Multilayer electrode for secondary battery and method of manufacturing same

Abstract

To provide a multilayer electrode for a secondary battery and a method for manufacturing the multilayer electrode for a secondary battery.SOLUTION: A method for manufacturing a multilayer electrode includes the steps of: (a) preparing at least two types of electrode slurries including an electrode active material, a binder, and a solvent and heating at least one type selected from the prepared at least two types of electrode slurries at a temperature lower than a boiling temperature (Tb) of the solvent in the selected at least one electrode slurry; (b) applying the at least two types of electrode slurries on a current collector; and (c) cooling the applied at least two types of electrode slurries.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a multilayer electrode for a secondary battery and a method for producing the same. [Background technology]

[0002] Recently, with the increasing demand for electronic devices such as mobile devices, there has been an expansion in the development of lightweight and compact electrochemical batteries (secondary batteries) to increase the portability of electronic devices. In line with this trend, the growth of the electric vehicle (EV) market is accelerating due to the trend toward stricter regulations related to automobile fuel efficiency and exhaust gas emissions worldwide, and there is a demand for the development of high-power, large-capacity batteries for use in such EVs.

[0003] Recently, multilayer electrodes have been studied as a way to improve the quality and performance of batteries. Specifically, by fabricating a two-layer electrode in which the lower electrode active material layer (which contacts the current collector) contains a higher binder content than the upper layer, it is possible to fabricate an electrode with improved adhesion and resistance between the current collector and the electrode active material layer (lower layer). However, in the fabrication method of a multilayer electrode, the problem of interlayer mixing of the binder in the slurry occurs during the process of laminating and drying electrode slurries with different compositions. That is, binder migration occurs, and after electrode fabrication, the binder distribution in the upper and lower electrode active material layers becomes mixed rather than maintaining the binder distribution in the initial slurry, resulting in the loss of a multilayer structure. Furthermore, if the degree of mixing is severe, the upper and lower layers will completely mix to form a single layer, thereby eliminating the aforementioned benefits of a multilayer electrode.

[0004] Therefore, research and development is required to ensure the fluidity of the electrode slurry to ensure that the thickness of each electrode active material layer in the multilayer electrode is uniform, and to minimize interlayer mixing by suppressing the migration phenomenon of the binder in the slurry during the drying process. Summary of the Invention [Problem to be solved by the invention]

[0005] The present invention aims to solve a problem caused by binder migration due to the flow of low-viscosity solvent during drying in the process of forming a laminated electrode active material layer by applying two or more types of electrode slurries onto a current collector and drying the mixture, i.e., a problem related to a decrease in adhesive strength between the current collector and the electrode active material layer. [Means for solving the problem]

[0006] The present invention provides a method for manufacturing an electrode electrode, comprising: (a) preparing two or more types of electrode slurries each containing an electrode active material, a binder, and a solvent; At least one selected from the two or more prepared electrode slurries is mixed at a temperature equal to or higher than the boiling point (T b ) heating at a temperature lower than (b) applying the two or more electrode slurries onto a current collector; and (c) a step of cooling the two or more types of electrode slurries that have been applied.

[0007] The two or more electrode slurries are characterized by having different binder contents relative to the slurry solid content.

[0008] The two or more electrode slurries may be characterized in that an electrode slurry having a higher binder content relative to slurry solids is applied closer to the current collector.

[0009] The step (b) may involve simultaneously or sequentially applying the two or more electrode slurries onto a current collector.

[0010] The method may further comprise the step (d) of drying the cooled two or more electrode slurries.

[0011] The drying can be carried out at a temperature of 90°C or higher and lower than 180°C for 45 seconds to 5 minutes.

[0012] The heating temperature (T) in the heating step can satisfy the following relational expression 1. [Equation 1] 0.3T b <T<0.9T b In the above-mentioned relational formula 1, T b is the boiling point of the solvent contained in the selected electrode slurry.

[0013] The selected electrode slurry is characterized in that it satisfies the following relational expressions 2 and 3, and the viscosity (A1) of the electrode slurry before heating is at least 20,000 cp or more. [Equation 2] 1.3≦A1 / A2≦12 [Equation 3] 1.1≦A3 / A2≦10 In the relational expressions 2 and 3, A1 is the viscosity of the selected electrode slurry before heating, A2 is the viscosity of the selected electrode slurry after heating, and A3 is the viscosity of the selected electrode slurry after cooling.

[0014] The cooling can be carried out at a temperature of 10 to 30°C for 1 to 60 seconds.

[0015] The two or more electrode slurries may each have a solids content of more than 50% by weight and not more than 80% by weight.

[0016] The present invention also provides A current collector; a first electrode layer formed on the current collector and containing a first electrode active material and a first binder; a second electrode layer formed on the first electrode layer and containing a second electrode active material and a second binder; a content (wt%) of the first binder relative to the total weight of the first electrode layer is higher than a content (wt%) of the second binder relative to the total weight of the second electrode layer; The multilayer electrode for a secondary battery is provided, wherein the adhesive strength of the first electrode layer to the current collector is 0.2 N / cm or more.

[0017] The multi-layer electrode may have a difference between a maximum loading value and a minimum loading value of the electrode layer at at least five positions spaced apart from each other in a lengthwise direction, the difference being 10% or less of an overall average loading value.

[0018] The first electrode layer and the second electrode layer may satisfy the following relational expression 4. [Equation 4] 0.5 <R e / R s <1 In the above-mentioned relational formula 4, R s is the binder content (B) relative to the solid content in the first electrode slurry applied onto the current collector to form the first electrode layer. 1s ) (wt %) and the binder content (B ) relative to the solid content in the second electrode slurry applied onto the first electrode slurry to form the second electrode layer. 2s ) (weight%) and the difference (B 1s -B 2s ) and R e is the binder content (B 1e ) (wt%) and the binder content of the second electrode layer (B 2e ) (weight%) and the difference (B 1e -B 2e )

[0019] The present invention also provides a multilayer electrode according to one embodiment of the present invention; A separator; and an electrolyte solution. [Effects of the Invention]

[0020] The method for manufacturing a multilayer electrode for a secondary battery according to the present invention provides an electrode for a secondary battery having improved adhesion between a current collector and an electrode active material layer, and is effective in preventing problems such as reduced fluidity and uneven electrode thickness that may be caused by high viscosity. [Brief explanation of the drawings]

[0021] [Figure 1] 1A and 1B are diagrams showing scanning electron microscope images of cross sections of multilayer electrodes of Example 1 and Comparative Example 3. [Figure 2] 1 shows the results of analyzing the cross sections of the multilayer electrodes of Example 1 and Comparative Example 3 using a scanning electron microscope with energy dispersive X-ray spectroscopy (SEM-EDS), illustrating the distribution of binder in the thickness direction of the electrode layer. [Figure 3] This is a schematic diagram of a test piece obtained by selecting five locations at predetermined intervals in the length direction (width direction) of the electrode layer and punching them into a circle with a diameter of 38 mm to evaluate the homogeneity of the electrode layer. [Figure 4] FIG. 1 is a graph showing the results of normalizing the weights of the electrode layers (loading amounts of the electrode layers) measured at different positions in the length direction (width direction) of the multilayer electrodes according to Example 1 and Comparative Example 1 by dividing them by the total weight of the electrode layers. DETAILED DESCRIPTION OF THE INVENTION

[0022] The advantages and features of the present invention, as well as methods for achieving them, will become clearer with reference to the following detailed description of the embodiments in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be embodied in various different forms. However, these embodiments are provided to fully disclose the present invention and to fully convey the scope of the invention to those skilled in the art to which the present invention pertains. The present invention is defined only by the scope of the claims. Specific details for implementing the present invention will be described in detail below with reference to the accompanying drawings. The same element numbers refer to the same components regardless of the drawing, and "and / or" includes each and every combination of one or more of the referenced items.

[0023] Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used in the same manner as commonly understood by a person of ordinary skill in the art to which this invention pertains. When a part of the entire specification "comprises" a certain element, this does not mean that it may further include other elements, but does not exclude other elements, unless otherwise specified. In addition, the singular form includes the plural form unless otherwise specified in the phrase.

[0024] In this specification, when a layer, film, region, plate, or other part is said to be "on" or "on top of" another part, this includes not only the case where it is "directly on top of" the other part, but also the case where there is another part in between.

[0025] In this specification, the "viscosity" refers to the viscosity measured at a shear rate of 1 s using a Brookfield rotational viscometer at the slurry temperature in each step. -1 The value is measured with a tolerance of ±500 cP.

[0026] The present invention provides a method for producing an electrode slurries comprising: (a) preparing two or more electrode slurries each containing an electrode active material, a binder, and a solvent; and (b) heating at least one selected from the two or more electrode slurries to a temperature below the boiling point (T b (b) heating the electrode at a temperature lower than that of the electrode prepared in step (b), (b) applying the two or more types of electrode slurries onto a current collector, and (c) cooling the applied two or more types of electrode slurries.

[0027] (a1) First, two or more electrode slurries containing an electrode active material, a binder, a conductive material, and a solvent are prepared. Here, the two or more electrode slurries may have different binder contents (wt%) relative to the slurry solids (electrode active material, binder, and conductive material) other than the solvent.

[0028] The solid content in the electrode slurry may be greater than 50% by weight, specifically greater than 50% by weight and not greater than 80% by weight, and more specifically greater than 55% by weight and not greater than 65% by weight, based on the electrode slurry.

[0029] Generally, in the process of manufacturing electrodes for secondary batteries, a slurry needs to have a viscosity of approximately 1,000 cP to 10,000 cP at 25°C to facilitate the slurry coating process and produce an electrode with a uniform thickness. However, when the electrode slurry is prepared using a conventional method in which the solvent content is reduced to a solids content of 60 wt% or more, the viscosity of the slurry exceeds 50,000 cP, resulting in a rapid decrease in the fluidity of the slurry, which reduces workability, produces an electrode with an uneven thickness, and reduces the adhesive strength between the current collector and the electrode active material layer in the electrode. For this reason, conventional electrode slurries contain a solids content of approximately 50 wt%.

[0030] On the other hand, in the present invention, the fluidity problem can be improved by heating a high-viscosity electrode slurry. However, if the weight exceeds the above-mentioned range, the electrode slurry does not contain the minimum amount of solvent necessary to disperse the solid particles during production, and the shear force for particle dispersion is insufficient, which may result in the solid particles in the slurry being applied to the current collector in an unevenly dispersed state. However, if the weight is below the above-mentioned range, the fluidity of the slurry is already sufficient, making the technology proposed in the present invention unnecessary.

[0031] The viscosity (A1) of the two or more electrode slurries before heating can be at least 20,000 cP or more at 25° C., preferably 20,000 to 70,000 cP, and more preferably 30,000 to 60,000 cP.

[0032] If the solvent content in the electrode slurry increases and the viscosity (A1) of the slurry is less than 20,000 cP at 25°C, the amount of solvent contained in the slurry is large, which may cause binder migration, weakening the adhesion between the electrode active material layer and the current collector, and reducing long-term stability.

[0033] When the viscosity (A1) of the electrode slurry exceeds 70,000 cP at 25°C, even if the content of the solid components (especially the electrode active material and the binder) increases, the increase in the adhesion force between the electrode active material layer and the current collector is negligible.

[0034] Specifically, the weight ratio of the electrode active material to the binder in the solid component (content ratio) can be 1:0.01 to 0.1, specifically 1:0.01 to 0.08, and more specifically 1:0.02 to 0.05.

[0035] The electrode active material can be used without limitation as long as it is an electrode active material usually used in secondary batteries. As an example of the negative electrode active material, it can be a carbon-based negative electrode active material, a silicon-based negative electrode active material, or a mixture thereof, but is not limited thereto. The carbon-based negative electrode active material can be one or more selected from artificial graphite, natural graphite, and hard carbon. The silicon-based negative electrode active material can be Si, SiO x (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Si-carbon composite, or a mixture of at least one of these and SiO2. In the case of the positive electrode active material, it can be a composite oxide of a metal selected from cobalt, manganese, nickel, and combinations thereof and lithium, but is not limited thereto.

[0036] The binder is not particularly limited as long as it is a conventional binder that can play a role of well adhering electrode active material particles to each other and well adhering the electrode active material to the current collector. As an example, the binder can be a water-soluble binder, specifically, styrene-butadiene rubber, acrylated styrene-butadiene rubber, polyvinyl alcohol, sodium polyacrylate, a copolymer of propylene and an olefin having 2 to 8 carbon atoms, a copolymer of (meth)acrylic acid and (meth)acrylic acid alkyl ester, or a combination thereof.

[0037] When using the water-soluble binder, the water-soluble binder is preferable because it does not affect the viscosity of the slurry and can effectively bind the electrode active material to the current collector. However, since the slurry tends to gel due to the fine particle electrode active material and conductive material, a thickener may be further included to impart viscosity to the slurry and produce a stable slurry. For example, the thickener may be a mixture of one or more cellulose-based compounds, specifically, carboxymethylcellulose, hydroxypropylmethylcellulose, methylcellulose, or alkali metal salts thereof. The alkali metal may be sodium, potassium, or lithium.

[0038] The solvent may be any solvent typically used in electrode slurries. Specifically, the negative electrode solvent may be at least one selected from the group consisting of water, pure water, deionized water, distilled water, ethanol, isopropanol, methanol, acetone, n-propanol, and t-butanol, but is not limited thereto. The positive electrode solvent may be at least one selected from the group consisting of amines such as N,N-dimethylaminopropylamine and diethyltriamine; ethers such as ethylene oxide and tetrahydrofuran; ketones such as methyl ethyl ketone; esters such as methyl acetate; and aprotic polar solvents such as dimethylacetamide and N-methyl-2-pyrrolidone, but is not limited thereto.

[0039] The conductive material is used to impart conductivity to the electrode and is not particularly limited as long as it is a conventional electron-conductive material that does not cause chemical changes in the battery. Examples include, but are not limited to, natural graphite, artificial graphite, carbon black, acetylene black, ketjen black, carbon fiber, carbon nanotubes, and combinations thereof.

[0040] The current collector may be, but is not limited to, a material selected from the group consisting of copper foil, nickel foil, stainless steel foil, titanium foil, nickel foam, copper foam, a polymer substrate coated with a conductive metal, and combinations thereof.

[0041] (a2) Next, at least one selected from the two or more high-viscosity electrode slurries produced is mixed with a solvent having a boiling point (T b ) and heat at a lower temperature.

[0042] Therefore, the selected high-viscosity electrode slurry can be heated at a temperature below its boiling point, thereby making it easy to work with and reducing the viscosity to a level that allows the formation of an electrode with a uniform thickness. Therefore, the method for manufacturing a multilayer electrode for a secondary battery according to the present invention uses two or more high-viscosity electrode slurries containing a high solid content, and in this case, at least one electrode slurry selected from the two or more high-viscosity electrode slurries can be heated to reduce its viscosity and then applied onto a current collector.

[0043] However, if the heating temperature of the electrode slurry is equal to or higher than the boiling point of the solvent, strong vapor pressure may be generated inside the piping or storage container for transporting the slurry during transportation or storage of the heated electrode slurry, which may result in risks such as water leakage and explosion.

[0044] Therefore, the heating can be performed at a temperature lower than the boiling point of the solvent contained in the electrode slurry.

[0045] Specifically, the heating temperature (T) can satisfy the following relational expression 1.

[0046] [Equation 1] 0.3T b <T<0.9T b

[0047] In the above-mentioned relational formula 1, T bis the boiling point of the solvent contained in the selected electrode slurry.

[0048] In the above relational expression 1, 0.35T b <T<0.9T b , preferably 0.4T b <T<0.85T b , more preferably 0.5T b <T<0.8T b It could be.

[0049] The viscosity (A2) of the heated slurry may be 5,000 to 10,000 cP, preferably 7,000 to 9,800 cP, and more preferably 8,000 to 9,500 cP at the temperature of each slurry.

[0050] In step (b), the two or more electrode slurries are applied onto a current collector, and the application can be performed without heating (elevating the temperature) the slurries and the substrate (current collector).

[0051] As a non-limiting example, the coating may be performed using any coating method generally known to be used for coating a liquid phase to form a film, such as, but not limited to, spray coating, dip coating, spin coating, gravure coating, slot die coating, doctor blade coating, roll coating, inkjet printing, flexography printing, screen printing, electrohydrodynamic printing, microcontact printing, imprinting, reverse offset printing, bar coating, and gravure offset printing.

[0052] The electrode slurry having a higher binder content relative to the slurry solids content of the two or more electrode slurries may be coated closer to the current collector, and the two or more electrode slurries may be coated simultaneously or sequentially onto the current collector. For example, a first electrode slurry including a first electrode active material, a first binder, and a first solvent and a second electrode slurry including a second electrode active material, a second binder, and a second solvent may be prepared, and then the heated first electrode slurry may be coated onto the current collector, and the heated second electrode slurry may be coated onto the first electrode slurry. In this case, the content (wt%) of the first binder relative to the total weight of the solids content of the first electrode slurry may be higher than the content (wt%) of the second binder relative to the total weight of the solids content of the second electrode slurry.

[0053] In step (c), the two or more types of electrode slurries that have been applied are cooled.

[0054] The cooling may be performed at a temperature of 10 to 30°C, specifically 10 to 25°C, more specifically 15 to 25°C, for 1 to 60 seconds, specifically 2 to 30 seconds, more specifically 5 to 10 seconds, immediately after at least one electrode slurry selected from the two or more electrode slurries is heated and applied onto the current collector.

[0055] The heated electrode slurry can regain its high viscosity upon cooling. Therefore, the method for manufacturing a multilayer electrode for a secondary battery according to the present invention involves using two or more high-viscosity electrode slurries containing a high solid content. The method involves heating at least one of the two or more electrode slurries to reduce its viscosity, applying it to a current collector, and then cooling the applied slurry composition immediately after application. This allows the electrode slurry to regain its high viscosity, thereby preventing binder migration during the subsequent drying process. Therefore, the present invention can provide a multilayer electrode with improved adhesion between the current collector and the electrode active material layer, and can prevent problems such as reduced fluidity and uneven electrode thickness that can be caused by high viscosity.

[0056] The viscosity (A3) of the cooled electrode slurry of the selected electrode slurry may be 10,000 cP to 60,000 cP, preferably 13,000 cP to 55,000 cP, more preferably 25,000 cP to 50,000 cP, and even more preferably 35,000 cP to 45,000 cP at the relevant temperature of each slurry.

[0057] That is, the selected electrode slurry is characterized by satisfying the following relational expressions 2' and 3'.

[0058] [Equation 2'] A2 <A1

[0059] [Equation 3'] A2 <A3

[0060] In the relational formula 2' and the relational formula 3', A1 is the viscosity of the selected electrode slurry before heating, A2 is the viscosity of the selected electrode slurry after heating, and A3 is the viscosity of the selected electrode slurry after cooling.

[0061] However, even if the heated electrode slurry is cooled within a limited time, it may not be cooled down to the temperature of the electrode slurry before heating. Therefore, the viscosity of the cooled and recovered electrode slurry may be lower than the viscosity of the electrode slurry before heating. Thus, the relational expressions 2' and 3' can be summarized as A2 < A3 ≤ A1, but are not limited thereto.

[0062] In one embodiment, the viscosity of the selected electrode slurry can be reduced to 1 / 3 or less after heating and then can recover to a viscosity similar to the initial one during cooling.

[0063] That is, the relational expressions 2' and 3' can also be summarized as the following relational expressions 2 and 3.

[0064] [Relational Expression 2] 1.3 ≤ A1 / A2 ≤ 12

[0065] In the relational expression 2, 2 ≤ A1 / A2 ≤ 11, specifically 3 ≤ A1 / A2 ≤ 10, and more specifically 5 ≤ A1 / A2 ≤ 7 may be applicable.

[0066] [Relational Expression 3] 1.1 ≤ A3 / A2 ≤ 10

[0067] In the relational expression 3, 1.5 ≤ A3 / A2 ≤ 9, specifically 2.5 ≤ A3 / A2 ≤ 7, and more specifically 4 ≤ A3 / A2 ≤ 5 may be applicable.

[0068] Further, the method for manufacturing a multilayer electrode for a secondary battery according to the present invention can further include a step (d) of drying the cooled electrode slurry. At this time, the drying can be performed at a temperature of 45 seconds to 5 minutes, preferably 1 to 4 minutes, more preferably 1 to 3 minutes, 90°C or higher and lower than 180°C, preferably 95 to 160°C, more preferably 100 to 140°C, and most preferably 100 to 135°C.

[0069] The present invention also provides a multilayer electrode for a secondary battery, comprising: a current collector; a first electrode layer formed on the current collector and containing a first electrode active material and a first binder; and a second electrode layer formed on the first electrode layer and containing a second electrode active material and a second binder, wherein the content (wt %) of the first binder relative to the total weight of the first electrode layer is higher than the content (wt %) of the second binder relative to the total weight of the second electrode layer, and the adhesive strength of the first electrode layer to the current collector is 0.2 N / cm or more.

[0070] Generally, in a multilayer electrode having a structure in which electrode layers containing different binder contents are stacked, interlayer mixing occurs due to a diffusion phenomenon between the layers during the drying step of the electrode manufacturing process, and the difference in binder content between the electrode layers of the manufactured electrode is significantly smaller than the difference in binder content in the electrode slurry before the formation of each electrode layer. In other words, the multilayer structure of the electrode cannot be maintained, and the adhesive strength between the electrode layer and the current collector and the effect of improving resistance, which are achieved by the multilayer structure, are reduced or eliminated.

[0071] However, by maintaining a binder content distribution similar to that of the electrode slurries corresponding to each electrode layer in the multilayer electrode for a secondary battery according to the present invention, the adhesive strength of the electrode layer to the current collector can be significantly increased. Specifically, the adhesive strength of the first electrode layer to the current collector can be 0.2 N / cm or more, specifically 0.20 to 1.5 N / cm, more specifically 0.22 to 0.5 N / cm.

[0072] The first electrode layer and the second electrode layer may satisfy the following relational expression 4.

[0073] [Equation 4] 0.5 <R e / R s <1

[0074] In the above-mentioned relational formula 4, R s is the binder content (B) relative to the solid content in the first electrode slurry applied onto the current collector to form the first electrode layer. 1s) (wt %) and the binder content (B ) relative to the solid content in the second electrode slurry applied onto the first electrode slurry to form the second electrode layer. 2s ) (weight%) and the difference (B 1s -B 2s ) and R e is the binder content (B 1e ) (wt%) and the binder content of the second electrode layer (B 2e ) (weight%) and the difference (B 1e -B 2e )

[0075] In the above-mentioned relational formula 4, 0.6 <R e / R s <1, specifically 0.7 <R e / R s <1, more specifically 0.75 <R e / R s It can be <1.

[0076] The multi-layer electrode according to an embodiment of the present invention may have electrode layers formed uniformly in the length direction (width direction), specifically, the loading amount of the electrode layers may be uniform.

[0077] Specifically, the multilayer electrode may have a difference between the maximum and minimum electrode layer loading values ​​at at least five positions spaced apart in the longitudinal direction that is 10% or less of the average overall loading value, e.g., 9%, 8%, or 7%, thereby further improving the above-mentioned effects.

[0078] In this case, the loading may refer to the weight of the solid content of the electrode active material slurry or the weight of the electrode layer of the finally formed electrode after the electrode slurry coated on the electrode current collector is dried. As a non-limiting example, a test piece may be obtained by punching out at least five circular portions at predetermined intervals in the longitudinal direction (width direction) of the electrode, and then the weight of the solid content of the electrode active material slurry or the electrode layer in the test piece may be measured.

[0079] Therefore, the multilayer electrode according to the present invention preferably has an improved adhesive force to a base material (current collector) by having a solid content homogeneously dispersed in the electrode layer.

[0080] The present invention also provides a secondary battery including the multilayer electrode, a separator, and an electrolyte.

[0081] Specifically, the multilayer electrode according to the present invention can have improved base material adhesion and thickness uniformity. Thereby, the secondary battery including the multilayer electrode preferably has more improved long-term stability.

[0082] The multilayer electrode can be a positive electrode or a negative electrode depending on the type of electrode active material contained in the electrode layer. The electrode active material can be used without limitation as long as it is an electrode active material commonly used in secondary batteries. Examples of the negative electrode active material include, but are not limited to, carbon-based negative electrode active materials, silicon-based negative electrode active materials, or mixtures thereof. The carbon-based negative electrode active material can be one or more selected from artificial graphite, natural graphite, and hard carbon. The silicon-based negative electrode active material can be Si, SiO x (0 < x < 2), Si-Q alloy (where Q is an element selected from the group consisting of alkali metals, alkaline earth metals, Group 13 elements, Group 14 elements, Group 15 elements, Group 16 elements, transition metals, rare earth elements, and combinations thereof, and is not Si), Si-carbon composite, or a mixture of at least one of these and SiO2. In the case of the positive electrode active material, it can be a composite oxide of a metal selected from cobalt, manganese, nickel, and combinations thereof and lithium, but is not limited thereto.

[0083] The separator is not particularly limited as long as it is a separator known in the art. For example, it can be selected from glass fiber, polyester, polyethylene, polypropylene, polytetrafluoroethylene, or combinations thereof, and can be in the form of a non-woven fabric or a woven fabric, and optionally can be used as a single-layer or multi-layer structure.

[0084] The electrolyte solution contains a non-aqueous organic solvent and an electrolyte salt. The non-aqueous organic solvent may be, but is not limited to, ethylene carbonate (EC), propylene carbonate (PC), dimethyl carbonate (DMC), diethyl carbonate (DEC), ethyl methylcarbonate (EMC), 1,2-dimethoxyethane (DME), γ-butyrolactone (BL), tetrahydrofuran (THF), 1,3-dioxolane (DOL), diethyl ether (DEE), methyl formate (MF), methyl propionate (MP), sulfolane (S), dimethyl sulfoxide (DMSO), acetonitrile (AN), or a mixture thereof. The electrolyte salt is dissolved in the non-aqueous organic solvent and serves as a source of electrolyte metal ions within the battery, enabling basic secondary battery operation and promoting the movement of electrolyte 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)(C y F 2y+1 The electrolyte salt may be, but is not limited to, SO2 (where x and y are natural numbers), LiCl, LiI, or a mixture thereof. In addition, the electrolyte salt may be a known substance used at a concentration appropriate for the purpose, and may further contain a known solvent or additive to improve charge / discharge characteristics, flame retardancy, etc., as needed.

[0085] Example 1 Step 1: Preparation of first negative electrode slurry A first negative electrode slurry (solid content of the total slurry: 60 wt%) was prepared by adding water to 93.4 wt% of artificial graphite, 3.0 wt% of carbon black conductive material, 2.4 wt% of SBR binder, and 1.2% of CMC at room temperature and mixing for 120 minutes. The prepared first negative electrode slurry was stirred for 30 minutes in a stirring tank heated to 70°C, maintaining the slurry temperature at 70°C. The viscosity of the prepared first negative electrode slurry was 60,000 cP, and the viscosity after heating to 70°C was 9,000 cP (the viscosities of the first negative electrode slurry before and after heating were measured using a rotational viscometer at a shear rate of 1 s for each slurry at the temperature). -1 Measured at.)

[0086] Step 2: Preparation of second negative electrode slurry A second negative electrode slurry (60 wt.% solids content based on the total slurry) was prepared by adding water to 95.2 wt.% artificial graphite, 3.0 wt.% carbon black conductive material, 0.6 wt.% SBR binder, and 1.2 wt.% CMC and mixing them at room temperature for 120 minutes. The second negative electrode slurry was stirred for 30 minutes in a stirring tank heated to 70°C, maintaining the slurry temperature at 70°C. The viscosity of the second negative electrode slurry was 60,000 cP, and the viscosity after heating to 70°C was 9,000 cP (the viscosities of the second negative electrode slurry before and after heating were measured using a rotational viscometer at a shear rate of 1 s for each slurry at the temperature after preparing the negative electrode slurry at the temperature). -1 Measured at.)

[0087] Step 3: Fabricating the anode The first negative electrode slurry was transferred to a coating die through piping and a pump heated to 70°C, and coated on both sides of a copper foil (Cu foil) (thickness: 6 μm) in a thickness of 50 μm to form a preliminary first electrode layer. At this time, the coating die from which the first negative electrode slurry was discharged was also maintained at 70°C by the warm water circulating inside.

[0088] The second negative electrode slurry was then transported to a coating die through piping and a pump heated to 70°C and coated on each of the preliminary first electrode layers to a thickness of 50 μm to form a preliminary second electrode layer. The preliminary first and second electrode layers formed on the copper foil (Cu foil) were cooled at room temperature for 10 seconds immediately after coating, and then dried for 2 minutes in a drying oven heated with hot air at 130°C to produce a negative electrode having a current collector / first electrode layer / second electrode layer structure and a final thickness of 60 μm.

[0089] Evaluation example 1: Measurement of cross-sectional SEM image of negative electrode and evaluation of homogeneity of negative electrode active material layer (Comparative Example 1) Negative electrodes were manufactured in the same manner as in Example 1, except that the applied first and second negative electrode slurries were not heated or cooled, but were dried immediately after application.

[0090] (Comparative Example 2) Negative electrodes were manufactured in the same manner as in Example 1, except that the first and second negative electrode slurries were not cooled and were immediately dried after application.

[0091] (Comparative Example 3) Negative electrodes were manufactured in the same manner as in Example 1, except that the solid content of each of the first and second negative electrode slurries was set to 50 wt %; the prepared first and second negative electrode slurries were applied at room temperature without heating; and the applied first and second negative electrode slurries were dried immediately after application without cooling.

[0092] (Evaluation method) *Measurement of cross-sectional SEM-EDS image of negative electrode Cross sections of the negative electrodes prepared in Example 1 and Comparative Example 3 were cut using an ion milling device, and the SBR binder distribution in the negative electrodes was measured using energy dispersive X-ray spectroscopy (SEM-EDS) using a scanning electron microscope. The results are shown in Figures 1 and 2 and Table 1. To detect only the SBR binder, the SBR binder in the negative electrodes was pretreated with osmium (Os) before SEM-EDS analysis. The binder is shown in red in Figure 1.

[0093] 1, it can be seen that in Example 1, the distribution of the binder decreases in the thickness direction of the electrode as it moves away from the current collector. This indicates that the binder contents in the first and second slurries before coating the current collector (2.4 wt % and 0.6 wt %, respectively) are well maintained in the first and second electrode layers after coating and drying.

[0094] On the other hand, in Comparative Example 3, it was confirmed that the binder was distributed uniformly across the entire thickness of the electrode. That is, the binder contents in the first and second slurries before coating the current collector (2.4 wt % and 0.6 wt %, respectively) were uniformly distributed across the entire thickness because the binder mixing between the layers (first and second electrode layers) occurred during the coating and drying process, and the initial binder distribution could no longer be maintained.

[0095] The binder content values ​​in the thickness direction of the electrode in FIG. 2 are summarized in Table 1 below. Specifically, B 1s is the binder content (wt%) relative to the solid content in the first slurry, B 2s is the binder content (wt%) relative to the solid content in the second slurry, B 1e is the binder content (wt%) in the first electrode layer, B 2e indicates the binder content (wt %) in the second electrode layer.

[0096] [Table 1]

[0097] As shown in Table 1, in Example 1, the difference in binder content between the first and second electrode layers (R e ): The difference in binder content between the initial first and second slurries (R S ) ratio was 0.78, and even after electrode formation, the binder content was almost similar to the binder distribution in the initial slurry. This result is believed to be due to the effect of suppressing interlayer mixing of the binder in the slurry, which restores the viscosity of the slurry to its initial viscosity of 60,000 cP through the heating and cooling process. It is believed that the suppression of interlayer mixing of the binder in each electrode layer maintained the binder content in the initial slurry.

[0098] In Comparative Example 3, a 50 wt% solids slurry with a viscosity of 8,500 cP was used to ensure the fluidity of the slurry and ensure a uniform coating. It was immediately applied to the current collector and dried without a separate heating or cooling process. Therefore, due to the fluidity of the slurry, the binders in the first and second slurries applied during the drying process were mixed, resulting in a significant reduction in the difference in binder content between the first and second electrode layers after drying compared to the initial slurries. This reduction in the difference in binder content between the first and second electrode layers ultimately significantly reduces the adhesive strength between the first electrode layer and the current collector, potentially increasing the resistance of the electrode and reducing the performance of the battery containing the electrode.

[0099] *Evaluation of coating uniformity across the width of the negative electrode active material layer To analyze the widthwise coating uniformity of the negative electrode active material layers prepared in Example 1 and Comparative Example 1, five locations were selected at predetermined intervals along the length of the negative electrode active material layer, and circular specimens with a diameter of 38 mm were punched out from the specimens, as shown in Figure 3. The weight of the negative electrode active material layer in each punched specimen was measured, and the loading weight of the negative electrode active material layer composition in the negative electrode was analyzed. The weight of the negative electrode active material layer (loading amount of the negative electrode active material layer composition) measured at each location was divided by the total weight of the negative electrode active material layer, and the normalized value is shown in Figure 4.

[0100] As can be seen from FIG. 4, in Example 1, in which the slurry was applied in a state where its viscosity was reduced by heating, and then cooled and dried, the difference in loading value (maximum value minus minimum value) of the negative electrode active material layer was a very low 7% of the overall average loading value measured at five points. In contrast, in Comparative Example 1, in which the slurry was applied in a state where its viscosity was high without heating, and then immediately dried, the difference in loading value of the negative electrode active material layer was 17% of the overall average loading value measured at five points, indicating a non-uniform distribution in which the loading amount was high in the center and low at the periphery.

[0101] Evaluation Example 2: Evaluation of adhesive strength between active material layer and current collector depending on change in solid content of negative electrode slurry (Examples 2 to 4, Comparative Examples 2 to 3) Negative electrodes were manufactured in the same manner as in Example 1, except that the solid content and the heating temperature of the slurry were varied when preparing the first and second negative electrode slurries, and the viscosities of the negative electrode slurries were set as shown in Table 2 below. In this case, the heating temperatures in Examples 2 to 4 were adjusted so that the viscosities of the respective slurries were 8,500 to 9,500 cP.

[0102] (Evaluation method) *Evaluation of the interfacial adhesion between the negative electrode active material layer and the current collector The negative electrodes prepared in Examples 2 to 4 and Comparative Examples 2 to 3 were cut to a size of 18 mm wide x 150 mm long. An 18 mm wide tape was attached to the foil layer of the negative electrode, and then the tape was firmly attached using a roller with a 2 kg load. The negative electrode active material layer was attached to one side of a tensile tester using double-sided tape. The tape attached to the foil was fastened to the other side of the tensile tester, and the adhesive strength was measured. The results are shown in Table 2 below. Here, the negative electrode active material layer refers to the first and second electrode layers.

[0103] [Table 2]

[0104] In Table 2, the solid content (wt%) refers to the solid content of the first and second negative electrode slurries, and the solid content of the first and second negative electrode slurries has the same value within a margin of error of ±3%. The viscosity of the negative electrode slurries refers to the viscosity of the first and second negative electrode slurries, and the viscosity of the first and second negative electrode slurries has the same value within a margin of error of ±5%. The viscosities of the negative electrode slurries before, after, and after cooling were measured at a shear rate of 1 s for each slurry at the corresponding temperature using a rotational viscometer after preparing the negative electrode slurries at the corresponding temperatures. -1 was measured.

[0105] As shown in Table 2, in Examples 1 to 4 where the solid content is 53 to 60% after the heating and cooling process of the slurry, the viscosity of the applied slurry is high and the fluidity is reduced, which can suppress interlayer mixing due to binder migration during the drying process, and it can be confirmed that the adhesive strength between the negative electrode active material layer, specifically, the first electrode layer and the current collector is increased.

[0106] On the other hand, in the case of Comparative Example 2, the slurry was heated to reduce its viscosity and then immediately dried without cooling. This meant that the interlayer mixing of the binder due to the increased fluidity of the slurry could not be suppressed, resulting in a decrease in adhesive strength.

[0107] In addition, it was confirmed that Comparative Example 3 had very low adhesive strength. This is because the negative electrode was formed using a room temperature slurry that did not undergo heating and cooling steps, and the solid content in the slurry was not high enough, and it contained a relatively high content of solvent. This is analyzed to be because, even under the same drying conditions, a concentration gradient with a high binder content on the surface of the negative electrode was formed due to the binder migration phenomenon.

[0108] On the other hand, Examples 3 and 4 had a relatively low solid content in the slurry, and thus tended to have slightly lower adhesive strength compared to Examples 1 and 2. However, it was confirmed that the preferred solid content in the slurry composition for a negative electrode is more than 50 parts by weight and not more than 80 parts by weight per 100 parts by weight of the slurry composition for a negative electrode.

[0109] Evaluation Example 3: Evaluation of adhesive strength between active material layer and current collector due to changes in drying conditions of negative electrode active material slurry (Examples 5 to 8) A negative electrode was produced in the same manner as in Example 1, except that in step 3 of Example 1, the drying process for the applied preliminary first and second electrode layers was changed to that shown in Table 3 below.

[0110] Comparative Example 4 A negative electrode was produced in the same manner as in Comparative Example 3, except that the drying step in Comparative Example 3 was changed to that shown in Table 3 below.

[0111] (Evaluation method) *Evaluation of the interfacial adhesion between the negative electrode active material layer and the current collector The same procedure as in Evaluation Example 2 was carried out, and the results are shown in Table 3 below.

[0112] [Table 3]

[0113] In Table 3, the solid content (wt %) is the solid content of the first and second negative electrode slurries, and the solid content of the first and second negative electrode slurries has the same value within an error range of ±3%.

[0114] As shown in Table 3, when the drying temperature is low, ie, 130°C or less (Examples 1, 5, and 6), the drying time increases to evaporate the solvent, but the binder migration phenomenon is suppressed, and the adhesion strength between the active material layer and the current collector increases.

[0115] On the other hand, when the drying temperature was higher than 150°C (Examples 7 and 8), rapid drying caused active material particles to be exposed to the surface of the solvent during the drying process, which exacerbated migration due to capillary action and reduced adhesion between the active material layer and the current collector. Furthermore, the conventional slurry with a solid content of 50 wt% (Comparative Example 4) showed the lowest adhesion despite undergoing the same drying process as Example 6, which had the best adhesion.

Claims

1. (a) preparing two or more electrode slurries containing an electrode active material, a binder, and a solvent; At least one selected from the two or more prepared electrode slurries is mixed with the solvent contained in the selected electrode slurry at a temperature of 1000 K or less, the boiling point (T ｂ ) heating at a temperature lower than (b) applying the two or more electrode slurries onto a current collector; (c) cooling the two or more electrode slurries applied; The viscosity (A1) of the electrode slurry before heating is at least 20,000 cp or more, The method for manufacturing a multilayer electrode for a secondary battery, wherein the electrode slurry having a higher binder content relative to slurry solids content is applied closer to the current collector.

2. The method of claim 1, wherein the two or more electrode slurries have different binder contents relative to slurry solids.

3. The method for manufacturing a multilayer electrode for a secondary battery according to claim 1 , wherein in step (b), the two or more types of electrode slurries are sequentially applied onto a current collector.

4. The method for producing a multilayer electrode for a secondary battery according to claim 1 , further comprising a step (d) of drying the cooled two or more electrode slurries.

5. 5. The method for producing a multilayer electrode for a secondary battery according to claim 4, wherein the drying is carried out at a temperature of 90° C. or higher and lower than 180° C. for 45 seconds to 5 minutes.

6. The method for manufacturing a multilayer electrode for a secondary battery according to claim 1 , wherein the heating temperature (T) in the heating step satisfies the following relational expression 1: [Relationship 1] 0.3T ｂ <T<0.9T ｂ In the above-mentioned relational formula 1, T ｂ is the boiling point of the solvent contained in the selected electrode slurry.

7. A method for producing the multilayer electrode for a secondary battery according to claim 1, comprising the steps of: The method for manufacturing a multilayer electrode for a secondary battery, wherein the selected electrode slurry satisfies the following relational expressions 2 and 3: [Relationship 2] 1.3≦A １ / A ２ ≦12 [Relationship 3] 1.1≦A ３ / A ２ ≦10 In the above-mentioned Relational Formula 2 and Relational Formula 3, A １ is the viscosity of the selected electrode slurry before heating, A ２ is the viscosity of the heated electrode slurry of the selected electrode slurry, A ３ is the viscosity of the cooled electrode slurry of the selected electrode slurry.

8. 2. The method for producing a multilayer electrode for a secondary battery according to claim 1, wherein the cooling is performed at a temperature of 10 to 30° C. for 1 to 60 seconds.

9. The method for producing a multilayer electrode for a secondary battery according to claim 1 , wherein each of the two or more electrode slurries has a solid content of more than 50 wt % and not more than 80 wt %.

10. A multilayer electrode for a secondary battery, comprising: A current collector; a first electrode layer formed on the current collector and containing a first electrode active material and a first binder; a second electrode layer formed on the first electrode layer and containing a second electrode active material and a second binder; a content (wt%) of the first binder relative to the total weight of the first electrode layer is higher than a content (wt%) of the second binder relative to the total weight of the second electrode layer; the adhesive strength of the first electrode layer to the current collector is 0.2 N / cm or more; The multilayer electrode for a secondary battery, wherein the first electrode layer and the second electrode layer satisfy the following relational expression 4: [Relationship 4] 0.5<R e / R s <1 In the above relational expression 4, R s is the difference (B 1s - B 2s ) between the binder content (B 1s ) (wt %) relative to the solid content in the first electrode slurry applied on the current collector to form the first electrode layer and the binder content (B 2s ) (wt %) relative to the solid content in the second electrode slurry applied on the first electrode slurry to form the second electrode layer, and R e is the difference (B 1e - B 2e ) between the binder content (B 1e ) (wt %) of the first electrode layer in the multilayer electrode and the binder content (B 2e ) (wt %) of the second electrode layer.

11. 11. The multilayer electrode for a secondary battery according to claim 10, wherein a difference between a maximum loading value and a minimum loading value of the electrode layer at at least five positions spaced apart from each other in a longitudinal direction of the multilayer electrode is 10% or less of an average loading value of the entire multilayer electrode.

12. A multilayer electrode according to claim 10; A separator; and an electrolyte.

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