Electrode assembly and secondary battery containing the same

Abstract

The present invention relates to an electrode assembly and a secondary battery including the same, and more particularly to an electrode assembly in which the deformation of the core portion is improved by modifying the design of the electrode assembly, and to an electrode assembly and a cylindrical secondary battery including the same.

Description

[Technical Field]

[0001] The present invention relates to an electrode assembly and a secondary battery including the same, and more particularly to an electrode assembly in which the deformation of the core portion is improved by modifying the design of the electrode assembly, and to a cylindrical secondary battery including the same. This application claims the benefits as of the filing date of Korean Patent Application No. 10-2023-0149978, filed with the Korean Intellectual Property Office on 2 November 2023, the contents of which are contained herein. [Background technology]

[0002] In the case of cylindrical batteries, a long electrode of a fixed width is rolled up to manufacture an electrode assembly in the form of a jelly roll. When such an electrode assembly is inserted into a battery case, the electrodes in the cylindrical battery undergo repeated contraction and expansion during charging and discharging. In particular, if a tab is located in the core of the electrode assembly or if a silicon-based active material is added to the negative electrode, the degree of contraction and expansion of the electrode assembly increases, and the pressure acting on the core of the electrode assembly increases significantly.

[0003] On the other hand, the core of a cylindrical battery is the space where the winding core used for winding the electrode assembly is located, and there is an empty space, i.e., a hollow core, used in the assembly process of the cylindrical battery, such as the insertion process of the electrode assembly into the battery case and the welding process.

[0004] Recently, as the design of low resistance / high capacity increases, the electrode assembly increasingly contains multiple tabs or silicon-based active materials are added. As a result, the possibility of core deformation of the electrode assembly due to contraction / expansion of the electrode assembly increases. In particular, due to the phenomenon of core collapse where the hollow of the core part cannot maintain a circular shape and collapses, the battery life deteriorates. Also, due to the phenomenon of core impingement where the end of the positive electrode located in the core part and the negative electrode adjacent to it are deformed beyond a certain level and the separator located between the positive and negative electrodes is damaged, there is a problem that the positive and negative electrodes come into direct contact and heat generation and ignition occur due to an internal short circuit.

[0005] In order to solve the problems of deterioration of battery life, damage to the separator, and occurrence of an internal short circuit due to deformation of such an electrode assembly, there is a need for technological development to improve the phenomenon where the core part hollow in this area cannot maintain a circular shape and collapses, and to suppress the occurrence of an internal short circuit due to damage to the separator.

Summary of the Invention

Problems to be Solved by the Invention

[0006] An object of the present invention is to provide an electrode assembly with an improved core deformation by changing the design of the electrode assembly and a secondary battery including the same.

[0007] However, the problems to be solved by the present invention are not limited to the above problems, and other problems not mentioned will be clearly understood by those skilled in the art from the following description.

Means for Solving the Problems

[0008] One embodiment of the present invention may be defined by an independent claim, and further features of a specific embodiment may be as described in each dependent claim.

[0009] One embodiment of the present invention provides an electrode assembly in which a first electrode, a separator membrane, and a second electrode are laminated and wound around a winding shaft. The first electrode may include a first surface facing the winding shaft and a second surface opposite to the first surface. Inside or partially inside the core portion of the electrode assembly, the flatness fraction of the first electrode may be more than 3% and not more than 13.5%. Throughout this specification, the flatness fraction may be determined as described below.

[0010] Throughout this specification, the terms "first electrode" and "second electrode" may be defined according to the meaning used in the technical field related to secondary batteries, particularly from the perspective of battery design and manufacturing. The first electrode and / or the second electrode may be configured to accommodate and store electrons and / or ions, particularly charge carriers such as lithium ions. Since the first electrode and the second electrode can have opposite polarities, they can respectively accommodate or release charge carriers during the charging process of the electrode assembly. Conversely, during the discharging process of the electrode assembly, they can also respectively release or accommodate charge carriers. Unless otherwise specified, the materials and functions of the first electrode and the second electrode may be those known in the technical field.

[0011] For example, the first electrode may be a positive electrode and the second electrode may be a negative electrode. In another example, the first electrode may be a negative electrode and the second electrode may be a positive electrode. Generally, the positive electrode may include a current collector provided as a plate, sheet, or film, on which a positive electrode active material is deposited. For example, the positive electrode active material may include a lithium metal oxide such as lithium cobalt oxide or lithium iron phosphate, or other suitable substances. The positive electrode current collector may be made of an electrically conductive material such as aluminum. Generally, the negative electrode may include a current collector provided as a plate, sheet, or film, on which a negative electrode active material is deposited. For example, the negative electrode active material may include a carbon-based material such as graphite. The negative electrode current collector may be made of an electrically conductive material such as copper.

[0012] Separation membranes are used in secondary battery-related technologies, particularly from the standpoint of battery design and manufacturing. Specifically, separation membranes may be porous membranes configured to be impermeable to solid materials but allow charge carriers to pass through. These charge carriers may be, for example, ions, lithium ions, and / or electrons. The separation membrane is placed between a first electrode and a second electrode to prevent short circuits between them.

[0013] The transfer of charge carriers between the first electrode and the second electrode may be made possible by an electrolyte that may be present at least partially between the first electrode and the second electrode. The electrolyte may be injected after the electrode assembly is housed in the battery case. More specifically, the electrode assembly may include such an electrolyte that is present between the first electrode and the second electrode.

[0014] The first electrode, the separation membrane, and the second electrode may be provided in layers (or sheets). The first electrode, the separation membrane, and the second electrode may be stacked in a specific order. Specifically, the second electrode, the separation membrane, the first electrode, and other separation membranes may be stacked in sequence to form a mono-cell. In other examples, one or more mono-cells may be stacked repeatedly on top of the mono-cell in sequence to form a multi-cell.

[0015] The electrode assembly may include a monocell or multicell wound into a roll. The roll-shaped electrode assembly is sometimes called a jelly roll. Specifically, in a monocell, the first electrode, separation membrane, second electrode, and selectively another separation membrane may be wound together around a winding shaft. Therefore, the electrode assembly may have a cylindrical shape, particularly a cylindrical shape with a helical or spiral cross-section. The first electrode, separation membrane, and second electrode of the electrode assembly may also be stacked in order and wound together around a winding shaft. Therefore, the electrode assembly generally has a cylindrical symmetrical structure with respect to the winding shaft. In cylindrical symmetry, the axial direction may be parallel to the winding shaft of the electrode assembly. The radial direction may be perpendicular to the winding shaft and perpendicular to the outer surface of the electrode assembly. The circumferential direction may be perpendicular to the axial and radial directions and may mean the direction along a circular path around the winding shaft. Furthermore, the winding direction of the electrode assembly may mean the direction along at least one of the first electrode, the separator membrane, and the second electrode in a plan view, and thus the winding direction may mean the direction along a helical path around the winding axis. Depending on the context, the winding direction may approximate a circle around the winding axis, similar to the circumferential direction, from the viewpoint of rotational direction.

[0016] Throughout this specification, unless otherwise specified, the azimuth angle can be expressed as the angle between two lines in a plane perpendicular to the winding shaft. The two lines forming the azimuth angle may intersect at the winding shaft.

[0017] Unless otherwise specified, geometric features such as lines, angles, and curvatures used throughout this specification may be defined in plan views, i.e., cross-sections taken from a viewpoint parallel to the winding axis of the electrode assembly. Therefore, unless otherwise specified, these geometric features may represent the approximate cylindrical geometric shape of the electrode assembly when wound around the winding axis. Furthermore, unless otherwise specified, all lines used herein may represent straight lines.

[0018] Throughout this specification, cylindrical symmetry may be an approximation in that the helical plan view and / or helical cross-section of the electrode assembly are considered to be approximately circular plan view and / or circular cross-section, respectively. The plan view of the electrode assembly may show a cross-section from a viewpoint parallel to the winding shaft. The electrode assembly may have helical ends facing the winding shaft, since the first electrode, the separator membrane, and the second electrode are wound around the winding shaft. On the other hand, the cross-section of the electrode assembly may show a cross-section perpendicular to the winding shaft. Since the first electrode, the separator membrane, and the second electrode are wound around the winding shaft, the cross-section of the electrode assembly may be helical around the winding shaft.

[0019] At least one of the first electrode, the separation membrane, and the second electrode may have a rectangular shape in the plan view before winding. The winding shaft may be parallel to at least one of the ends of the first electrode, the separation membrane, and the second electrode. In a specific example, the first electrode, the separation membrane, and the second electrode may each have a rectangular shape in their respective plan views, and the ends of the first electrode, the separation membrane, and the second electrode may be aligned parallel to each other.

[0020] An electrode assembly having the aforementioned features can achieve any of the technical effects mentioned herein. More specifically, one, some, or all of the electrodes of the electrode assembly may contract and / or expand during charging and discharging of the battery, which may lead to deformation of the electrode assembly. Due to the deformation of the electrode assembly, the hollow core of the electrode assembly may not maintain its circular shape and may collapse.

[0021] In one embodiment of the present invention, the electrode assembly can adjust the flatness fraction of the first electrode to a specific range, thereby suppressing the phenomenon in which the hollow core of the electrode assembly collapses due to deformation of the electrode assembly, preventing it from maintaining its circular shape.

[0022] Therefore, the electrode assembly according to one embodiment of the present invention can contribute to preventing damage to the first electrode, the separator membrane, and the second electrode. Furthermore, the electrode assembly according to one embodiment of the present invention can contribute to preventing internal short circuits between the first electrode and the second electrode. This can contribute to improving the stability and life characteristics of the battery.

[0023] One embodiment of the present invention provides a method for manufacturing an electrode assembly. The manufacturing method, by the manner described, embodies the flatness ratio of the first electrode and enables a continuous manufacturing process using, for example, conventional roll-to-roll process equipment, thereby increasing the productivity and economics of the battery. This method can also achieve the aforementioned and / or later-described technical effects.

[0024] According to one embodiment of the present invention, the core portion of the electrode assembly may be a region within 3 turns from the inner end of the first electrode.

[0025] Furthermore, the point of maximum curvature of the first electrode may be located inside or in part of the electrode assembly, within 3 turns (Turn) from the inner end of the first electrode in the winding direction of the electrode assembly, as described later.

[0026] The core portion may refer to a part of the electrode assembly enclosed by the third turn from the inner end of the first electrode. The core portion may correspond to the aforementioned core portion. The core portion can be embodied through some or all of the features described later.

[0027] Alternatively or additionally, the core portion may be a region within three turns from one longitudinal end of the first electrode of the electrode assembly. The longitudinal direction may be defined relative to the state of the electrode assembly before winding, or it may correspond to the winding direction used herein after winding. The one end may correspond to the inner end of the first electrode.

[0028] In one embodiment of the present invention, the first electrode includes a first electrode current collector and a first electrode active material layer provided on at least one surface of the first electrode current collector, wherein the first electrode active material layer extends to the inner end of the first electrode current collector, and the inner end of the first electrode current collector corresponds to the inner end of the first electrode.

[0029] In the plan view, as the first electrode is wound in the winding direction around the winding shaft, the first electrode extends from its inner end to its outer end. For example, the inner end of the first electrode may be located in a radially internal position in the region adjacent to the winding shaft. For example, the outer end of the first electrode may be located in a radially external position in the region adjacent to the outer circumferential surface of the electrode assembly. Alternatively or additionally, the same may apply to the second electrode.

[0030] In other words, the first electrode may include a first electrode current collector and a first electrode active material layer provided on the first electrode current collector. The first electrode extends from one longitudinal end in the direction opposite to the winding direction, forming the inner end of the other longitudinal end at the same position as the longitudinal end of the first electrode current collector. That is, with respect to the direction opposite to the winding direction, the first electrode cannot extend beyond the first electrode current collector.

[0031] As mentioned above, the active material layer may refer to a layer containing or composed of electrode active material. For example, the first electrode may be a positive electrode, and the first electrode active material layer may be a positive electrode active material layer.

[0032] According to one embodiment of the present invention, the first electrode includes a blank portion of the first electrode where the first electrode active material layer is not provided. The electrode assembly may also include a first electrode tab provided on or physically connected to the blank portion of the first electrode.

[0033] The blank portion of the first electrode may mean a part of the first electrode, particularly a part of the first electrode current collector, or a part of the first electrode where the first electrode active material layer is not provided, that is, a part of the first electrode current collector where the active material of the first electrode is not provided.

[0034] Electrode tabs may be formed from or on the first electrode blank portion, or at least one may be provided on the first electrode blank portion. The first electrode blank portion may be cut or notched. For example, at least one first electrode tab can be formed by forming one or more slits, notches, cuts, or similar features from the edge of the first electrode blank portion to the first electrode blank portion. Alternatively or additionally, at least one electrode tab may be provided separately and attached to the first electrode current collector or the first electrode blank portion.

[0035] According to one embodiment of the present invention, the angle formed by the extension line connecting the winding shaft and the inner end of the first electrode, that is, the extension line connecting the first straight line and the point on the winding shaft and the first electrode where the curvature is 1 or less, may be greater than 0°.

[0036] The mathematical definition of curvature may be as described above, or it may be determined as described below. In particular, the point where the curvature of the first electrode is 1 or less may be located within 3 turns from the inner end of the first electrode, i.e., in the core of the electrode assembly. The angle may mean the azimuth angle described above, and the vertex may coincide with or correspond to the winding axis.

[0037] According to one embodiment of the present invention, the flatness fraction of the first electrode may be determined after the activation of the electrode assembly.

[0038] Throughout this specification, activation may mean the process of preparing an electrode assembly or a battery cell containing the electrode assembly for use in a secondary battery by applying specific temperature and charge / discharge conditions. Activation may be referred to as formation. Activation may include aging, charging, and discharging stages, which may be performed in a specific order or selectively repeated in a specific order. The aging stage may be performed to allow the electrolyte to penetrate the first and second electrodes. For example, the aging stage may be performed by storing the electrode assembly at a specific temperature, e.g., 30°C, for a specific period, e.g., 30 minutes, 1 hour, 2 hours, or 3 hours. The charging stage may be performed to allow the electrolyte to decompose on the negative electrode surface to form a Solid Electrolyte Interphase (SEI), i.e., a solid electrolyte interface. The charging stage may include the process of charging the electrode assembly to a specific degree. After the charging stage, a high-temperature aging step may be selectively performed at a high temperature, e.g., 40°C, 50°C, 60°C, or 70°C. Subsequently, the electrode assembly may be discharged at a specific C-rate, such as 0.1C, 0.2C, 0.5C, or 1.0C. Optionally, a degassing step may be performed to remove gases generated during the activation step.

[0039] The first electrode and / or the second electrode may contract and / or expand during the activation step. Therefore, the technical effects achieved by the present invention may be particularly advantageous in the electrode assembly after at least some of the activation steps have been completed.

[0040] According to one embodiment of the present invention, the flatness ratio of the first electrode may be determined after 50 charge and discharge cycles under the conditions of 25°C, 1C charge, and 1C discharge.

[0041] Therefore, the flatness ratio may be determined after at least 50 or more charge and discharge cycles of the electrode assembly. This allows for a more accurate determination of the state or quality of the electrode assembly.

[0042] According to one embodiment of the present invention, the angle formed by the extension line connecting the winding shaft and the inner end of the first electrode, and the extension line connecting the winding shaft and the point of maximum curvature, may be greater than 40° and less than or equal to 98°. In particular, the point of maximum curvature may be located inside or in part of the core portion of the electrode assembly.

[0043] According to one embodiment of the present invention, a straight line connecting the winding shaft and the inner end of the first electrode, i.e., a first straight line, may be drawn with reference to a cross section perpendicular to the winding shaft of the electrode assembly. For example, with reference to a cross section perpendicular to the winding shaft of the electrode assembly, the first straight line may be a virtual line extending from the winding shaft through the inner end of the first electrode. As described later, the first straight line may be drawn to determine the angle with an extension line connecting the winding shaft and the point of maximum curvature, i.e., a second straight line.

[0044] According to one embodiment of the present invention, a second straight line may be drawn, connecting the winding shaft and the point of maximum curvature of the first electrode, based on a cross-section perpendicular to the winding shaft of the electrode assembly. For example, based on a cross-section perpendicular to the winding shaft of the electrode assembly, the second straight line may be a virtual line extending from the winding shaft through the point of maximum curvature of the first electrode. The second straight line may be drawn to determine the angle with the extension line connecting the winding shaft and the inner end of the first electrode, i.e., the first straight line.

[0045] According to one embodiment of the present invention, the angle formed by the extension line connecting the winding shaft and the inner end of the first electrode, and the extension line connecting the winding shaft and the point of maximum curvature, i.e., the angle formed by the first and second lines, from a viewpoint parallel to the winding shaft, may be greater than 40° and less than or equal to 98°. Here, the angle may be any of the ranges of values ​​disclosed herein. As stated above, the viewpoint parallel to the winding shaft may mean the viewing angle in the axial direction of the cylindrical symmetry of the electrode assembly.

[0046] According to one embodiment of the present invention, the point of maximum curvature of the first electrode may be the point where the curvature of the first electrode is maximum, with respect to a cross-section perpendicular to the winding shaft of the electrode assembly. The point of maximum curvature of the first electrode may be determined in a region with an azimuth angle of 0° to 180° extending from the inner end of the first electrode in the opposite direction to the winding direction, with respect to the winding shaft of the electrode assembly. The point of maximum curvature may be located inside or in part of the core portion of the electrode assembly.

[0047] Throughout this specification, the term curvature may be used in the sense commonly used in mathematics. In particular, curvature can represent a measure of how much a curve deviates from a straight line. Additionally or alternatively, curvature can represent a measure of how much a surface deviates from a plane. Here, curvature can represent how much the direction of the curve changes over a small distance for all parts of the curve, for example, the angle per unit distance. Curvature may also be a measure of the change in direction of a point along a curve. In particular, curvature may represent the instantaneous rate of change of the unit tangent vector to the curve at point P as the point P moves along the curve at a unit velocity. In a concrete example, the position of the point P(s) may be a function of a parameter s, for example, this may be time or the arc length relative to a given origin. Also, T(s) may be the unit tangent vector of the curve at P(s), which may be the derivative of P(s) with respect to s. Here, the derivative of T(s) with respect to s may be a vector perpendicular to the curve, and its length may be the curvature.

[0048] Throughout this specification, a curve can mean the contour and / or contour lines of the cross-section of the first electrode in a plan view, i.e., a view parallel to the winding axis. In particular, since the first electrode can be supplied as a sheet or layer, the first electrode is perceived as a thick curve in the plan view. Additionally or alternatively, similar content can be applied to the second electrode and / or separator membrane. Since the curve can be continuously differentiated near P, the tangent can change continuously along the curve. Since the curve can be differentiated twice at any P, for example, curvature can exist along the curve as the derivative of T(s) with respect to s.

[0049] Throughout this specification, curvature may be determined in terms of the oscillating circle of a curve. The oscillating circle of a curve at point P may be a circle having the same tangent and curvature as the curve. The tangent may be an approximation of the curve at point P. The oscillating circle may also be an approximation of the curve at point P. The curvature of a straight line may be zero. If the curvature of a point is not zero, the reciprocal of the curvature is considered to be the radius of curvature, i.e., the radius of the oscillating circle. The center of the oscillating circle may be considered the center of curvature and may be drawn by indicating the radius of curvature perpendicular to the tangent of the curve in the direction in which the curve bends.

[0050] Alternatively, the curvature used here may be determined from the derivative (or derivative value) of the central angle between the center of the contacting circle and the arc of the contacting circle.

[0051] As one method for determining the curvature of the first electrode as described above, the curvature may be determined from a plan view image or a cross-sectional image of the electrode assembly taken from a viewpoint parallel to the winding shaft. In particular, the curve may be determined using an image of the electrode assembly taken from a viewpoint parallel to the winding shaft, that is, a visualized cross-sectional view of the first electrode, and the curvature may be determined from the said curve.

[0052] Throughout this specification, the point of maximum curvature is the point in the plan view of the electrode assembly where the curvature of the first electrode or the second electrode is greatest, and may indicate a point within or on part of the first electrode or the second electrode. Therefore, the point of maximum curvature may be a point on the first electrode where the first electrode exhibits the greatest curvature in the plan view of the electrode assembly. Additionally or alternatively, the point of maximum curvature may be the point on the first electrode where the contact circle of the first electrode is smallest. Additionally or alternatively, the same can be applied to the second electrode and / or the separator membrane.

[0053] According to one embodiment of the present invention, the curvature of the first electrode can generally decrease as the first electrode extends from the winding shaft toward the outer circumference of the electrode assembly. In other words, the curvature of the first electrode can generally decrease along the radial outer circumference, the helical outer circumference, or the winding direction of the first electrode. Since the first electrode, the separator membrane, and the second electrode are wound together around the winding shaft, the curvature of the first electrode can generally decrease along the radial outer circumference or the winding direction. Additionally or alternatively, the curvature of the second electrode and / or the separator membrane can generally decrease along the radial outer circumference.

[0054] At the same time, the point of maximum curvature can be located closer to the winding shaft than to the outer surface of the electrode assembly. In a plan view of the electrode assembly, the "core" refers to the part of the electrode assembly that is closer to the winding shaft than to the outer surface. The core may be further defined as described below. The point of maximum curvature may be located inside or in part of the core of the electrode assembly. The point of maximum curvature can be applied to the first electrode, but the same may be applied to the second electrode and / or the separator membrane, additionally or alternatively.

[0055] Throughout this specification, as defined above, “winding direction” may mean the winding direction of the first electrode and / or the second electrode and the separation membrane, which may be a helical path around the winding axis. That is, with respect to the plan view and / or a cross section perpendicular to the winding axis, the winding direction of the first electrode may extend from the inner end of the first electrode along the winding direction to the outer end of the first electrode. However, in determining the azimuth angle, the winding direction of the first electrode may mean the circular direction as the direction of the side in the rotational direction around the winding axis.

[0056] The azimuth angle may be determined from a cross-section perpendicular to the winding shaft of the electrode assembly or from a plan view of the electrode assembly, where the vertex of the azimuth angle may coincide with the winding shaft. In the aforementioned region, with respect to the cross-section perpendicular to the winding shaft of the electrode assembly, one side of the azimuth angle may be the extension line connecting the winding shaft and the inner end of the first electrode, i.e., the first straight line. With respect to the cross-section perpendicular to the winding shaft of the electrode assembly, the other side of the azimuth angle may be the extension line connecting the winding shaft to the boundary of the aforementioned region.

[0057] The region may extend to or span azimuth angles greater than 0° and less than or equal to 180°. In other words, the region may cover the entire area between one boundary corresponding to an azimuth angle of 0° and another boundary corresponding to an azimuth angle of 180°. The boundary corresponding to an azimuth angle of 0° and the other boundary corresponding to 180° may intersect at the winding shaft, and the region may have another boundary on the outer circumference of the electrode assembly. The extension line connecting the winding shaft and the inner end of the first electrode, i.e., the first straight line, may correspond to an azimuth angle of 0°. The region may extend in the direction opposite to the winding direction; that is, it may cover the area opposite to the portion of the first electrode extending from the inner end in the winding direction. When the region extends over an azimuth angle range greater than 0° and less than or equal to 180°, the region may cover half of the cross-section of the electrode assembly perpendicular to the winding shaft and / or half of the plan view of the electrode assembly, and may be demarcated by the extension line connecting the winding shaft and the inner end of the first electrode. In other words, the region may be semicircular or similar in shape.

[0058] According to one embodiment of the present invention, the circularity of the first electrode may be 89% or more inside or in part of the core portion of the electrode assembly.

[0059] Throughout this specification, roundness can mean mathematical or geometric circularity or roundness. In particular, roundness may be determined according to ISO 1101. Throughout this specification, roundness may be determined by two-dimensional parameters from a plan view of the electrode assembly. Alternatively or additionally, roundness may be determined as described below.

[0060] In one embodiment of the present invention, on the first surface of the first electrode, the first extension line may be drawn by extending a straight line connecting two points within a distance of 5 mm from the inner end of the first electrode where the curvature direction changes. On one surface of the second electrode facing the first surface of the first electrode, the second extension line may be drawn by extending a straight line connecting two points at a distance of 5 mm from the inner end of the first electrode. The first extension line and the second extension line may form an angle of 25° or less.

[0061] According to one embodiment of the present invention, the first electrode may be free from cracks or wrinkles inside or in part of the core portion of the electrode assembly.

[0062] One embodiment of the present invention provides a secondary battery. The secondary battery according to one embodiment of the present invention may include an electrode assembly according to one embodiment of the present invention.

[0063] According to one embodiment of the present invention, the secondary battery may include a battery case, particularly a cylindrical battery case, that houses an electrode assembly.

[0064] One embodiment of the present invention provides an electrode assembly in which a first electrode, a separation membrane, and a second electrode are laminated and wound together, wherein the first electrode includes a first surface facing the winding shaft of the electrode assembly and a second surface opposite to the first surface, and in the core portion of the electrode assembly, the flatness fraction of the first electrode is greater than 3% and less than or equal to 13.5%.

[0065] Another embodiment of the present invention provides a secondary battery including the electrode assembly and a battery case for housing the electrode assembly. [Effects of the Invention]

[0066] An electrode assembly according to one embodiment of the present invention improves the phenomenon in which the hollow core collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during battery charging and discharging, thereby preventing damage to the second electrode and the separator membrane, and preventing internal short circuits between the first and second electrodes, thus improving the stability and lifespan characteristics of the battery.

[0067] Furthermore, the secondary battery according to the present invention improves the phenomenon in which the hollow core collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during charging and discharging, thereby preventing damage to the second electrode and the separator membrane, and preventing internal short circuits between the first electrode and the second electrode. As a result, the stability and lifespan characteristics of the battery can be improved.

[0068] The effects of the present invention are not limited to those described above, and any effects not mentioned will be clearly understood by those skilled in the art from this specification and the accompanying drawings. [Brief explanation of the drawing]

[0069] [Figure 1] This figure shows the correlation between the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum, the flatness fraction of the first electrode, and the deformation of the core. [Figure 2] This graph shows the correlation between the flatness fraction of the first electrode and the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum. [Figure 3] This image shows a CT image of the electrode assembly according to Example 1 and an image showing the curvature of the first electrode extracted from the CT image. [Figure 4] This image shows a CT image of the electrode assembly according to Example 2 and an image showing the curvature of the first electrode extracted from the CT image. [Figure 5] This image shows a CT image of the electrode assembly according to Comparative Example 1 and an image showing the curvature of the first electrode extracted from the CT image. [Figure 6] This image shows a CT image of the electrode assembly according to Comparative Example 2, and an image showing the curvature of the first electrode extracted from the CT image. [Figure 7] This image shows a CT image of the electrode assembly according to Comparative Example 3 and an image showing the curvature of the first electrode extracted from the CT image. [Figure 8] These are CT images of the electrode assembly and images of crack formation in the first electrode core according to Comparative Example 4. [Figure 9] These are CT images of the electrode assembly and images of crack formation in the first electrode core according to Comparative Example 5. [Figure 10] This diagram schematically illustrates a method for evaluating whether or not core impingement has occurred. [Figure 11] These are CT images showing the sliding range of the longitudinal end of the first electrode of a secondary battery, as shown in Reference Experiment Example 1 and Reference Experiment Example 2. [Modes for carrying out the invention]

[0070] Throughout this specification, when a part "includes" a component, this means, unless otherwise stated, that it may include other components rather than excluding them.

[0071] Throughout this specification, when a member is described as being "on top of" another member, this includes not only cases where one member is in contact with another member, but also cases where another member is present between the two members.

[0072] One embodiment of the present invention provides an electrode assembly in which a first electrode, a separation membrane, and a second electrode are laminated and wound together, wherein the first electrode includes a first surface facing the winding shaft of the electrode assembly and a second surface opposite to the first surface, and in the core portion of the electrode assembly, the flatness fraction of the first electrode is greater than 3% and less than or equal to 13.5%.

[0073] Here, the flatness ratio may be the ratio of the number of measurement points with a curvature of 1 or less, based on 100% of the number of measurement points located on the first surface of the first electrode.

[0074] Furthermore, the curvature may be calculated from a plurality of measurement points located on the first surface of the first electrode, which are extracted from computed tomography (CT) images, as described later.

[0075] An electrode assembly according to one embodiment of the present invention improves the phenomenon in which the hollow core cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during battery charging and discharging, by adjusting the flatness fraction of the first electrode. This prevents damage to the second electrode and the separator membrane, and prevents internal short circuits between the first and second electrodes, thereby improving the stability and lifespan characteristics of the battery.

[0076] In more detail, during battery charging and discharging, the electrodes contained in the electrode assembly repeatedly contract and expand. Because the rigidity of the battery case surrounding the outer surface of the electrode assembly limits outward expansion, stress may concentrate in the hollow space located in the core. This can increase the possibility of core deformation of the electrode assembly.

[0077] The deformation of the core can be classified into two phenomena: core collapse, in which the hollow core of the electrode assembly collapses when the internal stress reaches a certain level or higher, and core impingement, in which the end of the first electrode located in the core and the adjacent second electrode deform beyond a certain level, causing the separation membrane located between the first and second electrodes to break and the first and second electrodes to come into direct contact.

[0078] In particular, the collapse of the core portion may be concentrated in areas of the electrode assembly that are vulnerable to internal stress. Specifically, the collapse of the core portion may be concentrated in areas where the curvature of the first electrode is below a certain value, i.e., flat areas. That is, in the core portion of the electrode assembly, deformation due to internal stress may begin in areas where the curvature of the first electrode is below a certain value, and beyond a certain point, the hollow core portion may no longer be able to maintain its circular shape and may completely collapse. In an electrode assembly where the core portion has collapsed, the transfer of lithium ions between the first electrode and the second electrode cannot proceed smoothly any further, which may lead to a deterioration in battery life.

[0079] Here, the curvature of the first electrode may be affected by the step difference due to the thickness of the electrode, tab, etc., the shape of the winding core, and the tension acting on the electrode assembly during winding. For example, the winding core may include a pair of winding core sections separated around a separation section into which the separation membrane is inserted, and when a certain level of tension is applied, twisting of the winding core occurs, which inevitably creates a region in which the curvature of the first electrode is below a certain value, i.e., a flat region, and collapse of the core may occur in the region in which the curvature of the first electrode is below a certain value.

[0080] In this case, by adjusting the flatness fraction of the first electrode to a specific range, the phenomenon in which the hollow core collapses and fails to maintain its circular shape, i.e., the core collapse phenomenon, can be improved. This prevents damage to the second electrode and the separator membrane, and prevents internal short circuits between the first and second electrodes, thereby improving the stability and lifespan characteristics of the battery.

[0081] Figure 1 shows the correlation between the angle between the longitudinal end of the first electrode of the present invention and the point where the curvature of the first electrode is maximum, the flatness fraction of the first electrode, and the deformation of the core portion.

[0082] More specifically, Figures 1(a) to 1(c) show CT images of an electrode assembly according to one embodiment of the present invention and the curvature of the first electrode extracted from the CT images. More specifically, Figure 1(a) shows the flat region of the electrode assembly in relation to the winding core, Figure 1(b) shows the point where the curvature of the electrode assembly is maximum in relation to the winding core, and Figure 1(c) shows the flat region and the point where the curvature is maximum of the electrode assembly in relation to the winding core, respectively. On the other hand, Figure 1(d) is an image showing core deformation occurring in the flat region of the electrode assembly as shown in Figures 6(a) to 1(c) after the cycle has progressed.

[0083] More specifically, Figures 1(a) to 1(c) above represent, in color coordinates, the percentage of measurement points with a curvature below an arbitrarily set reference value relative to the curvature measured from each measurement point. Here, the set reference value is a specific value within the range of 0.5 to 1.0, and the range of the color index expressed to the right of the color coordinates is 0 to 0.8 in Figure 1(a) and 1 to 1.5 in Figure 1(b).

[0084] More specifically, Figure 1(a) visualizes the flat areas and flatness fraction by representing the regions where low curvature k values ​​are distributed using color coordinates when the color distribution range of the curvature measured at each measurement point is set to 0 to 0.8, and Figure 1(b) visualizes the point where the curvature is maximum by representing the regions where high curvature k values ​​are distributed using color coordinates when the color distribution range of the curvature measured at each measurement point is set to 1 to 1.5. In this case, each measurement point was extracted from the CT image of the first electrode as described above, and it can be confirmed that the longitudinal end of the first electrode located in the core is located at the 6 o'clock position.

[0085] According to one embodiment of the present invention, the winding may be performed using a winding core that includes a separation portion into which a separation membrane is inserted; a first winding core provided on one side of the separation portion, and a second winding core provided on the other side of the separation portion, having a different cross-sectional area from the first winding core.

[0086] More specifically, the winding core may include a separation portion into which a separation membrane is inserted, and a first winding core may be provided on one side of the separation portion, while a second winding core having a different cross-sectional area from the first winding core may be provided on the other side. By providing the separation portion in the winding core, the laminate, such as a separation membrane inserted into the separation portion, can be wound in the opposite direction to the rotation of the winding core as the winding core rotates.

[0087] In this case, since the winding core includes a first winding core portion and a second winding core portion having a different cross-sectional area from the first winding core portion, when high tension is applied to the winding core, twisting may occur in the relatively smaller winding core portion. As a result, a point where the curvature of the first electrode is maximum can be located adjacent to the relatively larger winding core portion with respect to the separation portion, and a flat region of the first electrode can be located adjacent to the relatively smaller winding core portion.

[0088] Furthermore, with respect to the longitudinal end of the first electrode, the region in the opposite direction to the winding direction of the electrode assembly, i.e., the rotation direction of the winding core, which is more than 0° and less than or equal to 180°, has a relatively smaller number of turns of the first electrode up to the outermost layer compared to the region more than 180° and less than or equal to 360°. Because the distance from the sliding longitudinal end of the first electrode is relatively far, concentrated stress is not relieved, and therefore the possibility of deformation of the core in the hollow direction at that location may be higher.

[0089] In other words, the winding core may include a pair of winding core sections separated around a separation section into which a separation membrane is inserted, and when a certain level of tension is applied, twisting of the winding core occurs, which inevitably causes a region where the curvature of the first electrode is below a certain value, i.e., a flat region, and the core can collapse in the region where the curvature of the first electrode is below a certain value.

[0090] In this case, when adjusting the flatness fraction of the first electrode in the core of the electrode assembly, the phenomenon in which the hollow of the core collapses and fails to maintain its circular shape, i.e., the core collapse phenomenon, can be improved. This prevents damage to the second electrode and the separator membrane, and prevents internal short circuits between the first and second electrodes, thereby improving the stability and lifespan characteristics of the battery.

[0091] According to one embodiment of the present invention, in the core portion of the electrode assembly, the flatness fraction of the first electrode may be greater than 3% and less than or equal to 13.5%. More specifically, in the core portion of the electrode assembly, the flatness fraction of the first electrode may be greater than 3% and less than or equal to 13.5%, 5% or more and less than or equal to 13%, 5% or more and less than or equal to 10%, 7% or more and less than or equal to 10%, or 7.5% or more and less than or equal to 10%.

[0092] When the flatness ratio range of the first electrode is met as described above, the phenomenon in which the hollow of the core cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during battery charging and discharging can be improved, damage to the second electrode and separator membrane can be prevented, and internal short circuits between the first and second electrodes can be prevented, thereby improving the stability and life characteristics of the battery.

[0093] Here, the flatness fraction (%) can mean the proportion of the flat region of the first electrode that is the object of measurement, and the flat region can mean the region having a curvature less than or equal to a reference value.

[0094] In other words, the flatness ratio can mean the proportion of the region in the first electrode, which is the object of measurement, that exhibits a curvature value less than or equal to a reference curvature value. For example, the flatness ratio may be the proportion (%) of the number of measurement points with a curvature of 1 or less, based on 100% of the number of measurement points.

[0095] Since the flatness ratio of the first electrode can be determined according to the aforementioned reference curvature value, if the range of the aforementioned reference curvature value is met, the degree to which the core portion deviates from a circular shape can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode can be increased.

[0096] The following describes in more detail the target of curvature measurement, the method of curvature measurement, and the timing of curvature measurement in relation to the flatness fraction (%) of the first electrode of the present invention.

[0097] According to one embodiment of the present invention, the curvature may be measured in the core portion of the electrode assembly with respect to the first electrode.

[0098] Here, the "core portion" may be a hollow space located on the winding shaft of the electrode assembly, and a region including a part of the stacked structure of the wound-up separation membrane / second electrode / separation membrane / first electrode.

[0099] That is, the core portion may include a hollow space located on the winding shaft of the electrode assembly and may also include a region from the innermost shell where winding of the electrode assembly begins to one longitudinal end of the first electrode, i.e., a region that does not include the first electrode. Furthermore, the core portion may further include a region of a predetermined length containing the first electrode, extending from one longitudinal end of the first electrode in the direction in which winding of the first electrode is performed.

[0100] According to one embodiment of the present invention, the core portion may be a region within 3 turns from one longitudinal end of the first electrode. More specifically, the core portion may be a region within 1 to 2.5 turns or 1.5 to 2 turns from one longitudinal end of the first electrode.

[0101] Here, one turn refers to the length required to wind the electrode or separator membrane included in the electrode assembly 360° from a reference point, and the length may be determined by the outer diameter of the core used to wind the electrode assembly, the thickness of the separator membrane and electrode, and the number of turns of the separator membrane and electrode located inside the reference point. For example, one turn of the first electrode may refer to the length required to wind the first electrode 360° from the longitudinal end of the first electrode in the direction in which the electrode assembly is wound.

[0102] In other words, the core portion may refer to the region from one longitudinal end of the first electrode to a point three turns or less away, or it may refer to the region from one to 2.5 turns away from one longitudinal end of the first electrode, or to a point one.5 to 2 turns away.

[0103] Since the flatness ratio of the first electrode can be determined according to the range of the core portion, when the range of the core portion is met, the degree to which the core portion deviates from a circular shape can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode can be increased.

[0104] According to one embodiment of the present invention, the curvature may be measured in the core portion of the electrode assembly, measured relative to the first electrode in the core portion of the electrode assembly, or measured by extracting the first electrode from a CT image of the core portion of the electrode assembly.

[0105] When the object of curvature measurement is the first electrode located in the core of the electrode assembly, the reliability of the curvature measurement can be increased, and the flatness ratio of the first electrode can be adjusted more easily by adjusting the position of the longitudinal end of the first electrode. This reduces the deformation of the end of the first electrode located in the core and the adjacent second electrode, thereby improving the phenomenon of core impingement, i.e., the damage to the separation membrane located between the first and second electrodes.

[0106] According to one embodiment of the present invention, the curvature may be calculated from a plurality of measurement points located on the first surface of the first electrode extracted from a computed tomography (CT) image. More specifically, the curvature may be measured with reference to the first surface of the extracted first electrode facing the winding shaft of the electrode assembly, after the first electrode has been extracted from a CT image of the electrode assembly. More specifically, the curvature may be calculated from a plurality of measurement points located on the first surface of the first electrode at a certain distance apart. For example, the plurality of measurement points may be located at a distance of 2° from the winding shaft on the first surface of the extracted first electrode.

[0107] According to one embodiment of the present invention, the number of measurement points may be 180 or more. More specifically, the number of measurement points may be 180 to 720. More specifically, the number of measurement points may be 240 to 660, 300 to 600, or 360 to 540.

[0108] Since the flatness ratio of the first electrode can be determined according to the number of measurement points, when the aforementioned range of measurement points is met, the degree to which the core portion deviates from a circular shape can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode can be increased.

[0109] According to one embodiment of the present invention, in the aforementioned core portion, the curvature can be measured from separate measurement points within a certain distance or within a range of arbitrarily selected numbers of measurement points, and the point of maximum curvature and the flatness ratio can be determined.

[0110] According to one embodiment of the present invention, the curvature may be the curvature given by the following formula 1. More specifically, the curvature calculated from the measurement points may be calculated according to the following formula 1 using coordinate values ​​obtained from a plurality of measurement points located on the first surface of the first electrode extracted from the CT image, i.e., the x-coordinate and y-coordinate.

[0111] [Formula 1] k=(x'y''-y'x'') / (x' 2 +y' 2 ) 3 / 2

[0112] In equation 1 above, k is the curvature of the first electrode, x is the x-coordinate of the measurement point, y is the y-coordinate of the measurement point, x' is the first derivative with respect to x, y' is the first derivative with respect to y, x'' is the second derivative with respect to x, and y'' is the second derivative with respect to y.

[0113] In other words, the calculation of curvature can mean measuring the x and y coordinate values ​​at a plurality of measurement points located on the first surface of the first electrode at a certain distance apart, and calculating the curvature value using the measured x and y coordinates and Equation 1. More specifically, the curvature is calculated using a parameterized representation of a plane curve, and the larger the k value in Equation 1, the greater the curvature, i.e., the degree of curvature. More specifically, the curvature may be calculated using the formula via a Python® program, and the x and y coordinate values ​​may be in the range of -3 to +3. On the other hand, x' and y' may represent the slope of the tangent line at each coordinate, and x'' and y'' may represent the rate of change of the slope.

[0114] If necessary, the curvature value may be normalized using the radius of curvature of the electrode assembly corresponding to each measurement point.

[0115] According to one embodiment of the present invention, in the core portion of the electrode assembly, the angle between the longitudinal end of the first electrode and a point where the curvature of the first electrode is 1 or less, with respect to the winding shaft, may exceed 0°.

[0116] More specifically, in the core portion of the electrode assembly, the angle between the longitudinal end of the first electrode and a point where the curvature of the first electrode is 1 or less, with respect to the winding shaft, may be greater than 0° and less than 50°.

[0117] More specifically, in the core portion of the electrode assembly, the angle formed between the longitudinal end of the first electrode and a point where the curvature of the first electrode is 1 or less, with respect to the winding shaft, may be 5° or more and less than 50°, 10° or more and 45° or less, or 15° or more and 40° or less.

[0118] In other words, the region of the first electrode with a curvature of 1 or less, i.e., the flat region, may be located between the point where the curvature of the first electrode is maximum and the longitudinal end of the first electrode, and may not overlap with the longitudinal end of the first electrode.

[0119] According to one embodiment of the present invention, the flatness ratio of the first electrode may be measured after activation. More specifically, the flatness ratio of the first electrode may be measured after activation, under normal standby and normal use conditions, i.e., before core deformation occurs. For example, the flatness ratio of the first electrode may be measured after 50 charge and discharge cycles under 25°C, 1C charge and 1C discharge conditions.

[0120] Here, the term "after activation" can mean after a predetermined cycle for the manufacture of the secondary battery and the completion of the product has been completed. More specifically, "after activation" may include the period before the start of active use, including multiple cycles for power supply purposes, i.e., the storage conditions before and after sale, and may also include the period during storage in which self-discharge has occurred.

[0121] The aforementioned activation may refer to a stage in which the stability of the battery is confirmed by repeatedly performing aging and charging / discharging after the assembly of the electrode assembly and battery case. After assembly, the "state after activation and before core deformation" can be achieved in a simple manner by performing a predetermined cycle, for example, 50 cycles under conditions of 25°C, 1C charging, and 1C discharging, on a battery obtained at any point in time. However, the activation conditions are not limited to those described above, as long as they are within the scope used in the industry to achieve the same objective.

[0122] Since the flatness ratio of the first electrode can be determined according to the curvature measurement timing, when the aforementioned curvature measurement timing is met, the degree to which the core portion deviates from a circular shape can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode can be increased.

[0123] Figure 2 is a graph showing the correlation between the flatness fraction of the first electrode and the angle formed by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum. Specifically, Figure 2(a) is a scatter plot matrix in which arbitrary factors extracted from CT images of 27 identical cells before and after acceleration cycles (1C / 1C 50 cycles) are quantified, and Figure 2(b) is a graph showing the correlation between the flatness fraction of the first electrode and the angle formed by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum.

[0124] Referring to Figure 2 for details, the location where the core deformation occurs may be determined by the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum, or by the value of the flatness fraction of the first electrode. The location where the core deformation of the electrode assembly occurs may coincide with the region where the curvature of the first electrode is 1 or less, i.e., the flat region. In other words, the lower the flatness fraction, the lower the possibility of core deformation occurring in the flat region, which is the location where the core deformation occurs.

[0125] On the other hand, as an additional influencing factor related to the occurrence of deformation in the core portion, when the first electrode tab is positioned at 12 o'clock, the second electrode tab located in the outermost shell is preferably positioned between 5 o'clock and 8 o'clock or between 6 o'clock and 7 o'clock, the longitudinal end of the first electrode located in the core portion is preferably positioned between 5 o'clock and 9 o'clock or between 6.5 o'clock and 8 o'clock, and the longitudinal end of the first electrode located in the outermost shell is preferably positioned between 4 o'clock and 6 o'clock or between 4.5 o'clock and 5.5 o'clock. On the other hand, the area of ​​the hollow core portion is 5 mm 2 15mm or more 2 or less or 7mm 2 13mm or more 2The following is preferable. If each of the aforementioned additional influencing factors satisfies the aforementioned range, the deformation reduction effect of the core can be further improved. In this case, the unit "hour" indicates the relative position of each factor in a clockwise direction when the first electrode tab is positioned at 12 o'clock on the CT image, and may be measured in the opposite direction to the direction in which the electrode assembly is wound, i.e., in the rotational direction of the winding core used to wind the electrode assembly. For example, 6 o'clock means making an angle of 180° with the first electrode tab positioned at 12 o'clock.

[0126] According to one embodiment of the present invention, in the core portion of the electrode assembly, the first electrode may be free from cracks or wrinkles. Here, the crack may mean a fissure visible on the surface of the first electrode, and the wrinkle may mean a wrinkle or crease visible on the surface of the first electrode.

[0127] When an electrode assembly including the first electrode in which the aforementioned crack has occurred is inserted into a battery case, the possibility of low voltage generation and short circuit occurrence due to foreign matter increases significantly. On the other hand, since the electrode assembly according to one embodiment of the present invention does not include a first electrode in which a crack or wrinkle has occurred in the core portion, the stability of the battery can be improved.

[0128] According to one embodiment of the present invention, in the core portion of the electrode assembly, the flatness fraction of the first electrode may have a positive correlation with the angle formed between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum. Specifically, referring to Figure 2(b), the correlation coefficient between the flatness fraction of the first electrode and the angle formed between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum may be approximately 0.756.

[0129] According to one embodiment of the present invention, in the core portion of the electrode assembly, the angle formed between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum, with respect to the winding shaft, may be greater than 40° and less than or equal to 98°. More specifically, in the core portion of the electrode assembly, the angle formed between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum, with respect to the winding shaft, may be greater than 40° and less than or equal to 98°, 45° or more and less than or equal to 95°, 45° or more and less than or equal to 90°, 45° or more and less than or equal to 80°, 45° or more and less than or equal to 70°, 45° or more and less than or equal to 65°, 50° or more and less than or equal to 95°, 50° or more and less than or equal to 90°, 50° or more and less than or equal to 80°, 50° or more and less than or equal to 70°, 50° or more and less than or equal to 65°, 60° or more and less than or equal to 95°, 60° or more and less than or equal to 90°, 60° or more and less than or equal to 80°, 60° or more and less than or equal to 70°, or 60° or more and less than or equal to 65°.

[0130] According to one embodiment of the present invention, the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum may be measured in a region greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound, with respect to the longitudinal end of the first electrode. That is, the point where the curvature is maximum may be the point where the curvature is maximum in a region greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound, with respect to the longitudinal end of the first electrode.

[0131] In other words, the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum may be measured in a region of greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound, that is, in the rotational direction of the winding core used to wind the electrode assembly, with respect to the longitudinal end of the first electrode.

[0132] More specifically, the stress generated by the progression of the cycle is concentrated in the hollow direction of the core due to the rigidity of the battery case, which may cause deformation of the core in the relatively stress-vulnerable flat region. In this case, with reference to the longitudinal end of the first electrode, the region in the opposite direction to the winding direction of the electrode assembly, i.e., the rotation direction of the winding core, which is more than 0° and less than or equal to 180°, has a relatively smaller number of turns of the first electrode up to the outermost shell layer compared to the region more than 180° and less than or equal to 360°. Because the distance from the sliding longitudinal end of the first electrode is relatively farther, the concentrated stress is not relieved, and therefore the possibility of deformation in the hollow direction of the core at that location may be higher.

[0133] Furthermore, the tension applied to the electrode assembly during winding causes twisting in the winding core. If the winding core includes a separation section, a point where the curvature of the first electrode is maximum and a flat area may occur symmetrically in the separation section. However, flat areas located in the region between 0° and 180° in the rotational direction of the winding core, relative to the longitudinal end of the first electrode, are vulnerable due to deformation of the core, and the likelihood of core deformation occurring after the acceleration cycle may be higher.

[0134] Therefore, if the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum is measured in a region between 0° and 180°, relative to the longitudinal end of the first electrode, in the direction opposite to the winding direction of the electrode assembly, the point where the curvature of the first electrode is maximum can be determined more efficiently, and the effect of reducing deformation of the core by adjusting the angle can be further improved.

[0135] When the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum satisfies the aforementioned range, the phenomenon in which the hollow of the core collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during charging and discharging of the battery, preventing damage to the second electrode and the separator membrane, and preventing internal short circuits between the first and second electrodes can be improved, thereby improving the stability and lifespan characteristics of the battery.

[0136] According to one embodiment of the present invention, in the core portion of the electrode assembly, the circularity of the first electrode may be 89% or more. Specifically, in the core portion of the electrode assembly, the circularity of the first electrode may be 89% or more or 90% or more, and may be 89% to 99% or 90% to 98%.

[0137] Here, the circularity (%) may be the ratio of the minimum separation distance between the winding shaft and the first electrode to 100% of the maximum separation distance between the winding shaft and the first electrode. Specifically, the circularity is the ratio (%) of the minimum separation distance (R max ) between the winding shaft and the first electrode to the maximum separation distance (R min ) between the winding shaft and the first electrode.

[0138] When the above-mentioned circularity range is satisfied, the shape of the electrode assembly can be closer to a circle, and it can be excellent in resistance to stress acting on the core portion. Thereby, when the battery is charged and discharged, the phenomenon that the hollow of the core portion cannot maintain a circular shape and collapses due to the deformation of the electrode assembly caused by the contraction / expansion of the electrode is improved, damage to the second electrode and the separator is prevented, and an internal short circuit between the first electrode and the second electrode can be prevented, so that the stability and life characteristics of the battery can be improved.

[0139] However, the roundness may differ from the flatness fraction, which represents the percentage of the region in the first electrode being measured that exhibits a curvature below a reference value. For example, depending on the shape of the electrode assembly, there may be cases where the roundness satisfies the aforementioned range, but the flatness fraction does not. In this case, by adjusting the flatness fraction to the aforementioned range in addition to the roundness, it is possible to eliminate cases where the shape of the electrode assembly deviates from a circle at a specific location. This makes it easier to control the shape of the electrode assembly and improves the reduction effect on core deformation compared to simply adjusting the roundness. In other words, the flatness fraction can be a more accurate standard for how "close to a circle" the shape of the electrode assembly is.

[0140] According to one embodiment of the present invention, the first electrode includes a first electrode current collector and a first electrode active material layer provided on at least one surface of the first electrode current collector, wherein the first electrode current collector and the first electrode active material layer may have longitudinal ends at the same location. That is, one longitudinal end of the first electrode may be in the form of a free edge.

[0141] This reduces the area of ​​the unnecessary blank portion of the first electrode current collector, thereby ensuring cost-effectiveness. It also allows the slitting process to be performed after the active material layer is formed on the electrode, and enables the roll-to-roll process, including the slitting and winding processes, to be carried out more efficiently.

[0142] Here, "same position" means that the longitudinal ends are the same, and may include cases where the ends are formed at substantially the same position due to process errors that may occur in the slitting process or other processes.

[0143] According to one embodiment of the present invention, the first electrode includes a blank portion of the first electrode where the first electrode active material layer is not provided, and may further include a first electrode tab provided on the blank portion of the first electrode.

[0144] In other words, the first electrode current collector may include a first electrode holding portion to which the first electrode active material is applied, and a first electrode blank portion to which the first electrode active material is not applied, and may include a tab on the first electrode blank portion. More specifically, the first electrode current collector may include a first electrode blank portion, and may include a first electrode tab provided on the first electrode blank portion.

[0145] In other words, one longitudinal end of the first electrode may have a free-edge shape, the plain portion of the first electrode may be located between the longitudinal ends of the first electrode, and the first electrode tab provided on the plain portion of the first electrode may be a middle tab.

[0146] According to one embodiment of the present invention, the first electrode current collector is not particularly limited as long as it does not cause a chemical change in the battery and is conductive. More specifically, the first electrode current collector can be stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. In other words, the first electrode current collector may be provided in the form of surface-treated stainless steel, aluminum foil, etc.

[0147] Furthermore, the first electrode current collector may typically have a thickness of 3 to 50 μm, and fine irregularities can be formed on the surface of the current collector to enhance the adhesion of the first electrode active material. For example, it may be used in various forms such as film, sheet, foil, net, porous material, foam, or nonwoven fabric.

[0148] According to one embodiment of the present invention, the first electrode active material may be a commonly used first electrode active material. Specifically, the first electrode active material may be a layered compound such as lithium cobalt oxide (LiCoO2) or lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound with the chemical formula Li 1+x Mn 2-xLithium manganese oxides such as O4 (0 ≤ x ≤ 0.33), LiMnO3, LiMn2O3, LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides such as LiV3O8, V2O5, Cu2V2O7; chemical formula LiNi 1-y M y Ni-site type lithium nickel oxide represented as O2 (where M is at least one selected from the group consisting of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01 ≤ y ≤ 0.3); chemical formula LiMn 2-z M z Lithium manganese composite oxides represented as O2 (where M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, satisfying 0.01 ≤ z ≤ 0.1) or Li2Mn3MO8 (where M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4 in which part of the Li in the chemical formula is substituted with an alkaline earth metal ion, etc., are examples, but are not limited to these. The first electrode may be Li-Metal.

[0149] According to one embodiment of the present invention, the first electrode active material layer may further include a first electrode conductive material and a first electrode binder. The first electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations as long as it does not cause chemical changes and has electronic conductivity in the battery that is constructed. Specifically, the first electrode conductive material may be graphite such as natural graphite or artificial graphite; carbon-based materials such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal black, or carbon fiber; metal powder or metal fiber such as copper, nickel, aluminum, or silver; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxides such as titanium oxide; or conductive polymers such as polyphenylene derivatives, and one of these alone or a mixture of two or more may be used.

[0150] Furthermore, the first electrode binder plays a role in improving the adhesion between the first electrode active material particles and the adhesion between the first electrode active material and the first electrode current collector. Specific examples include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and one or more of these can be used.

[0151] According to one embodiment of the present invention, the second electrode may include a second electrode current collector and a second electrode active material layer provided on the second electrode current collector. More specifically, the second electrode may include a second electrode current collector and a second electrode active material layer formed on one or both sides of the second electrode current collector, and containing the second electrode active material. That is, the second electrode active material layer is formed on the second electrode holding portion of the second electrode current collector, and the surface on which the second electrode active material layer is not provided can be represented as a blank portion of the second electrode.

[0152] According to one embodiment of the present invention, the second electrode current collector may include a second electrode holding portion on which a second electrode active material layer is formed, and a second electrode blank portion on which the second electrode active material layer is not formed, and may include a tab on the second electrode blank portion. Specifically, the second electrode current collector may include a second electrode blank portion, and may include a second electrode tab formed on the second electrode blank portion. As a result, the manufactured electrode assembly may include one or more second electrode tabs.

[0153] According to one embodiment of the present invention, the second electrode active material layer may include a second electrode active material comprising one or more selected from the group consisting of silicon-based materials and carbon-based materials. The second electrode active material layer may further include a second electrode conductive material and a second electrode binder, and the second electrode active material, second electrode conductive material, and second electrode binder can be any material used in the industry without limitation.

[0154] According to one embodiment of the present invention, the second electrode current collector can be any material that does not cause a chemical change in the battery and is conductive, and is not particularly limited. For example, the second electrode current collector can be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel with a surface treatment of carbon, nickel, titanium, silver, etc. More specifically, transition metals that adsorb carbon well, such as copper and nickel, can be used as the second electrode current collector. The thickness of the second electrode current collector may be 5 μm or more and 30 μm or less, but the thickness of the second electrode current collector is not limited thereto.

[0155] According to one embodiment of the present invention, the second electrode binder may contain at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethylcellulose (CMC), starch, hydroxypropylcellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and substances in which the hydrogen atoms of these substances are substituted with Li, Na, or Ca, and may also contain various copolymers thereof.

[0156] According to one embodiment of the present invention, the second electrode conductive material is not particularly limited as long as it does not cause a chemical change in the battery and is conductive, but for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal black; conductive fibers such as carbon fibers and metal fibers; conductive tubes such as carbon nanotubes; metal powders such as fluorocarbon, 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 can be used.

[0157] According to one embodiment of the present invention, the electrode assembly may include a plurality of separation membranes. For example, the electrode assembly may have a structure in which separation membrane / second electrode / separation membrane / first electrode are stacked in that order. The separation membrane separates the first electrode and the second electrode and provides a passage for lithium ions to move. It can be used without particular limitations as long as it is the type of separation membrane that is normally used in secondary batteries, and it is especially preferable that it has low resistance to ion movement of the electrolyte while having excellent electrolyte moisture retention capacity. Specifically, porous polymer films, such as porous polymer films made from polyolefin polymers such as ethylene homopolymer, propylene homopolymer, ethylene / butene copolymer, ethylene / hexene copolymer, and ethylene / methacrylate copolymer, or stacked structures of two or more layers thereof can be used. Alternatively, ordinary porous nonwoven fabrics, such as nonwoven fabrics made of high-melting-point glass fibers or polyethylene terephthalate fibers, may be used. Furthermore, the separation membrane may be made by using the aforementioned separation membrane material as the base layer and coating the base layer with a slurry containing ceramic components or polymer substances to ensure heat resistance or mechanical strength, and may be used selectively in a single-layer or multi-layer structure. The thickness of the separation membrane may be 5 μm or more and 20 μm or less, but is not limited thereto.

[0158] According to one embodiment of the present invention, the angle between the first electrode and the second electrode may be 25° or less. Specifically, the first electrode includes a first surface facing the winding shaft of the electrode assembly and a second surface opposite the first surface, and the first extension line drawn by extending a straight line connecting two points on the first surface of the first electrode where the curvature direction changes within a distance of 5 mm from the longitudinal end of the first electrode, and the second extension line drawn by extending a straight line connecting two points on the surface of the second electrode facing the first surface of the first electrode where the distance from the longitudinal end of the first electrode is 5 mm, may form an angle of 25° or less. In this case, the angle between the first electrode and the second electrode may be measured, for example, after activation and 200 or more additional charge and discharge cycles at 40°C or higher.

[0159] When the aforementioned angular range is met, the end of the first electrode located in the core and the adjacent second electrode do not deform beyond a certain level. As a result, the separation membrane located between the first and second electrodes is not damaged, and the phenomenon of the first and second electrodes coming into direct contact, i.e., core impingement, can be suppressed. This prevents damage to the second electrode and the separation membrane, and prevents internal short circuits between the first and second electrodes, thereby improving battery stability and lifespan characteristics.

[0160] According to one embodiment of the present invention, the first electrode and the second electrode may be a positive electrode and a negative electrode, respectively. Specifically, the first electrode may be a positive electrode, and the second electrode may be a negative electrode.

[0161] One embodiment of the present invention provides a secondary battery including the electrode assembly and a battery case for housing the electrode assembly. More specifically, the secondary battery may include the electrode assembly according to the above embodiment and the battery case for housing the electrode assembly.

[0162] The secondary battery according to the present invention improves the phenomenon in which the hollow core collapses due to deformation of the electrode assembly caused by the contraction / expansion of electrodes during charging and discharging, thereby preventing damage to the second electrode and the separator membrane, and preventing internal short circuits between the first electrode and the second electrode, thus improving battery stability and lifespan characteristics.

[0163] According to one embodiment of the present invention, the battery case may be cylindrical. More specifically, the battery case may be cylindrical, rectangular, or pouch-shaped, depending on the application, but is not limited thereto.

[0164] According to one embodiment of the present invention, the inside of the battery case may contain an electrolyte. Specifically, the electrolyte may include, but is not limited to, organic liquid electrolytes, inorganic liquid electrolytes, solid polymer electrolytes, gel-type polymer electrolytes, solid inorganic electrolytes, or molten inorganic electrolytes that can be used in the manufacture of lithium secondary batteries. Specifically, the electrolyte may contain a non-aqueous organic solvent and a metal salt.

[0165] According to one embodiment of the present invention, as the non-aqueous organic solvent, aprotic organic solvents such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methylsulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl propionate, and ethyl propionate can be used.

[0166] According to one embodiment of the present invention, the metal salt may be a lithium salt, and the lithium salt is a substance that is easily soluble in the non-aqueous electrolyte, for example, the anion of the lithium salt is F - Cl - , I - NO3 - , N(CN)2 - BF4 - ClO4 - PF6 - (CF3)2PF4 - (CF3)3PF3 - (CF3)4PF2 - (CF3)5PF - (CF3)6P - CF3SO3 - CF3CF2SO3 - , (CF3SO2)2N - , (FSO2)2N - CF3CF2(CF3)2CO - (CF3SO2) 2CH - (SF5)3C - (CF3SO2)3C - CF3(CF2)7SO3 - CF3CO2 - CH3CO2 - SCN - and (CF3CF2SO2)2N - You may use one or more selected from the group consisting of the following:

[0167] According to one embodiment of the present invention, in addition to the components of the electrolyte, the electrolyte may further contain one or more additives for the purpose of improving the battery life characteristics, suppressing the decrease in battery capacity, and improving the discharge capacity of the battery, such as haloalkylene carbonate compounds such as difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ether, ethylenediamine, n-glyme, hexaphosphate triamide, nitrobenzene derivative, sulfur, quinone imine dye, N-substituted oxazolidinone, N,N-substituted imidazolidine, ethylene glycol dialkyl ether, ammonium salt, pyrrole, 2-methoxyethanol, or aluminum trichloride.

[0168] One embodiment of the present invention provides a battery module and a battery pack containing the secondary battery as a unit cell. Because the battery module and battery pack include the secondary battery with improved high capacity, high battery stability, and life characteristics, they can be used as a power source for medium to large devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0169] The present invention will be described in detail below with reference to examples. However, the examples of the present invention can be modified into various other forms, and the scope of the present invention should not be construed as being limited to the examples described below. The examples herein are provided to give a more complete explanation of the present invention to a person of average skill in the art.

[0170] Examples

[0171] Example 1

[0172] Manufacturing of electrode assemblies

[0173] Li(Ni) is used as the first electrode active material. 0.89 Co 0.07 Mn 0.04A slurry of the first electrode active material was prepared by adding O2, CNTs as the first electrode conductive material, and polyvinylidene fluoride (PVdF) as the first electrode binder to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 97.92:0.5:1.58. The slurry of the first electrode active material was coated onto an aluminum current collector with a thickness of 15 μm and a width of 63.9 mm, and then dried and rolled to form a first electrode active material layer, thereby preparing a first electrode with a thickness of 135 μm.

[0174] Next, natural graphite (C, average particle size 17 μm) was prepared as the second electrode active material, and the second electrode active material, carbon black as the second electrode conductive material, and styrene butadiene rubber (SBR) as the second electrode binder were mixed in a weight ratio of 97.7:1.3:1.0 to produce the second electrode active material composition. Then, 7.8 g of distilled water was added to 5 g of the second electrode active material composition and stirred to produce a second electrode active material slurry. The second electrode active material slurry was applied to a copper (Cu) metal thin film, which was the second electrode current collector, with a thickness of 8 μm and a width of 65.2 mm, and dried (drying temperature 120°C, 1 minute) to form a second electrode with an average thickness of 166 μm. At this time, the temperature of the circulating air was 60°C.

[0175] Subsequently, two separation membranes were arranged sequentially and wound onto a 3.2 mm diameter core, and then the second electrode was inserted between the two separation membranes and wound further. In this process, the core used included a separation section into which the separation membranes were inserted, a first core section, and a second core section with a different cross-sectional area from the first core section, with the cross-sectional areas of the first and second core sections being 5.4 mm² each. 2 , 2.7mm 2 That was the case.

[0176] After the separation membrane and the second electrode have been wound up for approximately 3 turns, the first electrode is inserted and wound up, and a PET sealing tape is attached to the end where the winding ends, surrounding the outer circumferential surfaces of the upper and lower ends of the electrode assembly.

[0177] In this process, the positions of the longitudinal ends and tabs located in the first electrode core and outermost shell were adjusted as shown in Table 1 below, based on the first core located at the 6 o'clock position and the second core located at the 12 o'clock position of the electrode assembly. Here, FE (Free-edge) means that the electrode current collector and the electrode active material layer have their longitudinal ends at the same position, and the unit 'hours' indicates the relative positions of each factor in a clockwise direction when the first electrode tab is positioned at the 12 o'clock position on the CT image.

[0178] The area and roundness of the hollow core, measured from computed tomography (CT) images, are shown in Table 1 below. The angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum was determined by comparing the curvature measured at each measurement point on the CT image. Using the longitudinal end of the first electrode as a reference, the measurement point where the curvature is maximum in the region between 0° and 180° in the direction opposite to the winding direction of the electrode assembly was defined as the point where the curvature of the first electrode is maximum (curvature Max.). The angle between the point where the curvature of the first electrode is maximum and the longitudinal end of the first electrode, centered on the winding axis, was measured and is shown in Table 2 below. In the following examples, comparative examples, and experimental examples, the point where the curvature of the first electrode is maximum (curvature Max.) was measured using the same method.

[0179] [Table 1]

[0180] Manufacturing of rechargeable batteries

[0181] After inserting the electrode assembly into a cylindrical battery case, ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) are mixed in a volume ratio of 20:5:75 to obtain a LiPF6 solution of 1.4 M (mol / dm³). 3 A rechargeable battery was manufactured by injecting an electrolyte solution dissolved to the desired state, sealing the cylindrical battery case with a cap assembly, and then adding the electrolyte solution.

[0182] In this case, after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode was maximum was 60.8°, and the flatness ratio of the first electrode was 7.72%.

[0183] Example 2

[0184] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 65°, and the length of the winding separation membrane and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 9.6%.

[0185] Comparative Example 1

[0186] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 98.9°, and the length of the winding separation membrane and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 13.9%.

[0187] Comparative Example 2

[0188] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 112.4°, and the length of the separation membrane wound up and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 17.0%.

[0189] Comparative Example 3

[0190] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 129.3°, and the length of the winding separation membrane and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 16.49%.

[0191] Comparative Example 4

[0192] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 40°, and the length of the winding separation membrane and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 3.0%.

[0193] Comparative Example 5

[0194] An electrode assembly and a secondary battery were manufactured in the same manner as in Example 1, except that after 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions, the angle between the longitudinal end of the first electrode extracted from the core CT image and the point where the curvature of the first electrode is maximum was 20°, and the length of the winding separation membrane and the position of the first electrode at the time of insertion were adjusted so that the flatness ratio of the first electrode was 1.0%.

[0195] Experimental example

[0196] Experimental Example 1: Flatness Fraction Evaluation

[0197] The flatness ratios of Example 1, Example 2, and Comparative Examples 1 to 5 to secondary batteries were evaluated using the following method, and the results are shown in Table 2 and Figures 3 to 9 below, respectively.

[0198] 1) Extract the first electrode from the CT image of the electrode assembly.

[0199] 2) On the first surface of the first electrode, measurement points are set at 2° intervals from one end in the longitudinal direction of the first electrode, and the x and y coordinates of the measurement points are measured.

[0200] 3) Using the x and y coordinates of the measurement points, the curvature (k) is calculated for each measurement point according to the following formula 1.

[0201] [Formula 1] k=(x'y''-y'x'') / (x' 2 +y' 2 ) 3 / 2

[0202] In equation 1 above, k is the curvature of the first electrode, x is the x-coordinate of the measurement point, y is the y-coordinate of the measurement point, x' is the first derivative with respect to x, y' is the first derivative with respect to y, x'' is the second derivative with respect to x, and y'' is the second derivative with respect to y.

[0203] 4) If the curvature value calculated at each measurement point is 1 or less, that measurement point is evaluated as a flat area, and the percentage of measurement points evaluated as flat areas is evaluated as the flatness fraction (%) based on 100% of the total number of measurement points.

[0204] Experimental Example 2: Core Collapse Evaluation

[0205] The presence or absence of Core Collapse in the secondary batteries of Example 1, Example 2, and Comparative Examples 1 to 5 was evaluated using the method described below, and the presence or absence of Core Impingement was re-evaluated every 200 cycles. The results are shown in Table 2 and Figures 3 to 9 below, respectively.

[0206] 1) Extract the first electrode from the CT image of the electrode assembly.

[0207] 2-1) Measure the area of ​​the hollow core surrounded by the first electrode, from one end of the first electrode in the longitudinal direction to a point corresponding to one turn in the longitudinal direction of the first electrode.

[0208] 2-2) Core Collapse was evaluated as having occurred if the hollow area of ​​the core was less than 100% of the total cross-sectional area of ​​the separated portion, the first core portion, and the second core portion of the core used to wind the electrode assembly, based on 100% of the total cross-sectional area.

[0209] 3-1) The minimum diameter of the hollow core portion of the first electrode extracted on the first surface of the first electrode (D min ) and maximum value (D max Construct extensions of the lines corresponding to the given lines, and use the intersection points of each line as the central axis, i.e., the winding axis, to construct concentric circles.

[0210] 3-2) From one end of the first electrode in the longitudinal direction to a point corresponding to one turn in the longitudinal direction of the first electrode, the maximum separation distance (R) between the winding shaft and the first electrode. max ) and the minimum separation distance (R) between the winding shaft and the first electrode min ) are measured separately.

[0211] 3-3) The maximum separation distance (R) between the winding shaft and the first electrode max The minimum separation distance (R) between the winding shaft and the first electrode relative to ) minThe percentage (%) of the hollow core was calculated, i.e., the roundness of the hollow core. If the calculated roundness of the hollow core was less than 89%, it was determined that a core collapse had occurred.

[0212] On the other hand, the method for evaluating whether or not Core Collapse occurs can be applied by first evaluating whether or not Core Collapse occurs at the time of acquisition of an unknown secondary battery (Unknown Cell), re-evaluating whether or not Core Collapse occurs every 200 cycles, and comparing and analyzing the results with the Core Collapse conditions of the secondary battery according to the embodiment of the present invention.

[0213] Experimental Example 3: Core Impingement Evaluation

[0214] The presence or absence of core impingement in the secondary batteries of Example 1, Example 2, and Comparative Examples 1 to 5 was evaluated using the following method, and the presence or absence of core impingement was re-evaluated every 200 cycles. The results are shown in Table 2 and Figures 3 to 9 below, respectively.

[0215] Figure 10 schematically illustrates the method for evaluating whether or not core impingement occurs. Specifically, Figure 10(a) schematically illustrates the method for evaluating whether or not core impingement occurs when deformation occurs in the second electrode, and Figure 10(b) schematically illustrates the method for evaluating whether or not core impingement occurs when no deformation occurs in the second electrode.

[0216] 1) On the first surface of the first electrode 300, extend the straight line connecting the longitudinal end 310 of the first electrode and a point 5 mm away from the end to draw the first extension line E1.

[0217] 2-1) If deformation occurs in the second electrode

[0218] On the core portion of the electrode assembly, on the surface of the second electrode 100 facing the first surface of the first electrode, a straight line is drawn from the longitudinal end 310 of the first electrode, connecting two points where the curvature direction changes within a distance of 5 mm, to create a second extension line E2.

[0219] 2-2) When no deformation has occurred in the second electrode

[0220] On the core portion of the electrode assembly, on the surface of the second electrode 100 facing the first surface of the first electrode, a straight line is drawn by extending a line connecting two points 5 mm apart from the longitudinal end 310 of the first electrode, thereby creating a second extension line E2.

[0221] 3) Core impingement was determined to have occurred if the angle from the first extension line E1 to the second extension line E2 in a counterclockwise direction, centered on the intersection of the first extension line E1 and the second extension line E2, exceeded 25°.

[0222] On the other hand, the method for evaluating whether or not Core Impingement occurs can be applied by first evaluating whether or not Core Impingement occurs at the time of acquisition of an unknown secondary battery (Unknown Cell), re-evaluating whether or not Core Impingement occurs every 200 cycles, and comparing and analyzing the results with the Core Impingement conditions of the secondary battery according to the embodiment of the present invention.

[0223] Experimental Example 4: Evaluation of Core Crack Occurrence

[0224] The presence or absence of core cracks in the electrode assemblies of Example 1, Example 2, and Comparative Examples 1 to 5 was evaluated using the method described below, and the results are shown in Table 2, Figure 8, and Figure 9, respectively.

[0225] In detail, electrode assemblies for Examples 1 and 2 and Comparative Examples 1 to 5 were prepared, and the manufactured electrode assemblies were disassembled to visually check for the presence or absence of cracks or wrinkles in the core. If cracks or wrinkles were found in the core, it was evaluated that a core crack had occurred.

[0226] [Table 2]

[0227] Figure 8 shows a CT image of the electrode assembly according to Comparative Example 4 and an image of crack occurrence in the first electrode core, and Figure 9 shows a CT image of the electrode assembly according to Comparative Example 5 and an image of crack occurrence in the first electrode core. Specifically, Figures 8(a) and 9(a) are CT images of the electrode assembly according to Comparative Example 4 and Comparative Example 5, respectively, and Figures 8(b) and 9(b) are exploded images showing cracks that occurred in the first electrode of the core portion of the electrode assembly according to Comparative Example 4 and Comparative Example 5. Here, the circles indicate the region where core impingement occurred and the region where cracks occurred in the first electrode of the core portion, corresponding to the longitudinal end of the first electrode, respectively. Referring to Table 2, Figure 8(b), and Figure 9(b), it can be seen that in the electrode assemblies of Comparative Example 4 and Comparative Example 5, the flatness fraction of the first electrode in the core portion of the electrode assembly is 3% or less, and the stress generated by the twisting of the winding core is concentrated at the longitudinal end of the first electrode, which causes wrinkles or cracks in the core portion.

[0228] During the winding process, if cracks occur in the first electrode and resulting foreign matter develop in the core of the electrode assembly, the defect rate may increase and process efficiency may decrease. Therefore, by excluding the input of the first electrode, which is expected to develop cracks, from the manufacturing process, a decrease in the productivity of the electrode assembly and secondary battery can be prevented.

[0229] In more detail, when an electrode assembly with cracks is inserted into a battery case, the possibility of low voltage and short circuits occurring due to foreign matter on the first electrode within the battery case increases significantly. Therefore, the possibility of cracks occurring on the first electrode can be reduced by adjusting the flatness fraction of the first electrode during the winding process. In other words, the electrode assembly and the method for manufacturing the electrode assembly according to one embodiment of the present invention are suitable for continuous processes using existing roll-to-roll process equipment, and it can be seen that productivity and economic efficiency of the electrode assembly including the first electrode and the secondary battery can be ensured.

[0230] Reference experiment example: Evaluation of electrode sliding range

[0231] Reference Experiment Example 1

[0232] The secondary battery described in Example 1 was prepared and subjected to 50 charge and discharge cycles under the conditions of 25°C, 1C charge, and 1C discharge.

[0233] Under 4.25V-2.5V, 25℃, and 1C charging and discharging conditions, the system underwent 200 cycles from SOC 0% to SOC 100%. The position of the longitudinal end of the first electrode at SOC 0% and SOC 100% was extracted and recorded from CT images, and the sliding range of the longitudinal end of the first electrode was measured. Subsequently, the sliding range of the longitudinal end of the first electrode was evaluated from CT images before activation and at SOC 100%, and is shown in Figure 11 below.

[0234] Reference Experiment Example 2

[0235] The sliding range of the longitudinal end of the first electrode was measured in the same manner as in Reference Experiment Example 1, except that the secondary battery of Example 2 was used. Subsequently, the sliding range of the longitudinal end of the first electrode was evaluated from CT images before activation and at SOC 100%, and is shown in Figure 11 below.

[0236] Figure 11 is a CT image showing the sliding range of the longitudinal end of the first electrode of the secondary battery according to Reference Experiment Example 1 and Reference Experiment Example 2. More specifically, Figure 11 shows the CT images of the secondary battery before activation and at 100% SOC, and the sliding range of the longitudinal end of the first electrode according to Reference Experiment Example 1 and Reference Experiment Example 2. More specifically, Figure 11 shows the sliding range by comparing the position of the longitudinal end of the first electrode before activation and at 100% SOC, with the second electrode tab located at the outermost shell as the reference point.

[0237] Referring to Figure 11, it was confirmed that in Reference Experiment 1, a sliding motion of 18.41°(SOC100)-16.72°(before activation)=1.69° occurred, and in Reference Experiment 2, a sliding motion of 55.08°(SOC100)-53.30°(before activation)=1.78° occurred. In other words, in both Reference Experiment 1 and Reference Experiment 2, the measurement results confirmed that sliding occurred in the range of 1° to 3° relative to the initial position of the longitudinal end of the first electrode.

[0238] This shows that, in the secondary batteries according to Example 1 and Example 2, first electrode sliding may occur due to electrode contraction / expansion before activation and at SOC 100%, but sliding occurs within a specific range (1°~3°).

[0239] In other words, the winding process is carried out while maintaining a tension above a certain value, and the position of the point where the curvature of the first electrode is maximum is kept constant. Therefore, even as the cycle progresses, the range of the angle between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum is maintained within the sliding range (1° to 3°) of the longitudinal end of the first electrode.

[0240] Figures 3 and 4 show CT images of the electrode assemblies according to Example 1 and Example 2, and images showing the curvature of the first electrode extracted from the CT images.

[0241] Figures 5 to 7 show CT images of the electrode assemblies according to Comparative Examples 1 to 3, and images showing the curvature of the first electrode extracted from the CT images.

[0242] In detail, Figures 3 to 7 above show, in black and white coordinates, the proportion of measurement points with a curvature below an arbitrarily set reference value relative to the curvature measured at each measurement point. Here, the set reference value is a specific value within the range of 0.5 to 1.0, and the black and white index range expressed to the right of the black and white coordinates is 0 to 0.8 in Figures 3 to 7 (a) and 1 to 1.5 in Figures 3 to 7 (b).

[0243] More specifically, Figures 3 to 7(a) visualize the flat areas and flatness fraction by representing the regions where low k values ​​are distributed in dark gray when the black and white distribution range of the curvature measured at each measurement point is set to 0 to 0.8, and Figures 3 to 7(b) visualize the points where the curvature is maximum by representing the regions where high k values ​​are distributed in dark gray when the black and white distribution range of the curvature measured at each measurement point is set to 1 to 1.5. In this case, each measurement point was extracted from the CT image of the first electrode as described above, and it can be confirmed that the longitudinal end of the first electrode located in the core is located at the 6 o'clock position.

[0244] Referring to Table 2, Figures 3-7, Figure 8(a), and Figure 9(a), the electrode assemblies according to Example 1 and Example 2 underwent 50 cycles under 4.25V-2.5V, 25℃, 1C charging, and 1C discharging conditions. The flatness ratios of the first electrode extracted from the core CT image were 7.72% and 9.6%, respectively. It was also confirmed that no core collapse or core impingement occurred even after 500 cycles under 4.2V (0.25C)-2.85V (0.33C), 40℃ conditions.

[0245] On the other hand, as in Comparative Examples 1 to 3, if the flatness ratio of the first electrode extracted from the core CT image exceeds 10%, or as in Comparative Examples 4 and 5, if the flatness ratio of the first electrode extracted from the core CT image is 3% or less and does not meet the aforementioned range of flatness ratios, then the angle between the first extension line and the second extension line is 25° or more, and core impingement occurs. Alternatively, even if core impingement does not occur, the area of ​​the hollow core is less than 100% or the roundness of the hollow core is less than 89% of the total cross-sectional area of ​​the winding core used for winding, and core collapse occurs.

[0246] As a result, the electrode assembly and secondary battery containing the same according to one embodiment of the present invention improve the phenomenon in which the hollow of the core portion collapses due to deformation of the electrode assembly caused by the contraction / expansion of the electrodes during battery charging and discharging, by adjusting the flatness ratio of the first electrode extracted from the core portion CT image to a specific range. This prevents damage to the second electrode and the separator membrane, and prevents internal short circuits between the first and second electrodes, thus improving the stability and life characteristics of the battery.

[0247] The above detailed description is illustrative and explanatory of the present invention. Furthermore, the foregoing describes preferred embodiments of the present invention, and as stated above, the present invention can be used in a variety of other combinations, modifications, and environments, and can be modified or altered within the scope of the concepts of the present invention disclosed herein, the scope equivalent to the foregoing disclosures, and / or within the scope of the art or knowledge of those skilled in the art. Accordingly, the above detailed description of the invention is not intended to limit the present invention to the disclosed embodiments. Furthermore, the appended claims should be construed to include other embodiments as well. [Explanation of symbols]

[0248] 100...2nd electrode 300...1st electrode 310 ···Longitudinal end of the first electrode E1 ··· First extension line E2 ··· Second extension line A···The core part of the 1st volume B... Volume 2 Core

Claims

1. First electrode; an electrode assembly in which a separation membrane and a second electrode are stacked and wound around a winding shaft, The first electrode includes a first surface facing the winding shaft of the electrode assembly and a second surface that is opposite to the first surface. An electrode assembly in which the flatness fraction of the first electrode is greater than 3% and less than or equal to 13.5% within or in part of the core portion of the electrode assembly.

2. The electrode assembly according to claim 1, wherein the core portion of the electrode assembly is a region within three turns from the inner end of the first electrode.

3. The first electrode includes a first electrode current collector; and a first electrode active material layer provided on at least one surface of the first electrode current collector. The first electrode active material layer extends to the inner end of the first electrode current collector, The electrode assembly according to claim 1, wherein the inner end of the first electrode current collector corresponds to the inner end of the first electrode.

4. The first electrode includes a blank portion of the first electrode where the first electrode active material layer is not provided. The electrode assembly according to claim 3, further comprising a first electrode tab provided on or physically connected to the plain portion of the first electrode.

5. An extension line connecting the winding shaft and the inner end of the first electrode; and An extension line connecting the winding shaft and the point where the curvature of the first electrode is 1 or less; The electrode assembly according to claim 1, wherein the angle formed by the electrodes is greater than 0°.

6. The electrode assembly according to claim 1, wherein the flatness ratio of the first electrode is determined after activation of the electrode assembly.

7. The flatness ratio of the first electrode is The electrode assembly according to claim 1, determined after 50 charge and discharge cycles under the conditions of 25°C, 1C charge, and 1C discharge.

8. An extension line connecting the winding shaft and the inner end of the first electrode; and The extension line connecting the winding shaft and the point of maximum curvature of the first electrode The electrode assembly according to claim 1, wherein the angle formed is greater than 40° and less than or equal to 98°.

9. Using the cross-section of the electrode assembly perpendicular to the winding shaft as a reference, The electrode assembly according to claim 8, wherein the point of maximum curvature of the first electrode is the point where the curvature of the first electrode is maximum in the region where the azimuth angle of the electrode assembly is greater than 0° and less than or equal to 180° in the direction opposite to the winding direction of the electrode assembly, with reference to the inner end of the first electrode.

10. The electrode assembly according to claim 1, wherein the circularity of the first electrode is 89% or more inside or in part of the core portion of the electrode assembly.

11. A first extension line drawn by extending a straight line connecting two points on the first surface of the first electrode where the curvature direction changes within a distance of 5 mm from the inner end of the first electrode; and On the surface of the second electrode facing the first surface of the first electrode, the second extension line drawn by extending a straight line connecting two points that are 5 mm apart from the inner end of the first electrode is, The electrode assembly according to claim 1, having an angle of 25° or less.

12. The electrode assembly according to claim 1, wherein the first electrode is free from cracks or wrinkles inside or in part of the core portion of the electrode assembly.

13. A secondary battery comprising an electrode assembly according to any one of claims 1 to 12.

14. The battery case for housing the electrode assembly further includes, The secondary battery according to claim 13, wherein the battery case is cylindrical.

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