Electrode assembly and secondary battery comprising same

Abstract

The present invention relates to an electrode assembly in which a first electrode, a separator, and a second electrode are stacked and wound around a winding axis, wherein the first electrode comprises a first surface facing the winding axis of the electrode assembly and a second surface opposite to the first surface, and the flatness fraction of the first electrode is more than 3% and 13.5% or less inside or in a portion of a core part of the electrode assembly.

Description

Electrode assembly and secondary battery including the same

[0001] The present invention relates to an electrode assembly and a secondary battery including the same, and more particularly, to an electrode assembly having improved core deformation by changing the design of the electrode assembly, and a cylindrical secondary battery including the same. This application claims the benefit of Korean Patent Application No. 10-2023-0149978, filed with the Korean Intellectual Property Office on November 2, 2023, the entire contents of which are incorporated herein by reference.

[0002] In the case of cylindrical batteries, a long electrode with a fixed width is rolled into a jelly-roll-shaped electrode assembly. The cylindrical battery manufactured by inserting this electrode assembly into a battery case experiences repeated contraction / expansion of the electrode during charging and discharging. In particular, when a tab is located in the core of the electrode assembly or a silicon-based active material is added to the negative electrode, thereby increasing the degree of contraction / expansion of the electrode assembly, the pressure acting on the core of the electrode assembly increases significantly.

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

[0004] Recently, as low-resistance / high-capacity designs have increased, the number of cases in which electrode assemblies include multiple tabs or silicon-based active materials is increasing. Accordingly, the possibility of core deformation of the electrode assembly due to shrinkage / expansion of the electrode assembly increases. In particular, there has been a problem that the battery life is deteriorated due to a phenomenon in which the hollow core part cannot maintain its circular shape and collapses (core collapse), and the end of the positive electrode located in the core part and the adjacent negative electrode are deformed to a certain degree or more, resulting in a phenomenon in which the separator located between the positive and negative electrodes is damaged (core impingement), causing the positive and negative electrodes to come into direct contact and cause an internal short, resulting in heat generation and ignition.

[0005] In order to solve the problems of deterioration of battery life, damage to the separator, and occurrence of internal short circuits due to deformation of the electrode assembly, it is necessary to develop a technology that can improve the phenomenon in which the hollow core of the relevant area fails to maintain its circular shape and collapses, and suppress the occurrence of internal short circuits due to damage to the separator.

[0006] The present invention aims to provide an electrode assembly in which core deformation is improved 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 problems mentioned above, and other problems not mentioned will be clearly understood by those skilled in the art from the description below.

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

[0009] One embodiment of the present invention provides an electrode assembly comprising a first electrode, a separator, and a second electrode, which 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 the first surface. The flatness fraction of the first electrode in the interior or a portion of the core portion of the electrode assembly may be greater than 3% and less than or equal to 13.5%. Throughout the present 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 their meanings used in the secondary battery-related technical field, particularly from the perspective of battery design and manufacturing. The first electrode and / or the second electrode may be configured to receive and store charge carriers such as electrons and / or ions, particularly lithium ions. The first electrode and the second electrode may have opposite polarities, such that they may receive and release charge carriers, respectively, during a charging process of the electrode assembly, and conversely, they may likewise release and receive charge carriers, respectively, during a discharging process of the electrode assembly. The materials and functions of the first electrode and the second electrode may be those known in the technical field, unless otherwise specified.

[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, which may be 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 material. 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, which may be 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] Separators are used in secondary battery-related technologies, particularly from the perspective of battery design and manufacturing. In particular, separators are generally impermeable to solid materials, but may be porous membranes configured to allow charge carriers to pass through them. Charge carriers may be, for example, ions, lithium ions, and / or electrons. Separators are provided between the first and second electrodes to prevent short circuits between them.

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

[0014] The first electrode, the separator, and the second electrode may be provided as layers (or sheets). The first electrode, the separator, and the second electrode may be laminated in a specific order. Specifically, the second electrode, the separator, the first electrode, and the other separator may be sequentially laminated to form a mono-cell. In another example, one or more mono-cells may be sequentially and repeatedly laminated on a mono-cell to form a multi-cell.

[0015] The electrode assembly may include a mono-cell or multi-cell wound in a roll shape. The roll-shaped electrode assembly may also be referred to as a jelly roll. Specifically, in the mono-cell, the first electrode, the separator, the second electrode, and optionally another separator may be wound together about a winding axis. Accordingly, the electrode assembly may have a cylindrical shape, particularly a cylindrical shape having a spiral or spiral cross-section. In addition, the first electrode, the separator, and the second electrode of the electrode assembly may be sequentially stacked and wound together about the winding axis. Accordingly, the electrode assembly generally has a cylindrical symmetry about the winding axis. In the cylindrical symmetry, the axial direction may be parallel to the winding axis of the electrode assembly. The radial direction may be perpendicular to the winding axis and perpendicular to the outer peripheral surface of the electrode assembly. In addition, the circumferential direction may be perpendicular to the axial direction and the radial direction, and may mean a direction along a circular path centered on the winding axis. In addition, the winding direction of the electrode assembly may mean a direction along at least one of the first electrode, the separator, and the second electrode in a plan view, and accordingly, the winding direction may mean a direction along a spiral path centered on the winding axis. Depending on the context, the winding direction may be approximated as a circle centered on the winding axis, similar to the circumferential direction, from the perspective of the rotational direction.

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

[0017] Unless otherwise specified, geometric features such as lines, angles, and curvatures generally used throughout this specification may be defined in a plan view, i.e., a cross-section from a perspective parallel to the winding axis of the electrode assembly. Accordingly, unless otherwise specified, the geometric features may represent the approximately cylindrical geometry of the electrode assembly when wound about the winding axis. Furthermore, unless otherwise specified, all lines used herein may represent straight lines.

[0018] Throughout this specification, cylindrical symmetry may be approximate 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 represent a cross-section from a perspective parallel to the winding axis. The electrode assembly may have a helical end facing the winding axis since the first electrode, the separator, and the second electrode are wound around the winding axis. Meanwhile, the cross-section of the electrode assembly may represent a cross-section perpendicular to the winding axis. Since the first electrode, the separator, and the second electrode are wound around the winding axis, the cross-section of the electrode assembly may be spiral about the winding axis.

[0019] At least one of the first electrode, the separator, and the second electrode may have a rectangular shape in a plan view before winding. The winding axis may be parallel to at least one of the ends of the first electrode, the separator, and the second electrode. In a specific example, the first electrode, the separator, and the second electrode may each have a rectangular shape in a plan view, and the ends of the first electrode, the separator, and the second electrode may be arranged so as to be aligned parallel to each other.

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

[0021] An electrode assembly according to one embodiment of the present invention can suppress a phenomenon in which a hollow core portion of the electrode assembly fails to maintain a circular shape and collapses due to deformation of the electrode assembly by controlling the flatness fraction of the first electrode within a specific range.

[0022] Therefore, the electrode assembly according to one embodiment of the present invention can contribute to preventing damage to the first electrode, separator, and 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. Through this, it can contribute to improving battery stability and life characteristics.

[0023] One embodiment of the present invention provides a method for manufacturing an electrode assembly. This method achieves the flatness ratio of the first electrode in the manner described above, thereby enabling a continuous manufacturing process using, for example, existing roll-to-roll processing equipment, thereby increasing battery productivity and economic efficiency. This method can also achieve the aforementioned and / or the following technical effects.

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

[0025] In addition, the maximum curvature point of the first electrode may be provided on the inside or part of the electrode assembly within 3 turns in the winding direction of the electrode assembly from the inner end of the first electrode, as described later.

[0026] The core portion may refer to a portion of an electrode assembly surrounded by the third turn from the inner end of the first electrode. The core portion may correspond to the core portion described above. The core portion may be implemented through some or all of the features described below.

[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 based on the state of the electrode assembly before winding, and may correspond to the winding direction used in the present specification 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 an inner end of the first electrode current collector, and the inner end of the first electrode current collector may correspond to an inner end of the first electrode.

[0029] In the plan view, as the first electrode is wound in the winding direction around the winding axis, the first electrode extends from the inner end to the outer end. For example, the inner end of the first electrode may be located at a radially inner position in a region adjacent to the winding axis. For example, the outer end of the first electrode may be located at a radially outer position in a region adjacent to the outer circumferential surface of the electrode assembly. Alternatively or additionally, the same may be applied 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 a direction opposite to the winding direction, and forms an inner end, which is the other longitudinal end, at the same position as the longitudinal end of the first electrode current collector. That is, the first electrode cannot extend beyond the first electrode current collector in the direction opposite to the winding direction.

[0031] The active material layer may refer to a layer including or composed of an electrode active material as described above. 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 first electrode non-conductive portion that does not have a first electrode active material layer. In addition, the electrode assembly may include a first electrode tab that is provided on or physically connected to the first electrode non-conductive portion.

[0033] The first electrode non-conductive portion may mean a part of the first electrode, particularly a first electrode current collector, and may mean a part of the first electrode that is not provided with a first electrode active material layer, i.e. a part of the first electrode current collector that is not provided with an active material of the first electrode.

[0034] The electrode tab may be formed from or at least one from the first electrode uncoated portion, or may be provided on the first electrode uncoated portion. The first electrode uncoated portion may be cut or notched. For example, at least one first electrode tab may be formed by forming one or more slits, notches, cuts, or the like from an edge of the first electrode uncoated portion to the uncoated portion of the first electrode. Alternatively or additionally, at least one electrode tab may be provided separately and attached to the first electrode current collector or the first electrode uncoated 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 where the curvature of the winding shaft and the first electrode is 1 or less, may be greater than 0°.

[0036] The mathematical definition of curvature may be as described above or 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 portion of the electrode assembly. The angle may refer to the azimuth angle described above, and the vertex or apex 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 activation of the electrode assembly.

[0038] Throughout this specification, activation may refer to a process of preparing an electrode assembly for use in a secondary battery by applying specific temperature and charge / discharge conditions to an electrode assembly or a battery cell including the electrode assembly. Activation may be referred to as formation. Activation may include an aging step, a charging step, and a discharging step, which may be performed in a specific order and optionally repeated in a specific order. The aging step may be performed so that the electrolyte penetrates into the first electrode and the second electrode. For example, the aging step may be performed by storing the electrode assembly at a specific temperature, for example, 30° C., for a specific period of time, for example, 30 minutes, 1 hour, 2 hours, or 3 hours. The charging step may be performed so that the electrolyte decomposes on the surface of the negative electrode to form a solid electrolyte interphase (SEI). The charging step may include a process of charging the electrode assembly to a specific degree. After the charging step, a high temperature aging process may optionally be performed at a high temperature, for example, 40° C., 50° C., 60° C., or 70° C. Afterwards, the electrode assembly may be subjected to a discharging step at a specific C-rate, for example, 0.1 C., 0.2 C., 0.5 C., or 1.0 C. 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. Accordingly, the technical effects achieved by the present invention may be particularly advantageous in the electrode assembly after at least a portion of the activation step.

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

[0041] Therefore, the flatness ratio may be determined after performing at least 50 charge and discharge cycles of the electrode assembly. Accordingly, the condition or quality of the electrode assembly may be more accurately determined.

[0042] According to one embodiment of the present invention, an angle formed by 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 maximum curvature point may be greater than 40° and less than or equal to 98°. In particular, the maximum curvature point 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 axis and the inner end of the first electrode, i.e., a first straight line, can be drawn based on a cross-section perpendicular to the winding axis of the electrode assembly. For example, based on a cross-section perpendicular to the winding axis of the electrode assembly, the first straight line may be an imaginary line extending from the winding axis through the inner end of the first electrode. As described below, the first straight line may be drawn to determine an angle with an extension connecting the winding axis and a point of maximum curvature, i.e., a second straight line.

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

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

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

[0047] Throughout this specification, the term "curvature" may be used in its usual mathematical sense. In particular, curvature may represent a measure of how much a curve deviates from a straight line. Additionally or alternatively, curvature may represent a measure of how much a surface deviates from a plane. Here, curvature may represent how much the direction of a curve changes over infinitesimal distances for any portion of the curve, for example, angle per distance. Curvature may be a measure of the change in direction of a point along a curve. In particular, curvature may be a measure of the instantaneous rate of change of a unit tangent vector to a curve at a point P as the point P moves along the curve with unit velocity. In a specific example, the position P(s) of a point may be a function of a parameter s, which may be, for example, time or the length of an arc about a given origin. Furthermore, T(s) may be a unit tangent vector to 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 can be a vector perpendicular to the curve, and its length can be the curvature.

[0048] Throughout this specification, a curve may mean a cross-section, and / or an outline, a contour, etc., of the first electrode in a plan view, i.e., a view parallel to the winding axis. In particular, since the first electrode may be provided as a sheet or a layer, the first electrode is recognized as a thick curve in a plan view. Additionally or alternatively, the same may be applied to the second electrode and / or the separator. The curve may be continuously differentiable near P, so that the tangent may vary continuously along the curve. The curve may be differentiated twice at any P, so that a curvature may exist along the curve, for example, as a derivative of the aforementioned T(s) with respect to s.

[0049] Throughout this specification, curvature can be determined from the perspective of an osculating circle on a curve. The osculating circle of a curve at a point P can be a circle having the same tangent and the same curvature as the curve. The tangent line can be an approximation of the curve at a point P on the curve. Furthermore, the osculating circle can be an approximation of the curve at a point P. The curvature of a straight line can be zero. If the curvature of a point is not zero, the reciprocal of the curvature is considered the radius of curvature, i.e., the radius of the osculating circle. The center of the osculating circle is considered the center of curvature, and can be constructed by indicating the radius of curvature perpendicular to the tangent line of the curve in the direction in which the curve bends.

[0050] Additionally, the curvature used here can be determined as the derivative (or differential value) of the central angle for the arc of the contact circle at the center of the contact circle.

[0051] One method for determining the curvature of the first electrode described above may be to determine the curvature from a plan view image or cross-sectional image of the electrode assembly taken from a perspective parallel to the winding axis. In particular, a curve may be determined using an image of the electrode assembly taken from a perspective parallel to the winding axis, i.e., a cross-sectional view of the visualized first electrode, and the curvature may be determined from the curve.

[0052] Throughout this specification, a curvature maximum point is a point where the curvature of the first electrode or the second electrode is maximum in a plan view of the electrode assembly, and may represent a point on or inside the first electrode or the second electrode. Accordingly, the curvature maximum point may be a point on the first electrode where the first electrode exhibits the greatest curvature in a plan view of the electrode assembly. Additionally or alternatively, the curvature maximum point may be a point on the first electrode where the contact area of ​​the first electrode is the smallest. Additionally or alternatively, the same may be applied to the second electrode and / or the separator.

[0053] According to one embodiment of the present invention, the curvature of the first electrode may generally decrease as the first electrode extends from the winding axis in the outer circumferential direction of the electrode assembly. In other words, the curvature of the first electrode may generally decrease along the radial outer circumferential direction of the first electrode, the spiral outer circumferential direction of the first electrode, or the winding direction. Since the first electrode, the separator, and the second electrode are wound together around the winding axis, the curvature of the first electrode may generally decrease along the radial outer circumferential direction or the winding direction. Additionally or alternatively, the curvature of the second electrode and / or the separator may generally decrease along the radial outer circumferential direction.

[0054] At the same time, the maximum curvature point may be located closer to the winding axis than the outer circumferential surface of the electrode assembly. In the plan view of the electrode assembly, the 'core portion' means a portion closer to the winding axis than the outer circumferential surface of the electrode assembly. The core portion may be further defined as described below. The maximum curvature point may be provided in the interior or a part of the core portion of the electrode assembly. The maximum curvature point may be applied to the first electrode, but additionally or alternatively, the same may be applied to the second electrode and / or the separator.

[0055] Throughout this specification, as defined above, the 'winding direction' may mean the winding direction of the first electrode and / or the second electrode and the separator, which may be a spiral path centered on the winding axis. That is, based on the plan view and / or the 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, the winding direction of the first electrode may mean a circular direction as a directionality in the side of the rotational direction centered on the winding axis.

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

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

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

[0059] Throughout this specification, "circularity" may refer to circularity or roundness according to mathematical or geometrical principles. In particular, the roundness may be determined according to ISO 1101. Throughout this specification, the roundness may be determined as a two-dimensional parameter in the plan view of the electrode assembly. Alternatively or additionally, the 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 where the curvature direction changes within a distance of 5 mm from the inner end of the first electrode. On one surface of the second electrode opposite to the first surface of the first electrode, the second extension line may be drawn by extending a straight line connecting two points where the distance is 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 not include cracks or wrinkles in the interior or 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, in which an electrode assembly is accommodated.

[0064] One embodiment of the present invention provides an electrode assembly in which a first electrode, a separator, and a second electrode are laminated and wound, wherein the first electrode includes a first surface facing a winding axis of the electrode assembly and a second surface opposite the first surface, and a flatness fraction of the first electrode in a core portion of the electrode assembly 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 accommodating the electrode assembly.

[0066] An electrode assembly according to one embodiment of the present invention can improve a phenomenon in which a hollow core part cannot maintain a circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging by controlling the flatness fraction of the first electrode, and can prevent damage to the second electrode and separator and prevent internal short circuit between the first electrode and the second electrode, thereby improving battery stability and life characteristics.

[0067] In addition, the secondary battery according to the present invention can improve the phenomenon in which the hollow core part cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging, prevent damage to the second electrode and separator, and prevent internal short circuit between the first electrode and the second electrode, so that the battery stability and life characteristics can be improved.

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

[0069] Figure 1 shows the correlation between the angle formed by 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 portion.

[0070] Figure 2 is a graph showing the correlation 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.

[0071] FIG. 3 is an image showing a CT image of an electrode assembly according to Example 1 and the curvature of the first electrode extracted from the CT image.

[0072] Figure 4 is an image showing a CT image of an electrode assembly according to Example 2 and the curvature of the first electrode extracted from the CT image.

[0073] Figure 5 is an image showing a CT image of an electrode assembly according to Comparative Example 1 and the curvature of the first electrode extracted from the CT image.

[0074] Figure 6 is an image showing a CT image of an electrode assembly according to Comparative Example 2 and the curvature of the first electrode extracted from the CT image.

[0075] Figure 7 is an image showing a CT image of an electrode assembly according to Comparative Example 3 and the curvature of the first electrode extracted from the CT image.

[0076] Figure 8 is a CT image of an electrode assembly according to Comparative Example 4 and an image of a crack occurring in the first electrode core portion.

[0077] Figure 9 is a CT image of an electrode assembly according to Comparative Example 5 and an image of a crack occurring in the first electrode core portion.

[0078] Figure 10 schematically illustrates a method for evaluating whether core impingement has occurred.

[0079] 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 Experimental Example 1 and Reference Experimental Example 2.

[0080] Throughout this specification, whenever a part is said to "include" a component, this does not mean that it excludes other components, but rather that it may include other components, unless otherwise stated.

[0081] Throughout this specification, when it is said that an element is "on" another element, this includes not only cases where the element is in contact with the other element, but also cases where another element exists between the two elements.

[0082] One embodiment of the present invention provides an electrode assembly in which a first electrode, a separator, and a second electrode are laminated and wound, wherein the first electrode includes a first surface facing a winding axis of the electrode assembly and a second surface opposite the first surface, and a flatness fraction of the first electrode in a core portion of the electrode assembly is greater than 3% and less than or equal to 13.5%.

[0083] Here, the flatness ratio may be a ratio of the number of measurement points having a curvature of 1 or less based on 100% of the number of measurement points located on the first surface of the first electrode.

[0084] Additionally, the above 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 as described below.

[0085] An electrode assembly according to one embodiment of the present invention can improve a phenomenon in which a hollow core part cannot maintain a circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging by controlling the flatness fraction of the first electrode, and can prevent damage to the second electrode and separator and prevent internal short circuit between the first electrode and the second electrode, thereby improving battery stability and life characteristics.

[0086] Specifically, when the battery is charged and discharged, the electrodes included in the electrode assembly undergo repeated shrinkage / expansion, and since the expansion outward is limited by the rigidity of the battery case surrounding the outer surface of the electrode assembly, stress may be concentrated in the hollow direction, which is the empty space located in the core portion. Accordingly, the possibility of core deformation of the electrode assembly may increase.

[0087] The above core deformation may be classified into core collapse, a phenomenon in which the core cavity of the electrode assembly fails to maintain its circular shape and collapses when internal stress reaches a certain level or higher, and core impingement, a phenomenon in which the end of the first electrode located in the core and the second electrode adjacent thereto are deformed to a certain level or higher, the separator located between the first electrode and the second electrode is damaged, and the first electrode and the second electrode come into direct contact.

[0088] In particular, the core collapse may be concentrated in an area vulnerable to internal stress of the electrode assembly. Specifically, the core collapse may be concentrated in an area where the curvature of the first electrode is below a specific value, i.e., a flat area. That is, in the core of the electrode assembly, deformation due to internal stress may begin to occur from an area where the curvature of the first electrode is below a specific value, and after a certain point in time, the hollow core may no longer maintain its circular shape and may completely collapse. In an electrode assembly where core collapse has occurred, lithium ions can no longer be smoothly transferred between the first electrode and the second electrode, which may cause a deterioration in the battery life.

[0089] Here, the curvature of the first electrode may be affected by the step due to the thickness of the electrode, the tab, etc., the shape of the core, the tension applied to the electrode assembly during winding, etc. For example, the core may include a pair of core parts separated around a separation portion into which a separator is inserted, and when a certain level of tension is applied, the core may be distorted, and as a result, a region in which the curvature of the first electrode is lower than a specific value, i.e., a flat region, may inevitably be generated, and core collapse may occur in the region in which the curvature of the first electrode is lower than a specific value.

[0090] At this time, if the flatness fraction of the first electrode is adjusted to a specific range, the phenomenon of the core collapse, in which the hollow core part fails to maintain its circular shape and collapses, may be improved. This may prevent damage to the second electrode and separator and prevent internal short circuits between the first and second electrodes, thereby improving battery stability and life characteristics.

[0091] Figure 1 shows the relationship between the angle formed by 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.

[0092] Specifically, (a) to (c) of FIG. 1 illustrate a CT image of an electrode assembly according to an embodiment of the present invention and a curvature of a first electrode extracted from the CT image. More specifically, (a) of FIG. 1 illustrates a flat area of ​​the electrode assembly in relation to the core, (b) of FIG. 1 illustrates a point where the curvature of the electrode assembly is maximum in relation to the core, and (c) of FIG. 1 illustrates a flat area of ​​the electrode assembly and a point where the curvature is maximum in relation to the core, respectively. Meanwhile, (d) of FIG. 1 is an image in which core deformation occurs in a flat area of ​​the electrode assembly according to (a) to (c) of FIG. 6 after a cycle has been performed.

[0093] Specifically, (a) to (c) of the above Figs. 1 represent the ratio of measurement points having a curvature lower than an arbitrarily set reference value among the curvatures measured at each measurement point, expressed in color coordinates. Here, the set reference value is a specific value within the range of 0.5 to 1.0, and the color index range expressed on the right side of the color coordinates is 0 to 0.8 in (a) of Fig. 1, and 1 to 1.5 in (b) of Fig. 1.

[0094] More specifically, Fig. 1 (a) visualizes the flat area and flatness fraction by implementing the area where the low k value is distributed as a color coordinate when the color distribution range of the curvature measured at each measurement point is set to 0 to 0.8, and Fig. 1 (b) visualizes the point where the curvature is maximum by implementing the area where the high k value is distributed as a color coordinate when the color distribution range of the curvature measured at each measurement point is set to 1 to 1.5. At this time, each measurement point is 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 direction.

[0095] According to one embodiment of the present invention, the winding may be performed using a core including a separation portion into which a separator is inserted; a first core portion provided on one side of the separation portion with the separation portion as the center; and a second core portion provided on the other side of the separation portion and having a different cross-sectional area from the first core portion.

[0096] Specifically, the core may include a separation portion into which a separator is inserted, and a first core portion may be provided on one side of the separation portion, and a second core portion having a different cross-sectional area from the first core portion may be provided on the other side. By having the separation portion in the core, a laminate such as a separator inserted into the separation portion may be wound in a direction opposite to the rotational direction of the core by the rotation of the core.

[0097] At this time, since the core includes a first core portion and a second core portion having a different cross-sectional area from the first core portion, distortion may occur in the core portion having a relatively small size when a high tension is applied to the core portion. Accordingly, the point where the curvature of the first electrode is maximum may be located at a position adjacent to a relatively large core centered on the separation portion, and the flat area of ​​the first electrode may be located at a position adjacent to a relatively small core.

[0098] In addition, with respect to the longitudinal end of the first electrode, the region of 0° to 180° in the direction opposite to the winding direction of the electrode assembly, that is, the rotational direction of the core, has a relatively small number of turns of the first electrode up to the outermost layer compared to the region of 180° to 360°, and the distance from the longitudinal end of the first electrode, which is capable of sliding, is relatively long, so that the concentrated stress is not relieved, and therefore, the possibility of deformation in the hollow direction of the core portion at that location may be higher.

[0099] That is, the core may include a pair of cores separated with a separation portion in which a membrane is inserted as the center, and when a certain level of tension is applied, distortion of the core occurs, and as a result, a region in which the curvature of the first electrode is below a specific value, i.e., a flat region, may inevitably occur, and core collapse may occur in the region in which the curvature of the first electrode is below a specific value.

[0100] At this time, when the flatness fraction of the first electrode in the core portion of the electrode assembly is adjusted, the phenomenon of the core portion hollow portion failing to maintain its circular shape and collapsing, i.e., the core portion collapse phenomenon, may be improved. Through this, damage to the second electrode and separator can be prevented, and internal short circuit between the first electrode and the second electrode can be prevented, so that battery stability and life characteristics may be improved.

[0101] 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%. 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%, greater than or equal to 5% and less than or equal to 13%, greater than or equal to 5% and less than or equal to 10%, greater than or equal to 7% and less than or equal to 10%, or greater than or equal to 7.5% and less than or equal to 10%.

[0102] When the flatness ratio range of the first electrode described above is satisfied, the phenomenon of the core cavity not maintaining its circular shape and collapsing due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging can be improved, damage to the second electrode and separator can be prevented, and internal short circuit between the first electrode and the second electrode can be prevented, so that battery stability and life characteristics can be improved.

[0103] Here, the flatness fraction (%) may refer to the ratio of the flat area of ​​the first electrode, which is the measurement target, and the flat area may refer to an area having a curvature less than a reference value.

[0104] That is, the flatness ratio may refer to the ratio of an area in the first electrode, which is a measurement target, that exhibits a curvature value lower than or equal to a reference curvature value. For example, the flatness ratio may be the ratio (%) of the number of measurement points having a curvature of 1 or less based on 100% of the number of measurement points.

[0105] Since the flatness ratio of the first electrode may be determined based on the above-mentioned standard curvature value, when the range of the above-mentioned standard curvature value is satisfied, the degree to which the core part deviates from a circle can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode may be higher.

[0106] Hereinafter, with respect to the flatness fraction (%) of the first electrode of the present invention, the measurement target of curvature, the method of measuring curvature, and the time point of measuring curvature will be described in more detail.

[0107] According to one embodiment of the present invention, the curvature may be measured for the first electrode in the core portion of the electrode assembly.

[0108] Here, the 'core portion' may be a region including a hollow portion located on the winding axis of the electrode assembly and a part of the laminated structure of the wound separator / second electrode / separator / first electrode.

[0109] In other words, the core portion may include a hollow portion located on the winding axis of the electrode assembly, and may include a region up to one end in the longitudinal direction of the first electrode located at the innermost angle where winding of the electrode assembly begins, i.e., a region not including the first electrode. In addition, the core portion may further include a region of a predetermined length from one end in the longitudinal direction of the first electrode, including the first electrode in the direction in which the first electrode is wound.

[0110] 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. Specifically, the core portion may be a region within 1 to 2.5 turns or within 1.5 to 2 turns from one longitudinal end of the first electrode.

[0111] Here, 1 turn may mean the length required for 360° winding of an electrode or separator included in an electrode assembly from a reference point, and the length may be determined according to the outer diameter of the core used for winding the electrode assembly, the thickness of the separator and electrode, and the number of turns of the separator and electrode located inside the reference point. For example, 1 turn of the first electrode may mean the length required to wind the first electrode 360° from a longitudinal end of the first electrode in the direction in which the electrode assembly is wound.

[0112] In other words, the core portion may mean an area from one longitudinal end of the first electrode to a point spaced apart by 3 turns or less, an area from one longitudinal end of the first electrode to a point spaced apart by 1 to 2.5 turns, or an area from one longitudinal end of the first electrode to a point spaced apart by 1.5 to 2 turns.

[0113] Since the flatness ratio of the first electrode may be determined according to the range of the core portion, when the range of the core portion described above is satisfied, the degree to which the core portion deviates from a circle can be determined more efficiently, and the reliability of the flatness ratio of the first electrode determined may be higher.

[0114] According to one embodiment of the present invention, the curvature may be measured at the core portion of the electrode assembly, may be measured for the first electrode at the core portion of the electrode assembly, and may be measured by extracting the first electrode from a CT image of the core portion of the electrode assembly.

[0115] When the object of the above curvature measurement is the first electrode located in the core portion of the electrode assembly, the reliability of the curvature measurement may be higher, and by adjusting the position of the longitudinal end portion of the first electrode, the flatness ratio of the first electrode may be more easily adjusted. Through this, the deformation of the end portion of the first electrode located in the core portion and the adjacent second electrode may be reduced, and the effect of improving the phenomenon of the separator located between the first electrode and the second electrode being damaged, i.e., core impingement, may also be obtained.

[0116] 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. Specifically, the curvature may be measured based on the first surface of the extracted first electrode, which faces the winding axis of the electrode assembly, after extracting the first electrode from the CT image of the electrode assembly. More specifically, the curvature may be calculated from a plurality of measurement points located at a constant interval on the first surface of the first electrode. For example, the plurality of measurement points may be located at an interval of 2° on the first surface of the extracted first electrode with the winding axis as the center.

[0117] According to one embodiment of the present invention, the number of measurement points may be 180 or more. Specifically, the number of measurement points may be 180 or more and 720 or less. More specifically, the number of measurement points may be 240 or more and 660 or less, 300 or more and 600 or less, or 360 or more and 540 or less.

[0118] Since the flatness ratio of the first electrode may be determined based on the number of the above-described measurement points, when the range of the number of the above-described measurement points is satisfied, the degree to which the core part deviates from a circle can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode may be higher.

[0119] According to one embodiment of the present invention, the curvature may be measured at separate measurement points within a range of the aforementioned measurement points selected at regular intervals or arbitrarily from the aforementioned core portion, thereby determining a point of maximum curvature and a flatness ratio.

[0120] According to one embodiment of the present invention, the curvature may be a curvature according to the following Equation 1. Specifically, the curvature calculated at the measurement point may be calculated according to the following Equation 1 using coordinate values, i.e., x-coordinate and y-coordinate, obtained at a plurality of measurement points located on the first surface of the first electrode extracted from the CT image.

[0121] [Formula 1]

[0122] k=(x'y''-y'x'') / (x' 2 +y' 2 ) 3 / 2

[0123] In the above equation 1, 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 differential value of x, y' is the first differential value of y, x'' is the second differential value of x, and y'' is the second differential value of y.

[0124] In other words, the calculation of the curvature may mean measuring x-coordinate and y-coordinate values ​​at a plurality of measuring points having a constant interval located on the first surface of the first electrode, and calculating the curvature value using the measured x-coordinate and y-coordinate and Equation 1. Specifically, the curvature may be calculated as a parameter expression of a plane curve, and the curvature, i.e., the degree of bending, may increase as the k value according to Equation 1 increases. More specifically, the curvature may be calculated using the Equation through a Python program, and the x-coordinate and y-coordinate values ​​may have a range from -3 to +3. Meanwhile, the x' and y' may mean the slope of the tangent line at each coordinate, and the x'' and y'' may mean the rate of change of the slope.

[0125] If necessary, the above curvature value may be normalized using the radius of curvature of the electrode assembly for each measurement point.

[0126] According to one embodiment of the present invention, in the core portion of the electrode assembly, an angle formed between a 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 axis, may be greater than 0°.

[0127] 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 axis, may be greater than 0° and less than 50°.

[0128] 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 axis, may be 5° or more and less than 50°, 10° or more and 45° or less, or 15° or more and 40° or less.

[0129] In other words, the area where the curvature of the first electrode is 1 or less, i.e., the flat area, 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.

[0130] According to one embodiment of the present invention, the flatness ratio of the first electrode may be measured after activation. Specifically, the flatness ratio of the first electrode may be measured in a normal use standby state and a normal use state after activation, i.e., before core deformation occurs. For example, the flatness ratio of the first electrode may be measured after 50 cycles of charging and discharging under conditions of 25°C, 1C charging, and 1C discharging.

[0131] Here, the meaning of "post-activation" may refer to a period after a predetermined cycle for manufacturing and completing a secondary battery has been completed. Specifically, "post-activation" may include a storage state before the start of active use, including multiple cycles for power supply purposes, i.e., before and after sale, and may also include a state in which self-discharge occurs during storage.

[0132] The above activation may refer to a step of confirming the stability of the battery by repeating aging and charging / discharging after assembly of the electrode assembly and the battery case, and by performing a predetermined cycle, for example, 50 cycles under the conditions of 1C / 1C @25℃, on a battery obtained at any point after assembly, the 'state before core deformation after activation' can be achieved in a simple way. However, it is not limited to the above activation conditions as long as it is within the range used in the art to achieve the same purpose.

[0133] Since the flatness ratio of the first electrode may be determined according to the above-described curvature measurement time point, when the above-described curvature measurement time point is satisfied, the degree to which the core part deviates from a circle can be determined more efficiently, and the reliability of the determined flatness ratio of the first electrode may be higher.

[0134] Fig. 2 is a graph showing the correlation 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. Specifically, Fig. 2 (a) is a scatter plot matrix that quantifies arbitrary factors extracted from CT images before and after 27 acceleration cycles (1C / 1C 50 cycles) of the same cell, and Fig. 2 (b) is a graph showing the correlation 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.

[0135] Specifically, referring to FIG. 2, the location where the deformation of the core portion occurs may be determined based on the angle formed between the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum or the value of the flatness fraction of the first electrode, and the location where the deformation of the core portion of the electrode assembly occurs may coincide with an area where the curvature of the first electrode is 1 or less, i.e., a flat area. That is, the lower the flatness fraction, the lower the possibility of the deformation of the core portion occurring in the flat area, which is the location where the deformation of the core portion occurs.

[0136] Meanwhile, as an additional influencing factor related to the occurrence of the deformation of the core portion, when the first electrode tab is positioned at 12 o'clock, the second electrode tab located at the outermost portion is preferably positioned at 5 o'clock or more and 8 o'clock or less or 6 o'clock or more and 7 o'clock or less, and the longitudinal end of the first electrode located at the core portion is preferably positioned at 5 o'clock or more and 9 o'clock or less or 6.5 o'clock or more and 8 o'clock or less, and the longitudinal end of the first electrode located at the outermost portion is preferably positioned at 4 o'clock or more and 6 o'clock or less or 4.5 o'clock or more and 5.5 o'clock or less. Meanwhile, the area of ​​the hollow core portion is 5 mm 2 More than 15 mm 2 less than or equal to 7 mm 2More than 13 mm 2 The following is preferable. When the aforementioned additional influencing factors each satisfy the aforementioned range, the core deformation reduction effect may be more excellent. At this time, 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 winding direction of the electrode assembly, that is, the rotational direction of the core used for winding the electrode assembly. For example, 6 o'clock means forming a 180° angle with the first electrode tab positioned at 12 o'clock.

[0137] According to one embodiment of the present invention, in the core portion of the electrode assembly, the first electrode may not include a crack or wrinkle. Here, the crack may refer to a split that is visually visible on the surface of the first electrode, and the wrinkle may refer to a case where a wrinkle or fold that is visually visible occurs on the surface of the first electrode.

[0138] When an electrode assembly including a first electrode with a crack is inserted into a battery case, the possibility of low voltage and short circuits due to foreign substances increases significantly. On the other hand, the electrode assembly according to one embodiment of the present invention does not include a first electrode with a crack or wrinkle in the core portion, so battery stability can be improved.

[0139] 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 by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum. Specifically, referring to (b) of FIG. 2, the correlation coefficient 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 may be about 0.756.

[0140] 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 axis, may be greater than 40° and less than 98°. 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 axis, may be greater than 40° and less than 98°, greater than 45° and less than 95°, greater than 45° and less than 90°, greater than 45° and less than 80°, greater than 45° and less than 70°, greater than 45° and less than 65°, greater than 50° and less than 95°, greater than 50° and less than 90°, greater than 50° and less than 80°, greater than 50° and less than 70°, greater than 50° and less than 65°, greater than 60° and less than 95°, greater than 60° and less than 90°, greater than 60° and less than 80°, greater than 60° and less than 70°, or greater than 60° and less than 65°.

[0141] According to one embodiment of the present invention, the angle formed by the longitudinal end of the first electrode and the point at which the curvature of the first electrode is maximum may be measured in a region exceeding 0° and less than 180° in the opposite direction to the winding direction of the electrode assembly based on the longitudinal end of the first electrode. That is, the point at which the curvature is maximum may be a point at which the curvature is maximum in a region exceeding 0° and less than 180° in the opposite direction to the winding direction of the electrode assembly based on the longitudinal end of the first electrode.

[0142] In other words, the angle formed by 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 180° in the direction opposite to the direction in which the electrode assembly is wound, i.e., the rotational direction of the winding core used for winding the electrode assembly, based on the longitudinal end of the first electrode.

[0143] Specifically, since the stress generated as the cycle progresses is concentrated in the direction of the hollow core due to the rigidity of the battery case, deformation of the core may occur in a relatively stress-vulnerable flat region. At this time, with respect to the longitudinal end of the first electrode, in the direction opposite to the winding direction of the electrode assembly, that is, in the direction of rotation of the core, the number of turns of the first electrode up to the outermost layer is relatively small compared to the region exceeding 180° and 360°, and the distance from the longitudinal end of the first electrode, which is capable of sliding, is relatively large, so that the concentrated stress is not relieved, and therefore, the possibility of deformation in the direction of the hollow core may be higher at that location.

[0144] In addition, since the tension applied to the electrode assembly during winding causes distortion of the core, when the core includes a gap, the point where the curvature of the first electrode is maximum and the flat area may be symmetrically generated at the gap, but the flat area located in the area exceeding 0° and less than 180° in the rotational direction of the core based on the longitudinal end of the first electrode may be more vulnerable to core deformation and may have a higher possibility of core deformation occurring even after an acceleration cycle.

[0145] Accordingly, when the angle formed by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum is measured in a region of more than 0° and less than 180° in the direction opposite to the winding direction of the electrode assembly based on the longitudinal end of the first electrode, the point where the curvature of the first electrode is maximum can be determined more efficiently, and the effect of reducing core deformation according to angle adjustment can be more excellent.

[0146] When the angle formed by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum satisfies the above-mentioned range, the phenomenon in which the hollow core part cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging is improved, damage to the second electrode and separator is prevented, and internal short circuit between the first electrode and the second electrode is prevented, so that battery stability and life characteristics can be improved.

[0147] 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, or 89% or more and 99% or less, or 90% or more and 98% or less.

[0148] Here, the circularity (Circularity, %) may be a ratio of the minimum separation distance between the winding axis and the first electrode based on the maximum separation distance of 100% between the winding axis and the first electrode. Specifically, the circularity may be a ratio of the maximum separation distance (R) between the winding axis and the first electrode. max ) for the minimum separation distance (R) between the winding axis and the first electrode min ) may mean the ratio (%).

[0149] When the above-described circularity range is satisfied, the shape of the electrode assembly can be closer to a circle, and the resistance to stress applied to the core portion can be excellent. Accordingly, the phenomenon of the hollow portion of the core portion not maintaining its circular shape and collapsing due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging can be improved, damage to the second electrode and separator can be prevented, and internal short circuit between the first electrode and the second electrode can be prevented, so that battery stability and life characteristics can be improved.

[0150] However, the roundness may be different from the flatness fraction, which refers to the ratio of the area in the first electrode, which is the measurement target, that exhibits a curvature below a reference value. For example, depending on the shape of the electrode assembly, there are cases where the roundness satisfies the above-mentioned range, but the flatness fraction does not satisfy the above-mentioned range. In this case, by adjusting the flatness fraction in addition to the roundness to the above-mentioned range, the case where the shape of the electrode assembly deviates from a circle at a specific location can be excluded. Accordingly, compared to the case where the roundness is simply adjusted, shape control of the electrode assembly can be easier, and the effect of reducing core deformation can be more excellent. In other words, the flatness fraction can be a more accurate criterion for the 'degree to which the shape of the electrode assembly is close to a circle'.

[0151] 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, and the first electrode current collector and the first electrode active material layer may have longitudinal ends at the same position. In other words, one longitudinal end of the first electrode may be in the form of a free-edge.

[0152] Through this, the area of ​​the unnecessary first electrode current collector non-conducting portion can be reduced to secure economic feasibility, and since the slitting process can be performed after forming the active material layer on the electrode, the roll-to-roll process including the slitting process and the winding process can be performed more efficiently.

[0153] Here, 'same position' means that the longitudinal ends are the same, and may include cases where the ends are formed at substantially the same positions due to process errors that may occur during the slitting process, etc.

[0154] According to one embodiment of the present invention, the first electrode may include a first electrode non-conductive portion that does not have a first electrode active material layer, and may further include a first electrode tab provided on the first electrode non-conductive portion.

[0155] In other words, the first electrode current collector may include a first electrode holding portion to which a first electrode active material is applied and a first electrode non-conductive portion to which the first electrode active material is not applied, and may include a tab on the first electrode non-conductive portion. Specifically, the first electrode current collector may include a first electrode non-conductive portion, and may include a first electrode tab provided on the first electrode non-conductive portion.

[0156] That is, one longitudinal end of the first electrode may have a free-edge shape, the first electrode uncoated portion may be positioned between both longitudinal ends of the first electrode, and the first electrode tab provided on the uncoated portion of the first electrode may be a middle tab.

[0157] According to one embodiment of the present invention, the first electrode current collector is not particularly limited as long as it is conductive and does not cause a chemical change in the battery. Specifically, the first electrode current collector may be made of stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. That is, the first electrode current collector may be provided in the form of surface-treated stainless steel, aluminum foil, or the like.

[0158] In addition, the first electrode current collector may typically have a thickness of 3 to 50 μm, and may have fine unevenness formed on the surface of the current collector to increase the adhesive strength of the first electrode active material. For example, it may be used in various forms such as a film, sheet, foil, net, porous body, foam, or non-woven fabric.

[0159] 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), lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide such as LiFe3O4; or a compound having the chemical formula Li 1+x Mn 2-x Lithium 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 by O2 (wherein, 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 zLithium manganese composite oxide represented by O2 (wherein, M is at least one selected from the group consisting of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≤z≤0.1) or Li2Mn3MO8 (wherein, M is at least one selected from the group consisting of Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., in which a part of Li in the chemical formula is replaced with an alkaline earth metal ion, but is not limited thereto. The first electrode may be Li-metal.

[0160] 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 provide conductivity to the electrode, and in the battery to be formed, any material that does not cause a chemical change and has electronic conductivity may be used without particular limitation. 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, summer black, carbon fiber, etc.; metal powder or metal fiber such as copper, nickel, aluminum, silver, etc.; conductive whiskey such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; or conductive polymers such as polyphenylene derivatives, etc., and one of these may be used alone or a mixture of two or more may be used.

[0161] In addition, the first electrode binder serves to improve adhesion between the first electrode active material particles and the adhesive strength between the first electrode active material and the first electrode current collector. Specific examples thereof include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinyl pyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated-EPDM, styrene butadiene rubber (SBR), fluoroelastomer, or various copolymers thereof, and one of these may be used alone or a mixture of two or more thereof.

[0162] 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. Specifically, the second electrode may include a second electrode current collector and a second electrode active material layer formed on one or both surfaces of the second electrode current collector, and including a second electrode active material. In other words, 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 may be expressed as a second electrode non-coated portion.

[0163] 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 non-conductive portion on which the second electrode active material layer is not formed, and may include a tab on the second electrode non-conductive portion. Specifically, the second electrode current collector may include a second electrode non-conductive portion, and may include a second electrode tab formed on the second electrode non-conductive portion. Accordingly, the manufactured electrode assembly may include one or more second electrode tabs.

[0164] According to one embodiment of the present invention, the second electrode active material layer may include a second electrode active material including at least one selected from the group consisting of silicon-based materials and carbon-based materials. In addition, 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; the second electrode conductive material; and the second electrode binder may be any material used in the art without limitation.

[0165] According to one embodiment of the present invention, the second electrode current collector may be any conductive material that does not cause a chemical change in the battery, and is not particularly limited thereto. For example, the second electrode current collector may be made of copper, stainless steel, aluminum, nickel, titanium, calcined carbon, or aluminum or stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. Specifically, a transition metal that adsorbs carbon well, such as copper or nickel, may 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.

[0166] According to one embodiment of the present invention, the second electrode binder may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethylmethacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, polytetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene butadiene rubber (SBR), fluoroelastomer, polyacrylic acid, and a material in which hydrogens thereof are substituted with Li, Na, Ca, or the like, and may also include various copolymers thereof.

[0167] According to one embodiment of the present invention, the second electrode conductive material is not particularly limited as long as it is conductive and does not cause a chemical change in the battery, and for example, graphite such as natural graphite or artificial graphite; carbon black such as acetylene black, Ketjen black, channel black, paneth black, lamp black, thermal black; conductive fiber such as carbon fiber or metal fiber; conductive tube such as carbon nanotube; metal powder such as fluorocarbon, aluminum, nickel powder; conductive whiskers such as zinc oxide or potassium titanate; conductive metal oxide such as titanium oxide; conductive materials such as polyphenylene derivatives, etc. can be used.

[0168] According to one embodiment of the present invention, the electrode assembly may include a plurality of separators. For example, the electrode assembly may have a structure in which separator / second electrode / separator / first electrode are sequentially laminated. The separator separates the first electrode and the second electrode and provides a passage for lithium ions to move. Any separator commonly used in secondary batteries may be used without particular limitation, and in particular, a separator having low resistance to ion movement of the electrolyte and excellent electrolyte moisture retention capacity is preferable. Specifically, a porous polymer film, for example, a porous polymer film made of a polyolefin polymer such as an ethylene homopolymer, a propylene homopolymer, an ethylene / butene copolymer, an ethylene / hexene copolymer, and an ethylene / methacrylate copolymer, or a laminated structure of two or more layers thereof may be used. In addition, a conventional porous nonwoven fabric, for example, a nonwoven fabric made of high-melting-point glass fiber, polyethylene terephthalate fiber, etc. may also be used. In addition, the above-described separator may be a separator that uses the aforementioned separator material as a substrate layer and coats a slurry containing a ceramic component or a polymer material on the substrate layer to secure heat resistance or mechanical strength, and may optionally be used in a single-layer or multi-layer structure. The thickness of the above-described separator may be 5 ㎛ or more and 20 ㎛ or less, but is not limited thereto.

[0169] According to one embodiment of the present invention, the angle formed by the first electrode and the second electrode may be 25° or less. Specifically, the first electrode includes a first surface facing the winding axis of the electrode assembly, and a second surface opposite the first surface, and a first extension line drawn by extending a straight line connecting two points where the curvature direction changes within a distance of 5 mm from the longitudinal end of the first electrode on the first surface of the first electrode; and a second extension line drawn by extending a straight line connecting two points 5 mm apart from the longitudinal end of the first electrode on the surface of the second electrode opposite the first surface of the first electrode may form an angle of 25° or less. In this case, the angle formed by the first electrode and the second electrode may be measured after, for example, additional charging and discharging are performed 200 times or more at 40° C. or more after activation.

[0170] When the aforementioned angle range is satisfied, the end of the first electrode located in the core portion and the adjacent second electrode are not deformed beyond a certain level, so that the phenomenon in which the separator located between the first and second electrodes is damaged and the first and second electrodes come into direct contact, i.e., core impingement, is suppressed. Through this, damage to the second electrode and the separator can be prevented, and an internal short between the first and second electrodes can be prevented, so that battery stability and life characteristics can be improved.

[0171] According to one embodiment of the present invention, the first electrode and the second electrode may be an anode and a cathode, respectively. Specifically, the first electrode may be an anode, and the second electrode may be a cathode.

[0172] One embodiment of the present invention provides a secondary battery including the electrode assembly and a battery case for accommodating the electrode assembly. Specifically, the secondary battery may include the electrode assembly according to the above-described embodiment and a battery case for accommodating the electrode assembly.

[0173] The secondary battery according to the present invention can improve the phenomenon in which the hollow core part cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging, prevent damage to the second electrode and separator, and prevent internal short circuit between the first electrode and the second electrode, so that battery stability and life characteristics can be improved.

[0174] According to one embodiment of the present invention, the battery case may be cylindrical. Specifically, the battery case may be cylindrical, square, or pouch-shaped, depending on the intended use, but is not limited thereto.

[0175] According to one embodiment of the present invention, the interior of the battery case may include an electrolyte. Specifically, the electrolyte may include, but is not limited to, an organic liquid electrolyte, an inorganic liquid electrolyte, a solid polymer electrolyte, a gel polymer electrolyte, a solid inorganic electrolyte, or a molten inorganic electrolyte that can be used in the manufacture of a lithium secondary battery. Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0176] According to one embodiment of the present invention, the non-aqueous organic solvent may be, for example, an aprotic solvent such as N-methyl-2-pyrrolidinone, propylene carbonate, ethylene carbonate, butylene carbonate, dimethyl carbonate, diethyl carbonate, gamma-butyrolactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyl tetrahydrofuran, dimethylsulfoxide, 1,3-dioxolan, formamide, dimethylformamide, dioxolan, acetonitrile, nitromethane, methyl formate, methyl acetate, triester phosphate, trimethoxy methane, dioxolan derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolidinone, propylene carbonate derivatives, tetrahydrofuran derivatives, ethers, methyl pyropionate, ethyl propionate, etc. Organic solvents may be used.

[0177] 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 dissolved 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 - One or more selected from the group consisting of may be used.

[0178] According to one embodiment of the present invention, in addition to the electrolyte components, the electrolyte may further include one or more additives, such as, for example, a haloalkylene carbonate compound such as difluoroethylene carbonate, pyridine, triethylphosphite, triethanolamine, cyclic ether, ethylene diamine, n-glyme, hexaphosphoric acid 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, for the purpose of improving the life characteristics of the battery, suppressing battery capacity decrease, and improving the discharge capacity of the battery.

[0179] One embodiment of the present invention provides a battery module including the secondary battery as a unit cell and a battery pack including the same. The battery module and battery pack include the secondary battery with improved high capacity, high battery stability, and lifespan characteristics, and thus can be used as a power source for medium- to large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and power storage systems.

[0180] Hereinafter, the present invention will be described in detail using examples. However, the examples according to the present invention may be modified in various ways, and the scope of the present invention is not limited to the examples described below. The examples in this specification are provided to more fully explain the present invention to those of ordinary skill in the art.

[0181] Example

[0182] Example 1

[0183] Electrode assembly manufacturing

[0184] Li(Ni) as the first electrode active material 0.89 Co 0.07 Mn 0.04 )O2, CNT as a first electrode conductive material, and polyvinylidene fluoride (PVdF) as a first electrode binder were added to N-methyl-2-pyrrolidone (NMP) at a weight ratio of 97.92:0.5:1.58 to prepare a first electrode active material slurry. The first electrode active material slurry was coated on an aluminum current collector having a thickness of 15 μm and a widthwise length of 63.9 mm, and then dried and rolled to form a first electrode active material layer, thereby preparing a first electrode having a thickness of 135 μm.

[0185] Next, natural graphite (C, average particle size: 17 μm) was prepared as a second electrode active material, and the second electrode active material, carbon black as a second electrode conductive material, and styrene butadiene rubber (SBR) as a second electrode binder were mixed in a weight ratio of 97.7: 1.3: 1.0 to prepare a second electrode active material composition. Thereafter, 7.8 g of distilled water was added to 5 g of the second electrode active material composition and stirred to prepare a second electrode active material slurry. The second electrode active material slurry was applied to a copper (Cu) metal thin film as a second electrode current collector having a thickness of 8 μm and a widthwise length of 65.2 mm, and dried (drying temperature: 120°C, 1 minute), thereby forming a second electrode having an average thickness of 166 μm. At this time, the temperature of the circulating air was 60°C.

[0186] Afterwards, two sheets of membrane were sequentially placed and wound using a core having a diameter of 3.2 mm, and the second electrode was inserted between the two sheets of membrane and further wound. At this time, the core was used including a separation portion where the membrane was inserted, a first core portion, and a second core portion having a different cross-sectional area from the first core portion, and the cross-sectional areas of the first core portion and the second core portion were each 5.4 mm. 2 , 2.7 mm 2 It was.

[0187] After the above separator and the second electrode were wound for about 3 turns, the first electrode was inserted and wound, and at the end where the winding was completed, a seal tape made of PET material was attached and finished to wrap the upper and lower outer surfaces of the electrode assembly.

[0188] At this time, based on the first core portion located at the 6 o'clock direction and the second core portion located at the 12 o'clock direction of the electrode assembly, the positions of the longitudinal ends and the tabs located at the outermost portion of the first electrode core portion were adjusted as shown in Table 1 below. Here, FE (Free-edge) means that the electrode current collector and the electrode active material layer have longitudinal ends at the same position, and 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.

[0189] The area and circularity of the core hollow section measured from the computed tomography (CT) image were as shown in Table 1 below. At this time, the angle formed by the longitudinal end of the first electrode and the point where the curvature of the first electrode is maximum was determined by comparing the curvatures measured at each measurement point on the CT image, and the measurement point where the curvature has the maximum value in the region exceeding 0° and less than 180° in the opposite direction to the winding direction of the electrode assembly based on the longitudinal end of the first electrode was determined as the point where the curvature of the first electrode is maximum (curvature Max.), and the angle formed by the point where the curvature of the first electrode is maximum and the longitudinal end of the first electrode with the winding axis as the center was measured, and the results are shown in Table 2 below. The point where the curvature of the first electrode is maximum (curvature Max.) was measured using the same method in the following Examples, Comparative Examples, and Experimental Examples.

[0190] Influence factor control range 1st electrode core part FE position (hours) 6.5-8 1st electrode outermost part FE position (hours) 4.5-5.5 2nd electrode outermost tab position (hours) 6-7 Area of ​​core part hollow (mm) 2 )7-13 Circularity (%)89-92

[0191] Manufacturing of secondary batteries

[0192] After inserting the above electrode assembly into a cylindrical battery case, an electrolyte solution was injected in which ethylene carbonate (EC): ethyl methyl carbonate (EMC): diethyl carbonate (DEC) was mixed in a volume ratio of 20:5:75 to dissolve LiPF6 to a concentration of 1.4 M, and the cylindrical battery can was sealed with a cap assembly to manufacture a secondary battery.

[0193] At this time, after 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, 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 60.8°, and the flatness ratio of the first electrode was 7.72%.

[0194] Example 2

[0195] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, the length of the separator being wound and the position of the first electrode when inserted were adjusted so that 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 flatness ratio of the first electrode was 9.6%, and an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0196] Comparative Example 1

[0197] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, 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 is 98.9°, and the flatness ratio of the first electrode is 13.9%. Except that the length of the separator being wound and the position when the first electrode is inserted were adjusted, an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0198] Comparative Example 2

[0199] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, 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 is 112.4°, and the flatness ratio of the first electrode is 17.0%. Except that the length of the separator being wound and the position when the first electrode is inserted were adjusted, an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0200] Comparative Example 3

[0201] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, 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 is 129.3°, and the flatness ratio of the first electrode is 16.49%. Except that the length of the separator being wound and the position when the first electrode is inserted were adjusted, an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0202] Comparative Example 4

[0203] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, 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 is 40°, and the flatness ratio of the first electrode is 3.0%. Except that the length of the separator being wound and the position when the first electrode is inserted were adjusted, an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0204] Comparative Example 5

[0205] After 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, the length of the separator being wound and the position of the first electrode when inserted were adjusted so that 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 flatness ratio of the first electrode was 1.0%, and an electrode assembly and a secondary battery were manufactured in the same manner as in Example 1.

[0206] Experimental example

[0207] Experimental Example 1: Flatness Fraction Evaluation

[0208] The flatness ratios of the secondary batteries of Example 1, Example 2, and Comparative Examples 1 to 5 were evaluated using the following method, and the results are shown in Table 2 and Figures 3 to 9, respectively.

[0209] 1) The first electrode is extracted from the CT image of the above electrode assembly.

[0210] 2) On the first surface of the first electrode, measuring points are set at 2° intervals from one longitudinal end of the first electrode, and the x-coordinate and y-coordinate of the measuring points are measured.

[0211] 3) Using the x-coordinate and y-coordinate of the above measurement point, the curvature (k) for each measurement point is calculated using Equation 1 below.

[0212] [Formula 1]

[0213] k=(x'y''-y'x'') / (x' 2 +y' 2 ) 3 / 2

[0214] In the above equation 1, 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 differential value of x, y' is the first differential value of y, x'' is the second differential value of x, and y'' is the second differential value of y.

[0215] 4) If the curvature value calculated at each measurement point is 1 or less, the measurement point is evaluated as a flat area, and the ratio of the number of measurement points evaluated as a flat area based on 100% of the total number of measurement points is evaluated as the flatness fraction (%).

[0216] Experimental Example 2: Core Collapse Evaluation

[0217] The occurrence of core collapse in the secondary batteries of Example 1, Example 2, and Comparative Examples 1 to 5 was evaluated using the following method, and the occurrence of core impingement was re-evaluated every 200 cycles, and the results are shown in Table 2 and Figures 3 to 9, respectively.

[0218] 1) The first electrode is extracted from the CT image of the above electrode assembly.

[0219] 2-1) From one longitudinal end of the first electrode, the area of ​​the hollow core surrounded by the first electrode is measured to a point corresponding to one turn in the longitudinal direction of the first electrode.

[0220] 2-2) When the area of ​​the hollow core portion is less than 100% of the sum of the cross-sectional areas of the first core portion and the second core portion used for winding the electrode assembly, it was evaluated that Core Collapse occurred.

[0221] 3-1) On the first surface of the first electrode, the minimum value (D) of the diameter of the hollow core of the extracted first electrode min ) and maximum value (D max ) Draw an extension of the straight line corresponding to the line, and draw a concentric circle with the intersection of each straight line as the central axis, i.e. the winding axis.

[0222] 3-2) The maximum separation distance (R) between the winding shaft and the first electrode from one longitudinal end of the first electrode to a point corresponding to one turn in the longitudinal direction of the first electrode max ) and the minimum separation distance (R) between the winding shaft and the first electrode min ) are measured respectively.

[0223] 3-3) The maximum separation distance (R) between the above winding shaft and the first electrode max ) for the minimum separation distance (R) between the winding axis and the first electrode min ) was calculated as the ratio (%), that is, the circularity of the core hollow portion, and if the calculated circularity of the core hollow portion was less than 89%, it was evaluated that Core Collapse had occurred.

[0224] Meanwhile, the above method for evaluating whether or not Core Collapse has occurred can be applied by evaluating whether or not Core Impingement has occurred at the time of initial acquisition when an unknown secondary battery (Unknown Cell) is obtained, re-evaluating whether or not Core Collapse has occurred every 200 cycles, and comparing and analyzing the Core Collapse conditions of the secondary battery of the embodiment according to the present invention.

[0225] Experimental Example 3: Core Impingement Evaluation

[0226] The occurrence 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 occurrence of core impingement was re-evaluated every 200 cycles, and the results are shown in Table 2 and Figures 3 to 9, respectively.

[0227] Fig. 10 schematically illustrates a method for evaluating whether or not a core impingement has occurred. Specifically, Fig. 10 (a) schematically illustrates a method for evaluating whether or not a core impingement has occurred when deformation has occurred in the second electrode, and Fig. 10 (b) schematically illustrates a method for evaluating whether or not a core impingement has occurred when deformation has not occurred in the second electrode.

[0228] 1) On the first surface of the first electrode (300), a first extension line (E1) is drawn by extending a straight line connecting the longitudinal end (310) of the first electrode and a point 5 mm apart from the end.

[0229] 2-1) If deformation occurs in the second electrode

[0230] A second extension line (E2) is drawn by extending a straight line connecting two points where the bending direction changes within a distance of 5 mm from the longitudinal end (310) of the first electrode, on the surface of the second electrode (100) of the core portion of the electrode assembly, opposite the first surface of the first electrode.

[0231] 2-2) If no deformation occurs in the second electrode

[0232] A second extension line (E2) is drawn by extending a straight line connecting two points at a distance of 5 mm from the longitudinal end (310) of the first electrode on the surface of the second electrode (100) of the core portion of the electrode assembly, which faces the first surface of the first electrode.

[0233] 3) It was evaluated that core impingement occurred when the angle from the first extension line (E1) to the second extension line (E2) in a counterclockwise direction, centered at the intersection of the first extension line (E1) and the second extension line (E2), exceeded 25°.

[0234] Meanwhile, the above method for evaluating whether or not a core impingement has occurred can be applied by evaluating whether or not a core impingement has occurred at the time of initial acquisition when an unknown secondary battery (Unknown Cell) is obtained, re-evaluating whether or not a core impingement has occurred every 200 cycles, and comparing and analyzing the core impingement conditions of the secondary battery of the embodiment according to the present invention.

[0235] Experimental Example 4: Evaluation of Core Crack Occurrence

[0236] The occurrence of core cracks in the electrode assemblies of Example 1, Example 2, and Comparative Examples 1 to 5 was evaluated using the following method, and the results are shown in Table 2 and FIGS. 8 and 9, respectively.

[0237] Specifically, the electrode assemblies of Example 1, Example 2, and Comparative Examples 1 to 5 were prepared, and the manufactured electrode assemblies were disassembled to visually check whether cracks or wrinkles occurred in the core portion. At this time, if cracks or wrinkles were included in the core portion, it was evaluated that a core crack had occurred.

[0238] First electrode FE- Curvature Max. Angle (°) Flatness Fraction (%) Core Collapse Evaluation Result Core Impingement Evaluation Result Core Crack Occurrence Evaluation Result Example 160.87.720 / 20 / 20 / 2 Example 2659.60 / 20 / 20 / 2 Comparative Example 198.913.92 / 20 / 20 / 2 Comparative Example 2112. 417.02 / 21 / 20 / 2 Comparative Example 3129.316.492 / 21 / 20 / 2 Comparative Example 4403.01 / 21 / 21 / 2 Comparative Example 5201.01 / 21 / 21 / 2

[0239] Fig. 8 is a CT image of an electrode assembly according to Comparative Example 4 and an image of a crack occurrence in the first electrode core portion, and Fig. 9 is a CT image of an electrode assembly according to Comparative Example 5 and an image of a crack occurrence in the first electrode core portion. Specifically, Figs. 8 (a) and 9 (a) are CT images of an electrode assembly according to Comparative Example 4 and an electrode assembly according to Comparative Example 5, and Figs. 8 (b) and 9 (b) are exploded images showing a crack occurring in the first electrode of the core portion of an electrode assembly according to Comparative Example 4 and an electrode assembly according to Comparative Example 5. Here, the circle represents the core impingement occurrence area corresponding to the longitudinal end of the first electrode and the crack occurrence area of ​​the first electrode in the core portion, respectively. Referring to Table 2, (b) of FIG. 8, and (b) of FIG. 9, it can be seen that in the electrode assemblies according to Comparative Examples 4 and 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 core is concentrated on the longitudinal end of the first electrode, which causes wrinkles or cracks in the core portion.

[0240] During the winding process, if cracks and resulting foreign matter occur in the core portion of the electrode assembly, the defect rate may increase and the processability may deteriorate. Therefore, by excluding the input of the first electrode, which is expected to develop cracks, from the manufacturing process, a decline in the productivity of the electrode assembly and secondary battery can be prevented.

[0241] Specifically, when an electrode assembly with a crack is inserted into a battery case, the possibility of low voltage and short circuit due to foreign matter in the first electrode inside the battery case increases significantly. Accordingly, the possibility of crack occurrence in the first electrode may be reduced by adjusting the flatness fraction of the first electrode in the winding process. That is, it can be seen that the electrode assembly and the method for manufacturing the electrode assembly according to one embodiment of the present invention are suitable for a continuous process using existing roll-to-roll process equipment, thereby ensuring the productivity and economic feasibility of the electrode assembly and secondary battery including the first electrode.

[0242] Reference experiment example: Electrode sliding range evaluation

[0243] Reference Experiment Example 1

[0244] The secondary battery of Example 1 above was prepared and charged and discharged 50 times under conditions of 25°C, 1C charge, and 1C discharge.

[0245] The positions of the longitudinal end of the first electrode at SOC 0% and SOC 100% were extracted from the CT images and recorded while cycling 200 times from SOC 0% to SOC 100% under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, and the sliding range of the longitudinal end of the first electrode was measured. Thereafter, the sliding range of the longitudinal end of the first electrode was evaluated from the CT images before activation and at SOC 100%, and is shown in Fig. 11 below.

[0246] Reference Experiment Example 2

[0247] Except for using the secondary battery of Example 2, the sliding range of the longitudinal end of the first electrode was measured in the same manner as in Reference Experimental Example 1. Thereafter, the sliding range of the longitudinal end of the first electrode was evaluated from CT images before activation and at 100% SOC, and is shown in Figure 11 below.

[0248] Fig. 11 is a CT image showing the sliding range of the longitudinal end of the first electrode of the secondary battery according to Reference Experimental Example 1 and Reference Experimental Example 2. Specifically, Fig. 11 is a CT image of the secondary battery according to Reference Experimental Example 1 and Reference Experimental Example 2 before activation and at SOC 100% and an image showing the sliding range of the longitudinal end of the first electrode. More specifically, Fig. 11 shows the sliding range by comparing the position of the longitudinal end of the first electrode before activation and at SOC 100% of the secondary battery based on the second electrode tab located at the outermost part.

[0249] Referring to Fig. 11, it was confirmed that sliding occurred in the range of 18.41° (SOC 100) - 16.72° (before activation) = 1.69° in Reference Experimental Example 1, and in the range of 55.08° (SOC 100) - 53.30° (before activation) = 1.78° in Reference Experimental Example 2. That is, it was confirmed that sliding occurred in the range of 1° to 3° in the longitudinal end of the first electrode compared to the initial position in both Reference Experimental Examples 1 and 2.

[0250] Through this, it can be seen that the secondary batteries according to Examples 1 and 2 may experience first electrode sliding due to contraction / expansion of the electrode before activation and at SOC 100%, but sliding occurs within a specific range (1° to 3°).

[0251] That is, since the winding process is performed while maintaining a tension greater than a certain value, the position of the point where the curvature of the first electrode is maximum is maintained constant, so that even when the cycle is performed, it can be seen that the range of the angle formed by 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.

[0252] FIGS. 3 and 4 are CT images of electrode assemblies according to Examples 1 and 2 and images showing the curvature of the first electrode extracted from the CT images.

[0253] Figures 5 to 7 are CT images of electrode assemblies according to Comparative Examples 1 to 3 and images showing the curvature of the first electrode extracted from the CT images.

[0254] Specifically, the above FIGS. 3 to 7 represent the ratio of measurement points having a curvature lower than an arbitrarily set reference value with respect to the curvature measured at each measurement point, expressed in black and white color coordinates. Here, the set reference value is a specific value within the range of 0.5 to 1.0, and the black and white color index range expressed on the right side of the black and white color coordinates is 0 to 0.8 in (a) of FIGS. 3 to 7, and 1 to 1.5 in (b) of FIGS. 3 to 7.

[0255] More specifically, (a) of FIGS. 3 to 7 visualizes the flat area and the flatness fraction by implementing the area where the k value is low in dark gray when the black and white color distribution range of the curvature measured at each measurement point is set to 0 to 0.8, and (b) of FIGS. 3 to 7 visualizes the point where the curvature is maximum by implementing the area where the k value is high in dark gray when the black and white color distribution range of the curvature measured at each measurement point is set to 1 to 1.5. At this time, each measurement point is 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 direction.

[0256] Referring to Table 2 above, FIGS. 3 to 7, (a) of FIG. 8, and (a) of FIG. 9, the electrode assemblies according to Examples 1 and 2 had flatness ratios of the first electrode extracted from the core CT image of 7.72% and 9.6%, respectively, after 50 cycles under the conditions of 4.25 V-2.5 V 1 C / 1 C @25 ℃, and it was confirmed that core collapse and core impingement did not occur even after 500 cycles under the conditions of 4.2 V (0.25 C)-2.85 V (0.33 C), @40 ℃.

[0257] On the other hand, when the flatness ratio of the first electrode extracted from the core CT image is more than 10% as in Comparative Examples 1 to 3, or when the flatness ratio of the first electrode extracted from the core CT image is 3% or less as in Comparative Examples 4 and 5, and thus does not satisfy the flatness ratio range described above, it was confirmed that the angle formed by the first extension line and the second extension line is 25° or more, and thus Core Impingement occurs, or even if Core Impingement does not occur, the area of ​​the core hollow part is less than 100% or the roundness of the core hollow part is less than 89% based on the sum of 100% of the cross-sectional areas of the winding cores used for winding, and thus Core Collapse occurs.

[0258] Through this, it can be seen that the electrode assembly according to one embodiment of the present invention and the secondary battery including the same can improve the phenomenon in which the hollow core part cannot maintain its circular shape and collapses due to deformation of the electrode assembly caused by contraction / expansion of the electrode during battery charging / discharging by controlling the flatness ratio of the first electrode extracted from the core part CT image to a specific range, and can prevent damage to the second electrode and separator and prevent internal short between the first electrode and the second electrode, thereby improving battery stability and life characteristics.

[0259] The detailed description above is intended to illustrate and explain the present invention. Furthermore, the foregoing merely illustrates and describes preferred embodiments of the present invention. As described above, the present invention can be utilized in various other combinations, modifications, and environments, and variations or modifications can be made within the scope of the inventive concepts disclosed herein, the scope equivalent to the above-described disclosure, and / or the scope of technology or knowledge in the art. Therefore, the detailed description of the invention above is not intended to limit the present invention to the disclosed embodiments. Furthermore, the appended claims should be construed to include other embodiments.

[0260] [Explanation of symbols]

[0261] 100: Second electrode

[0262] 300: First electrode

[0263] 310: Longitudinal end of the first electrode

[0264] E1: First extension line

[0265] E2: Second extension line

[0266] A: First Division

[0267] B: Second Division

Claims

1. In an electrode assembly in which a first electrode, a separator, and a second electrode are laminated and wound around a winding axis, The first electrode includes a first surface facing the winding axis of the electrode assembly and a second surface opposite the first surface, An electrode assembly, wherein the flatness fraction of the first electrode in the core portion or a portion of the electrode assembly is greater than 3% and less than or equal to 13.5%.

2. In paragraph 1, An electrode assembly wherein the core portion of the electrode assembly is an area within 3 turns from the inner end of the first electrode.

3. In paragraph 1, The first electrode comprises 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, An electrode assembly wherein the inner end of the first electrode collector corresponds to the inner end of the first electrode.

4. In paragraph 3, The above first electrode includes a first electrode non-conductive portion that does not have a first electrode active material layer, An electrode assembly further comprising a first electrode tab provided on or physically connected to the first electrode non-conductive portion.

5. In paragraph 1, An extension line connecting the winding shaft and the inner end of the first electrode; and An extension line connecting the point where the curvature of the above-mentioned winding axis and the above-mentioned first electrode is 1 or less An electrode assembly wherein the angle formed is greater than 0°.

6. In paragraph 1, An electrode assembly wherein the flatness ratio of the first electrode is determined after activation of the electrode assembly.

7. In paragraph 1, The flatness ratio of the above first electrode is, An electrode assembly determined after 50 cycles of charging and discharging under conditions of 25 ℃, 1C charging and 1C discharging.

8. In paragraph 1, An extension line connecting the winding shaft and the inner end of the first electrode; and An extension line connecting the maximum curvature point of the above winding axis and the first electrode An electrode assembly wherein the angle formed is greater than 40° and less than 98°.

9. In paragraph 8, Based on the cross-section perpendicular to the winding axis of the above electrode assembly, An electrode assembly, wherein the maximum curvature point of the first electrode is a point where the curvature of the first electrode is maximum in a region of an azimuth greater than 0° and less than 180° in the direction opposite to the winding direction of the electrode assembly, based on the inner end of the first electrode.

10. In paragraph 1, An electrode assembly wherein the circularity of the first electrode in the core portion or a portion of the electrode assembly is 89% or more.

11. In paragraph 1, A first extension line drawn by extending a straight line connecting two points where the curvature direction changes within a distance of 5 mm from the inner end of the first electrode on the first surface of the first electrode; and A second extension line drawn by extending a straight line connecting two points 5 mm apart from the inner end of the first electrode on the surface of the second electrode opposite to the first surface of the first electrode, An electrode assembly having an angle of less than 25°.

12. In paragraph 1, An electrode assembly wherein the first electrode does not include cracks or wrinkles in the interior or 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. In paragraph 13, Further comprising a battery case for accommodating the above electrode assembly, The above battery case is a secondary battery having a cylindrical shape.

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