Battery cell activation device and battery cell activation method using the same

Abstract

According to an exemplary embodiment of the present invention, there is provided a battery cell activation device, the device comprising: a drive unit including a drive plate and a drive rod configured to move the drive plate in a first direction; and a support unit including a support plate and an elastic element coupled to the support plate, the support plate being spaced apart from the drive plate in the first direction and including a protrusion protruding in the first direction.

Description

[Technical Field]

[0001] The present invention relates to a battery cell activation device and a method for activating a battery cell using the same. This application claims the benefit of Korean Application No. 10-2023-0176240, filed on December 7, 2023, which is incorporated herein by reference in its entirety. [Background technology]

[0002] Unlike primary batteries, secondary batteries can be charged and discharged multiple times. Secondary batteries are widely used as energy sources for various wireless devices such as handsets, laptops, and wireless vacuum cleaners. In recent years, improvements in energy density and economies of scale have dramatically reduced the manufacturing cost per unit capacity of secondary batteries, and as the driving range of battery electric vehicles (BEVs) has increased to the same level as fuel-powered vehicles, the main use of secondary batteries has shifted from mobile devices to mobility.

[0003] The manufacturing of secondary batteries includes an electrode process including mixing, coating, roll pressing, slitting, and notching, an assembly process in which the electrode assembly is housed in a case, and an activation process in which the battery cells are electrically activated and stabilized. After the activation process, the battery cells can be stacked to form a cell stack. The cell stack can be mounted in a housing with a module frame or directly in the housing without a module frame. Summary of the Invention [Problem to be solved by the invention]

[0004] SUMMARY OF THE INVENTION An object of the present invention is to provide a battery cell activation device and a battery cell activation method using the same, which have improved reliability. [Means for solving the problem]

[0005] According to an exemplary embodiment of the present invention to solve the above-mentioned problems, there is provided a battery cell activation device, the device including: a drive unit including a drive plate and a drive rod configured to move the drive plate in a first direction; and a support unit including a support plate and an elastic element connected to the support plate, the support plate including a plurality of pressure plates spaced apart from the drive plate in the first direction, interposed between the drive unit and the support plate, and configured to apply pressure to a plurality of battery cells, the support plate including a protrusion protruding in the first direction.

[0006] The height of each of the protrusions in the first direction is in the range of 0.1 mm to 1.0 mm.

[0007] The height of each of the protrusions in the first direction is 0.6 mm or less.

[0008] Each of the protrusions comprises the same material as the support plate.

[0009] Each of the protrusions comprises a different material than the support plate.

[0010] The protrusion is spaced from the center of the support plate.

[0011] The protrusions are spaced apart from one another in a second direction parallel to the support plate, and the protrusions are spaced apart from a center of the support plate in the second direction.

[0012] The distance between the end of the support plate in the second direction and the protrusion is different from the distance between the protrusion and the center of the support plate in the second direction.

[0013] The distance between the end of the support plate in the second direction and the protrusion is smaller than the distance between the protrusion and the center of the support plate in the second direction.

[0014] Each of the pressure plates has a flat plate shape.

[0015] Each of the pressure plates does not include a protrusion.

[0016] Each of the pressure plates has a different surface geometry than the support plate.

[0017] According to an exemplary embodiment, there is provided a method for activating a secondary battery, the method including the steps of loading a plurality of pressure-sensitive papers and a plurality of battery cells into a battery cell activation device, performing an activation process on the plurality of battery cells, and evaluating the activation process of the plurality of battery cells based on the pressure-sensitive papers, wherein the battery cell activation device includes a drive unit including a drive plate and a drive rod configured to move the drive plate in a first direction, and a support unit including a support plate and an elastic element connected to the support plate, the support plate being spaced apart from the drive plate in the first direction and including a protrusion protruding in the first direction.

[0018] The activation process is evaluated based on the standard deviation of the pressure sensed by the pressure sensitive paper. [Effects of the Invention]

[0019] A battery cell activation device according to an exemplary embodiment of the present invention includes a support plate having a protrusion, which allows the battery cells to be evenly pressurized during an activation process, thereby improving the reliability of secondary battery manufacturing.

[0020] The effects that can be obtained from the exemplary embodiments of the present disclosure are not limited to the effects mentioned above, and other effects not mentioned can be clearly derived and understood from the following description by a person having ordinary skill in the art to which the exemplary embodiments of the present disclosure belong. In other words, unintended effects accompanying the implementation of the exemplary embodiments of the present disclosure can also be derived from the exemplary embodiments of the present disclosure by a person having ordinary skill in the art. [Brief explanation of the drawings]

[0021] [Figure 1] FIG. 1 is a plan view of a battery cell activation device according to an exemplary embodiment. [Figure 2] 1 is a partial plan view of a battery cell activation device according to an exemplary embodiment; [Figure 3] 1 is a flowchart illustrating a method for manufacturing a secondary battery according to an exemplary embodiment. [Figure 4] 1 illustrates pressure-sensitive paper according to an exemplary embodiment. [Figure 5] 1 illustrates pressure-sensitive paper according to an exemplary embodiment. DETAILED DESCRIPTION OF THE INVENTION

[0022] Hereinafter, preferred embodiments of the present invention will be described in detail with reference to the accompanying drawings. As a premise, the terms and words used in this specification and claims should not be interpreted as being limited to their ordinary or dictionary meanings, but should be interpreted as meanings and concepts that are consistent with the technical idea of ​​the present invention, based on the principle that an inventor can appropriately define the concept of terms in order to best describe his / her own invention.

[0023] Therefore, the embodiments described in this specification and the configurations shown in the drawings are merely the most preferred embodiments of the present invention and do not represent the entire technical idea of ​​the present invention, and there may be various equivalents and modifications that can replace them at the time of this application.

[0024] Furthermore, in the description of the present invention, if it is determined that a specific description of related publicly known configurations or functions may obscure the gist of the present invention, the detailed description will be omitted.

[0025] The embodiments of the present invention are provided to more completely explain the present invention to those skilled in the art, and therefore the shapes and sizes of components in the drawings may be exaggerated, omitted, or illustrated schematically for clearer explanation. Therefore, the sizes and proportions of each component do not completely reflect the actual sizes and proportions.

[0026] (First embodiment) FIG. 1 is a perspective view of a battery cell activation device 100 according to an exemplary embodiment.

[0027] Referring to FIG. 1, a battery cell activation device 100 may include a driving part 110, a supporting part 120, and a plurality of pressure plates .

[0028] According to an exemplary embodiment, the battery cell activation device 100 may be configured to perform an activation process of the battery cell BC, which may include repeating aging, charging, and discharging of the battery cell BC.

[0029] Repeated charging and discharging causes the electrolyte to decompose, forming an SEI (Solid Electrolyte Interphase) film on the surface of the negative electrode. SEI is a thin film that forms on the surface of the negative electrode material when the battery cell BC is charged for the first time after manufacturing. When the battery cell BC is charged, lithium ions in the battery cell BC move to the negative electrode, and in the process, the substances in the electrolyte are electrolyzed for the first time, resulting in the formation of an SEI film on the surface of the negative electrode material due to a chemical reaction that occurs. SEI can be a type of separator. SEI can prevent further decomposition of the electrolyte when lithium ions move from the positive electrode to the negative electrode to charge the battery.

[0030] In the aging process, the battery cell BC may be stored at room temperature for a predetermined time (for example, 30 minutes to 3 hours) to stabilize the charged or discharged battery cell BC. The key to the aging process is to evenly distribute the electrolyte within the pouch cell so that the electrolyte is evenly absorbed into both the positive and negative electrodes. The aging process can improve the mobility of lithium ions and the uniformity (e.g., thickness uniformity) of the SEI film.

[0031] The activation process of the battery cell BC may include degassing to remove gas generated inside the battery during aging and charging. Gas may be generated inside the battery cell BC during the charging / discharging and aging processes. In the degassing process, the gas inside the battery cell BC may be removed.

[0032] While the battery cell BC is being charged and discharged, the battery cell BC may be pressurized by a pressure plate. Uniform pressurization of the battery cell BC may prevent gas trapping and lithium plating, and may improve the uniformity of the SEI film. The battery cell BC including the uniform SEI film may have a relatively short charge or discharge time.

[0033] The battery cell BC is a basic unit of a lithium ion battery, i.e., a secondary battery. The battery cell BC may be pouch-shaped. The battery cell BC may include a pouch case PC and an electrode assembly housed in the pouch case PC. The electrode assembly housed in the pouch case PC includes a positive electrode, a negative electrode, and a separator interposed between the positive electrode and the negative electrode. The positive electrode may include a positive electrode tab connected to one of the electrode leads EL, and the negative electrode may include a negative electrode tab connected to one of the electrode leads EL.

[0034] A pouch case PC can be provided by performing a forming process and a sealing process on a pouch film. The pouch film can include an inner resin layer, a metal layer, and an outer resin layer. The inner resin layer can have thermal adhesive properties, which allows the pouch film to be sealed. The inner resin layer can include, for example, a polyolefin-based material. The metal layer can include any one of an alloy of iron, carbon, chromium, and manganese, an alloy of iron, chromium, and nickel, and aluminum.

[0035] The electrode assembly may be either a jelly roll type or a stack type, but is not limited thereto. A jelly roll type electrode assembly includes a wound positive electrode, a wound negative electrode, and a separator interposed therebetween. A stack type electrode assembly includes a plurality of sequentially stacked positive electrodes, a plurality of wound negative electrodes, and a plurality of separators interposed therebetween. The battery cell BC in FIG. 1 may be a workpiece that has undergone a pouch forming process and an electrolyte injection process but has not yet undergone an activation process.

[0036] The positive electrode may include a positive electrode current collector and a positive electrode active material, and the negative electrode may include a negative electrode current collector and a negative electrode active material.

[0037] The thickness of the positive electrode current collector may range from about 3 μm to about 500 μm. The positive electrode current collector may not induce chemical changes in the final secondary battery and may have high conductivity. The positive electrode current collector may include, for example, stainless steel, nickel, titanium, calcined carbon, and aluminum. The positive electrode current collector may also include stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The surface of the positive electrode current collector may include a micro-textured structure to enhance the adhesive strength of the active material. The positive electrode current collector may have the shape of a film, sheet, foil, net, porous material, foam, nonwoven fabric, or the like.

[0038] The thickness of the negative electrode current collector may range from about 3 μm to about 500 μm. The negative electrode current collector may not induce chemical changes in the final secondary battery and may have high conductivity. The negative electrode current collector may include copper, stainless steel, aluminum, nickel, titanium, calcined carbon, and aluminum-cadmium alloy. The negative electrode current collector may also include stainless steel surface-treated with carbon, nickel, titanium, silver, or the like. The surface of the negative electrode current collector may include a micro-textured structure to enhance the adhesion of the active material. The negative electrode current collector may have the shape of a film, sheet, foil, net, porous material, foam, nonwoven fabric, or the like.

[0039] The positive electrode active material is a material capable of undergoing an electrochemical reaction. The positive electrode active material can be a lithium transition metal oxide. Examples of the positive electrode active material include layered compounds such as lithium cobalt oxide (LiCoO2) and lithium nickel oxide (LiNiO2) substituted with one or more transition metals; lithium manganese oxide substituted with one or more transition metals; and lithium manganese oxide with the chemical formula LiNi 1-y M y Lithium nickel-based oxide represented by O2 (where M is any one of Co, Mn, Al, Cu, Fe, Mg, B, Cr, Zn, and Ga, and 0.01≦y≦0.7); Li 1+z Ni 1 / 3 Co 1 / 3 Mn 1 / 3 O2, Li 1+z Ni 0.4 Mn 0.4 Co 0.2 Li like O2 1+z Ni b Mn c Co 1-(b+c+d) M d O (2-e) A e(Here, -0.5 ≤ z ≤ 0.5, 0.1 ≤ b ≤ 0.8, 0.1 ≤ c ≤ 0.8, 0 ≤ d ≤ 0.2, 0 ≤ e ≤ 0.2, b + c + d < 1, M is any one of Al, Mg, Cr, Ti, Si, and Y, and A is any one of F, P, and Cl) lithium nickel cobalt manganese composite oxide; chemical formula Li 1+x M 1-y M' y PO 4-z X z (Here, M is a transition metal, more specifically, any one of Fe, Mn, Co, and Ni, M' is any one of Al, Mg, and Ti, X is any one of F, S, and N, -0.5 ≤ x ≤ +0.5, 0 ≤ y ≤ 0.5, 0 ≤ z ≤ 0.1) olivine-type lithium metal phosphate can be included.

[0040] The negative electrode active material can include carbon such as graphitizable carbon and graphite-based carbon. For example, the negative electrode active material can be Li x Fe2O3 (0 ≤ x ≤ 1), Li x WO2 (0 ≤ x ≤ 1), Sn x Me 1-x Me' y O z (Here, Me is any one of Mn, Fe, Pb, and Ge, Me' is any one of Al, B, P, Si, Group 1, Group 2, Group 3 elements of the periodic table, and halogen; 0 < x ≤ 1; 1 ≤ y ≤ 3; 1 ≤ z ≤ 8), etc. metal composite oxides can be included. The negative electrode active material can include, for example, lithium metal; lithium alloy; silicon-based alloy; tin-based alloy. The negative electrode active material can include, for example, metal oxides such as SnO, SnO2, PbO, PbO2, Pb2O3, Pb3O4, Sb2O3, Sb2O4, Sb2O5, GeO, GeO2, Bi2O3, Bi2O4, Bi2O5. The negative electrode active material can include, for example, conductive polymers such as polyacetylene; Li-Co-Ni-based materials, etc.

[0041] According to an exemplary embodiment, the drive unit 110 may include a drive plate 111 and a drive rod 116. The drive rod 116 may be configured to transmit an external drive force to the drive plate 111. The drive rod 116 may be coupled to, for example, a hydraulic cylinder or a linear servo motor. The drive plate 111 may have a generally flat plate shape. The drive plate 111 may be substantially perpendicular to the X direction. The drive plate 111 may be substantially parallel to the Y direction. The drive rod 116 may be configured to move the drive plate 111 in the X direction.

[0042] The support portion 120 may include a support plate 121 and an elastic element 126. The support plate 121 may be spaced apart from the drive plate 111 in the X direction. The support plate 121 may be substantially parallel to the drive plate 111. The support plate 121 may be substantially parallel to the Y direction. The support plate 121 may be substantially perpendicular to the X direction.

[0043] The elastic elements 126 may be coupled to the second surface 121S2 of the support plate 121. The axis of the elastic force of the elastic elements 126 may be substantially parallel to the X direction. That is, each of the elastic elements 126 may apply an X-direction elastic force proportional to the displacement in the X direction to the support plate 121. Each of the elastic elements 126 may apply pressure of an appropriate strength to the plurality of battery cells BC. The elastic elements 126 may be directly or indirectly connected to a sensor such as a load cell, and the pressure applied to the battery cells BC may be controlled based on the elastic force of the elastic elements 126.

[0044] The support plate 121 may include steps 121P. Each of the steps 121P may be spaced apart from the center of the support plate 121 (more specifically, the center in the Y direction). Each of the steps 121P may be located at an edge portion of the support plate 121. Each of the steps 121P may be adjacent to an end of the support plate 121 in the Y direction rather than the center of the support plate 121 in the Y direction.

[0045] The distance between the edge of the support plate 121 in the Y direction and the step 121P may be different from the distance between the center of the support plate 121 in the Y direction and the step 121P. The distance between the edge of the support plate 121 in the Y direction and the step 121P may be smaller than the distance between the center of the support plate 121 in the Y direction and the step 121P.

[0046] The distance between the step 121P and one of the Y-direction ends of the support plate 121 that is adjacent to the step 121P may be different from the distance between the step 121P and the center of the Y-direction of the support plate 121. The distance between the step 121P and one of the Y-direction ends of the support plate 121 that is adjacent to the step 121P may be smaller than the distance between the step 121P and the center of the Y-direction of the support plate 121.

[0047] The step 121P may protrude in the X direction from the first surface 121S1 of the support plate 121. The first surface 121S1 of the support plate 121 may face the driving plate 111. The first surface 121S1 of the support plate 121 may face the pressure plate 130. Thus, the step 121P may be configured to pressurize an edge portion of the battery cell BC interposed between the support plate 121 and the pressure plate 130 (i.e., the battery cell BC in contact with the pressure plate 130).

[0048] According to an exemplary embodiment, each of the steps 121P can include the same material as the support plate 121. For example, each of the steps 121P can be a continuous element integral with the support plate 121. For example, each of the steps 121P can include aluminum. In another example, the steps 121P can include a material having a lower hardness compared to the support plate 121. They can also be silicone pads. For example, each of the steps 121P can include silicone.

[0049] During the activation process, uniform pressure on the battery cells BC is one of the key factors for performance management of the battery cells BC because uniform pressure on the battery cells BC suppresses lithium plating. The pressure applied to the battery cells BC adjacent to the support plate 121 may be relatively non-uniform compared to the battery cells BC adjacent to the driving plate 111. Therefore, the pressure applied to the edge portions of the battery cells BC by the support plate 121 may be smaller than the pressure applied to the center portions of the battery cells BC by the support plate 121. According to an exemplary embodiment, the step 121P protruding from the edge of the support plate 121 allows the battery cells BC in contact with the support plate 121 to be uniformly pressurized, thereby improving the reliability of secondary battery manufacturing.

[0050] According to an exemplary embodiment, the height 122H of each step 121P may be in the range of approximately 0.1 mm to approximately 1 cm. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.2 mm or more. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.3 mm or more. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.4 mm or more. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.9 mm or less. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.8 mm or less. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.7 mm or less. According to an exemplary embodiment, the height 122H of each step 121P may be approximately 0.6 mm or less.

[0051] Here, the Y direction may be an extension direction of the pouch case PC of the battery cell BC. The main surface of the pouch case PC of the battery cell BC may be substantially parallel to the Y direction and substantially perpendicular to the X direction. The electrode leads EL of the battery cell BC may be spaced apart in the Y direction, but are not limited thereto.

[0052] According to an exemplary embodiment, the pressure plates 130 may be interposed between the drive plate 111 and the support plate 121. Each of the pressure plates 130 may be substantially parallel to the drive plate 111. Each of the pressure plates 130 may be substantially perpendicular to the X direction.

[0053] Each of the pressure plates 130 may be coupled to a shaft extending in the X direction, such that the pressure plates 130 may be moved along the shaft by driving the driving plate 111. A plurality of battery cells BC may be inserted between the pressure plates 130, and the plurality of battery cells BC may be compressed by the pressure plates 130, the driving plates 111, and the support plate 121.

[0054] Each of the pressure plates 130 may have a flat plate shape. This may eliminate any steps. Each of the pressure plates 130 may have two main surfaces different from the first surface 121S1 of the support plate 121.

[0055] The battery cell activation device 100 may further include a charging terminal configured to be electrically connected to the electrode leads EL of the plurality of battery cells BC. The charging terminal may be in contact with the electrode leads EL of the plurality of battery cells BC. The charging terminal may be configured to transfer an external power source to the plurality of battery cells BC.

[0056] (Second embodiment) FIG. 3 is a flowchart illustrating a method for manufacturing a secondary battery according to an exemplary embodiment.

[0057] FIG. 4 illustrates pressure sensitive paper according to an exemplary embodiment.

[0058] FIG. 5 illustrates pressure sensitive paper according to an exemplary embodiment.

[0059] 3 to 5, at P110, a plurality of pressure-sensitive sheets PSS and battery cells BC can be loaded into the battery cell activation device 100. The loading of the battery cells BC can be performed by a pick-and-place machine.

[0060] Pressure-sensitive paper PSS can contain microcapsules containing dyes. When pressure is applied to the pressure-sensitive paper PSS, the pressure can release the dyes contained in the capsules of the pressure-sensitive paper PSS, and the reaction between the dye and the color developer on the surface of the pressure-sensitive paper PSS allows the pressure-sensitive paper PSS to display characters or colors. As shown in Figures 4 and 5, the pressure-sensitive paper PSS can simultaneously display a color and a character indicating the applied pressure, or can display only one of the color and the character indicating the applied pressure.

[0061] Subsequently, the plurality of battery cells BC may undergo an activation process in P120, which may include multiple charge / discharge and aging processes as described above.

[0062] Next, in P130, the activation process of the plurality of battery cells BC can be evaluated. The activation process of the plurality of battery cells BC can be evaluated based on the pressure-sensitive paper PSS. While the plurality of battery cells BC are being processed, the pressure-sensitive paper PSS can be pressed together with the plurality of battery cells BC. As a result, after the activation process is completed, each pressure-sensitive paper PSS can represent the pressure applied to the surface of a corresponding one of the plurality of battery cells BC. For example, the pressure-sensitive paper PSS of FIG. 4 shows a more uniform pressure distribution than the pressure-sensitive paper PSS of FIG. 5, so the processing of the battery cell BC corresponding to the pressure-sensitive paper PSS of FIG. 4 is preferable to the processing of the battery cell BC corresponding to the pressure-sensitive paper PSS of FIG. 5.

[0063] According to an exemplary embodiment, the evaluation of the activation process may include evaluating the uniform pressurization of the plurality of battery cells BC. According to an exemplary embodiment, the evaluation of the activation process may be based on the standard deviation of the readings of the pressure-sensitive paper PSS. According to an exemplary embodiment, the evaluation of the activation process may include comparing the standard deviation of the readings of the pressure-sensitive paper PSS with a critical standard deviation. For example, if the standard deviation of the readings of the pressure-sensitive paper PSS is equal to or less than the critical standard deviation, the battery cells BC obtained as a result of the activation process may be determined to be good. For example, if the standard deviation of the readings of the pressure-sensitive paper PSS is greater than the critical standard deviation, the battery cells BC obtained as a result of the activation process may be determined to be bad.

[0064] Based on what is described herein, a person skilled in the art can easily arrive at an embodiment in which pressure-sensitive paper PSS is provided only on the battery cells BC between the support plate 121 and the pressure plate 130.

[0065] The present invention has been described in more detail above with reference to the drawings and embodiments, etc. However, the configurations shown in the drawings or embodiments in this specification are merely one embodiment of the present invention and do not represent all of the technical ideas of the present invention, and therefore, there may be various equivalents and modifications that can replace them at the time of filing this application.

Claims

1. a drive portion including a drive plate and a drive rod configured to move the drive plate in a first direction; a support portion including a support plate and an elastic element connected to the support plate, the support plate being spaced apart from the drive plate in the first direction; a plurality of pressure plates interposed between the driving portion and the support plate and configured to apply pressure to a plurality of battery cells; The support plate includes a protrusion protruding in the first direction.

2. 2. The battery cell activation device of claim 1, wherein the height of each of the protrusions in the first direction is in the range of 0.1 mm to 1.0 mm.

3. The battery cell activation device according to claim 1 , wherein the height of each of the protrusions in the first direction is 0.6 mm or less.

4. The battery cell activation device of claim 1 , wherein each of the protrusions comprises the same material as the support plate.

5. The battery cell activation device of claim 1 , wherein each of the protrusions comprises a different material than the support plate.

6. The battery cell activation device of claim 1 , wherein the protrusion is spaced from a center of the support plate.

7. the protrusions are spaced apart from one another in a second direction parallel to the support plate; The battery cell activation device of claim 1 , wherein the protrusion is spaced apart from the center of the support plate in the second direction.

8. 8. The battery cell activation device according to claim 7, wherein a distance between an end of the support plate in the second direction and the protrusion is different from a distance between the protrusion and the center of the support plate in the second direction.

9. 8. The battery cell activation device according to claim 7, wherein a distance between an end of the support plate in the second direction and the protrusion is smaller than a distance between the protrusion and the center of the support plate in the second direction.

10. The battery cell activation device according to claim 1 , wherein each of the pressure plates has a flat plate shape.

11. The battery cell activation device of claim 1 , wherein each of the pressure plates does not include a protrusion.

12. 10. The battery cell activation device of claim 1, wherein each of the pressure plates has a different surface shape than the support plate.

13. loading a plurality of pressure-sensitive papers and a plurality of battery cells into a battery cell activation device; performing an activation process on the plurality of battery cells; evaluating the activation process of the plurality of battery cells based on the pressure-sensitive paper; The battery cell activation device includes a driving unit including a driving plate and a driving rod configured to move the driving plate in a first direction, and a support unit including a support plate and an elastic element connected to the support plate, the support plate being spaced apart from the driving plate in the first direction; The support plate includes a protrusion protruding in the first direction.

14. The method for activating a secondary battery according to claim 13, wherein the activation process is evaluated based on a standard deviation of the pressure sensed by the pressure-sensitive paper.

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