1) of the slurry composition for a negative electrode is less than 12000 cP at 25°C due to an increase in the solvent content in the slurry composition for a negative electrode, the amount of the solvent contained in the slurry composition is large, so that the adhesive force between the negative electrode active material layer and the current collector is weakened due to the binder migration phenomenon, which may reduce long-term stability.

[0039] When the viscosity (A1) of the slurry composition for a negative electrode exceeds 70000 cP at 25°C, even if the content of solids (especially the negative electrode active material and the binder) increases, the increase in the adhesive force between the negative electrode active material layer and the current collector is insignificant.

[0040] Furthermore, as described below, the slurry composition for a negative electrode is coated on the current collector in a state where the viscosity of the slurry composition for a negative electrode is reduced by heating the slurry composition for a negative electrode, such that the binder in the coating film (the coated slurry composition for a negative electrode) does not migrate to the electrode surface. Therefore, even if the slurry composition for a negative electrode contains a small amount of binder, the slurry composition for a negative electrode can still have excellent adhesive force to the substrate (current collector). As a result, the slurry composition for a negative electrode can contain a smaller amount of binder than conventional slurry compositions for a negative electrode. Accordingly, the slurry composition for a negative electrode can contain a larger amount of negative electrode active material than conventional slurry compositions for a negative electrode, so as to provide a negative electrode with improved electrode capacity.

[0041] Specifically, the weight ratio (content ratio) of the negative electrode active material to the binder in the solids may be 1:(0.01-0.1), specifically 1:(0.01-0.08), more specifically 1:(0.02-0.05).

[0042] The negative electrode active material can be used without limitation, as long as it is a negative electrode active material conventionally used for secondary batteries. For example, the negative electrode active material may 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 may be one or more selected from artificial graphite, natural graphite and hard carbon. The silicon-based negative electrode active material may be Si, SiO x (0<x<2), Si-Q alloy (wherein Q is selected from 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 Si), Si-carbon composite, or a mixture of at least one of the foregoing and SiO2.

[0043] There are no particular limitations on the adhesive, as long as it is a conventional adhesive capable of effectively bonding the negative electrode active material to the current collector and simultaneously bonding the negative electrode active material particles together. For example, the adhesive can be a water-soluble adhesive, specifically a copolymer of styrene-butadiene rubber, styrene-butadiene acrylic rubber, polyvinyl alcohol, sodium polyacrylate, propylene and olefins having 2-8 carbon atoms, a copolymer of (meth)acrylic acid and (meth)acrylic acid alkyl esters, or a combination thereof.

[0044] When using water-soluble binders, the binder can bind the negative electrode active material to the current collector without affecting the slurry viscosity. However, because the negative electrode active material and the conductive material are fine particles, the slurry may easily gelle. Therefore, a thickener can be further included to stabilize the slurry by imparting viscosity. For example, the thickener can be a cellulose-based compound, specifically carboxymethyl cellulose, hydroxypropyl methyl cellulose, methyl cellulose, or one or more mixtures of their alkali metal salts. Na, K, or Li can be used as the alkali metal.

[0045] The solvent may be at least one selected from water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol and tert-butanol, but is not limited thereto.

[0046] Conductive materials are used to impart conductivity to the negative electrode, and there are no particular limitations, as long as they are conventional conductive materials that will 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 may be selected from copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, polymer substrate coated with conductive metal, and combinations thereof, but is not limited thereto.

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

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

[0050] However, when the heating temperature of the slurry composition used for the negative electrode is higher than or equal to the boiling point of the solvent, strong vapor pressure is generated in the pipelines or storage containers that transport the slurry composition when it is heated and transported and stored, which may cause dangers such as water leakage or explosion.

[0051] Therefore, heating can be carried out at a temperature lower than the boiling point of the solvent contained in the slurry composition used for the negative electrode.

[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, 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 corresponding temperature for each slurry composition, the viscosity (A2) of the heated slurry composition can be 5000-10000 cP, preferably 7000-9800 cP, and more preferably 8000-9500 cP.

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

[0059] As a non-limiting example, coating can be performed using any coating method known for forming films by typically coating a liquid phase. For example, coating can be performed using spraying, dip coating, spin coating, gravure coating, slot diecoating, doctor blade coating, roll coating, inkjet printing, flexography printing, screen printing, electrohydrodynamic 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 slurry composition for the negative electrode is cooled.

[0061] The slurry composition for the negative electrode is heated and coated onto the current collector and then immediately cooled at a temperature of 10-30°C, specifically 10-25°C, and more specifically 15-25°C for 1-60 seconds, specifically 2-30 seconds, and more specifically 5-10 seconds.

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

[0063] At the corresponding temperature of each slurry composition, the viscosity (A3) of the cooled slurry composition can be 10,000-60,000 cP, preferably 13,000-55,000 cP, more preferably 25,000-50,000 cP, and even more preferably 35,000-45,000 cP.

[0064] That is, the slurry composition for a negative electrode 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] wherein A1 is the viscosity of the slurry composition for a negative electrode before heating, A2 is the viscosity of the slurry composition for a negative electrode after heating, and A3 is the viscosity of the coated film (the slurry composition for a negative electrode) after cooling.

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

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

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

[0073] [Relational expression 2]

[0074] 1.3≤A1 / A2≤12

[0075] wherein 2≤A1 / A2≤11, specifically 3≤A1 / A2≤10, more specifically 5≤A1 / A2≤6.5.

[0076] [Relational expression 3]

[0077] 1.2≤A3 / A2≤10

[0078] wherein 1.5≤A3 / A2≤9, specifically 2.5≤A3 / A2≤7, more specifically 4≤A3 / A2≤5.

[0079] Furthermore, the method for manufacturing the negative electrode for a secondary battery according to the present invention may further include (d) drying the cooled slurry composition for the negative electrode. In this case, the drying may be carried out at a temperature above 90°C and below 135°C, preferably 95-130°C, more preferably 95-115°C, and more specifically 95-105°C for 20-150 seconds, preferably 50-130 seconds, more preferably 70-120 seconds, and most preferably 90-110 seconds.

[0080] Another embodiment of the present invention provides a negative electrode for a secondary battery manufactured according to the above manufacturing method, comprising: a current collector; and a negative electrode active material layer formed on the current collector and comprising a negative electrode active material, a conductive material and a binder.

[0081] Typically, by simply heating the slurry composition for the negative electrode to reduce its viscosity, coating it onto a current collector, and then drying it at a low viscosity, the lighter conductive material and binder, due to their relatively smaller particle size than the negative electrode active material, can easily migrate to the surface of the slurry layer for the negative electrode through the flow of the solvent. In this case, the adhesion of the negative electrode active material layer to the substrate (current collector) may be significantly reduced.

[0082] However, the negative electrode for a secondary battery manufactured according to the present invention is prepared by the following method: a slurry composition for the negative electrode is coated onto a current collector while the viscosity of the slurry composition for the negative electrode is reduced by heating a high-viscosity slurry composition for the negative electrode; the coated slurry composition for the negative electrode is cooled immediately after coating to restore the viscosity of the slurry composition for the negative electrode to a high viscosity; and then the cooled slurry composition for the negative electrode is dried, thereby suppressing the migration of solids (binders or conductive materials).

[0083] Therefore, the binder and conductive material may not have a concentration gradient in the thickness direction of the negative electrode active material layer. Specifically, when the negative 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 negative electrode active material layer, and a third region adjacent to the surface of the negative electrode active material layer, and is divided such that each region has the same thickness, the concentration deviation of the solid content (binder or conductive material) contained in each region may be less than 0.3% by weight, specifically 0.01-0.3% by weight, more specifically 0.05-0.25% 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] In addition, the present invention aims to solve the problems that the use of high-viscosity slurry can lead to reduced slurry fluidity and uneven thickness of the negative electrode. Compared with the prior art, although a high-viscosity slurry for the negative electrode is used, the fluidity of the slurry can be improved so that the negative electrode can be formed with a uniform thickness (uniformly loaded slurry) when the slurry is coated.

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

[0087] Specifically, in the negative electrode, the difference between the maximum and minimum load values ​​of the negative electrode active material layer at five or more locations with fixed intervals along the length direction can be less than 10% of the total average load, for example, less than 9%, less than 8%, or less than 7%. Therefore, the above-mentioned effect can be further improved.

[0088] In this case, the loading can refer to the weight of the solids in the negative electrode active material slurry or the weight of the negative electrode active material layer of the negative electrode finally formed after the negative electrode slurry is coated on the negative electrode current collector and dried. As a non-limiting example, after obtaining a sample by forming a circle with at least five fixed-spaced punching holes in the length direction (width direction) of the negative electrode, the weight of the solids in the negative electrode active material slurry or the negative electrode active material layer in the sample can be measured.

[0089] Therefore, since the solid is uniformly dispersed in the negative electrode active material layer, the negative 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 negative electrode active material layer to the current collector can be above 0.20 N / cm, specifically 0.20-1.5 N / cm, and more specifically 0.22-0.5 N / cm.

[0091] In addition, the present invention provides a secondary battery, comprising: a negative electrode; a positive electrode; a separator located between the negative electrode and the positive electrode; and an electrolyte.

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

[0093] The positive electrode may include a current collector and a negative electrode active material layer located on the current collector. The material of the current collector may be aluminum or copper, but is not limited to these.

[0094] The positive electrode active material can be used without restriction, as long as it is a commonly used positive electrode active material. For example, the positive electrode active material can be a composite oxide of a metal selected from cobalt, manganese, nickel, and combinations thereof with lithium, but is not limited to this.

[0095] There are no particular limitations on the diaphragm, as long as it is a diaphragm known in the art. For example, the diaphragm may be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene or combinations thereof, may be in the form of nonwoven or woven fabric, and may optionally be used in a single-layer or multi-layer structure.

[0096] The electrolyte comprises a non-aqueous organic solvent and an electrolytic salt. The non-aqueous organic solvent can be ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), methyl ethyl 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, serving as a supply source of electrolytic metal ions in the secondary battery to enable its basic operation and promote 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 salt may be LiPF6, LiBF4, LiTFSI, LiSbF6, LiAsF6, LiClO4, LiCF3SO3, Li(CF3SO2)2N, LiC4F9SO3, LiSbF6, LiAlO4, LiAlCl4, LiN(C x F 2x+1 SO2)(CyF 2y+1 SO2 (where x and y are natural numbers), LiCl, LiI, or mixtures thereof, but not limited thereto. Furthermore, the electrolytic salt can be a known substance used at a concentration suitable for the purpose, and, if desired, may further include known solvents or additives to improve charge / discharge characteristics, flame retardant properties, etc.

[0097] Example (Example 1)

[0098] Step 1: Preparation of negative electrode slurry

[0099] Water was added to 94 wt% artificial graphite, 3.0 wt% carbon black conductive material, 1.8 wt% SBR binder and 1.2 wt% CMC to prepare a negative electrode slurry (60 wt% solids based on total slurry). The prepared negative electrode slurry was stirred in a stirred tank heated to 70°C for 30 minutes to maintain the temperature of the negative electrode slurry at 70°C.

[0100] Step 2: Production of negative electrode

[0101] The negative electrode slurry prepared in step 1 (in its heated state to 70°C) was coated onto the copper current collector (copper foil with a thickness of 8 μm) using a slot die coater.

[0102] Then, the coated negative electrode slurry is placed at 25°C for 5 seconds, and then dried in a drying oven heated with hot air at 120°C for 1 minute to obtain the negative electrode active material layer.

[0103] In this case, the thickness of the negative electrode active material layer is set to 50 μm.

[0104] Evaluation Example [Evaluation Example 1]: Measurement of cross-sectional images of the negative electrode active material layer captured by SEM and evaluation of the adhesion between the active material layer and the current collector (Comparative Example 1)

[0105] The negative electrode was manufactured in the same manner as in Example 1, except that the heated negative electrode slurry was dried immediately after coating, instead of the cooling process of the coated negative electrode slurry in step 2 of Example 1.

[0106] (Comparative Example 2)

[0107] The same method as in Example 1 was used, except that the coated negative electrode slurry was dried and used as is, without performing the heating and cooling process of the negative electrode slurry in step 2 of Example 1.

[0108] (Evaluation Method)

[0109] Measurement of cross-sectional images of negative electrode active material layer captured by SEM-EDS

[0110] The cross-sections of the negative electrodes prepared according to Example 1 and Comparative Example 1 were cut using an ion milling apparatus. The distribution of the SBR binder in the negative electrodes was then measured by scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS), and the measurement results were recorded in [the relevant data]. Figure 1 The following comparisons and explanations are provided. To detect only the SBR binder, the SBR binder in the negative electrode was pretreated with osmium (Os) before SEM-EDS analysis.

[0111] * Evaluation of uniformity of negative electrode active material layer

[0112] To analyze the uniformity of the negative electrode active material layers manufactured in Example 1 and Comparative Example 1, such as Figure 2 As shown, five points with fixed intervals along the length of the negative electrode active material layer were punched to form a circle with a diameter of 38 mm. The weight of the negative electrode active material layer in the punched sample was then measured to analyze the loading level of the negative electrode active material layer. The weight of the negative electrode active material layer measured at each location (the loading level of the negative electrode active material layer composition) was divided by the total weight of the negative electrode active material layer for normalization. The normalization results are shown below. Figure 3 middle.

[0113] like Figure 3 As shown, when the negative electrode active material layer is fabricated, in the case where the viscosity of the negative electrode slurry is reduced to about 9000 cp by heating the negative electrode slurry (Example 1), the difference between the loading values ​​of the negative electrode active material layer measured at five points (maximum value - minimum value) is 7% of the total average loading value (very low). However, in the case where the negative electrode slurry is coated at a high viscosity of 55000 cp (Comparative Example 2), the difference between the loading values ​​of the negative electrode active material layer measured at five points is 17% of the total average loading value. This indicates a non-uniform distribution, with a high loading level in the central part and a low loading level in the outer part.

[0114] * Measurement of viscosity of negative electrode active material slurry

[0115] After preparing slurry compositions for the negative electrode at the corresponding temperatures, rotational viscometers were used to measure the viscosity at the corresponding temperatures of each slurry composition at 5 s / second (5 s). -1 The shear rate was measured to determine the viscosity of each slurry composition for the negative electrode before and after heating and cooling.

[0116] * Evaluation of interfacial adhesion between active material layer and current collector

[0117] The negative electrodes prepared in Example 1 and Comparative Example 1 were cut into pieces 18 mm wide and 150 mm long. 18 mm wide adhesive tape was applied to the foil layer of the negative electrode, and then a roller with a load of 2 kg was used to ensure full adhesion to the foil layer. Double-sided adhesive tape was used to attach the active material layer of the negative electrode to one side of a tensile testing machine. The adhesive 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.

[0118] [Table 1]

[0119]

[0120] Referring to Table 1, when observing the change in viscosity of the slurry composition used for the negative electrode with temperature, it can be seen that the viscosity of the slurry composition used for the negative electrode decreases immediately after the slurry composition is heated, and then recovers to a high viscosity after the slurry composition is cooled.

[0121] It can be seen that even when the negative electrode according to Example 1 is manufactured using a slurry composition for the negative electrode that has the highest viscosity due to its high solid content (60% by weight), the negative electrode according to Example 1 exhibits high adhesion to the substrate. It can be seen that the electrode according to Comparative Example 1 uses a slurry composition for the negative electrode with the same high solid content (60% by weight) as in Example 1. However, unlike Example 1, because a cooling process is not performed after coating the slurry composition for the negative electrode, and the slurry composition for the negative electrode, heated to have a reduced viscosity, is dried immediately, the adhesion to the substrate is reduced compared to Example 1. This is because when the slurry composition for the negative electrode is dried at a low viscosity, the binder and conductive material migrate to the electrode surface layer due to the flow of the solvent, resulting in an increase in the binder concentration in the region adjacent to the surface of the negative electrode active material layer compared to the binder concentration in the region adjacent to the substrate.

[0122] Meanwhile, the slurry of Comparative Example 2 contained the same amount of solids as in Example 1, resulting in a 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 produce a negative electrode. Therefore, it may be impossible to evaluate the adhesion to the substrate in Comparative Example 2.

[0123] Additionally, refer to Figure 1 It can be confirmed that there is essentially no concentration gradient of binder and conductive material in the thickness direction of the negative electrode active material layer in Example 1. This is believed to be 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 negative electrode active material layer compared to the concentration of binder and conductive material in the region adjacent to the substrate.

[0124] [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 negative electrode active material slurry (Examples 2 to 4 and Comparative Example 3).

[0125] The negative electrode was manufactured in the same manner as in Example 1, except that the viscosity of the negative electrode slurry was changed by altering the solid content and heating temperature of the negative electrode slurry in step 1 of Example 1, as shown in Table 2. In this case, the heating temperatures of Examples 2-4 were adjusted so that the viscosity of each slurry was 8500-9500 cP, and in Comparative Example 3, a slurry that was not heated at room temperature (25°C) was used.

[0126] (Evaluation Method)

[0127] The interfacial adhesion and viscosity between the active material layer and the current collector were evaluated using the same method as in Evaluation Example 1. The evaluation results are shown in Table 2.

[0128] [Table 2]

[0129]

[0130] Referring to Table 2, it can be seen that in the cases of Examples 1 and 2 with preferred solid content ranges, the fluidity of the solvent is reduced due to the high viscosity of the coated slurry, thereby suppressing the migration of the adhesive during the drying process. As a result, the adhesion between the substrate and the active material layer increases.

[0131] On the other hand, it can be confirmed that in Comparative Example 3, the adhesion is very low, and analysis shows that the negative electrode was formed at room temperature without a heating step using slurry. The solid content in the slurry is not high enough, resulting in a large amount of solvent in the slurry. Furthermore, due to the migration of the adhesive under the same drying conditions, a high concentration gradient of adhesive content is formed on the surface of the negative electrode.

[0132] Meanwhile, it can be confirmed that, due to the relatively low solids content in the slurry, Examples 3 and 4 tend to exhibit slightly lower adhesive strength than Examples 1-2. Based on 100 parts by weight of the slurry composition for the negative electrode, the solids content in the slurry composition for the negative electrode is greater than 55 parts by weight and less than 85 parts by weight.

[0133] [Evaluation Example 3]: The adhesion between the active material layer and the current collector was evaluated based on the changes in the drying conditions of the negative electrode active material slurry (Examples 5-8).

[0134] The negative electrode was manufactured in the same manner as in Example 1, except that the drying process of the coated negative electrode slurry in step 2 of Example 1 was changed as described in Table 3.

[0135] (Comparative Example 4)

[0136] The negative electrode was manufactured in the same manner as Comparative Example 3, except that the drying process of Comparative Example 3 was changed as described in Table 3.

[0137] (Evaluation Method)

[0138] 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.

[0139] [Table 3]

[0140]

[0141] Referring to Table 3, it can be confirmed that at low drying temperatures below 120°C (Examples 1, 5, and 6), increasing the drying time to evaporate the solvent suppresses adhesive migration, thereby 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, the migration caused by capillary action is exacerbated, thereby reducing the adhesion between the substrate and the active material layer. Furthermore, it can be confirmed that in the case of a conventional slurry with a solids content of 50% 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 also the lowest.

[0142] In the manufacturing method of the negative electrode for a secondary battery of the present invention, in order to solve the problem of conventional negative electrode manufacturing methods using low-viscosity slurry compositions for the negative electrode, namely the problem of reduced adhesion between the current collector and the negative electrode active material layer due to the migration of conductive material and binder to the surface of the negative electrode active material layer caused by the flow of low-viscosity solvent during drying of the slurry composition, a high-viscosity slurry composition for the negative electrode is used. However, the negative electrode is obtained by the following method: while reducing the viscosity of the high-viscosity slurry composition for the negative electrode by heating, the high-viscosity slurry composition for the negative electrode is coated onto the current collector, the coated slurry composition for the negative electrode is cooled to room temperature, and then the cooled slurry composition for the negative electrode is dried. Therefore, while providing a negative electrode for a secondary battery with improved adhesion between the current collector and the negative electrode active material layer, problems such as reduced flowability and uneven thickness of the negative electrode that may be caused by high viscosity can be prevented.

[0143] 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 the invention can be implemented in other specific forms without changing its technical spirit or essential characteristics. Therefore, it should be understood that the above embodiments are exemplary and not restrictive in all respects.

Claims

1. A method for manufacturing a negative electrode for a secondary battery, comprising: (a) Heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point T of the solvent. b Temperature; (b) The heated slurry composition for the negative electrode is coated onto the current collector; (c) Cooling the coated slurry composition for the negative electrode; and (d) The dried and cooled slurry composition for the negative electrode. Specifically, based on 100 parts by weight of the slurry composition for the negative electrode, the solid content of the slurry composition for the negative electrode is greater than 55 parts by weight and less than 85 parts by weight. The cooling process is carried out at a temperature of 10-30°C. The drying process is carried out at a temperature above 90°C and below 135°C. The slurry composition used for the negative electrode satisfies the following equations 2 and 3: [Relationship 2] 1.3 ≤ A1 / A2 ≤ 12 [Relationship 3] 1.2 ≤ A3 / A2 ≤ 10 Wherein, A1 is the viscosity of the slurry composition for the negative electrode before heating, A2 is the viscosity of the slurry composition for the negative electrode after heating, and A3 is the viscosity of the slurry composition for the negative electrode after cooling.

2. The method for manufacturing the negative electrode of a secondary battery according to claim 1, wherein, The slurry composition for the negative electrode has a viscosity A1 of 12,000-70,000 cP before heating.

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

4. The method for manufacturing the negative electrode of a secondary battery according to claim 1, wherein, The cooling process takes 1-60 seconds.

5. The method for manufacturing the negative electrode of a secondary battery according to claim 1, wherein, The drying process lasts for 20-150 seconds.

6. A negative electrode for a secondary battery, manufactured by a method for producing a negative electrode for a secondary battery, the method comprising: (a) Heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point T of the solvent. b Temperature; (b) The heated slurry composition for the negative electrode is coated onto the current collector; (c) Cooling the coated slurry composition for the negative electrode; and (d) The dried and cooled slurry composition for the negative electrode. Specifically, based on 100 parts by weight of the slurry composition for the negative electrode, the solid content of the slurry composition for the negative electrode is greater than 55 parts by weight and less than 85 parts by weight. The cooling process is carried out at a temperature of 10-30°C. The drying process is carried out at a temperature above 90°C and below 135°C. The slurry composition used for the negative electrode satisfies the following equations 2 and 3: [Relationship 2] 1.3 ≤ A1 / A2 ≤ 12 [Relationship 3] 1.2 ≤ A3 / A2 ≤ 10 Wherein, A1 is the viscosity of the slurry composition for the negative electrode before heating, A2 is the viscosity of the slurry composition for the negative electrode after heating, and A3 is the viscosity of the slurry composition for the negative electrode after cooling.

7. The negative electrode for a secondary battery according to claim 6, wherein, The adhesion force of the negative electrode active material layer to the current collector is above 0.20 N / cm.

8. The negative electrode for a secondary battery according to claim 6, wherein, In the negative 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 negative electrode active material layer is less than 10% of the total average load.

9. A secondary battery, comprising: negative electrode; positive electrode; The membrane located between the negative electrode and the positive electrode; and Electrolyte The negative electrode is manufactured using a method for manufacturing a negative electrode for a secondary battery, the method comprising: (a) Heating a slurry composition for a negative electrode containing a negative electrode active material, a binder, and a solvent to a temperature below the boiling point T of the solvent. b Temperature; (b) The heated slurry composition for the negative electrode is coated onto the current collector; (c) Cooling the coated slurry composition for the negative electrode; and (d) The dried and cooled slurry composition for the negative electrode. Specifically, based on 100 parts by weight of the slurry composition for the negative electrode, the solid content of the slurry composition for the negative electrode is greater than 55 parts by weight and less than 85 parts by weight. The cooling process is carried out at a temperature of 10-30°C. The drying process is carried out at a temperature above 90°C and below 135°C. The slurry composition used for the negative electrode satisfies the following equations 2 and 3: [Relationship 2] 1.3 ≤ A1 / A2 ≤ 12 [Relationship 3] 1.2 ≤ A3 / A2 ≤ 10 Wherein, A1 is the viscosity of the slurry composition for the negative electrode before heating, A2 is the viscosity of the slurry composition for the negative electrode after heating, and A3 is the viscosity of the slurry composition for the negative electrode after cooling.

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