Electrode assembly and secondary battery

Abstract

The present invention relates to an electrode assembly and a secondary battery. The electrode assembly includes a first electrode, a separator, and a second electrode stacked and wound around a winding axis. The first electrode has a first surface facing the winding axis and a second surface opposite the first surface. In an inner core portion of the electrode assembly or a portion thereof, a flat portion ratio is greater than 3% and less than or equal to 13.5%, the flat portion ratio being a ratio of a region of the first electrode in which a curvature is less than or equal to 1. The core portion is a portion of a region of the electrode assembly within 3 windings from an inner end portion of the first electrode. The design of this electrode assembly can solve core deformation.

Description

[0001] This application is a divisional application of the invention patent application filed on November 1, 2024, with application number 202480029073.5 (PCT / KR2024 / 017055) and entitled "Electrode Assembly and Secondary Battery Including the Electrode Assembly". Technical Field

[0002] This invention relates to electrode assemblies and secondary batteries including the electrode assemblies, and more particularly, to electrode assemblies that address core deformation by modifying the design of the electrode assembly, and cylindrical secondary batteries including the electrode assemblies. 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 disclosure of which is incorporated herein by reference. Background Technology

[0003] For cylindrical batteries, wound electrode assemblies are manufactured by winding long electrodes of a predetermined width into a roll. Cylindrical batteries, manufactured by inserting such electrode assemblies into a battery case, undergo repeated contraction / expansion of the electrodes during charging and discharging. In particular, the pressure acting on the core portion of the electrode assembly increases significantly when the tabs (internal tabs) are located in the core of the electrode assembly or when the degree of contraction / expansion of the electrode assembly is increased due to the addition of silicon-based active material to the negative electrode.

[0004] The core portion of a cylindrical battery is the space where the winding core (mandrel) for winding the electrode assembly is located, and it exists in the empty space used in the cylindrical battery assembly process, such as the insertion process of the electrode assembly into the battery box and the welding process, i.e., the core cavity.

[0005] Recently, with the increase in low-resistance / high-capacity designs, electrode assemblies have increasingly included multiple contacts or incorporated silicon-based active materials. Consequently, the possibility of core deformation of the electrode assembly due to shrinkage / expansion has increased. Specifically, the following problems exist: battery life deteriorates due to the core cavity failing to maintain its circular shape and collapsing (core collapse); and the separation between the positive and negative electrodes is damaged due to deformation of the positive electrode's end portion at the core and the adjacent negative electrode exceeding a certain level (core impact), leading to direct contact between the positive and negative electrodes and internal short circuits, resulting in overheating and fire.

[0006] To address the issues of battery life degradation, separator damage, and internal short circuits caused by electrode assembly deformation, a technology needs to be developed that can resolve the phenomenon of the core cavity in the corresponding region failing to maintain its circular shape and collapsing, and suppress the occurrence of internal short circuits due to separator damage. Summary of the Invention

[0007] Technical issues

[0008] The present invention aims to provide an electrode assembly and a secondary battery including the electrode assembly, the design of which has been modified to address core deformation.

[0009] However, the problems to be solved by the present invention are not limited to those described above, and those skilled in the art will clearly understand from the following description other problems not described.

[0010] Technical solution

[0011] This invention provides an electrode assembly comprising a first electrode, a spacer, and a second electrode stacked and wound around a winding axis. The first electrode may have a first surface facing the winding axis and a second surface opposite to the first surface. In or at a portion of the core portion of the electrode assembly, the planar portion of the first electrode may be greater than 3% and less than or equal to 13.5%. The planar portion percentage as used herein can be determined in detail below.

[0012] In accordance with the relevant technical fields of battery design and manufacturing, and particularly with respect to secondary batteries, the terms first electrode and second electrode are used herein. The first electrode and / or second electrode may be configured to receive and store charge carriers, such as electrons and / or ions (especially lithium ions). The first electrode and second electrode may have opposite polarities, such that they can respectively receive and release charge carriers during the charging process of the electrode assembly, and respectively release and receive charge carriers during the discharging process of the electrode assembly. Unless otherwise stated herein, the materials and functions of the first electrode and second electrode may be known in the art.

[0013] For example, the first electrode can be a positive electrode, and the second electrode can be a negative electrode. In other examples, the first electrode can be a negative electrode, and the second electrode can be a positive electrode. Typically, the positive electrode can include a current collector, which can be configured as a plate, sheet, or film, on which a positive electrode active material is deposited. For example, the positive electrode active material can include lithium metal oxide, such as lithium cobalt oxide or lithium iron phosphate, or any other suitable material. The current collector of the positive electrode can be made of a conductive material, such as aluminum. Typically, the negative electrode can include a current collector, which can be configured as a plate, sheet, or film, on which a negative electrode active material is deposited. For example, the negative electrode active material can include a carbon-based material, such as graphite. The current collector of the negative electrode can be made of a conductive material, such as copper.

[0014] In accordance with the relevant technical fields of battery design and manufacturing, and particularly with respect to secondary batteries, the term "separator" is used herein. Specifically, the separator can be a porous membrane configured to allow charge carriers (e.g., ions, lithium ions, and / or electrons) to pass through, while generally being impermeable to solid materials. The separator can be disposed between a first electrode and a second electrode to prevent short circuits between the two electrodes.

[0015] The transfer of charge carriers between the first and second electrodes can be achieved by an electrolyte, which may be at least partially present between the first and second electrodes. The electrolyte can be added to the electrode assembly after the electrode assembly is arranged in the battery case. In a specific example, the electrode assembly can be considered to include an electrolyte such that the electrolyte is present between the first and second electrodes.

[0016] The first electrode, separator, and second electrode can be configured as a layer (or sheet). The first electrode, separator, and second electrode can be placed on top of each other in a specific order. In a specific example, the second electrode, separator, first electrode, and another separator are arranged sequentially on top of each other to form a so-called single cell. In other examples, one or more single cells can be placed on top of a single cell in the aforementioned order (repeatedly) to form a multi-cell structure.

[0017] Electrode assemblies may include single (or multiple) cells wound into a roll. Such a rolled electrode assembly may also be referred to as a core. Specifically, in a single cell, a first electrode, a separator, a second electrode, and optionally another separator may be wound together about a winding axis. Therefore, the electrode assembly may have a cylindrical shape, and particularly a cylindrical shape with a helical (vortex-shaped) cross-section. Furthermore, the first electrode, separator, and second electrode of the electrode assembly may be wound together about a winding axis (i.e., wound in a stacked state in the aforementioned order), thereby giving the electrode assembly (approximately) cylindrical symmetry with respect to the winding axis. In this 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 (approximately) perpendicular to the outer peripheral surface of the electrode assembly; and the circumferential direction may be perpendicular to both the axial and radial directions and circular about the winding axis. Additionally, the winding direction of the electrode assembly may represent the direction along one or both of the first electrode, separator, and second electrode in a plan view. Therefore, the winding direction may extend along a helical path about the winding axis. Based on the context, the winding direction, in terms of the direction of rotation, can be approximated as a circle around the winding axis, similar to the circumferential direction.

[0018] Unless otherwise stated, the azimuth angle used herein can represent the angle between two lines in a plane perpendicular to the winding axis. The two lines forming the azimuth angle may intersect at the winding axis.

[0019] Generally, unless otherwise stated, geometric features involving lines, angles, curvatures, etc., as used herein refer to a plan view, which is a cross-sectional view of the electrode assembly when viewed parallel to the winding axis. Therefore, unless otherwise stated, such geometric features may refer to the (approximate) cylindrical geometry of the electrode assembly in its wound state about the winding axis. Furthermore, unless otherwise stated, any line used herein may represent a straight line.

[0020] Here, the cylindrical symmetry mentioned herein can be approximated, as the helical plan view and / or helical cross-section of the electrode assembly are respectively considered to be approximately circular plan views and / or circular cross-sections. The plan view of the electrode assembly can refer to a cross-sectional view parallel to the winding axis. When the first electrode, the spacer, and the second electrode are wound around the winding axis, the electrode assembly can have a helical end face facing the winding axis. The cross-section of the electrode assembly can refer to a cross-section perpendicular to the winding axis. When the first electrode, the spacer, and the second electrode are wound around the winding axis, the cross-section of the electrode assembly can be helical around the winding axis.

[0021] Before winding, at least one of the first electrode, the separator, and the second electrode may have a rectangular shape in the plan view. The winding axis may be parallel to at least one end 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 corresponding rectangular shape in the plan view and are arranged such that the corresponding ends of the first electrode, the separator, and the second electrode are aligned parallel to each other.

[0022] Such an electrode assembly having the features described above can achieve any of the technical effects mentioned herein. Specifically, one, some, or all of the electrodes of the electrode assembly can undergo contraction and / or expansion in response to charging and discharging processes, which may cause deformation of the electrode assembly. Such deformation may cause the wound core of the electrode assembly to fail to maintain its cylindrical shape and thus collapse.

[0023] The electrode assembly disclosed herein can exhibit a specific flat portion ratio of the first electrode to suppress the problematic phenomenon that the wound core of the electrode assembly cannot maintain its cylindrical shape due to deformation of the electrode assembly and thus collapses.

[0024] Therefore, the electrode assembly disclosed herein may help prevent damage to the first electrode, the second electrode, and the separator. The electrode assembly disclosed herein may also help prevent internal short circuits between the first and second electrodes. Thus, the claimed subject matter may contribute to increasing battery stability and overall lifespan.

[0025] This disclosure also relates to a method for manufacturing an electrode assembly. This method can achieve a flat portion ratio of the first electrode in the manner described above, thereby enabling a continuous manufacturing process, for example, using conventional roll-to-roll processing equipment, and thus improving productivity and economic efficiency. This method can also achieve any of the technical effects mentioned above and / or detailed below.

[0026] In an example implementation, the core portion may be a portion of the electrode assembly within three turns starting from the inner end of the first electrode.

[0027] Furthermore, the point of maximum curvature of the first electrode (see below) may be formed in or at the portion of the electrode assembly located within three turns of the first electrode along the winding direction of the electrode assembly, starting from the inner end of the first electrode.

[0028] The portion of the electrode assembly that is surrounded by the first electrode in a third loop from the inner end of the first electrode can be referred to as the core portion. The core portion can correspond to the core portion described above. The core portion can be implemented using some or all of the features described below.

[0029] Alternatively or additionally, the core portion may be a region of the electrode assembly extending within three turns from one end portion of the first electrode in the longitudinal direction. The longitudinal direction may refer to the state of the electrode assembly before winding and may correspond to the winding direction after winding as used herein. One end portion may refer to the inner end portion of the first electrode.

[0030] In an example implementation, the first electrode may include a first electrode current collector and a first electrode active material layer. The first electrode active material layer is disposed on at least one surface of the first electrode current collector such that the first electrode active material layer extends to the inner end of the first electrode current collector, and the inner end of the first electrode current collector corresponds to the inner end of the first electrode.

[0031] When the first electrode is wound around a winding axis, in a plan view, the first electrode extends in the winding direction between its inner end and its outer end. The inner end of the first electrode may be located at a radially inward position, for example, close to or near the winding axis. The outer end of the first electrode may be located at a radially outward position, for example, at or near the outer circumference of the electrode assembly. Alternatively or additionally, a similar situation may be applied to the second electrode.

[0032] In other words, the first electrode may include a first electrode current collector and a first electrode active material layer applied to the first electrode current collector. The first electrode terminates at the point where the first electrode current collector terminates (at the inner end of the first electrode) in the direction opposite to the winding direction. That is, the first electrode may not extend further than the first electrode current collector in the direction opposite to the winding direction.

[0033] The active material layer may refer to a layer comprising or made of the 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 layer made of a positive electrode active material.

[0034] In an example implementation, the first electrode may include an uncoated portion of the first electrode where no first electrode active material layer is disposed. The electrode assembly may also include a first electrode tab physically connected to or formed on the uncoated portion of the first electrode.

[0035] The uncoated portion of the first electrode can refer to the portion of the first electrode, and particularly the first electrode current collector, that lacks the active material layer of the first electrode (i.e., the active material of the first electrode).

[0036] At least one electrode tab may be formed from, on, or at the uncoated portion of the first electrode. The uncoated portion of the first electrode may be cut or slotted (e.g., by forming one or more slits, notches, or cuts from the edge of the uncoated portion into the uncoated portion) to form one or more first electrode tabs. Alternatively or additionally, at least one electrode tab may be provided separately and attached to the first electrode current collector or the uncoated portion of the first electrode.

[0037] The angle between the line extending from the winding axis through the inner end of the first electrode (i.e., the first line) and the line extending from the winding axis through the first electrode at a point with a curvature of less than or equal to 1 is greater than 0°.

[0038] The mathematical value of the curvature can be determined as detailed above and / or as described below. In particular, the point where the curvature of the first electrode is less than or equal to 1 can be within three turns from the inner end of the first electrode, i.e., in the core portion of the electrode assembly. The angle mentioned can refer to the azimuth angle as described above, with its vertex located on the winding axis (coinciding with the winding axis).

[0039] In the implementation example, the proportion of the flat portion of the first electrode is determined after the electrode assembly is activated.

[0040] As used herein, activation can refer to the process of applying specific temperatures and charge and discharge conditions to an electrode assembly (or a battery cell containing the electrode assembly) to prepare an electrode assembly for use in a secondary battery. Activation can also be referred to as formation. Activation can include aging, charging, and discharging processes, which can be performed in a specific order and optionally repeated in a specific order. The aging process can be configured to allow electrolyte to permeate into a first electrode and a second electrode. For example, the aging process can be performed by storing the electrode assembly at a specific temperature (e.g., 30°C) for a specific duration (e.g., 30 minutes, 1 hour, 2 hours, or 3 hours). The charging process can be configured to form an SEI layer, i.e., a solid electrolyte interface layer, by the decomposition of the electrolyte on the surface of the negative electrode. The charging process can include charging the electrode assembly to a certain level. Optionally, after the charging process, a high-temperature aging process can be performed at an elevated temperature (e.g., 40°C, 50°C, 60°C, or 70°C). The electrode assembly can then be discharged at a specific C-rate (e.g., 0.1 C, 0.2 C, 0.5 C, or 1.0 C). Optionally, a degassing process can be performed to remove the gases formed during the activation process described above.

[0041] The first and / or second electrodes can shrink and / or expand during activation. Therefore, the technical effects achieved by this invention may be particularly beneficial for electrode assemblies that have undergone (at least partially) the activation process.

[0042] In an example implementation, the proportion of the flat portion of the first electrode can be determined after 50 charge and discharge cycles at 25°C, 1C charging and 1C discharging.

[0043] Therefore, the flat portion ratio can be determined after at least 50 charge and discharge cycles (repetitions) of the electrode assembly. This allows for a more precise determination of the electrode assembly's condition or quality.

[0044] In an example implementation, the angle formed between a line extending from the winding axis and passing through the inner end of the first electrode and a line extending from the winding axis and passing through the point of maximum curvature can be greater than 40° and less than or equal to 98°. Specifically, the point of maximum curvature can be located within or at a portion of the core of the electrode assembly.

[0045] According to this disclosure, a straight line (first line) connecting the winding axis and the inner end of the first electrode can be drawn in a plane perpendicular to the winding axis. For example, such a first line extends from the winding axis through the inner end of the first electrode in a plane perpendicular to the winding axis of the electrode assembly. The first line can be imaginary and is specifically configured to determine the angle between it (i.e., the first line) and another line passing through the winding axis and the point of maximum curvature (i.e., the second line detailed below).

[0046] According to this disclosure, a straight line (second line) connecting the winding axis and the point of maximum curvature of the first electrode can be drawn in a plane perpendicular to the winding axis of the electrode assembly. For example, such a line extends from the winding axis through the point of maximum curvature of the first electrode in a plane perpendicular to the winding axis of the electrode assembly. The second line can be imaginary and is specifically configured to determine the angle between it (i.e., the second line) and the line extending through the inner end of the winding axis and the first electrode (i.e., the first line).

[0047] In this disclosure, the angle between a line extending from the winding axis through the inner end of the first electrode and a line extending from the winding axis through the point of maximum curvature (i.e., between the first and second lines), viewed from a perspective parallel to the winding axis, can be greater than 40° and less than or equal to 98°. Alternatively, this angle can be within any range of values ​​disclosed herein. As mentioned above, a perspective parallel to the winding axis can also mean a perspective in the axial direction of the cylindrical symmetry of the electrode assembly.

[0048] In an example implementation, the point of maximum curvature of the first electrode can be the point of maximum curvature in a region of the first electrode in a cross-section of the electrode assembly perpendicular to the winding axis of the electrode assembly. This region extends from the inner end of the first electrode in an azimuth angle greater than 0° and less than or equal to 180° in a direction opposite to the winding direction of the electrode assembly. The point of maximum curvature can be located inside or at a portion of the core portion of the electrode assembly.

[0049] Generally, the term curvature can be used in this document as it is commonly used in mathematics. Specifically, curvature can indicate a measure of a curve's deviation from a straight line. Alternatively or additionally, curvature can indicate a measure of a surface's deviation from a plane. As used herein, curvature can indicate how much the direction of a curve changes over a small distance (e.g., by angle per unit distance). Curvature can be a measure of the change in direction at a measurement point along the curve. Specifically, when point P moves along the curve at a unit velocity, curvature can be a measure of the (instantaneous) rate of change of the unit tangent vector of the curve at point P. In a specific example, the position P(s) of the point can be a function of a parameter s, which can be, for example, time or the arc length from a given origin; and T(s) can be the unit tangent vector of the curve at P(s), which is also the derivative of P(s) with respect to s. In this example, the derivative of T(s) with respect to s can be a vector orthogonal to the curve, and its length can be curvature.

[0050] The curve referred to herein can be the profile (and / or contour, shape, etc.) of the first electrode in a planar view (i.e., in a view parallel to the winding axis). Specifically, the first electrode can be configured as a sheet or layer such that it is perceived as a coarse curve in the planar view. Alternatively or additionally, this can also be applied to the second electrode and / or separator. The curve can be continuously differentiable near P, so that the tangent can vary continuously along the curve. The curve can be twice differentiable at any P, such that there exists curvature along the curve, for example as the derivative of T(s) with respect to s as defined above.

[0051] Alternatively or additionally, the curvature used herein can be determined based on the osculating circle at the curve. The osculating circle of the curve at point P can be a circle with the same tangent and curvature as the curve. The tangent can be an approximate tangent to the curve at point P. The osculating circle can be an approximate circle of the curve at point P. The curvature of a straight line can be zero. If the curvature at a point is not equal to zero, the reciprocal of the curvature is considered to be the radius of curvature, i.e., the radius of the osculating circle. The center of the osculating circle is considered to be the center of curvature and can be constructed by drawing a radius of curvature perpendicular to the tangent of the curve in the direction of the curve's curvature.

[0052] Furthermore, the curvature used in this paper can be determined as the derivative (or differential) of the central angle at the center of the osculating circle with respect to the arc at the osculating circle.

[0053] In any of the above-described options for determining the curvature of the first electrode, the curvature can be determined from a planar or cross-sectional image of the electrode assembly obtained from a view parallel to the winding axis. Specifically, the curve can be determined using an image of the electrode assembly obtained from a view parallel to the winding axis, i.e., a visually determined cross-sectional view of the first electrode. The curvature can then be determined from the curve.

[0054] The point of maximum curvature used herein can refer to a point within or a portion of the first or second electrode in a plan view of the electrode assembly, where the curvature of the first or second electrode is maximum (i.e., highest). Therefore, the point of maximum curvature can be a point on the first electrode where, in a plan view of the electrode assembly, the first electrode exhibits the greatest curvature. Alternatively or additionally, the point of maximum curvature can be a point on the first electrode where the closely spaced circle of the first electrode is minimum. Additionally or alternatively, this can also be applied to the second electrode and / or the separator.

[0055] In a specific example, the curvature of the first electrode typically decreases as it extends from the winding axis toward the outer circumference of the electrode assembly. In other words, the curvature of the first electrode typically decreases either radially outward or helically outward (winding) along the first electrode. The overall reduction in the curvature of the first electrode in the outward radial (or winding) direction may be due to the first electrode, the spacer, and the second electrode being wound together about the winding axis. Additionally or alternatively, the curvature of the second electrode and / or the spacer typically decreases radially outward.

[0056] Simultaneously, the point of maximum curvature may occur in a portion closer to the winding axis than to the outer circumference of the electrode assembly. In a plan view of the electrode assembly, the portion closer to the winding axis than to the outer circumference of the electrode assembly can be referred to as the core portion. The core portion can be further described in detail below. The point of maximum curvature (in a plan view) may occur or may be located in or within a portion of the core portion of the electrode assembly. This can apply to the first electrode. Additionally or alternatively, this can also apply to the second electrode and / or the separator.

[0057] As described above, the winding direction used herein can follow the winding of the first electrode (and / or the second electrode and the separator), and can be a helical path around the winding axis. Therefore, in a plan view (and / or in a cross-section perpendicular to the winding axis), the winding direction of the first electrode can begin from its inner end and follow the winding direction to its outer peripheral end. However, when determining the azimuth angle, the winding direction can be a circular direction around the winding axis in the winding direction of the first electrode (i.e., the direction according to the direction of rotation).

[0058] The azimuth angle can be determined in a cross-section of the electrode assembly perpendicular to its winding axis (or in a plan view of the electrode assembly), wherein the vertex of the azimuth angle coincides with the winding axis. In the case of the aforementioned region in the cross-section of the electrode assembly, one side of the azimuth angle can be given by a straight line (i.e., the first line) extending from the winding axis to the inner end of the first electrode. The other side of the azimuth angle can be given by a straight line extending from the winding axis to the boundary of the aforementioned region in a cross-section of the electrode assembly perpendicular to its winding axis.

[0059] The region extends through (or spans or covers) an azimuth angle greater than 0° and less than or equal to 180°. In other words, the region covers the entire area between a boundary corresponding to an azimuth angle of 0° and another boundary corresponding to an azimuth angle of 180°. These boundaries intersect at the winding axis, and the outer circumference of the electrode assembly provides another boundary. A straight line extending from the winding axis through the inner end of the first electrode, i.e., the first line as described above, can correspond to an azimuth angle of 0°. This region extends in the opposite direction to the winding direction, i.e., covering the area opposite to the initial portion of the first electrode extending from its inner end along the winding direction. When the region extends within an azimuth angle range greater than 0° and less than or equal to 180°, the region can cover half of the cross-section of the electrode assembly perpendicular to the winding axis of the electrode assembly (and / or half of the plan view of the electrode assembly), defined by a straight line extending through the winding axis and the inner end of the first electrode. Therefore, the region as described above can have a semi-circular shape or a similar shape.

[0060] In the implementation example, the roundness of the first electrode in or at the core portion can be greater than or equal to 89%.

[0061] As used herein, roundness can refer to the degree of roundness according to the teachings of mathematical geometry. In particular, roundness can be determined according to ISO 1101. As used herein, roundness can be determined as a two-dimensional parameter in a plan view of the electrode assembly. Additionally or alternatively, roundness can be determined in detail below.

[0062] In an example implementation, a first extension line can be drawn by extending a straight line from two points on the first surface connecting the first electrode, where the curvature direction changes within a distance of 5 mm from the inner end of the first electrode. A second extension line can be drawn by extending a straight line from two points on the surface of the second electrode facing the first electrode, where the curvature direction changes within a distance of 5 mm from the inner end of the first electrode. The first and second extension lines can form an angle of less than or equal to 25°.

[0063] In an example implementation, the first electrode does not include cracks or wrinkles in or at the core portion of the electrode assembly.

[0064] This disclosure also discloses a secondary battery. The secondary battery may include electrode assemblies as disclosed herein.

[0065] In an example implementation, the secondary battery may also include a battery case, particularly a cylindrical battery case, in which the electrode assembly is housed.

[0066] An exemplary embodiment of the present invention provides an electrode assembly in which a first electrode, a separator, and a second electrode are stacked and wound, wherein the first electrode has a first surface facing the winding axis of the electrode assembly and a second surface opposite to the first surface, and wherein, in the core portion of the electrode assembly, the flat portion of the first electrode accounts for more than 3% and less than or equal to 13.5%.

[0067] Another exemplary embodiment of the present invention provides a secondary battery comprising: the electrode assembly described above; and a battery case for housing the electrode assembly.

[0068] Technical effect

[0069] According to an exemplary embodiment of the present invention, the electrode assembly adjusts the proportion of the flat portion of the first electrode, making it possible to solve the problem that the core cavity cannot maintain its circular shape and collapses due to the deformation of the electrode assembly caused by the contraction / expansion of the electrode during the charging and discharging of the battery, thereby preventing damage to the second electrode and the separator, and preventing internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0070] In addition, the secondary battery according to the present invention can solve the problem that the core cavity cannot maintain its circular shape and collapses due to the deformation of the electrode assembly caused by the contraction / expansion of the electrode during the charging and discharging of the battery, prevent damage to the second electrode and the separator, and prevent internal short circuit between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0071] The effects of the present invention are not limited to those described above, and those skilled in the art will clearly understand from this specification and the accompanying drawings the effects not mentioned. Attached Figure Description

[0072] Figure 1 The correlation between the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode, the proportion of the flat portion of the first electrode, and the core deformation is shown.

[0073] Figure 2 It is a graph showing the correlation between the proportion of the flat portion of the first electrode and the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode.

[0074] Figure 3 It shows a CT image of the electrode assembly according to Example 1 and an image of the curvature of the first electrode extracted from the CT image.

[0075] Figure 4 It shows a CT image of the electrode assembly according to Example 2 and an image of the curvature of the first electrode extracted from the CT image.

[0076] Figure 5 It shows a CT image of the electrode assembly according to Comparative Example 1 and an image of the curvature of the first electrode extracted from the CT image.

[0077] Figure 6 It shows a CT image of the electrode assembly according to Comparative Example 2 and an image of the curvature of the first electrode extracted from the CT image.

[0078] Figure 7 It shows a CT image of the electrode assembly according to Comparative Example 3 and an image of the curvature of the first electrode extracted from the CT image.

[0079] Figure 8 It is based on the CT image of the electrode assembly of Comparative Example 4 and the image showing the appearance of the core crack of the first electrode.

[0080] Figure 9 It is based on CT images of the electrode assembly of Comparative Example 5 and an image showing the appearance of a core crack in the first electrode.

[0081] Figure 10 The method for assessing whether a core impact has occurred is illustrated schematically.

[0082] Figure 11 The image shows a CT image of the sliding range of the longitudinal end portion of the first electrode of a secondary battery according to Reference Experimental Example 1 and Reference Experimental Example 2. Detailed Implementation

[0083] Throughout this specification, when a part "comprises," "includes," or "has" a constituent element, unless otherwise specifically described, this does not mean that another constituent element is excluded, but rather that another constituent element may be included.

[0084] Throughout the specification, when a component is referred to as being "on" another component, the component may be in direct contact with the other component, or there may be an intermediate component present.

[0085] An exemplary embodiment of the present invention provides an electrode assembly in which a first electrode, a separator, and a second electrode are stacked and wound, wherein the first electrode has a first surface facing the winding axis of the electrode assembly and a second surface opposite to the first surface, and wherein, at the core portion of the electrode assembly, the flat portion of the first electrode accounts for more than 3% and less than or equal to 13.5%.

[0086] Here, the percentage of flat portions can be the percentage of the number of measurement points with a curvature of less than or equal to 1, based on the number of 100% of the measurement points located on the first surface of the first electrode.

[0087] Furthermore, the curvature can be calculated from multiple measurement points located on the first surface of the first electrode extracted from the computed tomography (CT) image, as described below.

[0088] According to an exemplary embodiment of the present invention, the electrode assembly adjusts the proportion of the flat portion of the first electrode, making it possible to solve the problem that the core cavity cannot maintain its circular shape and collapses due to the deformation of the electrode assembly caused by the contraction / expansion of the electrode during the charging and discharging of the battery, thereby preventing damage to the second electrode and the separator, and preventing internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0089] Specifically, when a battery is charged and discharged, the electrodes included in the electrode assembly undergo repeated contraction / expansion. However, the outward expansion is limited by the rigidity of the battery case surrounding the outer peripheral surface of the electrode assembly, so stress may concentrate in the direction of the cavity, which is the empty space located in the core portion. Therefore, the possibility of deformation of the electrode assembly core may increase.

[0090] Core deformation can be categorized into two phenomena: when the internal stress reaches a certain level or higher, the core cavity of the electrode assembly cannot maintain its circular shape and collapses, i.e., core collapse; and when the deformation of the end portion of the first electrode located in the core portion and the second electrode adjacent to the end portion exceeds a certain level, the separator between the first electrode and the second electrode is damaged, causing the first electrode and the second electrode to come into direct contact with each other, i.e., core collision.

[0091] Specifically, core collapse may concentrate in areas susceptible to internal stress in the electrode assembly. More specifically, core collapse may concentrate in areas where the curvature of the first electrode is at or below a certain value—that is, in flat areas. In other words, at the core portion of the electrode assembly, deformation caused by internal stress may begin in areas where the curvature of the first electrode is at or below a certain value, and after a certain point, the core cavity may no longer maintain its circular shape and may completely collapse. In electrode assemblies where core collapse has occurred, lithium ions can no longer transfer smoothly between the first and second electrodes, thus potentially degrading battery life.

[0092] Here, the curvature of the first electrode may be affected by factors such as the step portion caused by the thickness of the electrode, tabs, etc., the shape of the wound core, and the tension acting on the electrode assembly during winding. For example, the wound core may include a pair of wound core portions separated around a spacer portion with a separator inserted, and when a certain level of tension is applied, the wound core deforms, which inevitably creates a region where the curvature of the first electrode is a specific value or less, i.e., a flat region, and core collapse may occur in the region where the curvature of the first electrode is a specific value or less.

[0093] In this scenario, when the proportion of the flat portion of the first electrode is adjusted to a specific range, the phenomenon of the core cavity failing to maintain its circular shape and collapsing (i.e., core collapse) can be resolved. This prevents damage to the second electrode and the separator, and also prevents internal short circuits between the first and second electrodes, thus improving the battery's stability and lifespan characteristics.

[0094] Figure 1 The diagram illustrates the correlation between the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode, the proportion of the flat portion of the first electrode, and the core deformation.

[0095] Specifically, Figure 1 (a) to Figure 1 (c) shows a CT image of an electrode assembly according to an exemplary embodiment of the present invention and the curvature of a first electrode extracted from the CT image. More specifically, Figure 1 (a) shows the flat area of ​​the electrode assembly with respect to the wound core. Figure 1 (b) shows the point where the electrode assembly has the greatest curvature with respect to the winding core, and Figure 1 (c) shows the flat area and the point of maximum curvature of the electrode assembly with respect to the wound core. Note that... Figure 1 (d) is the result of the loop, based on Figure 1 (a) to Figure 1 Image of core deformation occurring in a flat region of the electrode assembly in (c).

[0096] Specifically, for the curvature measured at each measurement point, Figure 1 (a) to Figure 1 (c) represents the proportion of measurement points with curvature equal to or less than an arbitrary set reference value, 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 represented to the right of the color coordinates ranges from... Figure 1 In (a), it is 0 to 0.8, and in Figure 1 In (b), the value is 1 to 1.5.

[0097] More specifically, in Figure 1 In (a), when the color distribution range of the curvature measured at each measurement point is set to 0 to 0.8, the portion with low k-value regions is implemented as color coordinates to visually represent the proportion of flat areas and flat parts, and... Figure 1In (b), when the color distribution range of the curvature measured at each measurement point is set to 1 to 1.5, the portion with high k-value regions is implemented as color coordinates to visually represent the point with the maximum curvature. In this case, as described above, each measurement point is extracted from the CT image of the first electrode, and it can be confirmed that the longitudinal end portion of the first electrode located at the core portion is located in the 6 o'clock direction.

[0098] According to an exemplary embodiment of the present invention, winding can be performed using a winding core, the winding core including a spacer portion to which a separator is inserted, a first winding core portion A disposed on one side of the spacer portion relative to the spacer portion, and a second winding core portion B disposed on the other side of the spacer portion and having a cross-sectional area different from that of the first winding core portion A.

[0099] Specifically, the winding core may include a spacer portion with a separator inserted therein, a first winding core portion disposed on one side relative to the spacer portion, and a second winding core portion disposed on the other side and having a cross-sectional area different from that of the first winding core portion. Because the winding core has a spacer portion, stacked components, such as separators, inserted into the spacer portion can be wound in a direction opposite to the rotation direction of the winding core by rotating the winding core.

[0100] In this configuration, since the winding core comprises a first winding core portion and a second winding core portion, and the second winding core portion has a different cross-sectional area than the first winding core portion, the relatively smaller winding core portion may deform when high tension is applied. Therefore, with the spacer portion serving as the central portion, the point of maximum curvature of the first electrode can be located adjacent to the relatively larger winding core portion, and the flat area of ​​the first electrode can be located adjacent to the relatively smaller winding core portion.

[0101] Furthermore, based on the longitudinal end portion of the first electrode, in the region greater than 0° and less than or equal to 180° in the direction opposite to the winding direction of the electrode assembly—that is, in the rotational direction of the winding core—compared to the region greater than 180° and less than or equal to 360°, the first electrode has relatively fewer turns up to the outermost layer, and the distance from which it can slide to the longitudinal end portion of the first electrode is relatively longer. Therefore, the concentrated stress is not released. Consequently, the possibility of deformation along the core cavity direction at the corresponding location may be higher.

[0102] In other words, the wound core may include a pair of wound core portions that are separated around a spaced portion with a separator inserted, and when a certain level of tension is applied, the wound core deforms, which inevitably creates a region of curvature of the first electrode with a specific value or less, i.e., a flat region, and core collapse may occur in the region of curvature of the first electrode with a specific value or less.

[0103] In this configuration, by adjusting the proportion of the flat portion of the first electrode in the core section of the electrode assembly, the problem of the core cavity failing to maintain its circular shape and collapsing (i.e., core collapse) can be resolved. This prevents damage to the second electrode and the separator, and also prevents internal short circuits between the first and second electrodes, thereby improving the battery's stability and lifespan characteristics.

[0104] According to an exemplary embodiment of the present invention, in the core portion of the electrode assembly, the proportion of the flat portion of the first electrode can be greater than 3% and less than or equal to 13.5%. Specifically, in the core portion of the electrode assembly, the proportion of the flat portion of the first electrode can 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%.

[0105] When the flat portion ratio of the first electrode meets the above-mentioned range, the phenomenon that the core cavity cannot maintain a circular shape and collapses due to the deformation of the electrode assembly caused by the electrode contraction / expansion during battery charging and discharging can be solved, thereby preventing damage to the second electrode and the separator, and preventing internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0106] Here, the percentage of flat area (%) can refer to the proportion of the flat area of ​​the first electrode in the first electrode that is the measurement target, and the flat area can refer to the area with curvature equal to or less than the reference value.

[0107] In other words, the flatness percentage can refer to the proportion of the region with a curvature value equal to or less than a reference curvature value in the first electrode, which is the measurement target. For example, the flatness percentage can be the percentage (%) of the number of measurement points with a curvature of less than or equal to 1 out of 100% of the measurement points.

[0108] Since the proportion of the flat portion of the first electrode can be determined based on the reference curvature value, when the range of the above reference curvature value is met, the degree to which the core portion deviates from the circular shape can be determined more effectively, and the reliability of the determined proportion of the flat portion of the first electrode may be higher.

[0109] In the following text, the curvature measurement target, curvature measurement method, and curvature measurement time point will be described in more detail regarding the proportion of the flat portion of the first electrode of the present invention.

[0110] According to an exemplary embodiment of the present invention, the curvature of the first electrode can be measured at the core portion of the electrode assembly.

[0111] Here, "core portion" can be a region including a cavity located on the winding axis of the electrode assembly and a part of a stacked structure formed by the wound separator / second electrode / separator / first electrode.

[0112] In other words, the core portion may include a cavity located on the winding axis of the electrode assembly, and a region extending to the innermost end portion of the first electrode in the longitudinal direction, i.e., a region excluding the first electrode, where the innermost part is where the electrode assembly begins to be wound. Additionally, the core portion may also include a region of a predetermined length, encompassing the first electrode in the direction in which it is wound, starting from one end portion of the first electrode in the longitudinal direction.

[0113] According to an exemplary embodiment of the present invention, the core portion may be a region within 3 turns starting from one end portion of the first electrode in the longitudinal direction. Specifically, the core portion may be a region within 1 to 2.5 turns or 1.5 to 2 turns starting from one end portion of the first electrode in the longitudinal direction.

[0114] Here, "one turn" can refer to the length required to wind the electrode or spacer included in the electrode assembly 360° from a reference point, and this length can be determined based on the outer diameter of the winding core used to wind the electrode assembly, the thickness of the spacers and electrodes, and the number of turns of the spacers and electrodes positioned inside the reference point. For example, "one turn" of the first electrode can refer to the length required to wind the first electrode 360° from the longitudinal end portion of the first electrode in the direction in which the electrode assembly is wound.

[0115] In other words, the core portion can refer to the region extending up to a point less than or equal to 3 turns from one end portion of the first electrode in the longitudinal direction, the region extending up to a point ...

[0116] Since the proportion of the flat portion of the first electrode can be determined based on the range of the core portion, when the above-mentioned range of the core portion is met, the degree to which the core portion deviates from the circular shape can be determined more effectively, and the reliability of the determined proportion of the flat portion of the first electrode may be higher.

[0117] According to an exemplary embodiment of the present invention, curvature can be measured at the core portion of the electrode assembly, curvature can be measured for a first electrode at the core portion of the electrode assembly, and curvature can be measured by extracting the first electrode from a CT image of the core portion of the electrode assembly.

[0118] When the curvature measurement target is the first electrode located in the core portion of the electrode assembly, the reliability of the curvature measurement may be higher, and the proportion of the flat portion of the first electrode can be more easily adjusted by adjusting the position of the longitudinal end portion of the first electrode. As a result, the deformation of the end portion of the first electrode located in the core portion and the second electrode adjacent to that end portion is reduced, making it possible to resolve the phenomenon of damage to the separator located between the first electrode and the second electrode, i.e., core collision.

[0119] According to an exemplary embodiment of the invention, curvature can be calculated from a plurality of measurement points located on a first surface of a first electrode extracted from a computed tomography (CT) image. Specifically, curvature can be measured relative to a first surface of the extracted first electrode, which faces the winding axis of the electrode assembly, after the first electrode has been extracted from the CT image of the electrode assembly. More specifically, curvature can be calculated from a plurality of measurement points positioned at regular intervals on the first surface of the first electrode. For example, the plurality of measurement points can be positioned on the first surface of the extracted first electrode at 2° intervals around the winding axis.

[0120] According to an exemplary embodiment of the present invention, the number of measurement points can be greater than or equal to 180. Specifically, the number of measurement points can be greater than or equal to 180 and less than or equal to 720. More specifically, the number of measurement points can be greater than or equal to 240 and less than or equal to 660, greater than or equal to 300 and less than or equal to 600, or greater than or equal to 360 and less than or equal to 540.

[0121] Since the proportion of the flat portion of the first electrode can be determined based on the number of measurement points, when the above-mentioned range of the number of measurement points is met, the degree to which the core portion deviates from the circular shape can be determined more effectively, and the reliability of the determined proportion of the flat portion of the first electrode may be higher.

[0122] According to an exemplary embodiment of the present invention, curvature can be measured at separate measurement points within the range of the number of measurement points located at regular intervals in the core portion, or at arbitrarily selected measurement points within the range of the number of measurement points, and the point with the largest curvature and the proportion of flat portion can be determined.

[0123] According to an exemplary embodiment of the present invention, the curvature can be the curvature according to Formula 1 below. Specifically, the curvature calculated at the measurement point can be calculated using coordinate values, i.e., x-coordinates and y-coordinates, obtained from multiple measurement points located on the first surface of the first electrode extracted from the CT image, according to Formula 1 below.

[0124] [Formula 1]

[0125] k=(x'y''-y'x'') / (x' 2 +y' 2 ) 3 / 2

[0126] In Formula 1 above, k is the curvature of the first electrode, x is the x-coordinate of the measurement point, y is the y-coordinate of the measurement point, x' is the first derivative of x and y' is the first derivative of y, x'' is the second derivative of x and y'' is the second derivative of y.

[0127] In other words, calculating curvature might mean measuring the x-coordinate and y-coordinate values ​​at multiple measurement points positioned at regular intervals on the first surface of the first electrode, and using the measured x-coordinates, y-coordinates, and Equation 1 to calculate the curvature value. Specifically, curvature can be calculated as a parametric expression of a planar curve, and the curvature, i.e., the degree of bending, increases with the value of k according to Equation 1. More specifically, curvature can be calculated using the above formula via a Python program, and the x-coordinate and y-coordinate values ​​can range from -3 to +3. Note that x' and y' might mean the slope of the tangent at each coordinate, and x'' and y'' might mean the rate of change of the slope.

[0128] If needed, the curvature value can be standardized using the radius of curvature of the electrode assembly for each measurement point.

[0129] According to an exemplary embodiment of the present invention, at the core portion of the electrode assembly, the angle formed between the longitudinal end portion of the first electrode and a point where the curvature of the first electrode is less than or equal to 1 can be greater than 0°.

[0130] Specifically, at the core portion of the electrode assembly, the angle formed between the longitudinal end portion of the first electrode and a point where the curvature of the first electrode is less than or equal to 1 can be greater than 0° and less than 50°.

[0131] Specifically, at the core portion of the electrode assembly, the angle formed between the longitudinal end portion of the first electrode and a point where the curvature of the first electrode is less than or equal to 1 can be greater than or equal to 5° and less than 50°, greater than or equal to 10° and less than or equal to 45°, or greater than or equal to 15° and less than or equal to 40°.

[0132] In other words, the region where the curvature of the first electrode is less than or equal to 1, i.e., the flat region, can be located between the point where the curvature of the first electrode is the largest and the longitudinal end portion of the first electrode, and can not overlap with the longitudinal end portion of the first electrode.

[0133] According to an exemplary embodiment of the present invention, the proportion of the flat portion of the first electrode can be measured after activation. Specifically, the proportion of the flat portion of the first electrode can be measured after activation, in a standby state and a normal use state, i.e., before core deformation occurs. For example, the proportion of the flat portion of the first electrode can be measured after 50 charge and discharge cycles at 25°C, 1C charge and 1C discharge conditions.

[0134] Here, the term "post-activation" can mean after a predetermined number of cycles for manufacturing secondary batteries and completing the product. Specifically, "post-activation" can include the storage state before the start of effective use, including multiple cycles for power supply purposes, i.e., before and after sale, as well as the state during storage where self-discharge has already occurred.

[0135] Activation can refer to the process of checking battery stability by repeated aging and charge / discharge after assembling the electrode assembly and battery case. It can be achieved in a simple way, such as by cycling the obtained battery at 1 C / 1 C@25°C for a predetermined number of times at any point after assembly, for example, 50 cycles, to achieve the state "before core deformation occurs after activation." However, activation conditions are not limited to the above conditions, as long as they are within the scope of those used in the art to achieve the same purpose.

[0136] Since the proportion of the flat portion of the first electrode can be determined based on the curvature measurement time point, when the above curvature measurement time point is met, the degree to which the core portion deviates from the circular shape can be determined more effectively, and the reliability of the determined proportion of the flat portion of the first electrode may be higher.

[0137] Figure 2 This is a graph showing the correlation between the proportion of the flat portion of the first electrode and the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode. Specifically, Figure 2 (a) is a scatter plot matrix, where any factors extracted from CT images before and after 27 accelerated cycles (1 C / 1 C 50 cycles) of the same battery are quantified, and Figure 2 (b) is a graph showing the correlation between the proportion of the flat portion of the first electrode and the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode.

[0138] Specifically, refer to Figure 2 The location of core deformation can be determined based on the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode, or the percentage of the flat portion of the first electrode. Furthermore, the location of core deformation of the electrode assembly can coincide with the region of the first electrode where the curvature is less than or equal to 1, i.e., the flat region. In other words, the lower the percentage of the flat portion, the lower the probability of core deformation occurring in the flat region, which is the location where core deformation occurs.

[0139] Note that, as an additional influencing factor related to core deformation, when the first electrode tab is positioned at 12 o'clock, the second electrode tab at the outermost portion is preferably located between 5 o'clock and 8 o'clock or between 6 o'clock and 7 o'clock. The longitudinal end portion of the first electrode at the core portion is preferably located between 5 o'clock and 9 o'clock or between 6.5 o'clock and 8 o'clock, and the longitudinal end portion of the first electrode at the outermost portion is preferably located between 4 o'clock and 6 o'clock or between 4.5 o'clock and 5.5 o'clock. Note that the area of ​​the core cavity is preferably greater than or equal to 5 mm². 2 And less than or equal to 15 mm 2 or greater than or equal to 7 mm 2 And less than or equal to 13 mm 2 When the aforementioned additional influencing factors meet the ranges described above, the effect of reducing core deformation may be even better. Note that the unit "o'clock" indicates the relative position of each factor in a clockwise direction when the first electrode patch is positioned at 12 o'clock on the CT image, and can be measured in the opposite direction to the direction in which the electrode assembly is wound, i.e., in the rotational direction of the winding core used to wind the electrode assembly. For example, 6 o'clock means forming a 180° angle with the first electrode patch positioned at 12 o'clock.

[0140] According to an exemplary embodiment of the present invention, the first electrode may not include cracks or wrinkles at the core portion of the electrode assembly. Here, cracks may refer to cracks visible to the naked eye on the surface of the first electrode, and wrinkles may refer to visible wrinkles or folds that have appeared on the surface of the first electrode.

[0141] When an electrode assembly including a first electrode with a crack is inserted into a battery case, the likelihood of low voltage and short circuits due to foreign matter increases significantly. On the other hand, since the electrode assembly according to an exemplary embodiment of the present invention does not include a first electrode with a crack or wrinkle at the core portion, battery stability can be improved.

[0142] According to an exemplary embodiment of the present invention, in the core portion of the electrode assembly, the proportion of the flat portion of the first electrode can be positively correlated with the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode. Specifically, referring to... Figure 2 (b) The correlation coefficient between the proportion of the flat portion of the first electrode and the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode can be approximately 0.756.

[0143] According to an exemplary embodiment of the present invention, at the core portion of the electrode assembly, the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode around the winding axis can be greater than 40° and less than or equal to 98°. Specifically, at the core portion of the electrode assembly, the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode around the winding axis can be greater than 40° and less than or equal to 98°, greater than or equal to 45° and less than or equal to 95°, greater than or equal to 45° and less than or equal to 90°, greater than or equal to 45° and less than or equal to 80°, greater than or equal to 45° and less than or equal to 70°, greater than or equal to 45° and less than or equal to 65°, greater than or equal to 50° and less than or equal to 95°, greater than or equal to 50° and less than or equal to 90°, greater than or equal to 50° and less than or equal to 80°, greater than or equal to 50° and less than or equal to 70°, or greater than or equal to 60° and less than or equal to 65°.

[0144] According to an exemplary embodiment of the present invention, the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode can be measured in a region greater than 0° and less than or equal to 180° in the direction opposite to the winding direction of the electrode assembly, based on the longitudinal end portion of the first electrode. That is, the point of maximum curvature can be the point of maximum curvature in a region greater than 0° and less than or equal to 180° in the direction opposite to the winding direction of the electrode assembly, based on the longitudinal end portion of the first electrode.

[0145] In other words, the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode can be measured in a region greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound, i.e., in the rotational direction of the winding core used to wind the electrode assembly.

[0146] Specifically, the stress generated during cycling is concentrated in the direction of the core cavity due to the rigidity of the battery case, thus core deformation may occur in relatively stress-prone flat areas. In this case, based on the longitudinal end portion of the first electrode, in the region greater than 0° and less than or equal to 180° in the direction opposite to the winding direction of the electrode assembly, i.e., in the rotational direction of winding the core, compared to the region greater than 180° and less than or equal to 360°, the number of turns of the first electrode to the outermost layer is relatively small, and the distance that can slide from the longitudinal end portion of the first electrode is relatively long, so the concentrated stress is not released. Therefore, the possibility of deformation along the direction of the core cavity at the corresponding location may be higher.

[0147] Furthermore, the tension applied to the electrode assembly during winding causes deformation of the winding core. Therefore, when the winding core includes spacer portions, the point of maximum curvature of the first electrode and flat regions can appear symmetrically at the spacer portions. However, core deformation is more likely to occur in flat regions where the longitudinal end portion of the first electrode is located in an area greater than 0° and less than or equal to 180° in the rotational direction of the winding core, and the likelihood of core deformation may be higher in these flat regions even after accelerated cycling.

[0148] Therefore, when the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode is measured in a region greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound, based on the longitudinal end portion of the first electrode, the point of maximum curvature of the first electrode can be determined more effectively, and the effect of reducing core deformation by adjusting the angle may be better.

[0149] When the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode satisfies the above-mentioned range, the phenomenon that the core cavity cannot maintain its circular shape and collapses due to the deformation of the electrode assembly caused by the contraction / expansion of the electrode during the charging and discharging of the battery can be solved, thereby preventing damage to the second electrode and the separator, and preventing internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0150] According to an exemplary embodiment of the present invention, the roundness (roundness) of the first electrode in the core portion of the electrode assembly can be greater than or equal to 89%. Specifically, the roundness of the first electrode in the core portion of the electrode assembly can be greater than or equal to 89% or greater than or equal to 90%, and can be greater than or equal to 89% and less than or equal to 99%, or greater than or equal to 90% and less than or equal to 98%.

[0151] Here, roundness (%) can be the ratio of the minimum spacing distance between the winding axis and the first electrode to the maximum spacing distance between the winding axis and the first electrode at 100%. Specifically, roundness can refer to the minimum spacing distance (R) between the winding axis and the first electrode. 最小 ) and the maximum spacing distance (R) between the winding axis and the first electrode 最大 ) ratio (%).

[0152] When the above-mentioned roundness range is met, the shape of the electrode assembly can be closer to a circular shape, and its resistance to stress acting on the core portion can be excellent. Therefore, the phenomenon that the core cavity cannot maintain its circular shape and collapses due to the deformation of the electrode assembly caused by electrode contraction / expansion during battery charging and discharging can be solved, preventing damage to the second electrode and separator, and preventing internal short circuits between the first and second electrodes, thereby improving the stability and lifespan characteristics of the battery.

[0153] However, roundness can be different from the flattened portion ratio, which refers to the proportion of the area with curvature equal to or less than a reference value within the first electrode, which is the measurement target. For example, depending on the shape of the electrode assembly, there may be cases where the roundness meets the above range, but the flattened portion ratio does not. In this case, in addition to roundness, by adjusting the flattened portion ratio to the above range, it is also possible to eliminate the possibility that the shape of the electrode assembly deviates from a circular shape at a specific location. Therefore, compared to simply adjusting the roundness, the shape of the electrode assembly can be more easily controlled, and the effect of reducing core deformation may be better. In other words, the flattened portion ratio can be a more accurate reference for how "close to a circular shape" the shape of the electrode assembly is.

[0154] According to an exemplary embodiment of the present invention, the first electrode may include a first electrode current collector and a first electrode active material layer disposed 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 end portions at the same location. In other words, one end portion of the first electrode in the longitudinal direction may have a free edge shape.

[0155] This reduces the area of ​​unnecessary uncoated portions on the first electrode current collector to ensure economic efficiency, and allows for the slitting process to be performed after the active material layer is formed on the electrode, thereby enabling more efficient slitting and roll-to-roll processes, including the winding process.

[0156] Here, the description of "same position" means that the end portions in the longitudinal direction are the same, and may include cases where the end portions are formed in substantially the same position due to process errors that may occur in the cutting process, etc.

[0157] According to an exemplary embodiment of the present invention, the first electrode may include a first electrode uncoated portion where no first electrode active material layer is disposed, and may also include a first electrode tab disposed on the first electrode uncoated portion.

[0158] In other words, the first electrode current collector may include a first electrode coated portion coated with a first electrode active material and a first electrode uncoated portion not coated with the first electrode active material, and may include a contact located on the first electrode uncoated portion. Specifically, the first electrode current collector may include the first electrode uncoated portion and a first electrode contact disposed on the first electrode uncoated portion.

[0159] In other words, one end portion of the first electrode in the longitudinal direction may have a free edge shape, the uncoated portion of the first electrode may be located between the two longitudinal end portions of the first electrode, and the first electrode tab disposed on the uncoated portion of the first electrode may be an intermediate tab.

[0160] According to an exemplary embodiment of the present invention, the first electrode current collector is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. Specifically, the first electrode current collector can be made of stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surfaces are treated with carbon, nickel, titanium, silver, etc. That is, the first electrode current collector can be provided in the form of surface-treated stainless steel, aluminum foil, etc.

[0161] In addition, the first electrode current collector can typically have a thickness of 3 μm to 50 μm, and the surface of the current collector can be formed with microscopic irregularities to enhance the adhesion of the first electrode active material. For example, the first electrode current collector can be used in various forms such as membranes, sheets, foils, meshes, porous bodies, foams, and nonwoven fabrics.

[0162] According to an exemplary embodiment of the present invention, the first electrode active material can be a commonly used first electrode active material. Specifically, the first electrode active material can be a layered compound, such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2), or a compound substituted with one or more transition metals; lithium iron oxide, such as LiFe3O4; lithium manganese oxide, such as Li 1+x Mn 2-x O4 (0≤x≤0.33), LiMnO3, LiMn2O3 and LiMnO2; lithium copper oxide (Li2CuO2); vanadium oxides, such as LiV3O8, V2O5 and Cu2V2O7; and LiNi 1-y M yO2 (where M is at least one of Co, Mn, Al, Cu, Fe, Mg, B, and Ga, and satisfies 0.01 ≤ y ≤ 0.3) represents a nickel-site lithium nickel oxide; LiMn 2-z M z Lithium-manganese composite oxides represented by O2 (where M is selected from at least one of Co, Ni, Fe, Cr, Zn, and Ta, and satisfies 0.01≤z≤0.1) or Li2Mn3MO8 (where M is selected from at least one of Fe, Co, Ni, Cu, and Zn); LiMn2O4, etc., wherein a portion of the Li in the chemical formula is replaced by an alkaline earth metal ion, but is not limited thereto. The first electrode may be Li metal.

[0163] According to an exemplary embodiment of the present invention, the first electrode active material layer may further include a first electrode conductive material and a first electrode binder. The first electrode conductive material is used to impart conductivity to the electrode and can be used without particular limitations, as long as it does not cause a chemical change in the battery to be constructed and has electronic conductivity. Specific examples of the first electrode conductive material may include: graphite, such as natural or artificial graphite; carbon-based materials, such as carbon black, acetylene black, Ketjen black, channel black, furnace black, lamp black, thermal cracking black, and carbon fiber; metal powders or metal fibers, such as copper, nickel, aluminum, and silver; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; or conductive polymers, such as polyphenylene derivatives, and any one of them or a mixture of two or more of them may be used.

[0164] Additionally, the first electrode adhesive is used to improve the adhesion between particles of the first electrode active material and the adhesion between the first electrode active material and the first electrode current collector. Specific examples may include polyvinylidene fluoride (PVDF), vinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinyl alcohol, polyacrylonitrile, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, ethylene-propylene-diene polymer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, or various copolymers thereof, and any one of them or a mixture of two or more of them may be used.

[0165] According to an exemplary embodiment of the present invention, the second electrode may include a second electrode current collector and a second electrode active material layer disposed 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 comprising the second electrode active material. In other words, the second electrode active material layer may be formed on the second electrode coated portion of the second electrode current collector, and the surface on which the second electrode active material layer is not disposed may be referred to as the uncoated portion of the second electrode.

[0166] According to an exemplary embodiment of the present invention, the second electrode current collector may include a second electrode coated portion in which a second electrode active material layer is formed and a second electrode uncoated portion in which no second electrode active material layer is formed, and may include a tab located on the second electrode uncoated portion. Specifically, the second electrode current collector may include the second electrode uncoated portion and the second electrode tab formed on the second electrode uncoated portion. Therefore, the manufactured electrode assembly may include one or more second electrode tabs.

[0167] According to an exemplary embodiment of the present invention, the second electrode active material layer may include a second electrode active material, which includes one or more selected from the group consisting of silicon-based materials and carbon-based materials. Additionally, the second electrode active material layer may also include a second electrode conductive material and a second electrode binder, and there are no limitations on the materials used in the art for the second electrode active material, the second electrode conductive material, and the second electrode binder.

[0168] According to an exemplary embodiment of the present invention, the second electrode current collector is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery. For example, copper, stainless steel, aluminum, nickel, titanium, sintered carbon, or aluminum or stainless steel whose surfaces are treated with carbon, nickel, titanium, silver, etc., can be used for the second electrode current collector. Specifically, transition metals that readily adsorb carbon, such as copper and nickel, can be used for the second electrode current collector. The thickness of the second electrode current collector can be greater than or equal to 5 μm and less than or equal to 30 μm. However, the thickness of the second electrode current collector is not limited to this.

[0169] According to an exemplary embodiment of the present invention, the second electrode adhesive may include at least one selected from the group consisting of polyvinylidene fluoride-hexafluoropropylene copolymer (PVDF-co-HFP), polyvinylidene fluoride, polyacrylonitrile, polymethyl methacrylate, polyvinyl alcohol, carboxymethyl cellulose (CMC), starch, hydroxypropyl cellulose, regenerated cellulose, polyvinylpyrrolidone, tetrafluoroethylene, polyethylene, polypropylene, polyacrylic acid, ethylene-propylene-diene monomer (EPDM), sulfonated EPDM, styrene-butadiene rubber (SBR), fluororubber, polyacrylic acid, and materials in which hydrogen is replaced by Li, Na, Ca, etc., and may also include various copolymers thereof.

[0170] According to an exemplary embodiment of the present invention, the conductive material of the second electrode is not particularly limited, as long as it is conductive and does not cause chemical changes in the battery, and for example, graphite, such as natural or artificial graphite; carbon black, such as acetylene black, Ketjen black, channel black, furnace black, lamp black, and thermal cracking black; conductive fibers, such as carbon fibers and metal fibers; conductive tubes, such as carbon nanotubes; metal powders, such as fluorocarbons, aluminum and nickel powder; conductive whiskers, such as zinc oxide and potassium titanate; conductive metal oxides, such as titanium oxide; conductive materials, such as polyphenylene derivatives, etc., and similar materials can be used.

[0171] According to an exemplary embodiment of the present invention, the electrode assembly may include multiple separators. For example, the electrode assembly may have a structure in which separators / second electrodes / separators / first electrodes are stacked in sequence. The separators are used to separate the first electrode and the second electrode and provide a migration path for lithium ions. Any separator can be used without particular limitation, as long as it is typically used as a separator in a secondary battery. In particular, separators with high electrolyte retention capacity and low resistance to electrolyte ion migration can be preferably used. Specifically, porous polymer membranes can be used, for example, porous polymer membranes made of polyolefin-based polymers such as ethylene homopolymers, propylene homopolymers, ethylene / butene copolymers, ethylene / hexene copolymers, and ethylene / methacrylate copolymers, or having a stacked structure of two or more layers thereof. Alternatively, conventional porous nonwoven fabrics can be used, for example, nonwoven fabrics formed from high-melting-point glass fibers, polyethylene terephthalate fibers, etc. Additionally, as a separator, the following separator can be used, in which the aforementioned separator material is used as a base layer, and a slurry containing ceramic components or polymer materials is coated on the base layer to ensure heat resistance or mechanical strength. Separators with single-layer or multi-layer structures can be selectively used. The thickness of the separator can be greater than or equal to 5 μm and less than or equal to 20 μm. However, the thickness of the separator is not limited to this.

[0172] According to an exemplary embodiment of the present invention, the angle formed between the first electrode and the second electrode can be less than or equal to 25°. Specifically, the first electrode may include a first surface facing the winding axis of the electrode assembly and a second surface opposite to the first surface. An angle of less than or equal to 25° can be formed by extending a first extension line drawn by extending straight lines from two points on the first surface connecting the first electrode at a 5 mm interval from the longitudinal end portion of the first electrode, and a second extension line drawn by extending straight lines from two points on the surface connecting the second electrode facing the first electrode at a 5 mm interval from the longitudinal end portion of the first electrode. In this case, the angle formed by the first electrode and the second electrode can be measured, for example, after activation and an additional 200 or more charge-discharge cycles at a temperature of 40°C or higher.

[0173] When the aforementioned angle range is met, the end portion of the first electrode located at the core portion and the second electrode adjacent to that end portion will not deform beyond a certain degree. Therefore, damage to the separator between the first and second electrodes and direct contact between the first and second electrodes, i.e., core collision, can be suppressed. This prevents damage to the second electrode and the separator, and also prevents internal short circuits between the first and second electrodes, thereby improving the battery's stability and lifespan characteristics.

[0174] According to an exemplary embodiment of the present invention, the first electrode and the second electrode can be positive and negative electrodes, respectively. Specifically, the first electrode can be positive, and the second electrode can be negative.

[0175] An exemplary embodiment of the present invention provides a secondary battery, which includes the electrode assembly described above and a battery case for housing the electrode assembly. Specifically, the secondary battery may include the electrode assembly according to the exemplary embodiment described above and a battery case for housing the electrode assembly.

[0176] The secondary battery according to the present invention can solve the problem of the core cavity failing to maintain its circular shape and collapsing due to the deformation of the electrode assembly caused by the contraction / expansion of the electrodes during the charging and discharging of the battery, prevent damage to the second electrode and the separator, and prevent internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0177] According to an exemplary embodiment of the present invention, the battery box may have a cylindrical shape. Specifically, the battery box may have a cylindrical shape, a square shape, a pouch shape, etc., depending on the application, but is not limited thereto.

[0178] According to an exemplary embodiment of the present invention, the battery box 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 to manufacture lithium secondary batteries. Specifically, the electrolyte may include a non-aqueous organic solvent and a metal salt.

[0179] According to an exemplary embodiment of the present invention, as a non-aqueous organic solvent, for example, aprotic organic solvents such as N-methyl-2-pyrrolidone, propylene carbonate, ethylene carbonate, butyl carbonate, dimethyl carbonate, diethyl carbonate, γ-butyl lactone, 1,2-dimethoxyethane, tetrahydrofuran, 2-methyltetrahydrofuran, dimethyl sulfoxide, 1,3-dioxolane, formamide, dimethylformamide, dioxolane, acetonitrile, nitromethane, methyl formate, methyl acetate, triphosphate, trimethoxymethane, dioxolane derivatives, sulfolane, methyl sulfolane, 1,3-dimethyl-2-imidazolium ketone, propylene carbonate derivatives, tetrahydrofuran derivatives, diethyl ether, methyl propionate, or ethyl propionate can be used.

[0180] According to an exemplary embodiment of the present invention, lithium salts can be used as metal salts, and lithium salts are materials that are readily soluble in non-aqueous electrolyte solutions, wherein, for example, a selection from 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 substances in the lithium salt act as anions.

[0181] According to an exemplary embodiment of the present invention, for the purposes of improving battery life characteristics, suppressing battery capacity decline, and increasing battery discharge capacity, in addition to the above-mentioned electrolyte components, the electrolyte may also include one or more additives, such as compounds based on haloalkane carbonates like difluoroethylene carbonate, pyridine, triethyl phosphite, triethanolamine, cyclic ethers, ethylenediamine, n-glycol, triammonium hexaphosphate, nitrobenzene derivatives, sulfur, quinone imine dyes, N-substituted oxazolidinones, N,N-substituted imidazolidinanes, ethylene glycol dialkyl ethers, ammonium salts, pyrroles, 2-methoxyethanol, or aluminum trichloride.

[0182] Exemplary embodiments of the present invention provide a battery module comprising a secondary battery as a unit cell, and a battery pack comprising the battery module. Since the battery module and battery pack comprise a secondary battery with high capacity, high battery stability, and long lifespan characteristics, they can be used as a power source for medium and large-sized devices selected from the group consisting of electric vehicles, hybrid electric vehicles, plug-in hybrid electric vehicles, and energy storage systems.

[0183] In the following description, examples will be given in detail to specifically illustrate the invention. However, the examples according to the invention can be modified in many different forms, and the scope of the invention should not be construed as limited to the following examples. Examples are provided in this specification to explain the invention more fully to those skilled in the art.

[0184] Implementation of the invention

[0185] Examples

[0186] Example 1

[0187] Preparation of electrode components

[0188] By using the first electrode active material A first electrode active material slurry was prepared by adding CNTs (as the conductive material of the first electrode) and polyvinylidene fluoride (PVdF) (as the binder of the first electrode) to N-methyl-2-pyrrolidone (NMP) in a weight ratio of 97.92:0.5:1.58. The first electrode active material slurry was coated onto an aluminum current collector with a thickness of 15 μm and a length of 63.9 mm in the width direction, and then dried and rolled to form a first electrode active material layer, thereby preparing a first electrode with a thickness of 135 μm.

[0189] Next, a second electrode active material composition was prepared by using natural graphite (C, average particle size: 17 μm) as the second electrode active material, and mixing the second electrode active material, carbon black as the second electrode conductive material, and styrene-butadiene rubber (SBR) as the second electrode binder in a weight ratio of 97.7:1.3:1.0. Then, 7.8 g of distilled water was added to 5 g of the second electrode active material composition, followed by stirring to prepare a second electrode active material slurry. The second electrode active material slurry was coated onto a copper (Cu) metal film with a thickness of 8 μm and a width of 65.2 mm, serving as the second electrode current collector, and then dried (at 120°C for 1 minute) to form a second electrode with an average thickness of 166 μm. In this case, the temperature of the circulating air was 60°C.

[0190] Subsequently, the two separators are arranged sequentially and wound using a 3.2 mm diameter core. A second electrode is then introduced between the two separators and subsequently wound additionally. In this case, a core is used comprising a spacer portion in which the separators are inserted, a first wound core portion, and a second wound core portion with a cross-sectional area different from that of the first wound core portion. The cross-sectional areas of the first and second wound core portions are both 5.4 mm². 2 and 2.7 mm 2 .

[0191] After the separator and the second electrode are wound about three turns, the first electrode is introduced and wound, and a sealing tape made of PET is attached and trimmed to wrap around the upper and lower outer peripheral surfaces of the electrode assembly at the finished winding end portion.

[0192] In this case, based on the first wound core portion located at the 6 o'clock position of the electrode assembly and the second wound core portion located at the 12 o'clock position, the positions of the longitudinal end portions of the core portion and the outermost portion of the first electrode, as well as the positions of the tabs, are 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 end portions at the same location, and the unit "o'clock" means that when the first electrode tab is located at the 12 o'clock position on the CT image, the relative positions of each factor are shown in a clockwise manner.

[0193] The area and roundness of the core cavity measured from computed tomography (CT) images are shown in Table 1 below. In this case, regarding the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode, the curvature measured at each measurement point on the CT image is compared. Based on the longitudinal end portion of the first electrode, the measurement point with the maximum curvature in the region greater than 0° and less than or equal to 180° in the direction opposite to the direction in which the electrode assembly is wound is determined as the point of maximum curvature of the first electrode (maximum curvature). The angle formed between the point of maximum curvature of the first electrode and the longitudinal end portion of the first electrode is measured about the winding axis and is shown in Table 2 below. In the following examples, comparative examples, and experimental examples, the point of maximum curvature of the first electrode (maximum curvature) is measured using the same method.

[0194] [Table 1]

[0195] Preparation of secondary batteries

[0196] A secondary battery was prepared by inserting an electrode assembly into a cylindrical battery case, injecting an electrolyte solution, and sealing the cylindrical battery case with a cover assembly in which ethylene carbonate (EC), ethyl methyl carbonate (EMC), and diethyl carbonate (DEC) were mixed in a volume ratio of 20:5:75 and LiPF6 was dissolved to 1.4 M.

[0197] In this case, after 50 cycles at 4.25 V to 2.5 V and 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 60.8°, and the flat portion of the first electrode accounts for 7.72%.

[0198] Example 2

[0199] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 65° and the flat portion of the first electrode accounts for 9.6%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0200] Comparison Example 1

[0201] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 98.9° and the flat portion of the first electrode accounts for 13.9%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0202] Comparison Example 2

[0203] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 112.4° and the flat portion of the first electrode accounts for 17.0%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0204] Comparison Example 3

[0205] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 129.3° and the flat portion of the first electrode accounts for 16.49%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0206] Comparison Example 4

[0207] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 40° and the flat portion of the first electrode accounts for 3.0%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0208] Comparison Example 5

[0209] Except for adjusting the length of the separator winding and the position of the first electrode when it is introduced, such that after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the angle between the longitudinal end portion of the first electrode extracted from the CT image of the core portion and the point of maximum curvature of the first electrode is 20° and the flat portion of the first electrode accounts for 1.0%, the electrode assembly and the secondary battery are manufactured in the same manner as in Example 1.

[0210] Examples of experiments

[0211] Experimental Example 1: Evaluation of the Proportion of Flat Parts

[0212] The percentage of flattened portions of the secondary batteries in Examples 1 and 2, and in comparison Examples 1 to 5, were evaluated using the following methods, and are shown in Table 2 below. Figures 3 to 9 The results are shown in the figure.

[0213] 1) Extract the first electrode from the CT image of the electrode assembly.

[0214] 2) On the first surface of the first electrode, measurement points are set at 2° intervals from one end portion of the first electrode in the longitudinal direction, and the x and y coordinates of the measurement points are measured.

[0215] 3) Using the x and y coordinates of the measurement points, calculate the curvature (k) of each measurement point according to Formula 1 below.

[0216] [Formula 1]

[0217]

[0218] In Formula 1 above, k is the curvature of the first electrode, x is the x-coordinate of the measurement point, y is the y-coordinate of the measurement point, x' is the first derivative of x and y' is the first derivative of y, x'' is the second derivative of x and y'' is the second derivative of y.

[0219] 4) When the curvature value calculated at each measurement point is less than or equal to 1, the corresponding measurement point is evaluated as a flat region, and the ratio of the number of measurement points evaluated as flat regions based on 100% of the total number of measurement points is evaluated as the percentage of flat regions (%).

[0220] Experimental Example 2: Core Collapse Assessment

[0221] The following methods were used to evaluate whether core collapse occurred in the secondary batteries of Examples 1 and 2, and Comparative Examples 1 to 5, and the evaluation was repeated every 200 cycles. See Table 2 below for details. Figures 3 to 9 The results are shown in the figure.

[0222] 1) Extract the first electrode from the CT image of the electrode assembly.

[0223] 2-1) Measure the area of ​​the core cavity surrounded by the first electrode from one end portion of the first electrode in the longitudinal direction to a point corresponding to a circle of the first electrode in the longitudinal direction.

[0224] 2-2) Core collapse is assessed as having occurred when the area of ​​the core cavity is less than 100% of the sum of the cross-sectional areas of the spaced portion of the wound core used for winding the electrode assembly, the first wound core portion, and the second wound core portion.

[0225] 3-1) On the first surface of the first electrode, plot the minimum value (D) corresponding to the diameter of the core cavity of the extracted first electrode. 最小 ) and maximum value (D) 最大 The concentric circles are drawn by extending the straight lines and using the intersection of each straight line as the central axis, i.e., the winding axis.

[0226] 3-2) Measure the maximum distance (R) between the winding axis and the first electrode from one end portion of the first electrode in the longitudinal direction to a point corresponding to one turn of the first electrode in the longitudinal direction. 最大 ) and the minimum spacing distance (R) between the winding axis and the first electrode. 最小 ).

[0227] 3-3) Calculate the minimum spacing distance (R) between the winding axis and the first electrode. 最小 ) and the maximum spacing distance (R) between the winding axis and the first electrode 最大 The ratio (%) of the core cavity's roundness is used to determine the core cavity's roundness. When the calculated roundness of the core cavity is less than 89%, core collapse is considered to have occurred.

[0228] On the other hand, in the case of acquiring an unknown secondary battery (unknown battery), the above-described method for assessing whether core collapse has occurred can be applied in the following manner: assessing whether core collapse has occurred at the initial acquisition, reassessing whether core collapse has occurred every 200 cycles, and comparing and analyzing the results with the core collapse conditions of a secondary battery according to an exemplary embodiment of the present invention.

[0229] Experimental Example 3: Core Impact Assessment

[0230] The following methods were used to evaluate whether a core impact occurred in the secondary batteries of Examples 1 and 2, and Comparative Examples 1 to 5, with the evaluation re-evaluated every 200 cycles, as shown in Tables 2 and 3 below. Figures 3 to 9 The results are shown in the figure.

[0231] Figure 10The method for assessing whether a core impact has occurred is illustrated schematically. Specifically, Figure 10 (a) schematically illustrates a method for assessing whether a core impact has occurred when deformation occurs in the second electrode, and Figure 10 (b) schematically illustrates a method for assessing whether a core impact has occurred when no deformation has occurred in the second electrode.

[0232] 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 portion 310 of the first electrode and a point spaced 5 mm apart from the end portion.

[0233] 2-1) When the second electrode deforms

[0234] At the core portion of the electrode assembly, on the surface of the second electrode 100 facing the first electrode, a second extension line E2 is drawn by extending a straight line connecting two points whose curvature direction changes within a 5 mm interval from the longitudinal end portion 310 of the first electrode.

[0235] 2-2) When there is no deformation in the second electrode

[0236] At the core portion of the electrode assembly, on the surface of the second electrode 100 facing the first electrode, a second extension line E2 is drawn by extending a straight line connecting two points spaced 5 mm apart from the longitudinal end portion 310 of the first electrode.

[0237] 3) When the angle from the first extension line E1 to the second extension line E2 in a counterclockwise direction relative to the intersection of the first extension line E1 and the second extension line E2 exceeds 25°, it is assessed that a core impact has occurred.

[0238] On the other hand, in the case of acquiring an unknown secondary battery (unknown battery), the above-described method for assessing whether a core impact has occurred can be applied in the following manner: assessing whether a core impact has occurred at the time of initial acquisition, reassessing whether a core impact has occurred every 200 cycles, and comparing and analyzing the results with the core impact conditions of a secondary battery according to an exemplary embodiment of the present invention.

[0239] Experimental Example 4: Assessment of Core Crack Incidence

[0240] The electrode assemblies of Examples 1, 2, and Comparative Examples 1 to 5 were evaluated for core cracks using the following methods, as shown in Table 2 below. Figure 8 and Figure 9 The results are shown in the figure.

[0241] Specifically, electrode assemblies of Examples 1 and 2, and Comparative Examples 1 to 5, were prepared, and the manufactured electrode assemblies were disassembled for visual inspection to check for cracks or wrinkles at the core portion. In this case, when cracks or wrinkles were present at the core portion, core cracks were assessed as having occurred.

[0242] [Table 2]

[0243] Figure 8 Based on the CT images of the electrode assembly in Comparative Example 4 and the image showing the occurrence of a core crack in the first electrode, and Figure 9 This is based on CT images of the electrode assembly in Comparative Example 5 and an image showing the occurrence of a core crack in the first electrode. Specifically, Figure 8 (a) and Figure 9 (a) is a CT image of the electrode assembly based on Comparative Example 4 and Comparative Example 5, and Figure 8 (b) and Figure 9 (b) is an exploded image showing cracks appearing in the core portion of the first electrode in the electrode assembly according to Comparative Example 4 and the electrode assembly according to Comparative Example 5. Here, circles represent the core impact region corresponding to the longitudinal end portion of the first electrode and the crack initiation region of the first electrode, respectively. Refer to Table 2 and... Figure 8 (b) and Figure 9 In (b), in the case of the electrode assemblies according to Comparative Examples 4 and 5, it can be seen that in the core portion of the electrode assembly, the flat portion of the first electrode accounts for less than or equal to 3%, so the stress generated by the deformation of the winding core is concentrated on the longitudinal end portion of the first electrode, thereby generating wrinkles or cracks in the core portion.

[0244] If cracks and foreign matter appear in the core portion of the electrode assembly at the first electrode during the winding process, the defect rate may increase and the processing performance may deteriorate. Therefore, by eliminating the introduction of the first electrode, which is expected to crack during the manufacturing process, a decrease in the productivity of the electrode assembly and the secondary battery can be prevented.

[0245] Specifically, when a broken electrode assembly is inserted into the battery case, the likelihood of low voltage and short circuits occurring in the battery case due to foreign matter in the first electrode increases significantly. Therefore, during the winding process, the likelihood of cracks appearing in the first electrode can be reduced by adjusting the proportion of the flat portion of the first electrode. In other words, it can be seen that the electrode assembly and the method for manufacturing the electrode assembly according to an exemplary embodiment of the present invention are suitable for continuous processes using roller-to-roll process equipment in the related art, thereby ensuring the productivity and economic efficiency of the electrode assembly including the first electrode and the secondary battery.

[0246] Refer to the experimental example: Evaluation of electrode sliding range

[0247] Refer to Experiment Example 1

[0248] The secondary battery of Example 1 was prepared and subjected to 50 charge and discharge cycles at 25°C, 1 C charge and 1 C discharge conditions.

[0249] When 200 cycles from 0% state of charge to 100% state of charge were performed under conditions of 4.25 V to 2.5 V and 1 C / 1 C@25°C, the position of the longitudinal end portion of the first electrode at 0% and 100% state of charge was extracted and recorded from CT images, and the sliding range of the longitudinal end portion of the first electrode was measured. Subsequently, the results were evaluated from CT images before activation and CT images at 100% state of charge, and... Figure 11 The sliding range of the longitudinal end portion of the first electrode is shown in the figure.

[0250] Refer to Experiment Example 2

[0251] In addition to using the secondary battery of Example 2, the sliding range of the longitudinal end portion of the first electrode was measured in the same manner as in Reference Experiment Example 1. Subsequently, it was evaluated from CT images prior to activation and CT images at 100% state of charge, and... Figure 11 The sliding range of the longitudinal end portion of the first electrode is shown in the figure.

[0252] Figure 11 This is a CT image showing the sliding range of the longitudinal end portion of the first electrode of the secondary battery according to Reference Experimental Example 1 and Reference Experimental Example 2. Specifically, Figure 11 The images are based on CT images of the secondary batteries before activation and at 100% state of charge, as well as images showing the sliding range of the longitudinal end portion of the first electrode, according to Reference Experimental Example 1 and Reference Experimental Example 2. More specifically, Figure 11 The sliding range is shown based on the second electrode tab located at the outermost part, obtained by comparing the position of the longitudinal end portion of the first electrode before the secondary battery is activated and when the state of charge is 100%.

[0253] Reference Figure 11It was confirmed that a slip of 1.69° (=18.41° (state of charge of 100%) - 16.72° (before activation)) occurred in Reference Experiment Example 1, and a slip of 1.78° (=55.08° (state of charge of 100%) - 53.30° (before activation)) occurred in Reference Experiment Example 2. In other words, based on the measurements from both Reference Experiment Example 1 and Reference Experiment Example 2, it was confirmed that the longitudinal end portion of the first electrode slipped within a range of 1° to 3° relative to its initial position.

[0254] It can be seen that in the secondary batteries according to Examples 1 and 2, the slippage of the first electrode may occur due to the contraction / expansion of the electrode before activation and when it is at 100% charge, but the slippage occurs within a specific range (1° to 3°).

[0255] In other words, the winding process is performed while maintaining a specific or higher tension, so the position of the point of maximum curvature of the first electrode remains constant. Therefore, it can be seen that even when the cycle is in progress, the range of the angle formed between the longitudinal end portion of the first electrode and the point of maximum curvature of the first electrode remains within the sliding range (1° to 3°) of the longitudinal end portion of the first electrode.

[0256] Figure 3 and Figure 4 The images are CT images of the electrode assemblies according to Examples 1 and 2 and an image showing the curvature of the first electrode extracted from the CT images.

[0257] Figures 5 to 7 It is based on CT images of the electrode assemblies of Comparative Examples 1 to 3 and an image showing the curvature of the first electrode extracted from the CT images.

[0258] Specifically, Figures 3 to 7 In grayscale coordinates, this represents the ratio of the curvature of a measurement point that is equal to or less than an arbitrarily set reference value to the curvature measured at each measurement point. Here, the set reference value is a specific value within the range of 0.5 to 1.0, and the grayscale index represented on the right side of the grayscale coordinates ranges from... Figures 3 to 7 In (a), the range is 0 to 0.8. Figures 3 to 7 In (b), the values ​​are 1 to 1.5.

[0259] More specifically, in Figure 3 (a) to Figure 7 In (a), when the grayscale distribution range of the curvature measured at each measurement point is set to 0 to 0.8, the portion of the distribution with low k values ​​is rendered as a darker gray to visually represent the proportion of flat areas and flat parts, and Figure 3 (b) to Figure 7In (b), when the grayscale distribution range of the curvature measured at each measurement point is set to 1 to 1.5, the portion of the distribution with a high k value is rendered as a darker gray to visually represent the point of maximum curvature. In this case, 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 portion of the first electrode located in the core portion is located in the 6 o'clock direction.

[0260] Refer to Table 2 and Figures 3 to 7 , Figure 8 (a) and Figure 9 In (a), with the electrode assembly according to Examples 1 and 2, after 50 cycles at 4.25 V to 2.5 V, 1 C / 1 C@25°C, the percentage of flat portion of the first electrode extracted from the CT image of the core portion was 7.72% and 9.6%, respectively, and it was confirmed that even after 500 cycles at 4.2 V (0.25 C) to 2.85 V (0.33 C)@40°C, no core collapse or core impact occurred.

[0261] On the other hand, as in Comparative Examples 1 to 3, when the proportion of the flat portion of the first electrode extracted from the CT image of the core portion is greater than 13.5%, or as in Comparative Examples 4 and 5, when the proportion of the flat portion of the first electrode extracted from the CT image of the core portion is less than or equal to 3%, that is, when the above-mentioned flat portion proportion range is not met, it is confirmed that the angle formed between the first extension line and the second extension line is greater than or equal to 25°, and a core collision occurs. Or even when a core collision does not occur, a core collapse will occur because the area of ​​the core cavity is less than 100% of the sum of the cross-sectional areas of the wound core used for winding, or the roundness of the core cavity is less than 89%.

[0262] Thus, it can be seen that, according to an exemplary embodiment of the present invention, the proportion of the flat portion of the first electrode extracted from the CT image of the core portion is adjusted to a specific range, making it possible to solve the phenomenon that the core cavity cannot maintain a circular shape and collapses due to the deformation of the electrode assembly caused by the electrode contraction / expansion during the charging and discharging of the battery, thereby preventing damage to the second electrode and the separator, and preventing internal short circuits between the first electrode and the second electrode, thereby improving the stability and life characteristics of the battery.

[0263] The foregoing detailed description is intended to illustrate and explain the present invention. Furthermore, the foregoing description is merely to illustrate and explain preferred embodiments of the invention, and as mentioned above, the invention can be used in various other combinations, variations, and environments, and can be changed and modified within the scope of the concept of the invention disclosed in this specification, within the scope of equivalents to the foregoing disclosure, and / or within the scope of technology or knowledge in the art. Therefore, the foregoing detailed description of the invention is not intended to limit the invention to the disclosed embodiments. Moreover, the appended claims should be construed as including other embodiments as well.

[0264] [Explanation of reference numerals and symbols in the attached figures]

[0265] 100: Second electrode

[0266] 300: First electrode

[0267] 310: The longitudinal end portion of the first electrode

[0268] E1: First extension line

[0269] E2: Second extension line

[0270] A: First winding core section

[0271] B: Second winding core section

Claims

1. An electrode assembly comprising a first electrode, a spacer, and a second electrode stacked and wound around a winding axis. in, The first electrode has a first surface facing the winding axis and a second surface opposite to the first surface. Specifically, in the inner core of the electrode assembly or a portion thereof, the flat portion accounts for more than 3% and less than or equal to 13.5%, where the flat portion percentage is the ratio of the region of the first electrode with a curvature of less than or equal to 1. The core portion is the part of the electrode assembly located within three circles from the inner end of the first electrode.

2. The electrode assembly according to claim 1, in, The first electrode includes a first electrode current collector and a first electrode active material layer. The first electrode active material layer is disposed on at least one surface of the first electrode current collector such that the first electrode active material layer extends to the inner end of the first electrode current collector, and the inner end of the first electrode current collector corresponds to the inner end of the first electrode.

3. The electrode assembly according to claim 2, in, The first electrode includes an uncoated portion, in which no active material layer of the first electrode is disposed. The electrode assembly further includes a first electrode tab that is physically connected to or formed on the uncoated portion of the first electrode.

4. The electrode assembly according to claim 1, in, The angle between a line extending from the winding axis through the inner end of the first electrode and a line extending from the winding axis through a point on the first electrode with a curvature of less than or equal to 1 is greater than 0°.

5. The electrode assembly according to claim 1, in, After the electrode assembly is activated, the proportion of the flat portion of the first electrode is determined.

6. The electrode assembly according to claim 1, in, After 50 charge and discharge cycles at 25°C, 1C charge and 1C discharge conditions, the percentage of the flat portion of the first electrode is determined.

7. The electrode assembly according to claim 1, in, The angle formed between a line extending from the winding axis and passing through the inner end of the first electrode and a line extending from the winding axis and passing through the point of maximum curvature of the first electrode is greater than 40° and less than or equal to 98°.

8. The electrode assembly according to claim 7, in, The point of maximum curvature is the point of maximum curvature of the first electrode in a region of the cross-section of the electrode assembly perpendicular to the winding axis, the region extending from the inner end of the first electrode in an azimuth angle greater than 0° and less than or equal to 180° in a direction opposite to the winding direction of the electrode assembly.

9. The electrode assembly according to claim 1, in, The roundness of the first electrode is greater than or equal to 89% in or at the core portion.

10. The electrode assembly according to claim 1, in, The first extension line is drawn by extending a straight line from two points on the first surface connected to the first electrode whose curvature direction changes within a distance of 5 mm from the inner end of the first electrode. The second extension line is drawn by extending a straight line from two points within a distance of 5 mm from the inner end of the first electrode on the surface of the first surface facing the first electrode that connects the second electrode. Wherein, the first extension line and the second extension line form an angle of less than or equal to 25°.

11. The electrode assembly according to claim 1, wherein, The first electrode does not have cracks or wrinkles in or at the core portion of the electrode assembly.

12. A secondary battery, comprising: The electrode assembly according to any one of claims 1 to 11.

13. The secondary battery according to claim 12 further includes a battery case, particularly a cylindrical battery case, wherein the electrode assembly is housed in the battery case.

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