Patent Publication Number: US-2022216455-A1

Title: Electrode for Secondary Battery and Method of Manufacturing Electrode for Secondary Battery

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a national stage entry under 35 U.S.C. § 371 of International Application No. PCT/KR2020/015433 filed on Nov. 5, 2020, which claims priority from Korean Patent Application No. 10-2019-0160898 filed on Dec. 5, 2019 and Korean Patent Application No. 10-2020-0143369 filed on Oct. 30, 2020, all the disclosures of which are incorporated herein by reference. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to an electrode for a secondary battery and a method of manufacturing the electrode for a secondary battery, and more particularly, to a rolling device and a rolling method for a secondary battery that enhances a resistance against stress of non-coated portion. 
     BACKGROUND ART 
     The secondary battery can be formed by inserting an electrode assembly composed of a positive electrode plate, a negative electrode plate, and a separator into a case, and then sealing the can. A positive electrode plate or a negative electrode plate (hereinafter, referred to as “electrode plate”) can be configured by coating an active material slurry onto a positive conductive current collector or a negative conductive current collector to a predetermined thickness, interposing a separator between the positive electrode conductive current collector and the negative electrode conductive current collector, and winding the plate in a jelly-roll type many times or laminating it in a plurality of layers to form an electrode assembly. 
     The electrode plate may be formed of an active material coating layer coated with an active material slurry and a non-coated portion. The active material coating layer can include a roll process for increasing the adhesiveness to the electrode current collector and increasing the volume density of the active material. The rolled electrode plate can, after drying, be used by being passed through a cutter having a certain width and cut into a predetermined size. 
     The roll process has a problem that a compression deviation occurs due to a difference in thickness between the coating layer and the non-coated portion at the time of rolling the electrode plate. Due to such a deviation, unbalanced plastic deformation of the electrode current collector may occur, thereby causing a residual stress. In particular, the tensile residual stress may cause a reduction of fatigue durability and a reduction of fracture strength of components. 
       FIG. 1  is a schematic diagram showing a roll process using a conventional rolling device.  FIG. 2  is a plan view showing an electrode plate after rolling. 
     Referring to  FIG. 1 , a roll process of rolling a coated layer  30  and a non-coated portion  40  formed on an electrode current collector  20  by a rolling roll  10  may be performed. At this time, the pressure is concentrated on the coated layer  30 , and as shown in  FIG. 2 , a difference occurs between the degree of stretching of the coated portion  30 P and the degree of stretching of the non-coated portion  40 , and wrinkles may be generated in the non-coated portion  40 . Due to the wrinkles of the non-coated portion  40  generated during rolling, process defects such as electrode disconnection may occur in a subsequent process. In particular, while a high tensile residual stress remains at a boundary surface between the coated portion  30 P and the non-coated portion  40 , they can continuously receive weak stress due to the contraction and expansion of the electrode, and may become vulnerable to fracture. 
     DETAILED DESCRIPTION OF THE INVENTION 
     Technical Problem 
     It is an object of the present disclosure to provide an electrode for secondary batteries that increase a resistance against stress of the non-coated portion, and a method of manufacturing an electrode for a secondary battery. 
     However, the problem to be solved by embodiments of the present disclosure is not limited to the above-described problems, and can be variously expanded within the scope of the technical idea included in the present disclosure. 
     Technical Solution 
     According to one embodiment of the present disclosure, there is provided a method of manufacturing an electrode for a secondary battery, comprising the steps of: coating an active material onto an electrode current collector to form a coated portion and an uncoated portion, and applying a compressive residual stress to the surface of the uncoated portion, wherein the step of applying the compressive residual stress to the surface of the uncoated portion comprises a step of performing a peening process on the surface of the uncoated portion. 
     The step of applying the compressive residual stress to the surface of the uncoated portion is performed before a roll process performed along the moving direction of the electrode current collector, and the roll process may include at least one of a process of rolling a coated portion and an uncoated portion of the electrode current collector and a process of notching a coated portion and an uncoated portion of the electrode current collector. 
     The step of applying the compressive residual stress to the surface of the uncoated portion may include a step of performing a shot peening process. 
     The step of performing a shot peening process may include a step of forming a dimple portion on the surface of the uncoated portion. 
     A plastic deformation layer is formed on the surface of the dimple portion, so that the plastic deformation layer has a residual compressive stress, and the inside of the uncoated portion located outside the plastic deformation layer has a tensile residual stress. 
     The step of applying the compressive residual stress to the surface of the uncoated portion may include a step of performing an ultrasonic peening process. 
     The step of performing an ultrasonic peening process may include a step of forming a dimple portion on the surface of the uncoated portion. 
     The method of manufacturing an electrode for a secondary battery may further include a step of heat-treating the electrode current collector, after the step of performing a peening process on the surface of the uncoated portion. 
     According to another embodiment of the present disclosure, there is provided an electrode for a secondary battery comprising: an electrode current collector including a coated portion and an uncoated portion, and an active material layer located on the coated portion of the electrode current collector, wherein a dimple portion is formed on the surface of the uncoated portion, and a plastic deformation layer is formed on the surface of the dimple portion. 
     The electrode for a secondary battery further includes a plastic deformation layer located on the surface of the dimple portion, wherein the plastic deformation layer may have a compressive residual stress, and the inside of the uncoated portion located outside the plastic deformation layer has a tensile residual stress. 
     Advantageous Effects 
     According to the embodiments of the present disclosure, by applying a compressive residual stress to the surface of an uncoated portion before electrode rolling and/or notching process, the fatigue durability and fracture strength of the material can be improved and the resistance against stress can be enhanced. Therefore, it is possible to reduce the occurrence of fracture due to fatigue fracture at the boundary surface between the electrode and the uncoated portion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic diagram showing a roll process using a conventional rolling device. 
         FIG. 2  is a plan view showing an electrode plate after rolling. 
         FIG. 3  is a perspective view showing a rolling device according to one embodiment of the present disclosure. 
         FIG. 4  is a diagram schematically showing a state in which the rolling device of  FIG. 3  is viewed from the front. 
         FIG. 5  is a schematic diagram showing a notching device according to one embodiment of the present disclosure. 
         FIG. 6  is a schematic cross-sectional view showing a peening process of  FIGS. 4 and 5 . 
         FIG. 7  is a perspective view showing a shot peening process according to an embodiment of the present disclosure. 
         FIG. 8  is a cross-sectional view showing a state in which a dimple portion is formed by shot peening of  FIG. 7 . 
         FIG. 9  is a schematic cross-sectional view showing an aging process according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, various embodiments of the present disclosure will be described in detail with reference to the accompanying drawings so that those skilled in the art can easily implement them. The present disclosure may be modified in various different ways, and is not limited to the embodiments set forth herein. 
     Portions that are irrelevant to the description will be omitted to clearly describe the present disclosure, and like reference numerals designate like elements throughout the specification. 
     Further, in the figures, the size and thickness of each element are arbitrarily illustrated for convenience of description, and the present disclosure is not necessarily limited to those illustrated in the figures. In the figures, the thickness of layers, regions, etc. are exaggerated for clarity. In the figures, for convenience of description, the thicknesses of some layers and regions are shown to be exaggerated. 
     In addition, it will be understood that when an element such as a layer, film, region, or plate is referred to as being “on” or “above” another element, it can be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” another element, it means that other intervening elements are not present. Further, the word “on” or “above” means disposed on or below a reference portion, and does not necessarily mean being disposed on the upper end of the reference portion toward the opposite direction of gravity. 
     Further, throughout the specification, when a portion is referred to as “including” a certain component, it means that it can further include other components, without excluding the other components, unless otherwise stated. 
     Further, throughout the specification, when referred to as “planar”, it means when a target portion is viewed from the upper side, and when referred to as “cross-sectional”, it means when a target portion is viewed from the side of a cross section cut vertically. 
       FIG. 3  is a perspective view showing a rolling device according to one embodiment of the present disclosure. 
     Referring to  FIG. 3 , a method of manufacturing an electrode for a secondary battery according to one embodiment of the present disclosure includes the steps of: coating an active material onto an electrode current collector  300  to form a coated portion  400  and an uncoated portion  500 , and applying a compressive residual stress to the surface of the uncoated portion  500 . At this time, the step of applying the compressive residual stress to the surface of the uncoated portion  500  includes a step of performing a peening process on the surface of the uncoated portion  500 . When performing a peening process on the surface of the uncoated portion  500 , it is desirable to minimize a physical deformation on the surface of the uncoated portion  500 . That is, a compressive residual stress must be applied to the surface of the uncoated portion  500  by an action such as quenching through a peening process, which is different from applying an elongation force. According to this embodiment, there is no physical change on the surface of the uncoated portion  500  through the peening process, or even if there is a physical change, the change may occur uniformly. 
     The step of applying a compressive residual stress to the surface of the uncoated portion  500  is performed before a roll process performed along the moving direction of the electrode current collector  300 , and the roll process may include at least one of a process of rolling a coated portion  400  and an uncoated portion  500  of the electrode current collector  300  and a process of notching a coated portion  400  and an uncoated portion  500  of the electrode current collector  300 . 
       FIG. 4  is a diagram schematically showing a state in which the rolling device of  FIG. 3  is viewed from the front.  FIG. 5  is a schematic diagram showing a notching device according to one embodiment of the present disclosure. 
     Referring to  FIGS. 3 and 4 , the electrode rolling device  100  according to the embodiment of the present disclosure includes a first roller  101  which unwinds an electrode plate  250  having a coated portion  400  on which a coating material is formed on the electrode current collector  300  and an uncoated portion  500  corresponding to a non-coated portion, a second roller  102  which winds the electrode plate  250 , and a rolling roll  109  which is located between the first roller  101  and the second roller  102  and rolls the coated portion  400  and the uncoated portion  500  of the electrode plate  250  along the moving direction of the electrode plate  250 . The uncoated portion  500  may refer to a region excluding the coated portion  400  formed on the electrode current collector  300 . 
     The first roller  101  provides the electrode plate  250  to be rolled to the rolling device  100 , and moves the electrode plate  250  in a direction of arrow D 1  of  FIG. 4  in accordance with the clockwise rotation. The electrode plate  250  unwound by the first roller  101  passes between the rolling rolls  109  while moving along the direction of the arrow. The rolling rolls  109  are located on both sides with respect to the electrode plate  250 , respectively, and the electrode plate  250  that has passed between the two rolling rolls  109  is pressed. After that, the electrode plate  250  that has passed between the two rolling rolls  109  is rewound on the second roller  102 . 
     The method of manufacturing an electrode for a secondary battery according to the embodiment of the present disclosure includes a step of performing a peening process, before the electrode plate  250  having the coated portion  400  and the uncoated portion  500  is unwinded and then rolled by the rolling roll  109 . The peening process may be performed by a peening device PN located between the first roller  101  and the rolling roll  109  in the electrode rolling device  100  according to this embodiment. The peening process may apply a compressive residual stress to the uncoated portion  500  according to this embodiment. 
     Referring to  FIG. 5 , in a modified embodiment, the method of manufacturing an electrode for a secondary battery according to the embodiment of the present disclosure includes a step of performing a shot peening process, before the electrode plate  250  having the coated portion  400  and the uncoated portion  500  is unwinded and then notched by an upper mold  210  and a lower mold  220  for notching. The peening process may be performed by a peening device PN located between the first roller  201  and the notching molds  210  and  220  in the electrode notching device  200  according to the present embodiment. 
       FIG. 6  is a schematic cross-sectional view showing a peening process of  FIGS. 4 and 5 .  FIG. 7  is a perspective view showing a shot peening process according to an embodiment of the present disclosure.  FIG. 8  is a cross-sectional view showing a state in which a dimple portion is formed by shot peening of  FIG. 7 . 
     Referring to  FIG. 6 , the step of applying a compressive residual stress to the surface of the uncoated portion  500  according to the present embodiment may include a step of performing a shot peening process or a step of performing an ultrasonic peening process. In the following, the step of performing the shot peening process will be mainly described. 
     Referring to  FIGS. 6 to 8 , a shot ball  600  may be thrown onto the surface of the uncoated portion  500  at high speed to hammer the surface of the electrode current collector  300 . The shot ball  600  may be formed of a steel ball such as stainless steel. Specifically, in the shot peening process, the shot ball  600  collides with the surface of the uncoated portion  500  at high speed, and a kinetic energy of the shot ball  600  instantaneously causes a plastic deformation on the surface of the material, and the shot ball  600  separates from the surface. At this time, the uncoated portion  500  is formed with a dimple portion DP having an indented shape. According to this embodiment, a thin plastic deformation layer PDL is formed on the surface of the dimple portion DP, and the plastic deformation layer PDL may have a compressive residual stress. In contrast, the inside of the uncoated portion  500  located outside the plastic deformation layer PDL may have a tensile residual stress. Thus, a force to maintain the stretched surface in a state before stretching is applied to the plastic deformation layer PDL according to this embodiment, and a compressive residual stress and tensile residual stress are formed on the surface and inside of the uncoated portion  500  on which the dimple portion DP is formed, respectively, thereby achieving equilibrium. By leaving a compressive residual stress on the surface of the uncoated portion  500  by this shot peening process, the compressive residual stress is gradually canceled when repetitive tension is applied, and the fatigue life can be extended until the compressive residual stress disappears. 
     The compressive residual stress and tensile residual stress described above can be interpreted as cosine values of the residual stress measured using a residual stress tester. 
     Referring again to  FIG. 6 , in a modified embodiment, the step of applying a compressive residual stress to the surface of the uncoated portion  500  may include a step of performing an ultrasonic peening process. In the ultrasonic peening process, an ultrasonic device  700  is disposed on the upper end of the uncoated part  500 , so that ultrasonic waves collide with the surface of the uncoated part  500  at high speed, thereby causing plastic deformation. Specifically, in the ultrasonic peening process, the compressive residual stress can be controlled while ultrasonic energy is transmitted to the surface of the uncoated portion  500 , and high and low cycle fatigue can be improved. The ultrasonic peening process may be performed while controlling the altitude of the ultrasonic device  700  shown in  FIG. 6 . Through such an altitude control, the surface roughness of the uncoated portion  500  may be controlled. 
     The step of performing an ultrasonic peening process may replace the step of performing the above-described shot peening process. Even in the step of performing the ultrasonic peening process, a dimple portion may be formed on the surface of the uncoated portion  500 , and a plastic deformation layer having a compressive residual stress is formed on the surface of the dimple portion, and the inside of the uncoated portion located outside the plastic deformation layer may have a tensile residual stress. In the ultrasonic peening process, the physical deformation on the surface of the uncoated portion  500  may be relatively small compared to the above-described shot peening. 
       FIG. 9  is a schematic cross-sectional view showing an aging process according to an embodiment of the present disclosure. 
     Referring to  FIG. 9 , the method of manufacturing an electrode for a secondary battery according to the embodiment of the present disclosure may include a step of heat-treating the electrode current collector  300  after performing a peening process on the surface of the uncoated portion  500 . In the step of heat-treating the electrode current collector  300 , a heat treatment apparatus  800  is disposed on the upper end of the uncoated part  500  so that ultrasonic waves collide with the surface of the uncoated part  500  at high speed, thereby causing plastic deformation. When aging such as heat treatment is performed in addition to the above-mentioned peening process, a compressive residual stress may be further generated on the surface of the electrode current collector  300 . Accordingly, the level at which unbalanced plastic deformation occurs due to stress during rolling can be alleviated, and the level of tensile residual stress generated on the surface can be significantly alleviated. 
     According to the embodiment of the present disclosure, by applying a compressive residual stress of the electrode current collector  300  through a peening process, the electrode current collector  300  can be heat-treated in a state in which the fracture strength and fatigue durability are enhanced, thereby manufacturing an electrode for a secondary battery in which the internal grain of the electrode current collector  300  is stabilized. At this time, the heat treatment temperature can be set to a temperature and time within a range in which a large change in the physical properties of the material does not occur. 
     Although the preferred embodiments of the present disclosure have been described in detail above, the scope of the present disclosure is not limited thereto, and various modifications and improvements of those skilled in the art using the basic concepts of the present disclosure defined in the following claims also belong to the scope of rights. 
     DESCRIPTION OF REFERENCE NUMERALS 
       300 : electrode current collector 
       400 : coated portion 
       500 : uncoated portion 
       600 : shot ball 
     DP: dimple portion 
     PDL: plastic deformation layer