Electrode and electrochemical device including the same

Abstract

To provide an electrode having a multilayer structure formed by stacking a plurality of unit electrode active material layers.SOLUTION: An electrode according to the present invention is formed from granules comprising an electrode active material densely packed in each unit electrode active material layer, and even though the electrode active materials used in the unit electrode active material layers are different from each other, intermixing of the electrode active materials does not occur in the vicinity of an interface of the unit electrode active material layers, and thus, interference of electrochemical characteristics between different types of electrode active materials is small. In addition, porosity of the unit electrode active material layer disposed closer to the vicinity of the electrode surface is higher than that of the unit layer disposed closer to the vicinity of a current collector. Thus, wettability and ionic conductivity of an electrolyte of the electrode can be improved.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to an electrode manufactured by a dry method in which a plurality of granules containing an electrode active material and an electrode binder are compressed together, and an electrochemical device including the electrode.

[0002] This application claims priority based on Korean Patent Application No. 10-2022-0013757 filed on January 28, 2022, and Korean Patent Application No. 10-2022-0031451 filed on March 14, 2022, and the contents disclosed in the specifications and drawings of those applications are incorporated herein in their entirety. [Background technology]

[0003] With the rapid increase in fossil fuel use, there is an increasing demand for alternative and clean energy sources, and one of the most active research fields is the field of electrochemical power generation and storage. Currently, secondary batteries are a representative example of electrochemical devices that use electrochemical energy, and their range of use is expanding. Lithium secondary batteries, a representative example of such secondary batteries, are not only used as an energy source for mobile devices, but are also increasingly being used as a power source for electric vehicles and hybrid electric vehicles that can replace fossil fuel-powered vehicles such as gasoline and diesel vehicles, which are one of the major causes of air pollution. Their range of use is also expanding as an auxiliary power source for grid-connected vehicles.

[0004] Recently, various researches have been conducted on the development of batteries that use multiple electrode active materials to improve battery performance. When multiple electrode active materials are mixed to form a single electrode active material layer, the interference effect between the active materials can actually lead to a decrease in battery performance. Therefore, a method has been considered in which the electrode active material layer is constructed in multiple layers during battery manufacturing, and each layer has a specific function, thereby improving electrode characteristics and battery performance.

[0005] However, conventional electrode fabrication methods that use a slurry coating method for forming electrode active material layers have problems that make it difficult to achieve the improved effects of fabricating multilayer electrodes. The process of fabricating multilayer electrodes using the slurry coating method typically involves a wet-on-wet method, in which an upper slurry is applied to the surface of a lower slurry before the lower slurry dries. In this process, the fluidity of the slurry causes the upper and lower slurries to mix at the interlayer interface, making it difficult to completely prevent interference between the active materials. After the electrode slurry dries, a pressure process is performed to achieve the pre-designed target dimensions, such as the electrode thickness and porosity. When fabricating electrodes using the wet-on-wet slurry coating method, pressure is applied through the electrode surface. This can result in excessive pressure concentration near the electrode surface compared to other areas, resulting in excessive porosity near the electrode surface. This reduced porosity at the electrode surface can lead to reduced electrolyte wettability and ionic conductivity.

[0006] Therefore, in order to solve the above-mentioned problems, there is a demand for a multilayer electrode in which unit electrode active material layers are stacked, in which there is little mixing of the upper and lower electrode active materials at the interlayer interface, and in which the porosity of the upper part of the electrode is appropriately maintained in order to improve the inflow of electrolyte and ionic conductivity. Summary of the Invention [Problem to be solved by the invention]

[0007] An object of the present invention is to provide a multilayer electrode for an electrochemical device in which the interlayer electrode active material is less mixed in.

[0008] Another object of the present invention is to provide a multi-layer electrode having a high porosity in the upper part of the electrode.

[0009] Another object of the present invention is to provide a dry electrode containing granular particles as a multilayer electrode with little interlayer electrode active material contamination and high porosity in the upper part of the electrode.

[0010] The objects and advantages of the present invention can be realized by the means and combinations thereof set forth in the claims. [Means for solving the problem]

[0011] According to a first aspect of the present invention, there is provided an electrode for an electrochemical device, comprising a current collector and an electrode active material layer disposed on at least one surface of the current collector, The electrode active material layer includes a unit electrode active material layer in which a plurality of granules, each including an electrode active material and an electrode binder, are accumulated by compression bonding, and a first unit electrode active material layer to an n-th unit electrode active material layer are sequentially stacked on a surface of the current collector, where n is an integer of 2 or more; Among the unit electrode active material layers included in the electrode active material layer, the nth unit electrode active material layer disposed at the top has the highest porosity.

[0012] According to a second aspect of the present invention, in the first aspect, n is 3 or more and 10 or less.

[0013] According to a third aspect of the present invention, in the first or second aspect, the porosity of the nth unit electrode active material layer, which is the uppermost layer of the electrode active material layers, is 40 vol % or less.

[0014] According to a fourth aspect of the present invention, in any one of the first to third aspects, the first unit electrode active material layer has a porosity of 20 vol % or more.

[0015] According to a fifth aspect of the present invention, in any one of the first to fourth aspects, the porosity of each unit electrode active material layer increases with increasing distance from the surface of the current collector.

[0016] According to a sixth aspect of the present invention, in the first to fifth aspects, each unit electrode active material layer contains a plurality of types of electrode active materials, and the combination of components (types) of the plurality of electrode active materials in each unit electrode active material layer is the same, but the content ratio between each component in each unit electrode active material layer is different.

[0017] According to a seventh aspect of the present invention, in any one of the first to sixth aspects, the granules contained in each unit electrode active material layer contain multiple types of electrode active materials, and the combination of components (types) of the multiple electrode active materials in the granules of each unit electrode active material layer is the same, but the content ratio of the components in each granule of each unit electrode active material layer is different.

[0018] According to an eighth aspect of the present invention, in any one of the first to seventh aspects, in each unit electrode active material layer, the difference between the binder content (wt%) at the upper 100 wt% and the binder content (wt%) at the lower 100 wt% is 10 wt% or less, based on half the height of the unit electrode active material layer.

[0019] According to a ninth aspect of the present invention, in any one of the first to eighth aspects, n is 3 or more, and the difference in porosity between the first unit electrode active material layer at the bottom and the nth unit electrode active material layer at the top is 5 vol% to 20 vol%.

[0020] According to a tenth aspect of the present invention, in any one of the first to ninth aspects, the electrode is a negative electrode, and the electrode active material includes at least one selected from the group consisting of natural graphite, artificial graphite, and lithium (Li), silicon (Si), tin (Sn), a lithium alloy, a silicon alloy, a tin alloy, a silicon oxide, a tin oxide, a lithium-containing compound, a silicon-containing compound, and a tin-containing compound.

[0021] According to an eleventh aspect of the present invention, in any one of the first to tenth aspects, the granules contain an electrode conductive material.

[0022] According to a twelfth aspect of the present invention, in any one of the first to eleventh aspects, a primer layer is formed on at least one surface of the current collector, covering all or at least a part of the surface, and the primer layer includes a conductive material and a binder.

[0023] According to a thirteenth aspect of the present invention, there is provided a method for manufacturing an electrode according to any one of the first to twelfth aspects, the method comprising the steps of: coating an electrode powder on one surface of a current collector and pressing the coated powder to form a first unit electrode active material layer having a layered structure with a predetermined thickness; and then repeating the steps of coating an additional electrode powder on a surface of each unit electrode active material layer formed in the previous step and pressing the additional electrode powder to form a unit electrode active material layer (n-1 times) to manufacture an electrode including n unit electrode active material layers, wherein the electrode has the structure according to the first aspect, and n is an integer of 2 or more. [Effects of the Invention]

[0024] The electrode according to the present invention relates to a multilayer electrode formed by stacking a plurality of unit electrode active material layers. Each unit electrode active material layer is formed by accumulating granules containing an electrode material. Even if different electrode active materials are used in each unit layer, there is no contamination of the electrode active materials near the interface of the unit layers, resulting in minimal interference in electrochemical properties between the different electrode active materials. Furthermore, the electrode active material layers have higher porosity in the unit layers disposed closer to the electrode surface than in the unit layers disposed closer to the current collector. This improves the electrode's electrolyte wettability and ionic conductivity. Furthermore, the electrode according to the present invention does not require pressure adjustment during the calendering process for forming each unit electrode active material layer. Even when the same pressure is maintained, the electrode may have a porosity gradient between each unit electrode active material layer depending on the properties of the materials contained in each layer.

[0025] The following drawings attached to this specification illustrate preferred embodiments of the present invention and, together with the detailed description of the invention, serve to further understand the technical concept of the present invention, so the present invention should not be interpreted as being limited to the matters depicted in the drawings. [Brief explanation of the drawings]

[0026] [Figure 1] 1 is a schematic cross-sectional view of an electrode according to one embodiment of the present invention; [Figure 2]1 is a Nyquist plot of the batteries of each example and comparative example. [Figure 3] 1 is a diagram illustrating the surface and center portions of a granule 100 according to one embodiment of the present invention. [Figure 4] 1 is an SEM image of the first granules obtained by the above production. DETAILED DESCRIPTION OF THE INVENTION

[0027] The present invention will be described in detail below. However, the present invention is not limited to the following content, and each component may be modified in various ways or selectively mixed as needed. Therefore, it should be understood that all modifications, equivalents, and alternatives within the spirit and technical scope of the present invention are incorporated.

[0028] Throughout this specification, when a part "comprises" a certain element, it does not mean that it excludes other elements, but that it may further include other elements, unless otherwise specified.

[0029] The terms "about," "substantially," and the like used throughout this specification are used to mean a numerical value or a approximation thereof when inherent manufacturing and material tolerances are present in the stated meaning, and are used to prevent unscrupulous infringers from unfairly using disclosures in which precise or absolute numerical values ​​are recited to aid in the understanding of this application.

[0030] Throughout this specification, the phrase "A and / or B" means "A or B, or all of them."

[0031] Certain terminology used herein is for convenience only and is not limiting. Terms such as "top," "bottom," "left," "right," "front," "rear," "inside," and "outside" are used to describe relative positions and orientations between components, rather than absolute positions, or may indicate positions and orientations in the drawings to which reference is made. The above terms also include other words containing these terms, derivatives thereof, and words of similar meaning.

[0032] As used herein, the "glass transition temperature Tg" is measured by conventional methods known in the art, and may be measured, for example, by differential scanning calorimetry (DSC).

[0033] As used herein, the term "porosity" refers to the ratio of the volume occupied by pores to the total volume of a structure, and is expressed in units of %. It may be used interchangeably with terms such as void ratio and porosity. In the present invention, the method for measuring porosity is not particularly limited. In one example, porosity can be measured by, for example, the Brunauer-Emmett-Teller (BET) method using nitrogen gas or mercury penetrant (Hg porosimeter) and ASTM D-2873. Alternatively, the true density of the object to be measured can be calculated from the density (apparent density) of the object to be measured, the composition ratio of the materials contained in the object, and the density of each component, and the porosity can be calculated from the difference between the apparent density and the true density (net density).

[0034] The term "average particle size D50" used in the specification refers to the particle size at the 50% point of the cumulative particle number distribution by particle size, and the particle size may be measured using a laser diffraction method. Specifically, the powder to be measured is dispersed in a dispersion medium and then introduced into a commercially available laser diffraction particle size analyzer (e.g., Microtrac S3500). The particle size distribution is calculated by measuring the difference in diffraction pattern depending on particle size as the particles pass through a laser beam. The D50 particle size can be measured by calculating the particle diameter at the 50% point of the cumulative particle number distribution by particle size measured by the analyzer.

[0035] In this specification, the "thickness" of each layer included in an electrode may refer to a value measured by a known method for measuring thickness. The method for measuring thickness is not limited thereto, but may be, for example, a value measured using a thickness meter (Mitutoyo, VL-50S-B).

[0036] The "specific surface area" used herein may be a value measured by a known method for measuring specific surface area. The method for measuring specific surface area is not limited thereto, but may be, for example, a flow method or a stationary method.

[0037] The present invention relates to an electrode for an electrochemical device. The electrode may be a positive electrode or a negative electrode depending on the polarity of the electrode. The electrochemical device of the present invention includes all devices that perform electrochemical reactions, specifically, for example, all types of primary batteries, secondary batteries, fuel cells, solar cells, or capacitors such as supercapacitors. In the present invention, the electrochemical device is preferably a secondary battery, more preferably a lithium ion secondary battery. In a specific embodiment of the present invention, the electrochemical device may include the positive electrode, the negative electrode, or both of the positive electrode and the negative electrode according to the present invention.

[0038] An electrode according to one aspect of the present invention includes a current collector and an electrode active material layer disposed on at least one surface of the current collector.

[0039] The electrode active material layer is formed by introducing granules containing an electrode active material and an electrode binder. In a specific embodiment of the present invention, the electrode active material layer includes two or more unit electrode active material layers (hereinafter also referred to as unit layers) formed by compressing the plurality of granules.

[0040] In one embodiment of the present invention, the electrode active material layer may further contain free electrode active material particles, binder particles, conductive particles, etc., which may be derived from granules or not involved in the formation of granules. In a specific embodiment of the present invention, the electrode active material layer may preferably contain 80 wt% or more, 90 wt% or more, 95 wt% or more, or 99 wt% or more of the granules relative to 100 wt% of the electrode active material layer. Furthermore, each unit layer may preferably contain 80 wt% or more, 90 wt% or more, 95 wt% or more, or 99 wt% or more of the granules relative to 100 wt% of the unit layer.

[0041] The electrode has a first unit electrode active material layer to an nth unit electrode active material layer sequentially stacked on the surface of the current collector, where n is an integer of 2 or more, and the nth unit electrode active material layer disposed at the top of the unit electrode active material layers has the highest porosity.

[0042] In the present invention, the electrode active material layer has a multilayer structure in which n unit layers are sequentially stacked from the surface of the current collector, where n is an integer of 2 or more, and the unit layer disposed on the surface of the current collector among the n unit layers is referred to as the first unit layer, and the unit layer disposed farthest from the current collector is referred to as the nth unit layer.

[0043] In the present invention, the electrode active material layer has the highest porosity in the uppermost layer (nth unit layer) of the unit layers disposed at the top of the electrode. The uppermost layer refers to the unit layer disposed farthest from the current collector and facing the separator during electrode fabrication.

[0044] The electrode active material layer may have two or more, three or more, or five or more unit layers. The number of unit layers is not particularly limited, but may be 20 or less, 10 or less, 7 or less, or 5 or less.

[0045] Meanwhile, in one embodiment of the present invention, the thickness of each unit layer in the electrode active material layer may be in the range of 20% to 80% of the total thickness of the active material layer (100%).

[0046] In the electrode active material layer, the uppermost layer may have a thickness in the range of 20 to 80%.

[0047] In the electrode active material layer, the bottom layer may have a thickness in the range of 80 to 20%.

[0048] The electrode active material layer has pores formed by interstitial volumes, which are spaces between the granules, and may exhibit porous characteristics due to this structure. In one embodiment of the present invention, the electrode active material layer preferably has a porosity of 20 vol% to 40 vol%, taking into consideration electrolyte impregnation, morphological stability, ionic conductivity, and the like.

[0049] Meanwhile, in the present invention, the difference in porosity between unit layers in surface contact is 1 vol% or more, preferably 2 vol% or more. Meanwhile, the difference in porosity between unit layers is preferably 20 vol% or less. Specific examples include 10 vol% or less, or 5 vol% or less. In one embodiment of the present invention, the electrode active material layer preferably includes three or more unit layers so that the difference in interlayer porosity is not too steep.

[0050] In one embodiment of the present invention, the porosity of each unit layer may increase gradually with increasing distance from the surface of the current collector, with the uppermost layer having the highest porosity. This increase in porosity toward the uppermost layer may facilitate the movement of lithium ions and allow a uniform electrochemical reaction to occur in the thickness direction of the electrode active material layer.

[0051] Meanwhile, in one specific embodiment of the present invention, the porosity of the nth unit layer (top layer) may be 50 vol% or less, 40 vol% or less, 35 vol% or less, or 30 vol% or less. In one embodiment, the porosity of the nth unit layer may be 30 vol% to 40 vol%, or 33 vol% to 37 vol%. Also, the porosity of the first unit layer may be 20 vol% or more. If the porosity of the top layer is too high or the porosity of the first unit layer is too low, it is difficult to control the porosity of the entire electrode active material layer within an appropriate range. Therefore, it is preferable that the porosities of the top layer and the first unit layer be within the above ranges.

[0052] In one embodiment of the present invention, the electrode active material layer may include three or more unit layers, and in this case, the difference in porosity between the first unit layer and the uppermost unit layer is 5 vol% to 20 vol%, preferably 8 vol% to 12 vol%, more preferably 8 vol% to 12 vol%.

[0053] In one embodiment of the present invention, each unit layer has a uniform binder distribution in the thickness direction. In a specific embodiment, the difference between the binder content (wt%) in the upper 100 wt% and the binder content (wt%) in the lower 100 wt% of each unit layer may be 10 wt% or less, based on half the height of the unit layer. This binder distribution may be due to the fact that the electrode active material layer according to the present invention is manufactured by pressing granules. As described below, the electrode manufacturing method of the present invention is characterized by manufacturing granule particles containing an electrode material, applying the granule particles to a current collector, etc., and then applying pressure to accumulate the granules in a layered structure. In the case of an electrode manufacturing method involving slurry manufacturing, the evaporation of the solvent during drying of the slurry induces binder migration, resulting in the binder being distributed in a concentrated manner on the surface of the electrode. However, in the present invention, since a method of compressing and accumulating dry granules without using a solvent is applied, the binder migration phenomenon does not occur, and therefore the binder is uniformly distributed in the thickness direction in each unit layer.

[0054] In the present invention, the granules may be in the form of composite particles containing an electrode active material, an electrode binder, and optional components added as needed. Specifically, the granules may be secondary particles formed by granulating two or more electrode active material particles bound together by the electrode binder. In the present invention, the electrode active material may refer to an aggregate of particles. Here, the diameter of each particle of the electrode active material may be 0.05 μm to 2 μm.

[0055] In one specific embodiment of the present invention, the electrode active material may be 80 wt% or more or 90 wt% or more, and the electrode binder may be contained in an amount of 20 wt% or less or 10 wt% or less, based on 100 wt% of the total weight of the granules. In one embodiment of the present invention, the granules may further contain an electrode conductive material as an optional component, if necessary. The electrode conductive material may be contained in an amount ranging from 0.1 wt% to 20 wt%, preferably from 0.1 wt% to 10 wt%, based on 100 wt% of the granules. For example, the conductive material may be contained in an amount ranging from approximately 0.1 wt% to 5 wt% of the granules.

[0056] According to one embodiment of the present invention, the content of the electrode active material in the granules may be 85 wt% to 98 wt%. Within this range, the content of the electrode binder may be 0.5 wt% to 10 wt%, and the content of the electrode conductive material may be 0.5 wt% to 5 wt%. According to yet another embodiment, the content of the electrode active material may be 90 wt% to 98 wt%, the content of the electrode binder may be 0.5 wt% to 5 wt%, and the content of the electrode conductive material may be 0.5 wt% to 5 wt%.

[0057] Meanwhile, in one embodiment of the present invention, the electrode active material layer may contain the same or different electrode materials in each unit layer. For example, each unit layer of the electrode active material layer may contain different electrode active materials, or at least two unit layers that are in surface contact with each other may contain different electrode active materials. In one embodiment of the present invention, when the electrode active material layer is composed of a total of three unit layers, the first unit layer and the second unit layer may each contain different electrode active materials. Additionally or independently, the second unit layer and the third unit layer may each contain different electrode active materials.

[0058] When different types of electrode active materials are used in two opposing unit layers, interference between the electrochemical reactions of the electrode active materials does not occur at the interface between the unit layers, thereby further maximizing the effects of the multilayer electrode.

[0059] In yet another embodiment, each unit layer includes multiple types of electrode active materials, and the combination of components (types) of the multiple electrode active materials in each unit layer is the same, but the content ratio between the components in each unit layer may be different.

[0060] More preferably, the granules contained in each unit layer contain multiple types of electrode active materials, and the granules of each unit layer have the same combination of components (types) of the multiple electrode active materials, but the content ratio of each component in the granules of each unit layer may be different.

[0061] In a specific embodiment, when the electrode is a negative electrode, the electrode active material layer may include artificial graphite and / or natural graphite as the negative electrode active material. In this embodiment, the content of artificial graphite and natural graphite in the granules may vary from unit layer to unit layer. Specifically, the content of artificial graphite in the granules may increase from the first unit layer to the nth unit layer. Independently or in addition, the amount of natural graphite in the granules may decrease from the first unit layer to the topmost nth unit layer. In this way, the content of artificial graphite in the granules increases and the content of natural graphite decreases toward the top layer, and simultaneously or independently, the content of artificial graphite in the granules decreases and the content of natural graphite increases toward the bottom layer. Even if each layer is pressed at the same or similar pressure, the porosity of each unit layer will vary due to the difference in the inherent compression of each granule, and in particular, an electrode active material layer having the highest porosity in the top layer will be obtained.

[0062] In one specific embodiment of the present invention, a first unit layer, which is the lowest layer of the electrode active material layer, includes a plurality of first granules, and the first granules contain an electrode active material including natural graphite and artificial graphite, and the content of natural graphite in the first granules may be 50 wt% or more, or 70 wt% or more, relative to 100 wt% of the electrode active material. Preferably, the content of the first granules in the first unit layer may be 90 wt% or more, more preferably 99 wt% or more, relative to 100 wt% of the unit layer.

[0063] In one embodiment of the present invention, an nth unit layer, which is an uppermost layer of the electrode active material layer, includes a plurality of nth granules, and the nth granules may contain only artificial graphite as an electrode active material, or may contain both natural and artificial graphite, and the nth granules may contain 80 wt% or more, preferably 90 wt% or more, or 99 wt% or more, of artificial graphite relative to 100 wt% of the electrode active material. Preferably, the nth unit layer may contain 90 wt% or more, preferably 99 wt% or more, of nth granules relative to 100 wt% of the unit layer.

[0064] In the second layer, third layer, ... n-2 layer, n-1 layer disposed between the first layer and the nth layer, the content of artificial graphite in the granules may increase toward the nth layer, and the content of natural graphite in the granules may increase toward the first layer.

[0065] In a specific embodiment of the present invention, the electrode active material layer may have a structure in which a first unit layer, a second unit layer, and a third unit layer are stacked in order.

[0066] In one specific embodiment of the present invention, a first unit layer, which is the lowest layer of the electrode active material layer, includes a plurality of first granules, and the first granules contain an electrode active material made of natural graphite and artificial graphite, and the content of natural graphite (Nc1) in the first granules may be 50 wt% or more or 70 wt% or more relative to 100 wt% of the electrode active material. Preferably, the content of the first granules in the first unit layer may be 90 wt% or more, more preferably 99 wt% or more, relative to 100 wt% of the unit layer.

[0067] The second unit layer includes a plurality of second granules, and the second granules include an electrode active material made of natural graphite and artificial graphite. The content of natural graphite (Nc2) in the second granules relative to 100 wt% of the electrode active material may be adjusted to a range of 30 wt% to 70 wt%, preferably 30 wt% to 50 wt%, and is smaller than Nc1. Preferably, the content of the second granules in the second unit layer may be 90 wt% or more, more preferably 99 wt% or more, relative to 100 wt% of the unit layer.

[0068] And the third unit layer includes a plurality of third granules. The third granules contain only artificial graphite as the electrode active material or consist of natural graphite and artificial graphite. In the third granules, the content of artificial graphite (Ac3) can be more than 70 wt%, 80 wt% or more, preferably 90 wt% or more, or 99 wt% or more with respect to 100 wt% of the electrode active material. Preferably, the content of the third granules in the unit layer can be 90 wt% or more, preferably 99 wt% or more in 100 wt% of the unit layer. Also, the content of artificial graphite (Ac1) in the first granules with respect to 100 wt% of the electrode active material, the content of artificial graphite (Ac2) in the second granules with respect to 100 wt% of the electrode active material, and the content of artificial graphite (Ac3) in the third granules with respect to 100 wt% of the electrode active material are preferably such that Ac1 < Ac2 < Ac3.

[0069] FIG. 1 is according to an embodiment of the present invention and schematically shows a cross-section of an electrode including three unit layers. In the electrode 100, an electrode active material layer 110 is disposed on the surface of a current collector 120, and the electrode active material layer has the first unit layer 111, the second unit layer 112, and the third unit layer 113 sequentially laminated from the surface of the current collector. Here, the electrode active material layer has a porosity in the range of 20 vol% to 40 vol%. The porosity of the first unit layer is 20 vol% or more, the porosity of the third unit layer is 40 vol%, and the difference between the porosity of the first unit layer and the porosity of the third unit layer can be 5 vol% to 15 vol%. On the other hand, the difference in porosity between the first and second unit layers and the difference in porosity between the second and third unit layers can each independently be 1 vol% or more.

[0070] Also, in each of the first to third unit layers, based on half of the height of the unit electrode active material layer, the difference between the content (wt%) of the binder in the upper 100 wt% and the content (wt%) of the binder in the lower 100 wt% can be 10 wt% or less.

[0071] In the present invention, the amount of electrode binder contained in the surface portion 102 of the granule may be greater than the amount of electrode binder contained in the center portion 101 of the granule 100. The amount of binder may refer to weight or volume. Figure 3 is a schematic diagram illustrating the surface portion and center portion of a granule 100 according to one embodiment of the present invention.

[0072] In addition, or independently, the granules may contain an electrode binder in the center of the granules with a content (wt%) of B relative to the total weight of the granules (100 wt%). c / G t The content (wt%) of the electrode binder contained in the surface area of ​​the granules is B s / G t Here, the B c is the weight of the binder contained in the core, and B s is the weight of the binder contained in the surface area, and G t means the total weight of the granule particles.

[0073] In addition, or independently, the granules may have a content (vol%) B of electrode binder contained in the center of the granule relative to 100 vol% of the total volume of the granule. c / G t The content of the electrode binder in the surface area of ​​the granules is vol%B. s / G t Here, the B c is the volume of the binder contained in the center, and B s is the volume of binder contained in the surface area, and G t means the total volume of the granule particles.

[0074] Here, the surface portion may be a region near the surface of the granule from the surface of the granule to a predetermined depth toward the center of the granule. The central portion refers to a portion other than the surface portion. In one embodiment of the present invention, the surface portion may more specifically refer to a surface region extending from the surface of the granule toward the inside, specifically, up to 30% of the radius toward the center of the granule. In one embodiment of the present invention, the surface portion may refer to a surface region extending from the surface of the granule to 30%, up to 20%, up to 15%, up to 10%, or up to 5% of the radius. Preferably, the surface portion may refer to a surface region extending from the surface of the granule to 20% of the radius.

[0075] Meanwhile, in one embodiment of the present invention, the surface portion may refer to a region extending from the center of the granule to the outer periphery of the granule, covering 70% or more of the radius of the granule, and may refer to a region extending to 80% or more, 85% or more, 90% or more, or 95% or more of the radius of the granule.

[0076] In one embodiment of the present invention, the center of the granule may refer to a point halfway along the longest diameter of the granule. In the present invention, the radius may refer to the distance from the center of the granule to each point on the granule surface. In one embodiment of the present invention, the surface portion and the center portion may be distinguished based on points that are the same distance from each surface of the granule along each radius, specifically, points up to 30%, 20%, 10%, 5%, or 1% of the radius from the surface.

[0077] In one embodiment of the present invention, in a region from the center of the granule to the surface of the granule at a radius of 90% or more, the content of the electrode binder relative to the total weight of the granule in that region (100 wt%) may be 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, or 90 wt% or more.

[0078] In yet another embodiment of the present invention, in a region from the center of the granule to the surface of the granule at a radius of 95% or more, the content of the electrode binder relative to 100 wt% of the total weight of the granule in that region may be 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, or 90 wt% or more.

[0079] In yet another embodiment of the present invention, in a region from the center of the granule to a radius of 99% or more of the granule surface, the content of the electrode binder relative to 100 wt% of the total weight of the granule in that region may be 50 wt% or more, 60 wt% or more, 70 wt% or more, 80 wt% or more, or 90 wt% or more.

[0080] In one embodiment of the present invention, in a region from the center of the granule to the surface of the granule at a radius of 90% or more, the content of the electrode binder relative to the total volume of the granule in that region (100 vol%) may be 50 vol% or more, 60 vol% or more, 70 vol% or more, 80 vol% or more, or 90 vol% or more.

[0081] In yet another embodiment of the present invention, in a region from the center of the granule to the surface of the granule at a radius of 95% or more, the content of the electrode binder relative to 100 vol% of the total volume of the granule in that region may be 50 vol% or more, 60 vol% or more, 70 vol% or more, 80 vol% or more, or 90 vol% or more.

[0082] In yet another embodiment of the present invention, in a region from the center of the granule to the surface of the granule at a radius of 99% or more, the content of the electrode binder relative to the total volume of the granule in that region (100 vol%) may be 50 vol% or more, 60 vol% or more, 70 vol% or more, 80 vol% or more, or 90 vol% or more.

[0083] To describe the granule in more detail, the granule may have a center portion including a plurality of electrode active materials, and a surface portion including an electrode binder located on all or part of the outside of the center and binding the electrode active materials. That is, in the center of the granule, the plurality of electrode active materials may be in surface contact, line contact, point contact, or contact with one another through two or more of these to form aggregates, and in the surface portion of the granule, an electrode binder may be located on all or part of the outside of such aggregates to fix and bind the plurality of electrode active materials in the center of the granule.

[0084] According to an embodiment of the present invention, a small amount of electrode binder may also be included in the center portion to connect and fix the plurality of electrode active materials in the center portion. However, as described above, it is preferable that the content of the electrode binder is higher in the surface portion than in the center portion.

[0085] Meanwhile, in one embodiment of the present invention, the granules may have an aspect ratio of 0.5 to 1.0, preferably 0.75 to 1.0. The aspect ratio may refer to the ratio of the length of the minor axis to the length of the major axis of the granule. In yet another embodiment of the present invention, the average aspect ratio of the granules may be 0.5 to 1.0, preferably 0.75 to 1.0. In this case, the average aspect ratio may refer to the ratio of the average length of the shortened portions of the granule particles to the average length of the major axis. In this case, the average length of the shortened portions refers to the average length of the granules in the axial direction having the shortest length, and the average length of the major axis refers to the average length of the granules in the axial direction having the longest length. When the aspect ratio of the granules satisfies this range, it is advantageous in terms of having sufficient fluidity suitable for processing.

[0086] Meanwhile, in one embodiment of the present invention, the particle size of the granules may be in the range of 0.1 to 1,000 μm based on the longest diameter of the particle. In yet another embodiment of the present invention, the average particle size D50 of the granules may be in the range of 0.1 to 1,000 μm.

[0087] In one embodiment of the present invention, when the electrode is a negative electrode, the electrode active material, as a negative electrode active material, is, for example, a carbon material such as graphitizable carbon and / or non-graphitizable carbon; graphite-based carbon such as natural graphite and / or artificial graphite; Li x Fe2O3(0≦x≦1), Li x WO2(0≦x≦1), Sn x Me 1-x Me’ y O z (Me: Mn, Fe, Pb, Ge; Me’: Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, halogen; 0 < x ≦ 1; 1 ≦ y ≦ 3; 1 ≦ z ≦ 8) and other metal composite oxides; lithium metal; lithium alloy; silicon; silicon-based alloy; tin; tin-based alloy; silicon-based oxides such as SiO, SiO / C, SiO2; metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4 and Bi2O5; conductive polymers such as polyacetylene; Li-Co-Ni-based materials, etc. can be used. However, it is not limited to only these.

[0088] On the other hand, in one embodiment of the present invention, when the electrode is a positive electrode, the electrode active material, as a positive electrode active material, is a layered compound such as lithium manganese composite oxide (LiMn2O4, LiMnO2, etc.), lithium cobalt oxide (LiCoO2), lithium nickel oxide (LiNiO2), and a compound substituted with one or more transition metals; chemical formula Li 1+x Mn 2-x O4(where x is 0 to 0.33), lithium manganese oxides such as LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, LiV3O4, V2O5, Cu2V2O7; chemical formula LiNi 1-x M x O2(where M = Co, Mn, Al, Cu, Fe, Mg, B or Ga, and x = 0.01 to 0.3), Ni-site type lithium nickel oxide represented by; chemical formula LiMn 2-x M xLithium manganese composite oxides represented by Li2Mn3MO8 (where M = Co, Ni, Fe, Cr, Zn or Ta, and x = 0.01 to 0.1) or Li2Mn3MO8 (where M = Fe, Co, Ni, Cu or Zn); Li in which part of the Li in the chemical formula is replaced by aluminum ions. 1+x (Ni a Co b MncAl d ) 1-x O2 (x=0 to 0.03, a=0.3 to 0.95, b=0.01 to 0.35, c=0.01 to 0.5, d=0.001 to 0.03, a+b+c+d=1); disulfide compounds; and Fe2(MoO4)3, or a mixture of two or more thereof.

[0089] The electrode binder is not limited to a specific component as long as it has adhesiveness, is stable in electrochemical reactions, can bind the electrode active material and electrode materials such as the electrode conductive material to maintain a granular form, and can maintain a stable form by accumulating the granules in a layered structure by binding them together through compression.Non-limiting examples of such electrode binders include styrene butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), styrene butadiene styrene block polymer (SBS), styrene ethylene butadiene block polymer (SEB), styrene-(styrene butadiene)-styrene block polymer, natural rubber (NR), isoprene rubber (IR), ethylene-propylene diene terpolymer (EPDM), poly(ethylene-co-propylene-co-5-methylene-2-norbornene), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinyl chloride, polyvinylidene fluoride-hexafluoropropylene (PVdF), and polyvinylidene fluoride-hexafluoropropylene (PVdF). The polymer may include polyvinyl fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexylacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene, polypropylene, ethylene-co-vinyl acetate, polyethylene oxide, polypropylene oxide, polyarylate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, or two or more thereof.Specifically, the binder may be styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), polymethylmethacrylate, polyethylhexylacrylate, or polybutylacrylate. In one embodiment, the electrode binder may include one or more selected from the above.

[0090] The electrode conductive material is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. Non-limiting examples include graphite such as natural graphite and artificial graphite; carbon black-based carbon compounds such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives. In a specific embodiment, the electrode conductive material may include one or more selected from these.

[0091] The current collector is not particularly limited as long as it does not induce chemical changes in the battery and has high conductivity. Examples of the current collector include stainless steel, aluminum, nickel, titanium, calcined carbon, copper, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, etc. The current collector may also have fine irregularities on its surface to enhance adhesion of the active material, and may be in various forms, such as a film, sheet, foil, net, porous material, foam, or nonwoven fabric. In one embodiment of the present invention, the current collector may have a thickness of 10 μm to 50 μm, but is not particularly limited thereto. The current collector may have a thickness of, for example, 10 μm to 20 μm.

[0092] Meanwhile, in the present invention, a primer layer may be formed on at least one surface of the current collector, covering all or at least a portion of the surface. The primer layer may be introduced to improve the binding strength between the current collector and the electrode active material layer and electrical conductivity. The primer layer may include a second conductive material and a second binder resin. The second conductive material may be the same as the electrode conductive material described above. The second binder resin may be the same as the electrode binder described above.

[0093] Meanwhile, in yet another embodiment of the present invention, the primer layer may have the following structural features.

[0094] The primer layer includes a binder for the primer layer (hereinafter referred to as a second binder) and a conductive material for the primer layer (hereinafter referred to as a second conductive material), and the sum of the contents of the second binder and the second conductive material in the primer layer may be 90 wt% or more.

[0095] An electrode according to one embodiment of the present invention includes an electrode active material layer containing granules, and in this case, a primer layer including a second binder and a second conductive material, the sum of the contents of the second binder and the second conductive material being 90 wt % or more, thereby ensuring stability over time of the primer layer and thereby exhibiting excellent physical properties such as adhesive strength and life characteristics, but the present invention is not limited thereto.

[0096] According to one embodiment of the present invention, the primer layer includes a second binder and a second conductive material, and may further include a dispersant.

[0097] According to another embodiment of the present invention, the primer layer may include a second binder and a second conductive material, but may be substantially free of a dispersant.

[0098] According to an embodiment of the present invention, the second binder may be any known binder used in a primer layer without any particular limitation.

[0099] According to another embodiment of the present invention, the second binder may be a polymer that can ensure the stability of the primer layer over time. Specifically, the second binder may have a glass transition temperature (Tg) of 45°C or less.

[0100] According to another embodiment of the present invention, the second binder may be, for example, styrene butadiene rubber (SBR), butadiene rubber (BR), nitrile butadiene rubber (NBR), styrene butadiene styrene block polymer (SBS), styrene ethylene butadiene block polymer (SEB), styrene-(styrene butadiene)-styrene block polymer, natural rubber (NR), isoprene rubber (IR), ethylene-propylene-diene terpolymer (EPDM), poly(ethylene-co-propylene-co-5-methylene-2-norbornene), polytetrafluoroethylene (PTFE), polyvinylidene fluoride (PVdF), polyvinyl chloride, polyvinylidene fluoride-hexafluoropropylene (polyvinylidene The polymer may include polyvinyl fluoride-co-hexafluoropropylene, polyvinylidene fluoride-co-trichloroethylene, polymethylmethacrylate, polyethylhexylacrylate, polybutylacrylate, polyacrylonitrile, polyvinylpyrrolidone, polyvinylacetate, polyethylene, polypropylene, ethylene-vinyl acetate, polyethylene oxide, polypropylene oxide, polyarylate, cyanoethylpullulan, cyanoethylpolyvinylalcohol, or two or more thereof.Specifically, the binder may include styrene butadiene rubber (SBR), nitrile butadiene rubber (NBR), polymethylmethacrylate, polyethylhexylacrylate, polybutylacrylate, or two or more thereof.

[0101] According to yet another embodiment of the present invention, the second binder has the above-mentioned glass transition temperature value and may be one or a mixture of two or more selected from the above-mentioned types.

[0102] According to yet another embodiment of the present invention, the second binder may be styrene butadiene rubber (SBR) having a glass transition temperature Tg of -40°C to 45°C, nitrile butadiene rubber (NBR) having a glass transition temperature Tg of -40°C to 45°C, or a mixture thereof.

[0103] According to one embodiment of the present invention, the specific surface area of ​​the second conductive material is 30 m 2 / g~1,400m 2 / g and may be spherical. In this case, the size of the primary particles of the spherical conductive material may be, for example, 10 nm to 100 nm, specifically 15 nm to 70 nm, but is not limited thereto.

[0104] According to another embodiment of the present invention, the second conductive material has a specific surface area of ​​10 m 2 / g~400m 2 In this case, the diameter of the cross section of the tubular conductive material in the direction perpendicular to the longitudinal direction may be 0.1 to 3 nm, specifically 0.3 to 1.5 nm, but is not limited thereto.

[0105] The second conductive material is not particularly limited as long as it does not induce chemical changes in the battery and is conductive. For example, graphite such as natural graphite or artificial graphite; carbon black-based carbon compounds such as carbon black, acetylene black, ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fiber and metal fiber; metal powders such as carbon fluoride, aluminum, and nickel powder; conductive whiskers such as zinc oxide and potassium titanate; conductive metal oxides such as titanium oxide; and conductive materials such as polyphenylene derivatives may be used. Specifically, to ensure uniform mixing of the conductive materials and improve conductivity, the second conductive material may include activated carbon, graphite, carbon black, carbon nanotubes, or a mixture of two or more of these materials, and more specifically, activated carbon may be used.

[0106] According to an embodiment of the present invention, the primer layer may have the above-described composition and a thickness of 300 nm to 1.5 μm, specifically, 700 nm to 1.3 μm, but is not limited thereto.

[0107] Next, a method for producing an electrode according to the present invention will be described.

[0108] First, granules containing an electrode material are prepared. The granules can be prepared by mixing an electrode active material and an electrode binder with a solvent to prepare a fluidized slurry, and then spray-drying the slurry. In one embodiment of the present invention, the granules can further include an electrode conductive material, if necessary. Other additives for improving performance can also be included.

[0109] First, the electrode active material, the electrode binder, and optionally additional additives are dispersed or dissolved in a dispersion medium (solvent for the electrode binder) to obtain a slurry in which the electrode active material and the electrode binder are dispersed or dissolved.

[0110] The dispersion medium used to obtain the slurry is most preferably water, but organic solvents may also be used. Examples of organic solvents include alkyl alcohols such as methyl alcohol, ethyl alcohol, and propyl alcohol; alkyl ketones such as acetone and methyl ethyl ketone; ethers such as tetrahydrofuran, dioxane, and diglyme; amides such as diethylformamide, dimethylacetamide, N-methyl-2-pyrrolidone (hereinafter also referred to as NMP), and dimethylimidazolidinone; and sulfur-based solvents such as dimethyl sulfoxide and sulfolane. Alcohols are preferred. The use of an organic solvent with a boiling point lower than that of water can increase the drying rate during fluidized bed granulation. Furthermore, since the dispersibility or solubility of the negative electrode binder can be altered, the viscosity and fluidity of the slurry can be adjusted depending on the amount or type of dispersion medium, thereby improving production efficiency.

[0111] The amount of the dispersion medium used when preparing the slurry may be an amount that results in a solids concentration of the slurry generally in the range of 1 to 50 wt%, or 5 to 50 wt%, or 10 to 30 wt%.

[0112] The method or order for dispersing or dissolving the electrode active material and electrode binder in the dispersion medium is not particularly limited, and examples include a method in which the electrode active material and electrode binder are added to the dispersion medium and mixed, or a method in which the electrode binder is dissolved or dispersed in the dispersion medium and then the electrode active material is added and mixed. If a conductive material or additive is included, these components may be added when the electrode active material is added. Examples of mixing means include mixing equipment such as a ball mill, sand mill, bead mill, pigment disperser, stone mill, ultrasonic disperser, homogenizer, and planetary mixer. Mixing may be performed, for example, at room temperature to 80°C for 10 minutes to several hours.

[0113] Next, the slurry is spray-dried. The spray drying method is a method in which the slurry is sprayed into hot air and dried. Examples of spraying methods used in the spray drying method include a rotating disk method and a nozzle pressurization method. The rotating disk method is a method in which the slurry is introduced into approximately the center of a disk rotating at high speed, and when the slurry is removed from the disk by the centrifugal force of the disk, it is sprayed in a mist and dried. The rotation speed of the disk depends on the size of the disk, but is usually 5,000 to 35,000 rpm, preferably 15,000 to 30,000 rpm. On the other hand, the pressurization method is a method in which the slurry is pressurized and sprayed in a mist from a nozzle and dried.

[0114] The temperature of the sprayed slurry can be room temperature, but it is preferable to heat it to above room temperature in order to form a granular structure with a high electrode binder content on the surface. In one embodiment of the present invention, the hot air temperature (based on the temperature at the reactor inlet) during spray drying is usually 80°C to 250°C, preferably 100°C to 220°C, and more preferably 180°C to 220°C. In the spray drying method, the method of inhaling the hot air is not particularly limited, and examples include a method in which the hot air and the spray flow co-currently, a method in which the sprayed droplets are sprayed from the top of the drying tower and then descend with the hot air, a method in which the sprayed droplets and the hot air come into countercurrent contact, and a method in which the sprayed droplets flow co-currently with the initial hot air and then fall by gravity to come into countercurrent contact. Meanwhile, in one embodiment of the present invention, the reactor outlet temperature (the temperature of the hot air discharged from the reactor) during the spray drying can be controlled to 90°C to 130°C.

[0115] Optionally, the product obtained by spray drying, i.e., the granules, may be heat-treated to harden the surface thereof, and the heat-treatment temperature may be generally 80°C to 300°C.

[0116] Next, the granules are dispersed on the surface of an electrode current collector and pressed to form a layered first unit layer having a predetermined thickness. After the first unit layer is formed, additional granules are applied and pressed onto the top of the first unit layer to form additional unit layers. This application and pressing process is repeated until an electrode active material layer having a predetermined number of stacked unit layers is obtained. The pressure applied during the formation of each unit layer may be controlled, as described above, so that each unit layer has an appropriate porosity. Meanwhile, in one embodiment of the present invention, granules having different components or electrode active material contents (and / or content ratios) may be prepared for each unit layer, and the granules may be selected and introduced according to the characteristics of each unit layer. Each unit layer may be formed as follows.

[0117] As described above, a primer layer including a second conductive material and a second binder may be provided on all or part of at least one surface of the current collector.

[0118] According to one embodiment of the present invention, a plurality of prepared granules may be fed to a roll-type pressure molding device using a feeder such as a screw feeder to form unit layers. When forming the first unit layer, the current collector may be fed to the rolls of the pressure molding device simultaneously with the granules, so that the first unit layer is directly laminated on the current collector. Alternatively, the granules may be sprinkled on the current collector, the granules may be uniformly ground with a blade or the like to adjust the thickness, and then molded in a pressure molding device to form the first unit layer. In processes subsequent to the formation of the first unit layer, each unit layer is formed by sprinkling granules on the surface of the lower unit layer instead of the current collector, the granules may be uniformly ground with a blade or the like to adjust the thickness, and then molded in a pressure molding device.

[0119] In these methods, the temperature during roll pressure molding is typically 0°C to 200°C, preferably higher than the melting point or glass transition temperature of the negative electrode binder, and more preferably 20°C or more higher than the melting point or glass transition temperature. The molding speed during roll pressure molding can typically be 0.1 to 20 m / min, or 1 to 10 m / min. The linear pressure between the rolls can typically be 0.2 to 30 kN / cm, or 0.5 to 10 kN / cm.

[0120] To eliminate thickness variations among the formed unit layers and increase the density of the unit layers and the electrode active material layer to achieve high capacity, further pressurization may be performed as necessary. A common post-pressurization method is a roll pressing process. In the roll pressing process, two cylindrical rolls are set up parallel to each other with a small gap between them and rotated in opposite directions to sandwich and pressurize the electrode between them. The temperature of the rolls may be adjustable by heating or cooling.

[0121] The present invention has been described above with reference to the embodiments and drawings. However, a person having ordinary knowledge in the field to which the present invention pertains will be able to make various applications and modifications within the scope of the present invention based on the above content.

[0122] Example 1 (1) Preparation of current collector with primer layer Carbon black (specific surface area: 30 m 2 / g, particle size: 70 nm), 30 parts by weight, styrene butadiene rubber (SBR) (T g A primer layer slurry was prepared by mixing 69 parts by weight of acrylic resin (-15°C) and 1 part by weight of carboxymethyl cellulose (CMC) as a dispersant with water. The ratio of the conductive material, binder, and dispersant content in the slurry was the same as the ratio of the conductive material, binder, and dispersant content in the primer layer to be formed later. The solid content of the primer layer slurry was 7 wt%.

[0123] The prepared primer layer slurry was applied to one surface of a copper current collector (thickness: 10 μm) and dried at 130° C. to form a primer layer on the surface of the copper current collector.

[0124] (2) Preparation of granules for the negative electrode active material layer First, second and third granules were prepared, and the content ratio of natural graphite to artificial graphite in each granule was as shown in Table 1 below.

[0125] [Table 1]

[0126] Each granule was prepared as follows. The negative electrode active material, carbon black (Super C65), carboxymethyl cellulose (Daicel 2200, aqueous solution, solids concentration 1.5 wt%), and modified styrene-butadiene copolymer (Grade Name AX-B119) were mixed with water as a dispersant in a weight ratio of 95.6:1.0:1.1:2.3 to prepare a slurry with a viscosity of approximately 1,000 cPs using a homogenizer. The solid content of the slurry was 30 wt%. In this weight ratio, the carboxymethyl cellulose was calculated based on the solid content. The prepared slurry was then dried by feeding it into a spray dryer together with hot air at a pressure of -40 mmH2O. The spray dryer conditions were controlled at an inlet temperature of approximately 180°C, an outlet temperature of approximately 90°C, and a rotation speed of approximately 18,000 rpm. The resulting granules were sieved to remove coarse particles larger than 150 μm, and then fine particles smaller than 40 μm were separated. The separated fine particles were mixed with the granules from which only the coarse particles had been removed to prepare the final anode granules, which contained a higher amount of fine particles than the previous granules. The average particle size D50 of each granule was approximately 66.5 μm, and the aspect ratio was approximately 0.96. Figure 4 shows an SEM image of the first granules obtained by this process, with the bright areas indicated by arrows representing the binder resin. The first granules were stained with OsO4 to remove the binder. They were placed in glass containers using OsO4 crystals or an aqueous solution, and the reagents were evaporated to allow the reaction. The mixture was then mixed with epoxy resin, fixed in a silicone mold, and cured to obtain a granule matrix. Next, a cross-sectional sample was prepared from the granular matrix using an ion milling device (Hitachi IM5000, accelerating voltage: 6 kV), and then an SEM image was obtained.

[0127] (3) Manufacturing of the negative electrode The first granules were applied to one side of the current collector having the primer layer by using a thickness adjusting bar. 2 The first layer was formed by roll-to-roll hot rolling. The second granules were then applied to the surface of the first layer at a thickness of 25 cm. 2 The third granule was then evenly applied to the surface of the second layer at a density of 120 mg per layer and hot rolled to form a second layer. 2 The coating was uniformly applied at 120 mg per layer and hot rolled to form the third layer. During the formation of the first, second, and third layers, each roll was performed at approximately 60°C, with a pressure of 0.7 ton per cm and a speed of 2 m per minute.

[0128] Comparative Example 1 As negative electrode active materials, natural graphite with an average sphericity of 0.95 and artificial graphite with an average sphericity of 0.9 were prepared.

[0129] Next, slurries for forming the first, second, and third layers were prepared. Each slurry was prepared by mixing a negative electrode active material, carbon black (Super-C 65), styrene butadiene rubber (SBR), carboxymethyl cellulose (CMC), and water, and the solids content of the slurry was approximately 47 wt%. The negative electrode active material, carbon black, SBR, and CMC were mixed in a weight ratio of approximately 95.6:1.0:2.3:1.1. The artificial graphite to natural graphite content ratio for each layer was prepared as shown in Table 2 below.

[0130] [Table 2]

[0131] Next, the first layer slurry, the second layer slurry, and the third layer slurry were sequentially applied to the current collector prepared in Example 1 using a slot die. The first layer slurry, the second layer slurry, and the third layer slurry were then dried using a drying device equipped with a hot air blower and an IR (infrared) heater, and rolled using a roll pressing method to prepare a negative electrode having a negative electrode active material layer. The loading amount of the negative electrode active material layer per unit area was 360 mg / 25 cm. 2 The electrode active material layer was rolled at about 60° C. under a pressure of 0.7 ton per cm at a speed of 2 m per minute.

[0132] Comparative Example 2 A slurry for forming a first layer, a slurry for forming a second layer, and a slurry for forming a third layer were prepared in the same manner as in Comparative Example 1. Next, the slurry for forming the first layer was applied to the current collector prepared in Example 1 using a slot die and dried to form a first layer. Next, the slurry for forming the second layer was applied on top of the first layer and dried to form a second layer. Thereafter, the slurry for forming the third layer was applied on top of the second layer and dried. The drying was performed using a drying device equipped with a hot air blower and an IR (infrared) heater. Then, the resultant was rolled using a roll press method to manufacture a negative electrode having a negative electrode active material layer. The loading amount of the negative electrode active material layer per unit area was 360 mg / 25 cm. 2 The electrode active material layer was rolled at about 60° C. under a pressure of 0.7 ton per cm at a speed of 2 m per minute.

[0133] Comparative Example 3 A single-layer electrode was manufactured using the first granules prepared in Example 1. The first granules were applied to one side of the current collector prepared in Example 1 using a thickness adjusting bar to a thickness of 25 cm. 2 The electrode active material layer was formed by hot rolling at a pressure of 0.7 ton per cm at approximately 60°C and a speed of 2 m per minute.

[0134] Comparative Example 4 An electrode was manufactured using the first, second, and third granules prepared in Example 1. The electrode was manufactured in the same manner as in Example 1, except that the third granules were applied to the surface of the current collector to form a first layer, then the second granules were used to form a second layer, and the first granules were applied to the surface of the second layer to form a top layer.

[0135] Porosity measurement The porosity of the electrode active material layer was measured for each of Example 1, Example 2, and Comparative Examples 1 to 4. For the single-layer electrode, the porosity of the entire electrode active material layer was measured, and for the multilayer electrode, the overall porosity of the entire electrode active material layer was measured, and the porosity was measured for each layer.

[0136] The thickness of the entire electrode active material layer and the thickness of each unit layer were determined from SEM images of the cross section of the electrode active material layer. The porosity can be calculated by calculating the electrode active material layer from the composition ratio of the materials contained in the electrode active material layer and the density of each component, and then calculating the difference between the apparent density and the net density. Specifically, the porosity can be calculated using the following Equation 1:

[0137] [Formula 1] Porosity (vol%) = {1 - (apparent density / true density)} x 100

[0138] On the other hand, in the above formula, the apparent density can be calculated from the following formula 2.

[0139] [Formula 2] Apparent density (g / cm 3 ) = {Weight of electrode active material layer [g] / (Thickness of electrode active material layer [cm] × Area of ​​electrode active material layer [cm 2 ])}

[0140] The porosity calculated from each of the examples and comparative examples is as shown in Table 3 below.

[0141] [Table 3]

[0142] Battery manufacturing Batteries were manufactured using the negative electrodes obtained in Example 1 and Comparative Examples 1 to 4. LiNi 0.8 Co 0.1 Mn 0.1 O2, PVdF, and carbon black were mixed in a weight ratio of 97.0:1.5:1.5 and dispersed in 2-methyl-2-pyrrolidone to prepare a cathode slurry, which was then coated on an aluminum current collector, dried, and rolled to prepare a cathode. A battery was fabricated by interposing a porous polyethylene film between the anode and cathode. The electrolyte was a mixture of ethylene carbonate and ethyl methyl carbonate in a volume ratio of 1:2, to which LiPF6 was added at a concentration of 1M.

[0143] Capacity retention rate evaluation Each battery in Example 1 and Comparative Examples 1 to 4 was charged at 0.33 C until the voltage reached 0.01 V (vs. Li), and then discharged at the same current until the voltage reached 3 V. Subsequently, charging and discharging were repeated 200 times at the same current and voltage range. The charging and discharging were carried out at an ambient temperature of about 45°C.

[0144] The capacity retention rate can be calculated by the following formula 3.

[0145] [Formula 3] Capacity maintenance rate (%)=[200 th Discharge capacity of cycle / 2 nd Discharge capacity of cycles] x 100

[0146] [Table 4]

[0147] The battery of Example 1 was confirmed to have an excellent capacity retention rate. On the other hand, the battery of Comparative Example 2, although having a high porosity in the upper layer, exhibited a very low capacity retention rate. This is explained by the fact that resistance generated at the interlayer interface caused rapid deterioration of the upper layer, resulting in a decrease in capacity retention rate. On the other hand, when the porosity of the electrode surface was low, as in Comparative Examples 3 and 4, the capacity retention rate was confirmed to decrease.

[0148] EIS characterization Resistance was measured from electrochemical impedance spectroscopy using a Solaton 1470E cell test system and a frequency response analyzer 1255B at 25°C, with an amplitude of 10 mV and a scan range of 2000 Hz to 0.1 Hz. The resulting Nyquist plot is shown in Figure 2. From this, it was confirmed that the battery of Example 1 did not exhibit additional resistance. On the other hand, the batteries of Comparative Examples 2 and 4 exhibited additional resistance. In Comparative Example 2, the electrode surface had high porosity, but the binder component of the upper slurry infiltrated the pores of the lower unit layer when the slurry was applied to the upper part of each dried unit layer, blocking the pores and resulting in reduced interfacial resistance. In Comparative Example 4, the electrolyte did not easily infiltrate the electrode, resulting in reduced resistance.

Claims

1. An electrode for an electrochemical element comprising a current collector and an electrode active material layer disposed on at least one surface of the current collector, the electrode active material layer includes a unit electrode active material layer in which a plurality of granules including an electrode active material and an electrode binder are accumulated, and a first unit electrode active material layer to an n-th unit electrode active material layer are sequentially stacked on a surface of the current collector, where n is an integer of 2 or more; An electrode for an electrochemical device, wherein an uppermost unit electrode active material layer among the unit electrode active material layers included in the electrode active material layer has the highest porosity.

2. 2. The electrode for an electrochemical element according to claim 1, wherein n is 3 or more and 10 or less.

3. 2. The electrode for an electrochemical element according to claim 1, wherein the nth unit electrode active material layer, which is the uppermost layer among the electrode active material layers, has a porosity of 40 vol % or less.

4. The electrode for an electrochemical device according to claim 1 , wherein the porosity of the first unit electrode active material layer is 20 vol % or more.

5. The electrode for an electrochemical device according to claim 1 , wherein the porosity of each unit electrode active material layer increases with increasing distance from the surface of the current collector.

6. 2. The electrode for an electrochemical element according to claim 1, wherein each unit electrode active material layer contains a plurality of types of electrode active materials, and the combination of the components of the plurality of electrode active materials is the same in each unit electrode active material layer, but the content ratio between the components is different in each unit electrode active material layer.

7. 7. The electrode for an electrochemical element according to claim 6, wherein the granules contained in each unit electrode active material layer contain a plurality of types of electrode active materials, and the combination of the components of the plurality of electrode active materials is the same in the granules of each unit electrode active material layer, but the content ratio of the components in each granule of each unit electrode active material layer is different.

8. 2. The electrode for an electrochemical element according to claim 1, wherein a difference between the binder content (wt%) at the upper 100 wt% and the binder content (wt%) at the lower 100 wt% of each unit electrode active material layer is 10 wt% or less, based on half of the height of the unit electrode active material layer.

9. 2. The electrode for an electrochemical element according to claim 1, wherein n is 3 or more, and a difference in porosity between the first unit electrode active material layer at the bottom and the nth unit electrode active material layer at the top is 5 vol% to 20 vol%.

10. 2. The electrode for an electrochemical element according to claim 1, wherein the electrode is a negative electrode, and the electrode active material comprises at least one selected from the group consisting of natural graphite, artificial graphite, lithium (Li), silicon (Si), tin (Sn), a lithium alloy, a silicon alloy, a tin alloy, a silicon oxide, a tin oxide, a lithium-containing compound, a silicon-containing compound, and a tin-containing compound.

11. 2. The electrode for an electrochemical device according to claim 1, wherein the granules contain an electrode conductive material.

12. 2. The electrode for an electrochemical element according to claim 1, wherein the current collector has a primer layer formed on at least one surface thereof, the primer layer covering the entire surface or at least a portion of the surface, the primer layer including a conductive material and a binder.

13. 10. A method for manufacturing an electrode for an electrochemical device, comprising: forming a first unit electrode active material layer having a layered structure with a predetermined thickness by applying an electrode powder to one surface of a current collector and pressing the applied powder; and then forming a unit electrode active material layer by applying an additional electrode powder to a surface of each unit electrode active material layer formed in the previous step and pressing the additional electrode powder to form a unit electrode active material layer (n-1 times), thereby manufacturing an electrode including n unit electrode active material layers, wherein the electrode has the structure of the electrode for an electrochemical device according to claim 1, and n is an integer of 2 or more.

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